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1:250 000 tobermorey sf 53-12 - Northern Territory Government
NATIONAL GEOSCIENCE MAPPING ACCORD 1:250 000 Geological Map Series Explanatory Notes 2nd Edition TOBERMOREY SF 53-12 Northern Territory Geological Survey NORTHERN TERRITORY DEPARTMENT OF BUSINESS, INDUSTRY AND RESOURCE DEVELOPMENT NORTHERN TERRITORY GEOLOGICAL SURVEY DEPARTMENT OF INDUSTRY, TOURISM AND RESOURCES GEOSCIENCE AUSTRALIA SF 53-07 SF 53-08 SF 54-05 ELKEDRA SANDOVER RIVER URANDANGI SF 53-11 HUCKITTA 6253 ALGAMBA 6353 ALKEA 6453 TOBERMOREY 6352 MARQUA Sheet SF 53-12 SF 54-09 TOBERMOREY SF 53-12 6252 TARLTON TOBERMOREY, Northern Territory (Second Edition) GLENORMISTON 1:250 000 GEOLOGICAL MAP SERIES EXPLANATORY NOTES 6452 TOKO SF 54-13 MOUNT WHELAN NT SF 53-16 HAY RIVER PD KRUSE, AT BRAKEL, JN DUNSTER, and ML DUFFETT QLD SF 53-15 ILLOGWA CREEK Government Printer of the Northern Territory Darwin, October 2002 NORTHERN TERRITORY DEPARTMENT OF BUSINESS, INDUSTRY AND RESOURCE DEVELOPMENT MINISTER: Hon Paul Henderson, MLA CHIEF EXECUTIVE OFFICER: Peter Blake NORTHERN TERRITORY GEOLOGICAL SURVEY DIRECTOR: Dr R Dennis Gee BIBLIOGRAPHIC REFERENCE: Kruse PD, Brakel AT1, Dunster JN, and Duffett ML, 2002. Tobermorey, Northern Territory (Second Edition). 1:250 000 geological map series explanatory notes, SF 53-12. Northern Territory Geological Survey, Darwin and Geoscience Australia, Canberra (National Geoscience Mapping Accord). (1:250 000 geological map series, ISSN 0814-7485) Bibliography ISBN 0 7245 7049 7 559.429 KEY WORDS: Geological mapping, Geophysical interpretation, Structural geology, Economic geology, Sedimentary geology, Stratigraphy, Northern Territory, Tobermorey, Palaeoproterozoic, Neoproterozoic, Cambrian, Ordovician, Jurassic, Cretaceous, Cenozoic, Arunta Province, Georgina Basin, Tarlton Fault, Marqua Monocline, Toomba Fault Zone, Mopunga Group, Keepera Group, Toko Group, Igneous intrusions. For further information contact: Reference Geologist Northern Territory Geological Survey GPO Box 3000 Darwin NT 0801 Phone: +61 8 8999 5281 Web site: http://www.dbird.nt.gov.au/ntgs 1 Formerly Australian Geological Survey Organisation (Geoscience Australia) © Northern Territory Government 2002 Printed for the Northern Territory Geological Survey by the Government Printer of the Northern Territory Disclaimer This information is provided on the understanding that the user agrees to release and indemnify the Northern Territory, the Commonwealth of Australia, companies who supplied and acquired the data, and their employees, agents and contractors, in respect of all liability for actions, claims, costs, expenses, loss, damage or injury, which may be suffered by them, or any other persons, arising from the use of the data, or as a consequence of any unlawful or negligent act or omission of the user. ii Abstract TOBERMOREY1 includes portions of the Georgina Basin and Arunta Province. Outcrop of the Narwietooma Package of the Arunta Province is restricted to the southwestern corner and southern margin of TOBERMOREY. In the southwest, >1820 Ma migmatite (p_Cd) is widespread. Four recognised granite bodies intrude unit p_Cd. The Your Dam Metamorphics outcrop in the south-central map area; these are thought to have been metamorphosed in the period 1780-1720 Ma. Scattered Mount Tietkens Granite Complex intrudes near the southern sheet boundary. Limited geochronological data indicate an age of 1753 ± 7 Ma for one exposed granite. Granites penetrated in drillholes have yielded U-Pb SHRIMP zircon ages of 1846 ± 6 Ma, 1763 ± 4 Ma and 1749 ± 8 Ma. Unmetamorphosed Georgina Basin sediments cover the bulk of the map area. Except near major faults, they are little deformed. A Neoproterozoic succession commences with deposits of both major Cryogenian glaciations; the Yardida Tillite and Black Stump Arkose correlate, respectively, with lower (Sturtian) and upper (Marinoan) glacial deposits of the Adelaide Rift. The dominantly marine Wonnadinna Dolostone, Gnallan-a-Gea Arkose, Elyuah Formation and Grant Bluff Formation succeed these units. These sediments were deposited initially in northwest-trending grabens controlled by major faults, such as the Tarlton Fault and Marqua Monocline-Toomba Fault Zone. Cambrian deposition in TOBERMOREY commenced in the mid-Early Cambrian with the Red Heart Dolostone, a marine unit bearing calcimicrobial-archaeocyathan patch reefs. Two depositional sequences have been recognised in the Middle Cambrian. Sequence 1 is represented by the platformal marine Thorntonia Limestone. Sequence 2, which spans the remainder of the Middle Cambrian, is primarily Arthur Creek Formation. This unit includes a basal interval of anoxic, basinal pyriticcarbonaceous black shale, which constitutes a major potential petroleum source rock. The upper Arthur Creek Formation is of more typical aerobic carbonate deposited in normal marine waters. Quartzic dolostone and quartz sandstone of the Steamboat Sandstone complete sequence 2. The Late Cambrian Arrinthrunga Formation is primarily peritidal. Cambro-Ordovician deposits consist of two principal depofacies: peritidally influenced, platformal marine carbonate rocks of the Ninmaroo Formation in the east, interfingering with terrigenous quartz-glauconite sandstone of the marine Tomahawk Formation in the west. The corridor of interfingering falls within TOBERMOREY. A dominantly marine Early to Middle (to possibly Late) Ordovician succession is preserved in the Tarlton and Toko Ranges. From bottom to top, it consists of the Kelly Creek Formation, Coolibah Formation, Nora Formation, Carlo Sandstone, Mithaka Formation and Ethabuka Sandstone. This last formation may represent the initial pulse (Rodingan Movement) of the Alice Springs Orogeny. The Early-Middle Devonian Cravens Peak beds may denote a second, mid-Devonian pulse (Pertnjara-Brewer Movements) of the Alice Springs Orogeny. The Orogeny culminated in the Carboniferous and was responsible for much of the present structure in the region. Mesozoic (Jurassic-Cretaceous) quartz sandstone formerly blanketed the region, but is now confined to the southern map area as outliers of the Eromanga Basin. Continental Cenozoic deposits indicate pedogenic and lacustrine (Austral Downs Limestone) and fluviatile (Poodyea Formation) environments. The southern Georgina Basin is strongly prospective for petroleum, with excellent potential source rock and seal in the lower Arthur Creek Formation juxtaposed against potential reservoir dolostone in the underlying Thorntonia Limestone. The structurally complex southern basin margin has potential for Pb-Zn mineralisation and numerous occurrences have been identified. Diamonds are being actively sought in the region. In the Arunta Province, malachite has been recognised in the Mount Dobbie Granite and L Pgc granite. 1 Names of 1:250 000 and 1:100 000 mapsheets are shown in large and small capital letters, respectively, eg TOBERMOREY, Tarlton. Note: A change in the name of the 1:250 000 mapsheet from Tobermory to Tobermorey was agreed by the Northern Territory and Auslig in 2002 following a query from Auslig. The incorrectly spelled First Edition mapsheet name was derived from the homestead name, which has always been known as Tobermorey (since being established in 1913). The name on the Second Edition mapsheet (released early 2003) was changed to agree with the feature name, but the hard-copy version of these notes was released prior to notification of the name change and used the original spelling. The revised name has been used throughout this 2008 CD ROM version, which is otherwise unchanged from the original 2002 text. iii CONTENTS Geophysics ....................................................................... 38 Gravity....................................................................... 38 Magnetics................................................................... 39 Radiometrics.............................................................. 40 Downhole geophysics................................................ 40 Geological history .......................................................... 40 Economic geology . ......................................................... 43 Georgina Basin............................................................... 43 Petroleum................................................................... 43 Phosphate................................................................... 44 Diamonds................................................................... 44 Copper-lead-zinc........................................................ 45 Platinum group elements............................................ 46 Gold............................................................................ 47 Uranium..................................................................... 47 Manganese................................................................. 47 Groundwater.............................................................. 47 Arunta Province............................................................. 47 Copper-lead-zinc........................................................ 47 Gold............................................................................ 48 Tungsten..................................................................... 48 Diamonds................................................................... 48 Uranium..................................................................... 48 Other metals and non-metals..................................... 48 Acknowledgments............................................................ 48 References......................................................................... 48 Appendix - revised stratigraphic units.............................. 56 Abstract ........................................................................... iii Introduction........................................................................ 1 Terminology and classification.................................... 2 Location and access..................................................... 2 Climate......................................................................... 2 Physiography................................................................ 2 Vegetation.................................................................... 2 Previous investigations................................................ 2 Regional geological setting................................................ 3 Arunta Province................................................................. 4 Metamorphic rocks.......................................................... 4 Migmatite unit (p–Cd)................................................... 5 Your Dam Metamorphics (p–Cy).................................. 6 Intrusive rocks.................................................................. 6 Mount Tietkens Granite Complex (L Pg)........................ 6 Granite unit (L Pga)......................................................... 7 Granite unit (L Pgb)........................................................ 7 Granite unit (L Pgc)......................................................... 8 Granite unit (L Pgd)........................................................ 8 Quartz veins................................................................. 8 Georgina Basin stratigraphy............................................. 9 Neoproterozoic..................................................................... 9 Yardida Tillite (L Put)....................................................11 Keepera Group................................................................11 Black Stump Arkose (L Pus)..........................................11 Wonnadinna Dolostone (L Puw)................................... 12 Mopunga Group............................................................. 12 Gnallan-a-Gea Arkose (L Pun)...................................... 12 Elyuah Formation (L Pue)............................................. 12 Grant Bluff Formation (L Pua)...................................... 13 Cambrian............................................................................ 14 Red Heart Dolostone (–Cld)........................................ 14 Thorntonia Limestone (–Cmt)..................................... 15 Arthur Creek Formation (–Cma)................................. 17 Steamboat Sandstone (–Cms)...................................... 18 Arrinthrunga Formation (–Cua)................................... 19 Eurowie Sandstone Member (–Cue)............................ 20 Cambro-Ordovician........................................................... 20 Tomahawk Formation (–COt)...................................... 20 Ninmaroo Formation (–COn)...................................... 24 Ordovician.......................................................................... 27 Kelly Creek Formation (Olk)..................................... 27 Toko Group.................................................................... 29 Coolibah Formation (Olc).......................................... 29 Nora Formation (Oln)................................................ 31 Carlo Sandstone (Omc).............................................. 32 Mithaka Formation (Omm)........................................ 33 Ungrouped...................................................................... 33 Ethabuka Sandstone (Ome)........................................ 33 Devonian............................................................................ 34 Cravens Peak beds (Dc)............................................. 34 Mesozoic............................................................................ 34 Undifferentiated Jurassic-Cretaceous (JK)................ 34 Palaeogene-Neogene.......................................................... 35 Austral Downs Limestone (Cza)................................ 35 Poodyea Formation (Czp).......................................... 36 Palaeogene-Neogene-Quaternary...................................... 36 Quaternary.......................................................................... 37 Structure and tectonics ................................................... 37 FIGURES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 iv Location of TOBERMOREY...................................... 1 Regional geological setting......................................... 3 Simplified solid geology.............................................. 4 Lithostratigraphic column........................................... 5 Arunta Province terrane of southwestern TOBERMOREY.......................................................... 6 Migmatite unit p–Cd leucocratic phase........................ 6 Migmatite unit p–Cd melanocratic phase..................... 7 Granite unit L Pga: pale grey to pink granite.................. 7 Granite unit L Pgb: granite of leucocratic phase ........... 8 Granite unit L Pgb: melanocratic phase quartz diorite........................................................................... 8 Granite unit L Pgc: coarse leucogranite.......................... 9 Granite unit L Pgd: pale grey granodiorite..................... 9 Wonnadinna Dolostone: quartz-lithic intraclast dolopackstone to dolograinstone............................... 12 Grant Bluff Formation measured section.................. 13 Synaeresis cracks in Grant Bluff Formation............... 14 Lower Thorntonia Limestone: bioclast dolowackestone......................................................... 15 Medial Thorntonia Limestone: bioclast coquina....... 16 Medial-upper Thorntonia Limestone erosive contact.......16 Lower Arthur Creek Formation: pyriticcarbonaceous dololaminite........................................ 17 Upper Arthur Creek Formation: interbedded microsparstone and mudstone................................... 17 Outcrop of upper Arthur Creek Formation................ 18 Arrinthrunga Formation: stromatolite boundstone................................................................. 20 Cross-bed foreset azimuths in Tomahawk Formation and Ninmaroo Formation......................... 21 40 41 42 43 44 45 46 47 Carlo Sandstone: arthropod scratch marks................ 33 Mithaka Formation: Rusophycus............................... 34 Austral Downs Limestone: silicified limestone.................................................................... 35 Austral Downs Limestone: pedogenic alpha fabric .........................................................................36 Austral Downs Limestone: pedogenic beta fabric .........................................................................36 Schematic lithologs and downhole geophysical logs of TOBERMOREY drillholes....................... 42-43 Microdiamond and chromite grain locations in TOBERMOREY........................................................ 45 Lower Thorntonia Limestone: galena and sphalerite in vug fill................................................... 46 48 Medial Thorntonia Limestone: sphalerite and platy organic matter in vug fill.................................... 46 24 Tomahawk Formation: sublitharenite........................ 22 25 Tomahawk Formation partial measured section........ 23 26 Tomahawk Formation: deformation in quartz sandstone................................................................... 23 27 Tomahawk Formation-Kelly Creek Formation measured section....................................................... 24 28 Ninmaroo Formation: stromatolite boundstone and bioclast-lithoclast grainstone.............................. 25 29 Ninmaroo Formation: arthropod track in quartz sandstone........................................................ 25 30 Ninmaroo Formation: synaeresis cracks in dolomudstone............................................................ 26 31 Ninmaroo Formation: domical stromatolites............ 26 32 Ninmaroo Formation: ooid grainstone...................... 27 33 Measured section through Kelly Creek Formation, Nora Formation and Carlo Sandstone....................... 28 34 Kelly Creek Formation: granule conglomerate......... 30 35 Kelly Creek Formation: Cruziana and Rusophycus................................................................ 30 36 Kelly Creek Formation: ?Gyrolithes......................... 30 37 Kelly Creek Formation: quartzic dolosparstone........ 31 38 Coolibah Formation: intraclast-bioclast grainstone-rudstone................................................... 31 39 Nora Formation: basal ferruginous echinoderm grainstone-rudstone................................................... 32 TABLES 1 2 3 Pre-Quaternary lithostratigraphy of Georgina and Eromanga Basins........................................... 10-11 Rock densities of lithostratigraphic units.................. 39 TOBERMOREY drillhole locations and available downhole geophysical data....................................... 41 Introduction PD Kruse wrote the bulk of these explanatory notes and acted as compiler. AT Brakel contributed to CambroOrdovician stratigraphy and Structure and tectonics from his field experience. JN Dunster wrote most of Economic geology, although PD Kruse prepared the section on Petroleum and contributed to Phosphate and Diamonds. ML Duffett wrote Geophysics other than the Downhole geophysics section. Groundwater was prepared using material contributed by Bob Read (Natural Resources Division, Department of Infrastructure, Planning and Environment, Alice Springs). Cited locations in this publication are based on Map Grid of Australia zone 53 coordinates and the GDA94 map datum, and are deemed accurate to ± 50 m. Within the text, the Universal Grid Reference style is used as explained on the mapface (eg Niko Hill at MGA 745000E 7500000N is represented as QR450000). Full grid references are provided in tables. To convert from the AGD66 AMGs used on older maps, block shift all data 127 m to the east and 170 m to the north. During Second Edition field mapping of TOBERMOREY, the United States Department of Defence ceased selective availability of its Global Positioning System (GPS) signal at midnight on 1 May 2000. In principle, civil users can now position themselves with a hand-held GPS receiver to within ± 10 m or better in ideal conditions (Crettenden 2000). First Edition TOBERMOREY explanatory notes (Smith 1965a) and map (published 1966) were the result of fieldwork in 1958-1959 by the then Bureau of Mineral Resources (BMR)2, supplemented by stratigraphic drilling in 1962 (Milligan 1963) and 1964 (Smith 1972, Questa 1994). The Geophysical Branch of BMR contributed road and helicopter gravity surveys in 1957-1961 (Barlow 1965, 1966), and an aeromagnetic survey of the map area in 19631964 (Wells et al 1966). The present Second Edition map incorporates portions of the existing BMR 1:100 000 Toko Preliminary Edition (Simpson et al 1979) and BMR-Geological Survey of Queensland (GSQ) 1:250 000 Hay River-Mount Whelan First Edition Special maps (Simpson et al 1985). Together, these account for one quarter of TOBERMOREY, including all of Toko and the southern half of Marqua. The remaining area was collaboratively remapped in 19992000 by Northern Territory Geological Survey (NTGS) geologists PD Kruse (Tarlton, Tobermorey, northern Marqua and eastern Alkea) and JN Dunster (Arunta Province in southwestern Tarlton), and AGSO geologist AT Brakel (Algamba and western Alkea). Colour aerial photographs at 1:50 000 scale provided the field base for this remapping. 136˚30' 138˚00' e Cr AY AD By RGI bb y G HI GEO Georgina Downs HW NA ANGI Ammaroo URAND A T IS UN MO Urandangi Argadargada SAN 139˚30' 21˚00' O Annitowa Mount Guide ek R 135˚00' 21˚00' DOV RIVER ER DOV SAN BUNDE Y 22˚00' ER Walgra Ooratippra Arapunya W IGH NO HU HW AY Marqua Jervois Glenormiston a SIMPSON DESERT k ee Cr ER RIV Indiana Y HA TY EN u Marq PL Atula ER RIV 0 25 50 75 100 km Lake Caroline 136˚30' Major road Northern Territory Queensland 23˚00' 24˚00' 135˚00' Roxborough Downs E Tarlton Downs TY EN PL R Linda Downs DO H 22˚00' VE HIG Baikal Huckitta AY Lucy Creek Carandotta RI Manners Creek Tobermorey Derry Downs 23˚00' Herbert Downs m02-054.dgn 24˚00' 139˚30' 138˚00' Minor road Drainage Homestead Town, settlement Figure 1 Location of TOBERMOREY 2 Subsequently renamed Australian Geological Survey Organisation (AGSO), now Geoscience Australia (GA). in the northeast characteristically form low to moderate hills on the sand plains. There is an overall easterly fall in this northern area, so that the lowest elevation is around Tobermorey homestead. Flat sand plains also mask Arunta Province rocks in southwestern TOBERMOREY. Drainage in the southwest is centred on the Hay River, which flows south-southeastward, parallel to the prevailing seif dune trend of the Simpson Desert. Drainage from the southern flank of the Toko Range enters the Field River, which likewise debouches into the Simpson Desert. Runoff from the northern flank of the Toko Range and adjacent areas to the north drains into the Georgina River system and ultimately into channel country in southwestern Queensland. However, field experience during mapping was that the last two digits in both easting and northing could be unstable, and so quoted values for these digits in the present notes should not be relied upon. Terminology and classification Grainsize terminology follows Wentworth (1922). Sandstones are classified according to Folk (1974). Carbonate rock classification follows the revision of Wright (1992). The term quartzic, often used herein with regard to dolostones, indicates a significant but not dominant admixed detrital quartz component. Location and access Vegetation TOBERMOREY is bounded by latitudes 22°00’S and 23°00’S, and longitudes 136°30’E and 138°00’E (Figure 1). This is a remote and sparsely populated area, with small Aboriginal communities at Warlpeyangrere and Urlampe, and station homesteads at Tarlton Downs, Marqua, Tobermorey and Manners Creek. Beef cattle grazing is the principal land use. The unsealed east-west Plenty Highway traverses the central map area and links Alice Springs, some 500 km to the west with Urandangi, Boulia and other centres in western Queensland to the east. Station tracks provide reasonable access, although the northwestern quadrant is virtually devoid of tracks and bores. Four-wheel drive access to the more rugged parts of the Toko Range can be difficult. Fuel and a campground are available at Tobermorey homestead. Fuel may also be obtained en route from Alice Springs at Jervois homestead. Elsewhere in the region, it is available at Alpurrurulam on the Sandover Highway (weekdays only), and at Urandangi, Boulia, Mt Isa and Camooweal in Queensland. Two broad vegetation styles are recognised in TOBERMOREY (AUSLIG 1990). In both, the tallest stratum represents less than 10% of the total vegetation. In the area west of the Tarlton Range, the northwestern map area and the central Toko Range, tall open Acacia shrubland is developed on more sandy soils. Shrubs over 2 m high constitute the tallest stratum. These are dominantly Acacia, Eucalyptus, and some Hakea and Grevillea. Spinifex (especially Triodia pungens) is a common cover. The remainder of the map area is of low open Acacia woodland. The tallest woodland stratum is typically less than 10 m high, of mixed desert acacias, eucalypts (such as Eucalyptus argillacea and E. terminalis) and, on more calcareous soils, gidgee (Acacia georginae). E. microtheca is characteristic of floodplains. The understorey is mainly of Acacia, Cassia and Carissa shrubs, or grasses such as Astrebla (Mitchell grass) and Dichanthium. Previous investigations Climate Early geological investigations of the Georgina Basin, including TOBERMOREY, were summarised by Whitehouse (1936), who also commented on lithostratigraphy and expanded on an earlier initial biozonation (Whitehouse 1931). CT Madigan in 1935 was perhaps the first geologist to traverse TOBERMOREY (Madigan 1937). Hossfeld (1954), in reviewing the geology of the Northern Territory, noted the presence of Ordovician rocks and fossils in the Tarlton and Toko Ranges. Noakes and Traves (1954) reviewed the geology of the Barkly region, of which TOBERMOREY is a part, and Noakes (1956) and Sprigg (1963) also dealt briefly with local geology. JN Casey (BMR) was apparently the first to record Cambrian fossils from the sheet area, while accompanying a CSIRO field party in 1954 (Casey and Gilbert-Tomlinson 1956). The Georgina Basin was specifically surveyed by the then BMR and GSQ during 1957-1965. Within this programme, TOBERMOREY was systematically mapped in 1958-1959, resulting in the First Edition geological map (Smith and Vine 1960, Smith et al 1961, Smith 1965a). Smith (1972) summarised work in the Georgina Basin up to that date. Concurrently, petroleum companies conducted initial investigations in the region from the late 1950s. Mining The climate is arid with median annual rainfall of 150-200 mm. Most falls in the summer months of October to March, but totals can vary greatly from year to year. Summers are hot, with mean maximum and minimum January temperatures of 36‑39°C and 21‑24°C, respectively. Winters are mild with mean maximum and minimum July temperatures of 21‑24°C and 5‑7°C respectively (Bureau of Meteorology 1988). The winter months of April to September are the most congenial for fieldwork. Physiography Tarlton Range in the southwest and Toko Range in the southeast of the map area are prominent plateau ranges consisting primarily of Ordovician rocks. Carbonate rocks in the upper Tomahawk Formation outcrop as prominent plateaux rimming the northern Tarlton Range. Lesser uplands are present in south-central TOBERMOREY, eg the Umberumbera Hills comprising the Ninmaroo Formation and Mesozoic rocks. The northern half of the map area is dominated by sand plains, particularly in the northwestern quadrant where the Tomahawk Formation is the source of much colluvial sand. This unit and the Ninmaroo Formation companies commenced metalliferous exploration in the early 1970s, and diamond exploration in the 1980s (see Economic geology). The BMR pursued a fresh Georgina Basin project during the later 1970s and early 1980s (Druce 1974), including further drilling, selective remapping and directed research programmes. thought to be dominated by large mafic-intermediate intrusive bodies and younger non-magnetic granitoids. This region of thickened, stable crust with relatively low heat flow and low crustal temperatures underlies much of TOBERMOREY, SANDOVER RIVER, eastern HUCKITTA and northern HAY RIVER. In contrast with the Arunta Province to the south, the Altjawarra Domain resisted Proterozoic and Palaeozoic deformation and was only mildly reactivated during the Alice Springs Orogeny. The adjacent, much younger Irindina Package occupies most of ILLOGWA CREEK and southern HUCKITTA and, from aeromagnetic interpretation (Ahmad 2002), extends eastward into west-central HAY RIVER, and thence further into southeastern HAY RIVER (Figure 2). Buick et al (2001a, 2001b) obtained SHRIMP U-Pb zircon ages from Irindina Package rocks on ALCOOTA and ALICE SPRINGS that suggest deposition in the late Neoproterozoic to early Cambrian. This package underwent granulite facies metamorphism during the Larapinta Event (Hand et al 1999) at 480-460 Ma (late Early and Middle Ordovician; Pietsch 2001, Scrimgeour and Raith 2001). Regional geological setting TOBERMOREY incorporates a significant portion of the southern Georgina Basin, as well as a small area of exposed basement metamorphic rocks and granites assigned to the Arunta Province (Figures 2, 3). This is consistent with extrapolation of the interpretation of Pietsch (2001) that indicates basement continuity with the >1820 Ma Arunta Province Narwietooma Package. The remaining basement in the northern two thirds of TOBERMOREY is regarded as being part of the subsurface Altjawarra craton of Myers et al (1996), referred to herein as the Altjawarra Domain. This domain is based on geophysical interpretations by Myers et al (1996) and Teasdale and Pryer (2002), who recognised a structurally distinct basement package 136˚30' 135˚00' 21˚00' 138˚00' 139˚30' 21˚00 ' Cz C2 Lg P -- Cz Ll P C3 -Lu P Ll P C3 -C2 -- Lu P Lg P C2 -- Lu P Cz Lg P Lg P C3 -- Lg P 22˚00' 22˚00' Lg P CO -CO -- C3 -- D C3 -Lu P C2 -- Lg P Mz Cz O Lg P O O -Mz C3 -Lg P CO D C2 -- Cz C2 -- C1 CO D C3 O Lg P Ll P O Lu P Lu P LC P -- Mz D Lg P Mz Ll P Lu P C1 -- -- Lu P Lg P CO Lg P Ll P Lg P Lg P 23˚00 ' -- LC P Lg P -- Lg P C3 -- Lg P -- Ll P -- 23˚00' O C2 -- Lg P -- Lg P Ll P C3 Mz C1 -- 24˚00' 135˚00' ARUNTA PROVINCE, DAVENPORT PROVINCE, MOUNT ISA INLIER m02-063.dgn 24˚00' 139˚30 ' 138˚00' 136˚30' GEORGINA BASIN P LC -- Neoproterozoic-Cambrian metamorphic rocks C2 -- Middle Cambrian O P Lg Palaeoproterozoic granite C1 -- Lower Cambrian P Ll Palaeoproterozoic metamorphic rocks P Lu Neoproterozoic Ordovician Cz Cenozoic CO -- Cambro-Ordovician Mz Mesozoic C3 Upper Cambrian D Devonian -- Figure 2 Regional geological setting of TOBERMOREY 136˚30’ 22˚00’ 138˚00’ 22˚00’ Cza C On -- Cza Cza Cza C Ot -- C On -- C Ot -- Cza C Ot -- C On Cza -- Olk C Ot -- Omc Oln Olk Olk C On -- P L ua Cza -- C On Cza Olc Olk Olc p C d? Olc Oln -- Omc C ua -p Cy -- JK P Lg L us JK P P L un -P L uw C ma JK P L us P Lg C ma -- -- P L gc C ua C mt -- JK JK Olk C ms -- -- P L gd Oln JK C On JK Omc P L ga JK -- Olk C Ot -- P L ua C ua -- p Cd C ua JK Olc -- P L gb JK C Ot -- P L un P L uw Oln Olc Omm Omc Oln Czp Omc P L ut JK Omc P L us JK P L uw 23˚00’ 136˚30’ Czp Ome Dc Omm 23˚00’ m02-074.dgn 138˚00’ ARUNTA PROVINCE GEORGINA BASIN P L gd granite C ma -- Arthur Creek Formation Oln Nora Formation Czp Poodyea Formation P L gc granite C mt -- Thorntonia Limestone Olc Coolibah Formation Cza Austral Downs Limestone P L gb granite P L ua Grant Bluff Formation Olk Kelly Creek Formation JK undifferentiated Jurassic-Cretaceous P L ga granite P L un Gnallan-a-Gea Arkose C On -- Ninmaroo Formation Dc Cravens Peak beds P Lg Mount Tietkens Granite Complex P L uw Wonnadinna Dolostone C Ot -- Tomahawk Formation Ome Ethabuka Sandstone p Cy Your Dam Metamorphics P L us Black Stump Arkose C ua -- Arrinthrunga Formation Omm Mithaka Formation p Cd migmatite P L ut Yardida Tillite C ms Steamboat Sandstone Omc Carlo Sandstone -- -- -- Figure 3 Simplified solid geology of TOBERMOREY Arunta Province The southern margin of the Georgina Basin incorporates a principally marine stratigraphic succession ranging from mid-Neoproterozoic to Middle Ordovician, as well as Devonian, Jurassic-Cretaceous and Cenozoic rock units (Figure 4). This margin was deformed during multiple tectonic events, of which the Alice Springs Orogeny was most influential. Two prominent synclines, the Dulcie Syncline in HUCKITTA-ELKEDRAALCOOTA-BARROW CREEK and the Toko Syncline in TOBERMOREY-HAY RIVER-GLENORMISTONMOUNT WHELAN, preserve the Ordovician-Devonian succession. Major northwest-trending faults traverse the region. Northward, the central Georgina Basin has been little affected by tectonism, and flat-lying Middle Cambrian sediments overlie Early Cambrian volcanic rocks. Metamorphic rocks In the mid-1970s, the Arunta Province was subdivided into three principal tectonic terranes (northern, central and southern), and its inferred lithostratigraphy grouped into three major divisions, each of distinctive lithological and metamorphic facies character (Shaw and Stewart 1975, Stewart and Warren 1977). More recent studies (Zhao and Bennett 1995, Collins and Shaw 1995, Hand et al 1999) have highlighted significant problems with these lithostratigraphic subdivisions, primarily on the basis of new geochronological data. In particular, it has been shown that stratigraphic correlation or subdivision cannot be made on the basis of metamorphic grade and deformation alone. Pietsch (2001) thereby proposed a stratotectonic subdivision of the province based on protolith assemblages, relationships, similar tectonic history and geochronology. His analysis did not extend east of 136°30’E, but it appears from current gravity and aeromagnetic data that his Narwietooma Package embraces all of the exposed Arunta Province in TOBERMOREY and extends southward into northwestern HAY RIVER (unit A5 of Ahmad 2000). This package comprises >1820 Ma rocks of mafic and felsic composition intercalated with pelitic and psammitic metasediments, the whole of which was metamorphosed to granulite facies in the period 1780‑1720 Ma. In adjacent HUCKITTA (at least), coeval 1780‑1760 Ma granites (Jervois Granite, Xanten Granite; unit g6 of Ahmad 2000) occur within the Narwietooma Package. Migmatite unit (p –Cd) This unnamed unit extends into the southwestern corner of TOBERMOREY from southeastern HUCKITTA, where it was first recognised, and continues southward into northernmost HAY RIVER. First Edition mapping interpreted it as possibly Archaean, but it is now regarded as more likely to be Palaeoproterozoic (Freeman 1986). It is bounded on the east by the Tarlton Fault and is therefore a component of the Jervois Block of Warren (1981: 5). Freeman (1986) described it as quartzofeldspathic gneiss, grading into biotite gneiss, with migmatite as concordant and discordant leucosomes in both. Feldspathic quartzite, schistose muscovite-bearing quartzite, schistose quartzrich metasediment, biotite schist, layered magnetitequartz rock, calc-silicate rock and megacrystic granitic gneiss are minor constituents in HUCKITTA. As in HUCKITTA, most outcrops in TOBERMOREY are isolated exposures on extensive sand plains. The best outcrop is adjacent to the Tarlton Fault around PQ820680 to the south of Canyon Bore (Figure 5), where contiguous outcrop of leucocratic migmatite bears local rafts of black schistose amphibolite. Leucocratic rocks include deformed granodiorite and feldspar-quartz gneiss. In thin section, granodiorite from PQ617828 comprises a plagioclase−quartz−K‑feldspar mosaic ranging from coarsely to finely granular, with interstitial chlorite (at least some after biotite) and minor sericite. Plagioclase shows deformed twin lamellae and incipient sericitisation. Quartz shows strained extinction, especially in coarser crystals. Disseminated apatite and opaques are also present. Plagioclase is much more sericitised in a feldsparquartz gneiss from PQ568726 (Figure 6), in which it is intergrown with aggregates of quartz and microcline, with associated flaky biotite and muscovite. It forms relatively large crystals, with ragged margins, which are altered to sericite/clay and epidote with minor carbonate. Quartz, associated with microcline, constitutes a recrystallised, equigranular medium-grained mosaic with unit, undulose or semicomposite extinction of its components and showing a weak gneissic foliation. The dark amphibolite (Figure 7) comprises a weakly foliated intergrowth of plagioclase laths and xenoblasts, and recrystallised prismatic hornblende, together with minor Figure 4 Lithostratigraphic column for TOBERMOREY. Wavy lines denote unconformities Figure 5 View to north over Arunta Province terrane of southwestern TOBERMOREY: hill at left with surrounding foreground and middle ground is migmatite unit p–Cd; Keepera Ridges with Neoproterozoic succession on centre horizon; Tarlton Range with Ordovician succession prominent at right horizon; Tarlton Fault at base of range. Tarlton PQ828664, south of Canyon Bore aggregates of biotite and possibly retrograde muscovite. Plagioclase is variably altered to sericite, epidote and carbonate. Accessory, disseminated anhedral opaques are generally intergrown with hornblende or biotite. Foliationparallel and crosscutting thin veins are of K‑feldspar or quartz. These rocks are interpreted to represent basic igneous rocks, which have been metamorphosed to amphibolite grade. Quartzitic rocks are also present, as at the peak of a prominent hill at PQ682747. Your Dam Metamorphics (p –Cy) This unit was proposed by Warren in Stewart et al (1980) for scattered outcrops of muscovite-feldspar-quartz granofels, schistose gneiss and schist in the vicinity of Your Dam in south-central TOBERMOREY. As mapped by Simpson et al (1985), outcrops are confined to lows between overlying Mesozoic rocks. These metasediments comprise variable proportions of muscovite, biotite, quartz, rare K-feldspar and accessory tourmaline (Warren 1981: 6). Warren postulated a depositional age of greater than 1800 Ma. Intrusive rocks Three granite units have been recognised in the TOBERMOREY-HAY RIVER region; these are the Mount Dobbie Granite, L Pgt granite (both confined to HAY RIVER) and Mount Tietkens Granite Complex. Simpson et al (1985) implied that all three are approximately coeval. Four additional granite bodies (L Pga to P Lgd) were delineated during remapping in southwestern TOBERMOREY; these are described separately pending clarification of their genetic relationships. They were mapped as undifferentiated ?Palaeoproterozoic, dominantly coarse-grained muscovite granite by Smith (1965a). Mount Tietkens Granite Complex (P Lg ) The Mount Tietkens Granite Complex of Warren (in Stewart et al 1980; Teikens Granite Complex of Simpson et al 1985, Shergold 1985) is poorly defined. Its nominated reference area falls within a region mapped by Simpson et al (1985) as L P gt. Those authors do nevertheless map the complex: at 6 km south of Old Marqua and at Christmas Creek in the Marqua Monocline area, and much farther afield at the southern end of the Toomba Figure 6 Migmatite unit p–Cd: feldspar-quartz gneiss of leucocratic phase. Scale bar = 2 mm. Photomicrograph C72822, crossed nicols; Tarlton PQ568726, near Tarlton Downs-Jervois boundary Figure 7 Migmatite unit p–Cd: amphibolite of melanocratic phase. Scale bar = 2 mm. Photomicrograph C72818, crossed nicols; Tarlton PQ547872, near Tarlton Downs-Jervois boundary Figure 8 Granite unit L P ga: pale grey to pink granite. Scale bar = 2 mm. Photomicrograph C72817, crossed nicols; Tarlton PQ828666, near Tarlton Downs-Jervois boundary Fault in southwestern MOUNT WHELAN. The unit therefore appears to embrace all otherwise unassigned granite outcrops in the TOBERMOREY-HAY RIVERMOUNT WHELAN region. Warren in Stewart et al (1980: 38) included within the unit an ‘older phase’ of medium even-grained muscovite-biotite leucocratic granite, and a ‘younger phase’ of porphyritic leucocratic granite, with associated tourmaline leucogranite and late-stage pegmatites. She quoted a K-Ar date on muscovite from an associated pegmatite of 1726 Ma, and Shergold (1985) a K-Ar date on an associated pegmatite at Christmas Creek of 1719 Ma. An intrusive age close to 1800 Ma was postulated (Warren in Stewart et al 1980). In TOBERMOREY, possibly related granites have been penetrated in four drillholes. The closest to the mapped Mount Tietkens Granite Complex is BHD9 in the Marqua Monocline area, which intersected coarse, grey, feldspar-quartz-two mica granite from 537 m depth (McGeough and Shalley 1992). Grey feldspar-quartzmuscovite granite that was penetrated at 598 m depth in NTGS99/1, about 25 km to the west on Marqua Creek, has been dated by the SHRIMP U‑Pb zircon method at 1846 ± 6 Ma (Andrew Cross, ANU, pers comm 2001). Granites intersected in Hacking 1 at 1222 m (Weste 1989) and Owen 2 at 1150 m depth (Kress 1991) have ages of 1749 ± 8 Ma and 1763 ± 4 Ma, respectively (A Cross, pers comm, 2002), and are tentatively included in the Mount Tietkens Granite Complex. Granite unit (L Pga) This granite intrudes migmatite unit p–Cd, adjacent to the Tarlton Fault around PQ820680 to the south of Canyon Bore. It comprises medium-grained, pale grey to pink granite (Figure 8), which in thin section shows hypidiomorphic intergrowth of anhedral alkali feldspar (microcline) and quartz with subhedral plagioclase. Whereas microcline is relatively fresh, plagioclase is extensively altered to sericite, clay and minor epidote. Quartz exhibits both unit and undulose extinction. Biotite and muscovite occur as disseminated, randomly oriented flakes, and reddish-brown to black iron oxides as grains, aggregates and grain boundary and fracture linings. Apatite is present in trace amounts. Later-stage quartz ± feldspar veins of millimetre to decimetre scale probably relate to proximity to the Tarlton Fault. Granite unit (L Pgb) Granite unit L Pgb forms isolated outcrops on sand plains around PQ580850 in the vicinity of Boundary Hill. Rock types are generally of undeformed medium-grained leucocratic (white) and more melanocratic phases (grey), altogether ranging from granite to quartz diorite. In thin section, granite from PQ582851 (Figure 9) consists of an equigranular intergrowth of anhedral microcline and quartz with weakly prismatic plagioclase. Plagioclase tends toward mild alteration to sericite, clay and finely Figure 9 Granite unit L Pgb: granite of leucocratic phase. Scale bar = 2 mm. Photomicrograph C72819, crossed nicols; Tarlton PQ582851, Tarlton Downs-Jervois boundary Figure 10 Granite unit L Pgb: quartz diorite of melanocratic phase. Scale bar = 2 mm. Photomicrograph C72820, crossed nicols; Tarlton PQ582851, Tarlton Downs-Jervois boundary granular epidote, with intergrown trace carbonate, in its core regions. Quartz shows unit to weakly undulose extinction. Biotite flakes are partially to completely chloritised; minor muscovite is interstitial to quartz and feldspar or locally intergrown with the biotite. Accessory disseminated opaques are present, as are trace amounts of apatite and zircon. Associated quartz diorite (Figure 10) is distinguished by the absence of alkali feldspar. Plagioclase (labradorite) and quartz predominate, and biotite is much more common. Plagioclase cores are altered and biotite is chloritised, as in the granite. A more diverse suite of accessory minerals includes opaques, apatite, allanite, sphene and zircon. Among these, allanite forms larger, prismatic brown crystals, generally intergrown with biotite. Quartz and quartz-iron oxide veins occur locally. A sample of L Pgb from PQ582851 yielded a SHRIMP UPb zircon date of 1753 ± 7 Ma (A Cross, pers comm, 2002). Accessory minerals include fine anhedral apatite and bluegreen pleochroic tourmaline. Copper occurrences within this granite are denoted by secondary malachite within quartz-hematite veins, presumably related to the nearby Tarlton Fault. Granite unit (L Pgd) This unnamed, slightly to moderately foliated granite body outcrops on sand plains to the south of the Marshall River on the western margin of TOBERMOREY, around PQ540610. A pale grey medium-grained granodiorite from PQ546624 (Figure 12) consists of coarse-grained plagioclase and lesser microcline phenocrysts intergrown with finer polycrystalline quartz aggregates. Feldspar shows evidence of granulation along crystal boundaries; myrmekitic quartz intergrowths are present marginal to some microcline crystals. Plagioclase is incipiently altered to sericite and finely granular epidote. Quartz exhibits undulose extinction and sutured margins. Biotite and lesser muscovite occur singly between quartz and feldspar crystals, or together as intergrowths. Accessory minerals include opaques (disseminated as anhedral grains), apatite and zircon. Granite unit (L Pgc) A granite outcrop along Camel Creek is assigned to unit L Pgc. A sample from PQ887606 (Figure 11) is a coarse leucogranite comprising a granular mosaic of microcline, quartz and plagioclase. Plagioclase shows weakly prismatic crystals commonly altered to sericite, clay and finely granular epidote. Quartz shows weakly undulose extinction. Biotite is minor and muscovite even more so. Opaques (probably iron oxides) are present as fine grains and intergrowths. Quartz veins Quartz veins up to 10 m thick mark the trace of the Tarlton Fault to the south of Canyon Bore, where they outcrop as a northnorthwest-trending discontinuous wall up to 10 m high. Figure 11 Granite unit L Pgc: coarse leucogranite. Scale bar = 2 mm. Photomicrograph C72824, crossed nicols; Tarlton PQ887606, Camel Creek Figure 12 Granite unit P Lgd: pale grey granodiorite. Scale bar = 2 mm. Photomicrograph C72823, crossed nicols; Tarlton PQ546624, near Tarlton Downs-Jervois boundary Georgina Basin stratigraphy Elements of the two immediately overlying s u p e r s e q u e n c e s o u t c r o p i n T O B E R M O R E Y: Supersequence 2 (Yardida Tillite but not Mount Cornish Formation) and Supersequence 3 (Black Stump Arkose, Wonnadinna Dolostone, Gnallan-a-Gea Arkose, Elyuah Formation and Grant Bluff Formation, but not Oorabra Arkose, Sun Hill Arkose, Boko Formation, Elkera Formation, Central Mount Stuart Formation and Andagera Formation). Neither the Neoproterozoic nor the basal Cambrian constituents of Supersequence 4 (upper Central Mount Stuart Formation, possibly uppermost Elkera Formation; Mount Baldwin Formation, Adam Shale) are identified in TOBERMOREY. The First Edition TOBERMOREY map and explanatory notes (Smith 1965a) included all pre-Grant Bluff Formation strata in the now defunct Field River beds of Smith (1963). Neoproterozoic rocks in the vicinity of the Marqua Monocline in southeastern TOBERMOREY (Adam Trough) have been revised and remapped on the Hay River-Mount Whelan Special sheet (Simpson et al 1985, Shergold 1985). Following Second Edition remapping along the Tarlton Fault, such rocks are now known to outcrop at three additional localities in southwestern TOBERMOREY (Keepera Trough); these include the Keepera Ridges and two unnamed hills, one near the Plenty Highway at PQ571924 and another at PQ949573 near Limestone Creek. All three localities are bounded by the Tarlton Fault and its offshoots. Neoproterozoic The Georgina Basin is a component of the formerly contiguous Centralian Superbasin (Walter et al 1992, 1995), together with the Savory, Officer, Amadeus and Ngalia Basins. Sedimentation in the Centralian Superbasin and Adelaide Rift commenced at about 800 Ma (Cryogenian; Zhao et al 1994, Fanning et al 1986). Initial deposition in the southern Georgina Basin was in fault-bounded troughs, of which the Keepera and Adam Troughs (Walter 1980) are represented in TOBERMOREY (Table 1). Neoproterozoic rocks were conceptually excluded from the Georgina Basin (eg Smith 1972) until their developmental continuity with Cambrian and younger rocks was recognised by Walter (1980) and Shergold and Druce (1980). Currently accepted lithostratigraphy is primarily that of Walter (1980), who revised and amplified the nomenclature of Smith (1964) and subdivided the Neoproterozoic succession into four hiatus-bound tectosomes [modified as sequences by Walter et al (1992) and supersequences by Walter et al (1994) and Walter et al (1995)]. Stidolph et al (1988) and Haines et al (1991) recognised additional units in the southwestern Georgina Basin. The oldest supersequence, Supersequence 1 comprising the Yackah beds and Amesbury Quartzite, outcrops in adjacent HAY RIVER and HUCKITTA as well as further to the west, but not in TOBERMOREY. UNIT, MAP SYMBOL, MAX THICKNESS LITHOLOGY DEPOSITIONAL ENVIRONMENT BASAL CONTACT CHARACTERISTIC FEATURES Cobble and boulder conglomerate, pebbly sandstone Limestone, silicified to chert and chalcedony Fluviatile channel Unconformable on Oln, Omc, Omm, Ome, Dc Low- to high-angle cross-beds Terrestrial brackish lacustrine, pedogenic Unconformable on _COn, Olk Rhizoliths, alveolar texture, soil glaebules, circum- and intragranular cracks, brecciation; plateau-forming Quartz sandstone, minor mudstone and conglomerate Terrestrial, cool water marine Unconformable on most older units Cross-beds, ferruginisation, plant fossils; mesa-forming Calcareous sandstone and siltstone, limestone, minor conglomerate; grades upward into quartz sandstone and conglomerate Shallow subtidal marine, offshore sandbar, beach, nonmarine braided fluviatile Unconformable on Omm, Ome Fish and invertebrate fossils Quartzose and quartzofeldspathic sandstone, siltstone, claystone Gypsiferous shale and siltstone, calcareous siltstone, quartz sandstone, glauconitic and micaceous quartz sandstone, minor coquinite, granule conglomerate Quartzose sandstone, minor feldspathic sandstone High-energy barrier, lower-energy subtidal marine Conformable and gradational on Omm Clay pellets, cross-laminations, ripples, load casts, current structures, invertebrate fossils, ichnofossils Low-energy lagoonal marine Conformable and gradational on Omc Invertebrate fossils, conodonts, ichnofossils including enormous Rusophycus Littoral, high-energy shallow marine Conformable and gradational on Oln Micaceous and glauconitic siltstone, claystone, sandstone, minor dolostone; basal coquinite Limestone, quartzic limestone, dolostone, marl, siltstone, basal sandstone Quartzose and dolomitic quartz sandstone, siltstone, dolostone, partially dolomitised limestone, minor conglomerate Low-energy intertidal to shallow subtidal marine, possibly below wave base Intertidal to shallow subtidal marine Conformable on Olk, Olc Basal clay pellet bed; current structures, thick cross-bed sets, ripples, ichnofossils; caps Tarlton and Toko Ranges Invertebrate fossils; recessive, forms scarp of Tarlton and Toko Ranges Ninmaroo Formation (_COn) 795 m Limestone, dolostone, minor quartz sandstone Subtidal open marine, restricted marine, peritidal Disconformable on _Cua Tomahawk Formation (_COt) 190 m Quartzose and glauconitic sandstone, minor dolostone, limestone, dolomitic quartz sandstone, conglomerate Subtidal open marine, restricted marine, littoral to sublittoral Disconformable on _Cua Dolostone, limestone, minor quartz sandstone, siltstone, shale, marl, conglomerate Quartz sandstone, quartzic dolostone Peritidal, restricted shallow subtidal marine Conformable on _Cma, _Cms Stromatolites, thrombolites, nodular evaporites, gypsum crystals, fenestrae; local karst at top Intermittently emergent hypersaline shoreline Transitional within _Cua Abundant halite hopper casts; ripples, cross-laminations, gypsum crystals CENOZOIC Poodyea Formation (Czp) 10 m Austral Downs Limestone (Cza) 8 m MESOZOIC Undifferentiated Jurassic-Cretaceous (JK) 30 m DEVONIAN Cravens Peak beds (Dc) 280 m ORDOVICIAN Ethabuka Sandstone (Ome) 1147 m Mithaka Formation (Omm) 156 m Carlo Sandstone (Omc) 190 m Nora Formation (Oln) 250 m Coolibah Formation (Olc) 110 m Kelly Creek Formation (Olk) 290 m Peritidal to open subtidal marine CAMBROORDOVICIAN CAMBRIAN Arrinthrunga Formation (_Cua) 975 m Eurowie Sandstone Member (_Cue) 60 m Conformable on Olk Lenticular chert, silicified microbial mounds, stromatolites, invertebrate fossils, conodonts Conformable on _COt, Current structures, mud pebble and _COn in TOBERMOREY; cobble mould horizons, ripples, lowdisconformable on angle cross-beds, granule beds, karstified _COn, stromatolites, invertebrate fossils, Georgina Limestone in conodonts, ichnofossils; plateau- and GLENORMISTONmesa-forming MOUNT WHELAN Synaeresis features, flat-pebble conglomerate, cross-beds including herringbone, stromatolites, invertebrate fossils, conodonts, joints; laterally interfingers with _COt Cross-beds, ripples, current structures, flat-pebble conglomerate, ichnofossils, invertebrate fossils, conodonts, mesoscale deformation of sandstone beds; interfingers laterally with _COn Table 1 Pre-Quaternary lithostratigraphy of Georgina and Eromanga Basins in TOBERMOREY and environs 10 unIt, MaP syMBol, Max thIcKness lIthology dePosItIonal envIronMent steamboat sandstone (_Cms) 80 m Quartzose and calcareous/dolomitic quartz sandstone, quartzic dolostone High-energy marine barrier arthur creek Formation (_Cma) 457 m Upper: dolostone, limestone Lower: foetid pyriticcarbonaceous black shale, laminated dolostone Limestone, dolostone; medial pyriticcarbonaceous black shale interbeds; basal terrigenous sandstone, greywacke, mudstone Dolostone, minor quartzic dolostone, mudstone, conglomerate; basal dolomitic quartzofeldspathic sandstone Upper: open to restricted subtidal marine Lower: deeper anoxic marine Open to restricted subtidal marine thorntonia limestone (_Cmt) 03 m red heart dolostone (_Cld) 9 m Basal contact characterIstIc Features Conformable on _Cma; possibly disconformable on Quita Formation in URANDANGIGLENORMISTON Disconformable on _Cmt Some cross-beds Disconformable on _Cld or older units; nonconformable on L Pg Nodular chert, nodular evaporite, phosphate and glauconite lithoclasts, bitumen, disseminated pyrite, invertebrate fossils Subtidal marine Disconformable on Adam Shale, Mount Baldwin Formation or older units Archaeocyathan-calcimicrobial patch reefs, invertebrate fossils, pyrite and rare galena, mesophase bitumen; Diplocraterion in basal sandstone; microkarstic upper surface Marine Conformable on L Pun, Lue P Marine Disconformable on Luw, Oorabra Arkose P Ripples, cross-beds, synaeresis features, mud pebble horizons; forms prominent strike ridges Interfingers with L Pun Marine Disconformable on L Puw Cross-stratification Intertidal, shallow to deep marine Proximal glacial outwash Conformable and gradational on L Pus Disconformable on L Put Oncoids, fenestrae, stromatolites, flat pebbles Glaciogene, postglacial Disconformable on Yackah beds Nodular evaporite, shredded to brecciated texture, invertebrate fossils, disseminated pyrite neoProteroZoIc grant Bluff Formation (L Pua) 70 m elyuah Formation (L Pue) 20 m gnallan-a-gea arkose (L Pun) 450 m Wonnadinna dolostone (L Puw) 460 m Black stump arkose (L Pus) 700+ m yardida tillite (L Put) 2900 m Quartz sandstone/quartzite, glauconitic quartz sandstone, mudstone Mudstone, siltstone; thin basal sandstone or pebbly arkose Pebbly arkose, sandstone, siltstone, shale Dolostone, quartzic dolostone Micaceous (to pebbly) arkose, sandstone, micaceous mudstone Diamictite, siltstone, dolomitic shale, dolostone, minor quartz sandstone, arkose Table 1 (continued) Yardida Tillite (L Put) Keepera Group The glacigene Yardida Tillite (Walter 1980) is correlated with the Sturtian glaciation in the Adelaide Rift (Preiss et al 1978). It comprises mainly green-grey diamictite, laminated siltstone and minor fine to very coarse (rarely pebbly) quartz sandstone and arkose. Locally, the formation culminates in a 100 m-thick interval of grey, laminated dolomitic shale containing lenticular dolostone, interpreted as a postglacial cap dolostone. The formation outcrops extensively in the Field River Anticline, which extends southeastward into HAY RIVER. Using data derived from this latter area, Walter (1980) constructed a composite type section, incorporating measured sections, drillholes and seismic traverses, to yield an estimated thickness of 2900 m. In the absence of exposed contacts, Walter inferred a disconformable relationship with the underlying Yackah beds; the contact with the overlying Black Stump Arkose is likewise disconformable. Black Stump Arkose (L Pus) Red-brown to purple brown micaceous, fine to very coarse (to pebbly) arkose, sandstone and laminated micaceous mudstone of the Black Stump Arkose (Walter 1980) are correlated with the Marinoan glaciation in the Adelaide Rift. They are interpreted as proximal glacial outwash deposits resulting from rapid erosion and mass transport (Walter et al 1995, Walter and Veevers 1997). The composite type section of Walter (1980), which has an estimated minimum thickness of 700 m, is located in northeastern HAY RIVER. In TOBERMOREY, the unit is exposed to the south of the Marqua Monocline between Old Marqua and Boat Hill. At Boat Hill, it rests disconformably on the uppermost dolomitic shale of the Yardida Tillite (Walter 1980); elsewhere this interval may be absent. The upper boundary of the formation is gradational into the Wonnadinna Dolostone. G:\Geological Survey\Publishing\Production\NTGS_250k_explan_notes_update\Updated_NTGS_250k_explan_notes\Tobermory\Tobermory_Archive\Tob_PM\Tob_Table1_p2.doc 11 Wonnadinna Dolostone (L Puw) Gradationally overlying the Black Stump Arkose is the Wonnadinna Dolostone (Wonnadinna Dolomite of Walter 1980), a unit of purple-red to yellow-brown and green-grey, locally quartzic dolostone, interbedded with more or less dolomitic arkose, siltstone and shale. The dolostone locally bears abundant oncoids, fenestrae and possible columnar stromatolites, and according to Walter et al (1995), is unlike correlative postglacial cap dolostones elsewhere. Domical and saucer-shaped stromatolites and flat pebbles are present in dolostone along the Marqua Monocline, to the northwest of Boat Hill (QQ711658). A bed in the Keepera Ridges (PQ782756) includes abundant flat dolomudstone pebbles and associated clots of galena. The formation is 460 m thick in the composite type section in northeastern HAY RIVER and is at least 380 m near Boat Hill and in the Keepera Ridges in TOBERMOREY. In the type section, the lower boundary is at the base of the first prominent dolostone bed, 4.7 km north-northeast of Gnallan-a-Gea Bore (Walter 1980). Correlative cap dolostones in adjacent Centralian basins were interpreted by Kennedy (1996) as deeper water deposits (below storm wave base) resulting from an anomalous flux of inorganic carbonate to the sea floor during postglacial transgression. An example from the uppermost Olympic Formation of the Amadeus Basin (Figure 5A of Kennedy 1996) shows millimetre-scale graded quartzic dolomudstone beds interleaved with fibrous cement and cut by subvertical neptunian fractures. A lithologically similar dolostone from an unnamed hill near Limestone Creek (southwestern TOBERMOREY) shows a tendency toward grading in thin section, wherein dolomudstone beds with ghost peloidal textures and admixed quartz silt and minor muscovite coarsen toward their bases via the addition of angular to subangular, medium silt- to fine sand-sized detrital quartz. Although bedding-parallel cement layers are absent, subvertical, possibly neptunian fractures are present, now infilled by ferruginous ferroan dolomite and gypsum. Tabular dolomudstone clasts from such rocks can be reworked into quartz-lithic intraclast dolopackstone to dolograinstone (Figure 13). The Wonnadinna Dolostone may thus mark deeper water as well as intertidal to shallow subtidal deposition (fenestrae, oncoids). Figure 13 Wonnadinna Dolostone. Quartz-lithic intraclast dolopackstone to dolograinstone; brown tabular dolomudstone clasts at bottom. Scale bar = 2 mm. Photomicrograph C75142, crossed nicols; Tarlton PQ953575, near Blue Hills Bore The formation comprises brown to grey, fine to very coarse pebbly arkose, sandstone, siltstone and shale. Cross-stratification is common in the arkose and sandstone. The lowest pebbly arkose or sandstone denotes the lower boundary with the underlying Wonnadinna Dolostone, which Walter (1980) inferred from regional correlations to be disconformable. The Gnallan-a-Gea Arkose is gradational upward variously into the overlying Elyuah Formation or Grant Bluff Formation. At the northern end (PQ771763) of the prominent ridge of the Keepera Ridges to the east of Arthur Creek, the Grant Bluff Formation rests directly and conformably on metre-scale beds of orange-brown pebbly Gnallan-a-Gea Arkose; 1.3 km to the southeast along the same ridge, grey mudstone of the Elyuah Formation intervenes. Mopunga Group The term Mopunga Group was coined by Noakes (1956) without named constituent formations, and later redefined by Smith (1964) and Walter (1980). Elyuah Formation (L Pue) Gnallan-a-Gea Arkose (L Pun) The Elyuah Formation (Smith 1964) was redefined by Walter (1980) to exclude the then constituent Oorabra Arkose Member (Smith 1964 after Joklik 1955), which he elevated to formation status. As redefined, the Elyuah Formation is predominantly of laminated shale and siltstone with a thin but persistent basal sandstone or pebbly arkose. The 36 m-thick type section of Smith’s (1964: 23‑24) former ‘shale member’ is near the southeastern extremity of the Dulcie Range in HUCKITTA; Walter (1980) presented additional measured In TOBERMOREY the Gnallan-a-Gea Arkose (Walter 1980) outcrops in the Keepera Ridges and south of the Marqua Monocline, where Walter (op cit) measured thicknesses of 10 m and 40 m, respectively. It attains 345 m in the type section in the Field River Anticline (northeastern HAY RIVER), and at least 1450 m in the Bat Hills further south. 12 sections in the range 80-210 m thickness. In HUCKITTA the grey, green or red shale is commonly veneered by colluvium, so that the basal pebble conglomerate, of milky quartz and minor lithic pebbles in a quartz arenite matrix, is commonly the only outcropping portion of the formation (Freeman 1986). The formation is likewise obscured in the Keepera Ridges on TOBERMOREY, where Walter (1980) measured a maximum thickness of 210 m. There, colluvium shed from the conformably overlying Grant Bluff Formation veneers a discontinuous northeast-facing scarp slope with limited exposure of Elyuah Formation. Perhaps the best outcrop is toward the southern end of the prominent ridge to the east of Arthur Creek, at PQ779754: a 23 m interval of laminated to thinly bedded grey mudstone and minor purple micaceous quartzic siltstone, with thin interbeds of grey to brown, very fine to fine quartz sandstone. Conversely, at the northern end of the same ridge, around PQ771763, the scarp slope is predominantly of orange-brown pebbly Gnallan-a-Gea Arkose and there are only short outcrop intervals of Elyuah-type thinly bedded grey mudstone. These observations collectively suggest interfingering between the two formations. metres 250 FAULT PQ775749 light grey quartzite, medium bedded red/yellow-brown siltstone, weathering to dark brown, medium bed 55 225 cream to light grey quartzite, thickly bedded 200 white coarse to granule quartz sandstone 28 230 CLIFF: quartz sandstone, thinly to thickly bedded, planar-laminated to mm- to cm-scale rippled 42 230 light grey flat pebble conglomerate bed, 5 cm thick, weathering to redbrown GRANT BLUFF FORMATION 150 Grant Bluff Formation (Pua) Walter (1980) revised the Grant Bluff Formation of Smith (1964) to exclude the recessive upper interval, which he named the Elkera Formation. The thus restricted Grant Bluff Formation is nevertheless widespread; it extends from ALCOOTA to HAY RIVER and ranges in thickness from 50‑100+ m in HUCKITTA up to 1170 m in HAY RIVER. The unit outcrops as prominent strike ridges of distinctive laminated to medium undulosebedded quartz sandstone/quartzite with micaceous partings; the sandstone is medium to coarse and has common centimetre-scale (rarely up to decimetrescale), asymmetric and some symmetric ripples, small- to large-scale trough cross-beds and dewatering structures including synaeresis features (Freeman 1986). Feldspar grains are a local minor constituent, as is glauconite. Mud pebble and clay granule horizons are locally present. Jenkins et al (1992) recorded the ichnofossils Palaeophycus tubularis Hall and Planolites montanus Richter from the type section in the Elua Range (HUCKITTA); these simple horizontal trails are consistent with the terminal Neoproterozoic age assigned to the formation. In TOBERMOREY, the Grant Bluff Formation outcrops in two small fault-bounded blocks at an unnamed hill 2 km north of the Plenty Highway at PQ571924, and as the characteristic prominent strike ridges in the Keepera Ridges. A nearly complete section to the east of Arthur Creek in the latter area, which is truncated at the top by an offshoot of the Tarlton Fault, is 248 m thick (Figure 14). As in the Elua Range, it comprises two boldly outcropping sandstone ridges separated by a medial, relatively recessive interval of interbedded, thinly bedded, glauconitic fine quartz sandstone and mudstone. Large-scale synaeresis cracks are prominent on bedding surfaces of the stratigraphically higher sandstone ridge CLIFF: light grey fine quartz sandstone, weathering to red-brown, thinly bedded 22 265 100 50 fine glauconitic quartz sandstone, thinly bedded 70 265 PQ778751 CLIFF: light grey medium-coarse quartzite, thinly-thickly bedded 40 225 PQ771763 0 GNALLANA-GEA ARKOSE m02-045.dgn orange-brown pebbly arkose, thickly bedded ripples tabular intraclasts synaeresis features cross-beds Figure 14 Measured section through Grant Bluff Formation. Grid locations, and measured dips and dip azimuths are shown at left. Tarlton PQ771763 to PQ775749, Keepera Ridges 13 Figure 15 Grant Bluff Formation. Largescale synaeresis cracks in upper part of formation. Tarlton PQ776750, Keepera Ridges Red Heart Dolostone (–Cld) (Figure 15). A contact with the conformably underlying Gnallan-a-Gea Arkose is preserved at PQ771763, where pebbly Gnallan-a-Gea Arkose passes upward over 1.5 m through grey, medium bedded very fine quartz sandstone into grey, thinly bedded mudstone and thence into grey, thinly to medium bedded very fine quartz sandstone, with tabular mud pebble moulds, of the basal Grant Bluff Formation (Figure 14). Elsewhere, the Grant Bluff Formation rests conformably on the Elyuah Formation. In adjacent HUCKITTA and westward, it is conformably succeeded by the Elkera Formation. Walter (1980) modified the original Mount Baldwin Formation of Smith (1964) to exclude the upper interval, of archaeocyath-bearing dolostone and associated siliciclastic rocks. Freeman (1986) subsequently assigned this interval to the Errarra Formation. The Errarra Formation on HUCKITTA is equivalent to the Red Heart Dolomite of Walter et al (1979) on HAY RIVER, and these were synonymised as Red Heart Dolostone by Ambrose et al (2001). The type section in the Desert Syncline (HAY RIVER) spans the interval 52.6‑62.4 m in cored drillhole Hay River 11B and comprises 9.0 m of grey and tan stylolitic, mottled, brecciated and vuggy dolostone above a basal 0.8 m of dolomitic, coarse quartzofeldspathic sandstone with thin interbeds of green mudstone. Walter et al (1979) identified the U-shaped dwelling burrow Diplocraterion sp in outcrop of the basal sandstone. The lower contact of the formation is disconformable on the Adam Shale or older units; the upper contact is an irregular microkarst surface beneath theThorntonia Limestone or Arthur Creek Formation (Shergold 1985). The Red Heart Dolostone shares a common mid-Early Cambrian fauna with the Todd River Dolostone of the Amadeus Basin, of archaeocyaths (Kruse in Walter et al 1979, Kruse and West 1980, Kruse and Shi in Brock et al 2000), brachiopods, hyoliths, molluscs, fragmentary ?echinoderms and problematic skeletal fossils (Laurie and Shergold 1985, Laurie 1986). These are indicative of a late Atdabanian age in Siberian terms (Debrenne et al 1990). The Red Heart Dolostone is not exposed in TOBERMOREY, but is intersected in cored drillholes Hacking 1 (1209.9‑1221.9 m), BHD4 (350.9‑356.1 m) and BHD9 (528.8‑536.6 m depth). It is evidently also present in BHD5 (491.7‑510.6 m) and BHD6 (39.3‑49.1 m depth), where it is equivalent to unit Cl of McGeough and Shalley (1992). It thus has a thickness range in the map area of 5‑19 m. The basal beds in these intervals are variously of quartzic dolostone, quartz-intraclast dolomitic granule Cambrian Lower Cambrian rocks are not exposed in TOBERMOREY. Elsewhere along the southern margin of the Georgina Basin, sandstones of the Mount Baldwin Formation (Smith 1964) in HUCKITTA and the Octy and Neutral Junction Formations (Haines et al 1991) in ALCOOTA and BARROW CREEK bear a basal Cambrian ichnofauna that enables correlation with Arumbera Sandstone III and IV of the Amadeus Basin (Walter et al 1989). The Adam Shale is a lateral equivalent on HAY RIVER, which yields coeval acritarchs (Muir in Walter et al 1979). In MIM cored drillholes in the Boat Hill area of TOBERMOREY (McGeough and Shalley 1992), maroon and olive green mudstone below the Red Heart Dolostone (ie below 356.1 m depth in BHD4, and possibly also 510.6 m in BHD5 and 49.1 m in BHD6) may represent the Adam Shale. Southgate and Shergold (1991) outlined a sequence stratigraphy of the Middle Cambrian rocks of the Georgina Basin. They recognised two major Middle Cambrian sequences: sequence 1 (including an upper, tectonically enhanced sequence 1a) of Ordian-early Templetonian age; and a more substantial sequence 2 of late Templetonian to early Mindyallan age. Their scheme was modified by Gravestock and Shergold (2001), who transferred the former sequence 1a to sequence 2. 14 conglomerate, or marly mudstone with abundant admixed lithoclasts (including granite-derived quartz and feldspar in Hacking 1). In the overlying grey to pink dolostone, pyrite commonly (and galena rarely) fills vugs and lines fractures (see Economic geology). Hydrocarbon-bearing fluid inclusions in the Red Heart Dolostone in drillhole Hay River 11A (HAY RIVER) have homogenisation temperatures of 174‑225°C (Wilkins in Shergold 1979a, 1979b). Wilkins in Shergold (1979b) applied an unspecified pressure correction and calculated a minimum temperature of 250°C for crystal growth. The same samples also contain mesophase bitumen. This has been used as a palaeothermal indicator of 400‑470°C (Taylor in Shergold 1979a, Shergold 1985) or >200°C (Chapman 2001). However, it may also form partly as a result of natural radioactivity. In contrast, migrated bitumens studied in the Arthur Creek Formation and Thorntonia Limestone in Owen 2 are considerably less mature. Several contain humic acid that only occurs in very immature bitumens. The most mature pyrobitumen has a reflectivity of 1.38-1.44% (Glikson in Kress 1991). 1939, 1941, Öpik 1968, 1970, 1975, Jell 1975, Shergold 1975b, 1981, Schmitt and Southgate 1982, Kruse 1990; sequence 1 of Southgate and Shergold 1991). A maximum 103 m thickness has been reported in Morstone 1 petroleum exploration well in CAMOOWEAL (Stewart and Hoyling 1963). The formation has a gradational lower contact on the Colless Volcanics (Early Cambrian) or on a variously named basal terrigenous siliciclastic interval. Where these are absent, it rests unconformably on Proterozoic rocks (Shergold and Druce 1980). In addition to the limited outcrop, the Thorntonia Limestone is entirely intersected in six TOBERMOREY cored drillholes. From east to west these are Owen 2 (1053.3‑1150.2 m), BHD4 (301.9‑350.9 m), BHD5 (456.0‑491.7 m), BHD9 (484.9‑528.8 m), Tobermorey 14 (85.3‑109.5 m; Gibson 1984) and NTGS99/1 (554.7‑598.4 m depth), giving a thickness range in the map area of 24‑97 m. In these drillholes the unit rests disconformably on the Red Heart Dolostone or nonconformably on Palaeoproterozoic granite (see Mount Tietkens Granite Complex). A medial dark, siliciclastic mud-rich interval imparts a stratigraphically successive light-dark-light grey colouration to the carbonate portion of the formation and this is generally recognisable in drillholes across the southern Georgina Basin. All three intervals exhibit disseminated pyrite. A basal terrigenous interval is locally present (eg 5 m of dolomitic feldspathic greywacke with basal arkosic pebble conglomerate directly overlying granite in Owen 2). Thorntonia Limestone (–Cmt) Lower Middle Cambrian strata in TOBERMOREY are exposed along the Marqua Monocline in the Boat Hill area (Simpson et al 1985). They were initially assigned to the Marqua beds (Smith and Vine 1960, Smith 1963), a provisional unit that included all Middle Cambrian rocks in the immediate region, and later to the Hay River Formation (Shergold in Walter et al 1979). The type section of the latter is in cored drillholes Hay River 11, 11A and 11B in the Desert Syncline (HAY RIVER), where three constituent numbered ‘members’ were recognised. Shergold (1985) subsequently modified the formation concept to embrace a greater thickness of strata in three discrete packages, portrayed by Donnelly et al (1988) as an Ordian-age ‘Lower Hay River Formation’, an early Templetonian ‘Upper Hay River Formation Member 1’ and a Floran-Undillan ‘Upper Hay River Formation Member 2’. In the nomenclatural rationalisation of Ambrose et al (2001), the last unit was included as part of the Arthur Creek Formation (which see), whereas the first two were retained in the Thorntonia Limestone. The name Hay River Formation was abandoned. The Thorntonia Limestone (Öpik 1956) is extensive around the eastern (Queensland) margin of the Georgina Basin and extends as far south as cored drillhole GSQ Mount Whelan 1 (Green and Balfe 1980) in MOUNT WHELAN. No type section has been designated, but a type area is implied in the Thorntonia area in CAMOOWEAL. Reference sections have been nominated by Shergold and Southgate (1986: 19), who also provided detailed regional descriptions. The formation includes grey limestone, partially dolomitised limestone and dolostone with chert nodules and minor phosphorite. It is richly fossiliferous, with a varied biota of trilobites including Redlichia, Xystridura, Pagetia, Peronopsis and questionable Onaraspis, brachiopods, hyoliths including Guduguwan hardmani, bradoriides, echinoderms, molluscs, chancelloriids, calcimicrobes and stromatolites, to which Shergold et al (1985) assigned an Ordian (early Middle Cambrian) age (Whitehouse Figure 16 Thorntonia Limestone. Bioclast dolowackestone of lower light grey interval. Pyrite (black) is disseminated and stylolite- and fracture-associated. Scale bar = 2 mm. Photomicrograph C72846, crossed nicols; 588.1 m depth in drillhole NTGS99/1; Marqua QQ470743 15 NTGS99/1 and BHD holes) and in Hay River 11, 11A and 11B, it is a relatively thin (0.5‑6.2 m) phosphatic and glauconitic bioclast intraclast peloid dolopackstone to dolograinstone that has an erosive lower contact on the medial interval (Figure 18). Bioclasts include coarse sand and granules of lingulate brachiopods, fragmentary trilobites (including Xystridura, Pagetia and Peronopsis), echinoderm ossicles (prone to phosphatisation), and hyoliths (dominantly Guduguwan hardmani) and their phosphatic steinkerns. A Templetonian age was attributed by Shergold (1985) and an early Templetonian age by Donnelly et al (1988). This unit is now a dolosparstone, with bitumen occluding intercrystal porosity, particularly toward the top of the formation, where a microkarstic surface is locally evident. In Owen 2, the upper interval is apparently much thicker (27 m) and contains additional, locally amalgamated black terrigenous silt laminations, pyritic carbonaceous dolostone intervals and thin, sand to granule intraclast-bioclast conglomerate. Altogether, this upper light grey interval may correlate with the apparently coeval Bronco Stromatolith Bed (Southgate 1986), a distinctive, thin (1‑30 cm) marker bed of limonite-stained, stratiform to low-domical stromatolites that directly overlies an irregular and indurated erosion surface at the top of the Thorntonia Limestone in Figure 17 Thorntonia Limestone. Bioclast coquina of dark medial interval. Packstone of hyolith conchs and some opercula (mainly Guduguwan hardmani) at bottom. Wackestone of phosphatic hyolith steinkerns at top. Scale bar = 2 mm. Photomicrograph C72845, crossed nicols; 575.2 m depth in drillhole NTGS99/1; Marqua QQ470743 The lower light grey interval comprises planar- to nodular-bedded dolostone with ghost grainstone texture or admixed siliciclastic mud; bioclast (trilobite, brachiopod, echinoderm) dolowackestone to dolopackstone (Figure 16); brecciated dolostone including lithoclast dolopackstone; and basal quartzic (and locally glauconitic) dolostone with breccia of carbonate and granitic granules to pebbles. Some lithotypes are foetid, mottled, stylobedded or concretionary, and may enclose lithoclast conglomerate, bioclast coquina or nodular evaporite. The darker medial interval embraces a similar suite of carbonate lithotypes including nodular anhydrite, but with common interbeds of foetid dark grey to black marl and pyritic carbonaceous mudstone. Also present are thin bioclast coquinas (Figure 17), intraformational conglomerates of reworked tabular dolomudstone, and tracts of black (phosphatic?) sand to granules, together with evidence of submarine hardground development, such as thin phosphatised crusts. Elevated Total Organic Carbon (TOC) values in this interval indicate source rock potential (see Petroleum). This and the underlying light grey interval of the Thorntonia Limestone become more siliciclastic southward, as is evident in drillholes Hay River 11, 11A and 11B (Shergold 1985). The upper light grey interval equates with the ‘Upper Hay River Formation Member 1’ of Donnelly et al (1988). In the Marqua Monocline drillholes (Tobermorey 14, Figure 18 Thorntonia Limestone. Erosive contact between phosphatic and glauconitic, bioclast-intraclast-peloid dolograinstone of upper light grey interval (above) and marly dolomudstone of medial interval. Phosphatic grains in upper interval include originally phosphatic lingulate brachiopods (light brown) and phosphatised hyolith steinkerns (dark brown). Scale bar = 2 mm. Photomicrograph C72844, plane polarised light; 558.7 m depth in drillhole NTGS99/1; Marqua QQ470743 16 Figure 19 Arthur Creek Formation. Pyritic-carbonaceous dololaminite of lower interval. Laminations are highlighted by variations in proportions of organic matter and clays (dark) versus fine xenotopic dolomite spar, quartz silt and bedding-subparallel muscovite (light). Scale bar = 2 mm. Photomicrograph C72843, crossed nicols; 530.0 m depth in drillhole NTGS99/1; Marqua QQ470743 Figure 20 Arthur Creek Formation. Centimetre-scale sedimentaryboudinaged interbeds of lime mud (now pale microsparstone) and dolomitic-siliciclastic mudstone (dark) of upper interval. Rounded dolospar-filled areas in microsparstone are burrows. Scale bar = 2 mm. Photomicrograph C72851, crossed nicols; 835.6 m depth in drillhole Hacking 1; Marqua QQ054745 CAMOOWEAL. PN Southgate (quoted in Gravestock and Shergold 2001: 124) has most recently reinterpreted this bed as a product of submarine erosion. The present upper interval would then represent the former sequence 1a of Southgate and Shergold (1991), since transferred by Gravestock and Shergold (2001) to sequence 2. (451.5 m), and also includes the immediately underlying and overlying formations (Thorntonia Limestone and Steamboat Sandstone, respectively). Altogether, the formation is disconformably underlain by Thorntonia Limestone or where this is absent, by Red Heart Dolostone. Exceptionally, it is nonconformable on Palaeoproterozoic granite in Randall 1 (HUCKITTA). The maximum known thickness of the Arthur Creek Formation (483+ m) is intersected in BHD9 in TOBERMOREY. The Arthur Creek Formation replaces the former Sandover beds in ELKEDRA (Stidolph et al 1988), and in TOBERMOREY, the Marqua Formation and ‘Upper Hay River Formation Member 2’ of Donnelly et al (1988; Ambrose et al 2001). However, these two latter units do correspond to two informal subdivisions of the Arthur Creek Formation (cf Shergold 1985: 10‑11); these are a lower interval of deeper water, foetid pyritic-carbonaceous black shale with planar to undulose dolostone laminations (anaerobic facies of Stidolph et al 1988); and an upper interval of paler carbonate exhibiting dolomudstone to dolograinstone textures (aerobic facies of Stidolph et al 1988). The transition is abrupt in NTGS99/1, in which the lower and upper intervals are respectively 272.1 m and 179.4 m thick; elsewhere the transition may be gradational or interdigitating. Overall, the formation denotes an initial rapid Arthur Creek Formation (–Cma) Smith (1964) coined the Arthur Creek beds to encompass all strata between the Mount Baldwin Formation (then understood to include what is now the Red Heart Dolostone) and the Arrinthrunga Formation in HUCKITTA. Freeman (1986) revised this interval as the Arthur Creek Formation, using as type section two cored drillholes NTGS HUC1 and NTGS HUC2; these gave a notional thickness of 417.6 m. However, comparison with more recent Pacific Oil and Gas drillholes casts some doubt on the claimed stratigraphic overlap of HUC1 and HUC2. In addition, the interval immediately above 70.7 m depth in HUC2 is here regarded as Steamboat Sandstone. A replacement holostratotype section is therefore nominated here and spans the interval 103.2‑554.7 m in cored drillhole NTGS99/1 (see Appendix). This drillhole is one of the thickest available intersections of the formation 17 transgression and the onset of anoxic conditions, succeeded by gradual regression into more aerobic marine conditions above wave base. The lower, black shale interval is readily oxidised and leached, and consequently is poorly exposed. In some areas, as for example flanking the Davenport Province in ELKEDRA, near-surface chertification leaves tabular rubble, in which the trilobite Xystridura is prominent (Plate 6 of Stidolph et al 1988). Pristine laminites in this interval (Figure 19), as intersected in cored drillholes, comprise millimetre-scale laminations highlighted by variations in the proportions of organic matter and clays (dark) versus fine xenotopic dolomite spar, quartz silt and bedding-subparallel muscovite (light). Grading within lamination couplets and the absence of bioturbation led Morris (1986) to interpret these rocks as anoxic rhythmites. Patchy early diagenetic carbonate forms remnant dolostone nodules within these otherwise burial-compressed laminites. The proportion of carbonate decreases toward the base of the formation, where foetid pyritic-carbonaceous black shale constitutes viable petroleum source rock (see Petroleum). Pelagic agnostine trilobites and benthic lingulate brachiopods and sponge spicules are locally present. Disseminated pyrite of microbial origin (Ferris 2000) occurs throughout the interval. Some pyrite replaces sponge spicules or occurs as epigenetic nodules and veinlets. Superimposed thin (centimetre-scale) granule to pebble intraclast, intraclastbioclast and quartz sandstone beds, some with scoured bases, have been interpreted as turbidity current deposits (Morris 1986). The upper, paler carbonate interval represents subtidal restricted marine shelf deposition above wave base. It is predominantly of light grey dolomicrosparstone and dolosparstone with recurring ghost grainstone textures, alternating with more or less nodular (sedimentary boudinaged) centimetre-scale calcimudstone-siliciclastic mudstone interbeds (Figure 20). Sedimentary and diagenetic features include admixed quartz silt to sand, wisps to thin interbeds of darker siliciclastic silt, bioturbation, stylobeds, disseminated and fracture-lining pyrite, nodular evaporite, and shredded to brecciated textures due to evaporite dissolution and collapse. Bioclasts are common. The upper interval is also poorly exposed, but locally outcrops as light grey platy limestone or dolostone (Figure 21). Common quartzic interbeds in more easterly drillholes, such as NTGS99/1 and Owen 2, record intermittent terrigenous sand pulses that were presumably derived from a source further to the east. Quartz sand input peaks around the lower-upper transition in both drillholes. In Owen 2, the transition is marked by breccias of sand- to cobble-sized tabular dolostone in a darker grey siliciclastic matrix; these are interpreted as debris flows. Altogether, the Arthur Creek Formation is richly fossiliferous and yields a variety of miomeran and polymeran trilobites, lingulate and calciate brachiopods, hyoliths, molluscs, echinoderms and sponge spicules. Trilobites indicate an age ranging from Templetonian (pre-Triplagnostus gibbus Zone; Laurie 2000a) up to the Boomerangian Lejopyge laevigata Zone, ie latest Middle or early Late Cambrian (Marqua Formation of Shergold 1985: 11; Shergold 1995, cf Geyer and Shergold 2000). The formation thus spans most of the Middle Cambrian and constitutes a large part of sequence 2 (Southgate and Shergold 1991). Steamboat Sandstone (–Cms) Smith (1972) and Shergold et al (1985) have summarised the Steamboat Sandstone (Noakes et al 1959) of the URANDANGI-GLENORMISTON area. There, outcrop of the formation consists of 60-80 m thickness of calcareous sandstone and quartz sandstone with minor quartzic grainstone and dolostone, possibly disconformable on Quita Formation and undoubtedly conformable beneath Mungerebar Limestone. Agnostine and polymeran trilobites indicate an age within the terminal Middle Cambrian Lejopyge laevigata Zone, extending to the Middle-Late Cambrian passage Zone of Damesella torosa-Ascionepea janitrix (Shergold et al 1985). Correlative fine quartz sandstone, which is exposed between the Arthur Creek and Arrinthrunga Formations in the vicinity of Eurolley Bore in eastern HUCKITTA, is taken to be a decalcified equivalent of quartzic dolostone Figure 21 Arthur Creek Formation. Centimetre-scale interbeds of calcimudstone (pale grey) and dolomitic-siliciclastic mudstone (brown) of upper interval. Compare with Figure 20. Toko QQ705664, Marqua Monocline north of Boat Hill 18 sediment of the overlying unit, plus associated pisoids and cavity-fill breccia. However, the disconformity is difficult to identify where overlain by the Ninmaroo Formation, as the mixed carbonate rock types above and below it are similar. In drillholes BMR 12 Cockroach and PAP Netting Fence 1, it is recognised at a marked change in downhole geophysical log characteristics (Kennard 1981: 5). Kennard (1981) documented the following component rock types: microbial (‘algal’) (dolo)boundstone, including thrombolitic, columnar, columnar-domical and various laminated types; peloid (dolo)grainstone and lesser packstone and wackestone, including ripple cross-laminated peloid (dolo)grainstone; ooid (dolo)grainstone; minor flat-pebble conglomerate; minor calci/dolomudstone with variable terrigenous silt and clay content, as well as pseudomorphs of gypsum crystals and nodular evaporite; mixed mudstone (marl); mixed sandstone (quartzic lime/ dolostone); and minor quartz sandstone (see Eurowie Sandstone Member below). These constitute the elements of eight discrete lithofacies, of which four dominate exposures along the Marqua Monocline in TOBERMOREY; these are, from base to top, the peloid grainstone, peloidal lime-mud, and intermixed laminated microbial (‘algal’) boundstone and microbial (‘algal’) mound lithofacies (Kennard 1981: fig 14). A basal cross-stratified peloid grainstone and dolostone unit mapped by Simpson et al (1985) coincides with the lowest, and perhaps also part or all of the next succeeding lithofacies of Kennard (1981); Shergold (1985: 14) observed that this mapped basal unit is overlain by a mainly microbial boundstone lithofacies. Two complete measured sections of the formation in this area are respectively 777 m and 914 m in thickness (Kennard 1980). The basal beds of the formation are exposed in a northsouth transect, 9 km to the east of Old Marqua homestead at QQ497729. There, scattered blue-grey thin-bedded calcimudstone and grey/brown nodular limestone of the uppermost Arthur Creek Formation are overlain by much more closely spaced limestone beds, including some Steamboat Sandstone (quartzic carbonate rocks), then basal Arrinthrunga Formation mainly of locally centimetre-scale cross-bedded ooid-peloid grainstone (peloid grainstone lithofacies). About 4 km east of Marqua homestead, the succeeding peloidal lime-mud lithofacies is represented by stylobedded peloidal dolomudstone at QQ392754. Outcrops in the vicinity of Smiths Dam (QQ240830) illustrate a variety of lithofacies. At QQ242834, 500 m north of the bore, a 2 m-thick slope section exposes basal light grey, brown-mottled dolomicrobialite, which is succeeded by light grey grainstone, followed by intraclast wackestone at the top. A further 400 m to the north at QQ241839 (and also west of the bore at QQ180830; Figure 22), columnar stromatolites are well exposed on bedding planes. At QQ213828 to the west of the bore, microbial laminite with late-diagenetic siliceous nodules is exposed at the base of an isolated hill of Jurassic-Cretaceous quartz granule conglomerate and claystone. The Arrinthrunga Formation was deposited on an extensive, very shallow, restricted carbonate platform, which was bordered in the west and north by an evaporitic, mixed carbonate-siliciclastic shoreline (Kennard 1981: 26). occurring at the same stratigraphic level in numerous southern Georgina Basin drillholes, and is thus assigned here to the Steamboat Sandstone. The Steamboat Sandstone was formerly recognised in outcrop in the Marqua Monocline area of TOBERMOREY as unit –Cmm2 (Simpson et al 1985). This unit was stated by Shergold (1985) to be up to 60 m thick. It is also delineated in TOBERMOREY cored drillholes Hacking 1 (718.6-752.9 m), NTGS99/1 (86.4-103.2 m) and Owen 2 (643.5-692.7 m depth), giving a confirmed thickness range of 17‑49 m. This interval comprises generally recrystallised grey dolostone with admixed quartz sand to granules, dolomitic granule intraclasts, darker siliciclastic mud wisps to interbeds, smallscale cross-beds and planar to wavy stylolites. Arrinthrunga Formation (–Cua) The Arrinthrunga Formation (Smith 1964) is widespread across the southern Georgina Basin and extends from eastern BARROW CREEK eastward to TOBERMOREY, northern HAY RIVER and westernmost MOUNT WHELAN (Reynolds 1968). The Meeta beds (Smith 1972), formerly recognised in ELKEDRA, SANDOVER RIVER, FREW RIVER and AVON DOWNS, are synonymous (Kennard 1981). The Arrinthrunga Formation is primarily a peritidal unit; correlative marine sedimentary rocks from elsewhere in Queensland are referred to the Georgina Limestone, which laterally interfingers with it (see Shergold et al 1985: 14). The formation includes well bedded limestone and dolostone, and minor interbedded quartz sandstone, siltstone and shale. It attains 800-900 m thickness (maximum 975 m; Smith 1972: 104) in HUCKITTA. Quartz sandstone in the medial part of the formation is assigned to the Eurowie Sandstone Member (see below). The nominated type section in the Elua Range in HUCKITTA (Smith 1964) was determined by Kennard (1980) to be structurally complex, with a faulted base and with additional medial portions faulted out, so that only about one third of the total interval is represented. Kennard (1980) therefore nominated a reference section GEO 802 in the nearby southern Dulcie Range area. Due to its deposition in peritidal environments, the formation is very sparsely fossiliferous. Its stratigraphic position conformably and gradationally above the Arthur Creek Formation (or Steamboat Sandstone where present), and disconformably below the Tomahawk Formation or Ninmaroo Formation, brackets the unit within the Late, but not latest Cambrian. Among the rare trilobites, brachiopods, hyoliths and molluscs reported, the stratigraphically lowest appears to be a single trilobite pygidium from basal beds at Black Tank in the Marqua Monocline; this indicates an age close to the Middle-Late Cambrian boundary (Shergold 1985). The precise age of the uppermost beds of the Arrinthrunga Formation is uncertain, but the youngest fauna in the laterally interfingering Georgina Limestone provides a proxy upper age limit of Iverian (mid-Late Cambrian) Irvingella tropica Zone (Shergold in Green and Balfe 1980). Beneath the Tomahawk Formation, the top of the Arrinthrunga Formation is an at least locally karstic erosion surface as is shown by an irregular topography infilled by 19 Among drillholes in the region, the Arrinthrunga Formation is intersected in TOBERMOREY drillholes Hacking 1, NTGS99/1, Owen 2, Lucy Creek 1 and BMR 12 Cockroach. Of these, the thickest intersection is in Lucy Creek 1, which records a complete thickness of 715 m (Pemberton 1967). Hacking 1 preserves a 468 m cored interval of the lower and medial portions of the formation (including the Eurowie Sandstone Member). As in other drillhole intersections throughout the southern Georgina Basin, the formation in Hacking 1 is a compendium of thin lithotype intervals marking the geologically rapid lateral and vertical environmental shifts of a peritidal platform. These lithotypes include grey silty, mottled and stylonodular calcimudstone; intraclast and quartzic lime/dolostone; dark grey siliciclastic mudstone; and centimetre-scale interbeds of light grey calcimudstone and dark grey or green siliciclastic mudstone. Fenestrae, small-scale cross-beds, thin intraformational granule intraclast (including flat pebble) and lithoclast conglomerates, wavy microbial laminites and domical and columnar stromatolites are recurrent, as are nodular evaporites and disrupted, mottled and shredded evaporite dissolution textures. Coarser, bedspecific dolomitisation has rendered many intervals porous to vuggy. Kennard (1981: 3) also confirmed the presence of the Arrinthrunga Formation in BMR 12 Cockroach and Lucy Creek 1 in TOBERMOREY. Eastward, the formation is recognised in PAP Netting Fence 1 (GLENORMISTON; Smith 1972: 108). Figure 22 Arrinthrunga Formation. Columnar stromatolite boundstone. Interstitial sediment between columns is very fine sandsized quartz, fine to medium sand-sized ellipsoidal ?peloids (now microspar) and trace muscovite in what is now a calcite mosaic. Scale bar = 2 mm. Photomicrograph C75149, crossed nicols; Marqua QQ180830, near Dam A Marqua Eurowie Sandstone Member (–Cue) Quartz sandstone and quartzic dolostone in the medial Arrinthrunga Formation in eastern HUCKITTA (Freeman 1986) and southeastern ELKEDRA (Stidolph et al 1988) are attributed to the Eurowie Sandstone Member (Smith 1964). In HUCKITTA, this member is generally 40‑60 m thick (Kennard 1981: 24). It includes rippled and cross-laminated quartz sandstone with abundant hopper casts after halite, and outcrops as medium to thick beds and lenses. Gypsum crystals and laminae, and nodules probably after anhydrite are also present, as are bevelled ripples, dolostone pebble clasts and parting and streaming lineations. An intermittently emergent hypersaline shoreline depositional environment is indicated. Kennard (1981) reported the member in Lucy Creek 1 in TOBERMOREY, where it is 28 m thick. A 29 m thick, more or less quartzic dolostone with nodular evaporites in Hacking 1, in the interval 269.5-298.5 m depth, is also attributed to the Eurowie Sandstone Member. This platform was subjected to intermittent local emergence. The formation thus records a general shoaling from a more open subtidal setting (upper Arthur Creek Formation) through a probably low, yet wide seaward high-energy barrier (Steamboat Sandstone, ooid-peloid grainstone of the basal Arrinthrunga Formation in the Marqua Monocline area) into characteristically restricted, shallow subtidal to peritidal conditions. Hypersalinity on the platform limited grazing skeletal invertebrates and so permitted microbial mats to flourish, forming planar, wavy, domical and columnar stromatolites, according to depth and hydrodynamic conditions. Gypsum and nodular evaporites (anhydrite) precipitated in intertidal to supratidal flats. Siliciclastic mud to fine sand was shed onto the platform from the hinterland. Kennard (1981: 30) postulated a rapid sea-level rise in mid-Arrinthrunga time, leading to the wide establishment of the microbial mound lithofacies on the platform. Quartz sandstone and mixed ooid-quartz sandstone lithofacies are surmised to have migrated across the platform as shoaling resumed, culminating in intermittent emergence and the reestablishment of hypersalinity (Eurowie Sandstone Member). Stromatolites continued to propagate in areas bypassed by the terrigenous sediments. This cycle of sea-level rise and shoaling was repeated before regression exposed the platform and subjected its carbonate blanket to karstification. Cambro-Ordovician Tomahawk Formation (–COt) The Tomahawk beds (Smith 1964) are here formalised as the Tomahawk Formation. The type section and lower boundary stratotype is nominated in cored drillhole NTGS ELK6 in ELKEDRA and a reference section and upper boundary stratotype is in the Tarlton Range in TOBERMOREY (see Appendix). This stratotype is preferred to the eastern Dulcie Range reference section X30 of Smith (1964: 48-49, 52), 20 zircon and tourmaline, with interstitial coarse dolomite mosaic cement. At the surface, the glauconite is readily leached to produce the weathered outcrops of friable, brown to red-brown quartz sandstone that are characteristic of the formation. These appear as well sorted, locally dolomitic, fine to medium (to coarse) quartz arenite and minor quartzwacke, subarkose and sublitharenite, plus thin interbeds of micaceous siltstone and shale. Subarkoses include plagioclase, microcline and perthitic feldspars (Freeman 1986). The Tomahawk Formation dominates the northwestern quarter of TOBERMOREY, although outcrops are generally poor. There as elsewhere, the formation essentially comprises white to brown, thin to medium, well sorted, fine quartz arenite and minor granule and cobble conglomerate. Dolostone and minor dolomitic sandstone are interbedded in units 0.1‑30+ m thick. Sandstone typically exhibits horizontal planar lamination, but cross-beds also occur (including solitary and festoon sets at centimetre and decimetre scales, and rare herringbone cross-beds). Ripples include symmetric oscillation and asymmetric linear and interference types. Synaeresis features are characteristic. Current markings (normally current lineation, less frequently small flute moulds and current bounce or skip marks) are present throughout the region, although not abundant. Wrinkle marks (runzel marks) are also widespread. Seven foreset directions from sandstone cross-beds were measured which, when corrected for tectonic tilt, are scattered around a southeast-facing semicircle (Figure 23). Evidence of reversing tidal currents in the form of herringbone cross-beds is more common in the dolostone beds than in the sandstone. Other types of dolostone cross-bedding are more abundant, and include both planar because the basal contact with the Arrinthrunga Formation may be faulted there, and because it is now recognised that this section includes the Kelly Creek Formation in the upper part of the ‘Tomahawk beds’ interval (with the formation boundary within the 400-500’ span in his Fig 22). The Tomahawk Formation is thus dominantly a sandstone unit that contains minor siltstone interbeds, and at the top, thin carbonate intervals. A maximum recorded thickness of 225 m was measured in section X54 (Smith 1972) in the western Dulcie Range (northwestern HUCKITTA), but the upper portion of this interval may represent the Kelly Creek Formation, by analogy with section X30. The true thickness may be in the order of 150-190 m; 149 m was intersected in cored drillhole NTGS ELK6. The Tomahawk Formation is widespread in the southwestern Georgina Basin and interfingers laterally with the carbonate-rich Ninmaroo Formation in the southeast. This corridor of interfingering can be traced northward across central TOBERMOREY and into south-central SANDOVER RIVER. The Tomahawk Formation rests disconformably on the Arrinthrunga Formation and is overlain with apparent conformity by the Kelly Creek Formation. Trilobites and rostroconch molluscs in basal beds indicate an Iverian to Payntonian (medial Late Cambrian) age for initiation of the Tomahawk Formation (Casey and GilbertTomlinson 1956: 65, Jones et al 1971: 21, Pojeta et al 1977). Conodonts provide an upper age range, given by Shergold and Druce (1980: 161) as the late Datsonian Cordylodus oklahomensis‑C. lindstromi Zone. The Datsonian stage was then regarded as Early Ordovician, but is now assigned to the latest Cambrian (Shergold and Nicoll 1992). The recently ratified global Cambrian-Ordovician boundary in Newfoundland is correlated with the base of the succeeding Warendian stage in Australia (Cooper et al 2001). Other fossil groups represented in the formation are brachiopods, hyoliths, pelecypods, gastropods, echinoderms and possible sponge spicules (Freeman 1986, Stidolph et al 1988), although none has been described. In the Dulcie Range (HUCKITTA), Nicoll (1991b) noted the presence of the conodont Cordylodus caseyi, indicative of the late Warendian (Early Ordovician) Chosonodina herfurthi-Cordylodus angulatus Zone, and in succeeding beds, the younger conodont Ulrichodina deflexa, in an interval attributed to the uppermost Tomahawk beds, but these may well be Kelly Creek Formation. This assessment is supported by the report of Jones et al (1971: 21) of late Datsonian conodonts in the uppermost Tomahawk beds and Kelly Creek Formation in the Tarlton Range area. Conodonts Cordylodus proavus and ?Oneotodus gracilis, which indicate a Datsonian age, were identified from the newly remapped uppermost beds of the Tomahawk Formation in this latter area (Zhen Yongyi pers comm 2002). The Tomahawk Formation has thus yielded no fauna of confirmed Ordovician age, although an occurrence of putative Early Ordovician fossils was identified during Second Edition mapping (see below). Sandstone dominates the Tomahawk Formation. Fresh sandstone preserved above 149 m depth in cored drillhole NTGS ELK6 is a locally centimetre-scale cross-bedded, dolomitic, glauconite-quartz sedarenite of detrital glauconite pellets (up to 30%), quartz sand and trace to accessory Figure 23 Cross-bed foreset azimuths (corrected for tectonic tilt) in Tomahawk Formation and Ninmaroo Formation, western TOBERMOREY. Inner circle: directions from fine arenite; outer circle: directions from dolostone 21 solitary and festoon types in centimetre- and decimetrescale sets. They indicate palaeocurrents directed around the compass (Figure 23); northwest-southeast and east-west current trends include reversing tidal current pairs. A set of symmetric ripples in a dolostone bed at PR936063, which is 4 cm high and internally cross-laminated with foresets dipping in opposite directions, also indicates oscillating currents. Tabular intraclast beds within sandstone, generally 4‑15 cm thick, are minor. Intraclasts are preserved either as moulds or as the original fine sandstone clasts. They are well rounded in plan view and are generally about 1 cm across, but range up to 8 cm or more. Most hills surficially or entirely consist of deeply weathered material from a zone underlying a former geomorphic surface. Thus ferruginised and silicified rock occur beneath overlying (or formerly overlying) ferricrete and silcrete, respectively; some small areas mapped as the Triassic Tarlton Formation on the First Edition map (Smith 1965a) consist of such ferruginised rock. Manganese staining and encrustations up to centimetres thick are locally present. Ichnofossils are common, especially on planar and rippled bedding surfaces. These are mainly horizontal traces of Planolites type, as well as Palaeophycus, Teichichnus, bilobed trails, Corophioides, Skolithos, Arenicolites or Diplocraterion, and arthropod traces such as Monomorphichnus, Diplichnites, Rusophycus and Cruziana (Stidolph et al 1988 and personal observations), collectively indicative of littoral to open marine conditions [Skolithos and Cruziana ichnofacies of Seilacher (1964)]. A distinctive coiled trace (?Gyrolithes) is evident in the marginal central-western map area and in the Tarlton Range area; a single specimen found at QR034289 northwest of Olivers Bore is probably the most easterly known occurrence. The rostroconch mollusc Cymatopegma semiplicatum, of mid-late Paytonian (Late Cambrian) age, was discovered at PR926420 during the present Second Edition mapping and identified by Laurie (2000b). Upper levels of the formation contain poorly to moderately sorted, coarse quartz sandstone to granule and pebble conglomerate, interbedded with fine sandstone. Granules are variously rounded, whereas pebbles are consistently well rounded and usually of quartz, although a minority are of silicified fine quartz arenite. Coarser conglomerate is uncommon; however at PR167276 there are at least two beds, one with cobble and boulder clasts up to 35 cm in size, the other with a boulder, about 1 m long, of poorly sorted coarse sandstone. Rarely, conglomerates are steeply bidirectionally cross-bedded (eg high hill at QQ166913; Figure 24). Another notable occurrence, at PR941321, is characterised by subrectangular blocks of fine Tomahawk Formation arenite and well rounded cobbles and boulders of similar sandstone or quartz. Maximum clast size there is 0.7 m. The sandstone clasts have yielded the trilobite Lycophron sp, pelecypod Sthenodonta sp, indeterminate costate orthide brachiopods and indeterminate pseudoplanispiral gastropods of probable Early Ordovician age (Laurie, 2000b). Hence the deposit is younger than this. Siltstone, with white mica common on bedding surfaces, becomes interbedded with sandstone in these upper levels of the formation as a minor constituent, generally as thin, white beds. There are few sections of substantial thickness exposed in the region. The thickest measured, at PR557389 near the northwestern sheet margin, is only 23 m in thickness (Figure 25). Beds in this section, including a 15 cm dolostone band, can be recognised in a hill 40 km to the east-southeast at PR935287, testifying to the lateral extent of detailed stratigraphy. Medium to thick beds of grey to yellow-brown limestone or coarsely recrystallised dolostone, which are included in the Tomahawk Formation, commonly bear relict peloids and ooids, admixed quartz sand, accessory glauconite and trace tourmaline. Medium-bedded carbonate flatpebble conglomerate is extensive in the Dulcie Range area of HUCKITTA (Freeman 1986), and is known also in TOBERMOREY. Within the corridor of interfingering with the Ninmaroo Formation in central TOBERMOREY, where dolostone occupies all or most of a hill, it is mapped as Ninmaroo Formation (see below). West of this corridor, dolostone is depicted in Ninmaroo colour but included within the Tomahawk Formation. Although these carbonate interbeds are typically flatlying, associated sandstones tend to be structurally deformed at mesoscale (including kink folds; Freeman 1986). This deformation is widespread in HUCKITTA, ELKEDRA, SANDOVER RIVER and TOBERMOREY (Figure 26). Sandstone outcrop usually displays greatly varying dips, which can change significantly in magnitude and direction Figure 24 Tomahawk Formation. Granule conglomerate sublitharenite of angular to subrounded, medium silt- to granule-sized quartz in groundmass of quartz, muscovite and iron oxides ± clays. Scale bar = 2 mm. Photomicrograph C75148, crossed nicols; Marqua QQ166913, near Southern Cross Bore 22 metres 20 15 10 Figure 26 Tomahawk Formation. Mesoscale deformation in quartz sandstone: view along axis of local synclinal fold. Marqua QR126085, Atnetye 5 within metres. This renders it difficult to obtain reliable regional dips. Folding can be broad enough to occupy a moderate-sized hill, but commonly occurs on metre and decimetre scales. In most cases, beds dip into a hill on all sides. A good illustration at PR744261, where in a small hill only metres wide, dips of 15° toward 155° occur in the north side, but change to 13° toward 310° on the south side over a distance of only 3 m. Freeman (1986) invoked subsurface cavernous solution of underlying carbonate rock (Arrinthrunga Formation) by groundwater, with consequent progressive subsidence of lithified sandstone, to account for this lithology-selective deformation. This would account for many instances of sandstone and dolostone in close lateral juxtaposition. The uppermost Tomahawk Formation adjacent to the Tarlton Fault and on the eastern margin of the Tarlton Range is a series of prominent, thinly to thickly bedded, metre-scale grey limestone packages and intervening recessive marly mudstone to fine sandstone. A measured section through this interval is 32 m thick; the formation top is placed at the top of the uppermost prominent limestone bed (Figure 27). The interval has also been intersected beneath the northern Tarlton Range in cored drillhole BMR Grg 12 (Milligan 1963). These limestones include more or less recrystallised, partially dolomitised ooid and ooid-intraclast grainstone (some with admixed glauconite sand, minor lime mud intraclasts and bioclasts), ooid-bioclast grainstone, quartzicglauconitic-lithic limestone and intraclast-lithoclast limestone (including granule-sized flat pebbles). Some grainstones show 0 m02-042.dgn LEGEND fine-grained quartz arenite siltstone dolostone quartzic dolostone partially dolomitised limestone dolomitic quartz sandstone festoon crossbeds symmetric ripples synaeresis features intraclasts horizontal planar lamination ichnofossils small flute moulds Figure 25 Partial measured section in Tomahawk Formation, northwestern TOBERMOREY. Algamba PR557389, southwest of Lucy Creek No 15 Bore 23 metres 70 PQ920682 tops are planed off by overlying cross-bedded crinoid grainstone. Limestone beds in this uppermost Tomahawk Formation interval become selectively dolomitised adjacent to the Ninmaroo Formation to the northeast of Tarlton Range. This transition can be observed in a broad amphitheatre between Cockroach Creek and Whitewood Bore to the southeast. North of the Plenty Highway, this interval is totally dolomitised; these prominent beds are traceable westward at least to the mapsheet boundary. The Tomahawk Formation represents the episodic shedding of terrigenous sand, possibly from exposed Arunta Province basement to the west, onto a broad marine platform, which was otherwise dominated by the correlative Ninmaroo Formation in the east. Deposition took place in littoral to sublittoral environments that extended a considerable distance offshore, within restricted to open marine waters that were moderately deep yet still subjected to storm wave action. Ichnofossil elements of the Skolithos ichnofacies may represent occupation by opportunistic species after highenergy events, with more diverse Cruziana ichnofacies taxa populating the sediments in less stressful low-energy interludes (Ekdale et al 1984). Widespread symmetric oscillation ripples probably formed near approximate storm wave base. The common cross-bedded units represent migrating straight and lunate megaripples, and together with other current indicators, record episodes of current activity that were occasionally strong enough to cause scouring. Intraclast-bearing layers and hummocky cross-strata mark rare instances of severe storm activity extending to the seabed. Rare herringbone cross-beds indicate that reversing tidal currents played a role in megaripple migration. The introduction of coarse sand and gravel up to boulder size in the upper Tomahawk Formation implies shallowing into turbulent conditions of the foreshore, as a result of progradation in response to uplift and erosion of the hinterland. light grey fine-medium quartz sandstone: thinly to medium bedded, weathering to orangebrown KELLY CREEK FORMATION 60 LEDGE: medium quartz sandstone: thinly to medium bedded, weathering to maroon-brown 50 light grey-brown fine quartz sandstone: thinly to medium bedded light grey/brown strongly dolomitised intraclast-?bioclast grainstone: glauconitic, thickly bedded light grey coarse quartz sandstone: medium bedded, high-angle cross-beds 40 light grey intraclast limestone: bedding-parallel flat pebbles 30 TOMAHAWK FORMATION light grey intraclast limestone: bed of light grey flat pebble intraclast limestone, pebbles generally bedding-parallel light grey ooid-intraclast grainstone: thickly bedded, partially dolomitised 20 light grey, partially browndolomitised intraclast-lithoclastbioclast limestone: glauconitic 10 light grey, partially dolomitised, coarsely recrystallised glauconitic limestone light grey fine quartz sandstone; minor quartzic-lithic limestone, marl Ninmaroo Formation (–COn) light grey fine-medium quartzicglauconitic-lithic limestone light grey/brown patchily dolomitised ooid grainstone: glauconitic, calcimudstone intraclasts, bioclasts PQ930670 0 The Ninmaroo Limestones and Ninmaroo Series of Whitehouse (1936) were emended by Casey (1959) as the Ninmaroo Formation. The type section, which is 795 m thick, is at Black Mountain (BOULIA; Jones et al 1971) in the Burke River Structural Belt of the eastern Georgina Basin, where five formal members are recognised (Shergold and Druce 1980). These members are not identified in TOBERMOREY. Much work on conodont (Jones et al 1971, Druce and Jones 1971, Druce 1978, Druce et al 1982, Nicoll 1990, 1991a, Nicoll and Shergold 1991, Nicoll 1992, Nicoll et al 1992) and trilobite biostratigraphy (Shergold 1975a) has been conducted in the Burke River region, together with the documentation of rostroconch and polyplacophoran molluscs (Pojeta et al 1977, Runnegar et al 1979). The age of the Ninmaroo Formation at Black Mountain is thus well constrained and ranges from the mid-Payntonian (medial Late Cambrian) Hispidodontus appressus Assemblage Zone up to the Warendian (earliest Early Ordovician) Chosonodina herfurthi/Cordylodus angulatus Assemblage Zone (Shergold and Nicoll 1992). The conformably underlying Chatsworth Limestone is absent in TOBERMOREY, where the Ninmaroo Formation flat pebble intraclasts invertebrate fossils ichnofossils crossbeds m02-046.dgn Figure 27 Measured section through Tomahawk Formation-Kelly Creek Formation contact (reference section and upper boundary stratotype of Tomahawk Formation). Tarlton PQ930670 to PQ920682, near Lucky Bore, southern Tarlton Range millimetre- to centimetre-scale high-angle cross-bedding. A distinctive cliff exposure in an indent at PQ913668 reveals laterally linked columnar stromatolites 0.8 m in height (relief >1:1) with a surrounding narrow apron of coarse crinoidossicle packstone or grainstone, all of which is encased in possibly microbially-bound ribbon lime mud. The column 24 of Ninmaroo Formation dolostone. The northern slope of the next hill to the south is mostly of Ninmaroo Formation dolostone and dolomitic sandstone, but contains two tongues of Tomahawk Formation sandstone. Clearly exposed Tomahawk-Ninmaroo contacts are rare. One distinctive exception is at QQ118887, to the east of the Tarlton Range, where brown medium quartz sandstone of the Tomahawk Formation immediately overlies yellowbrown, partially quartzic, small-scale cross-bedded Ninmaroo Formation dolostone. A 3‑20 cm-thick, dark red-brown crust of silicified Ninmaroo Formation dolostone marks the contact. This crust can be traced laterally on an irregular surface, along which the crust varies in orientation from horizontal to near-vertical. Elsewhere (eg QQ220905), the contact may be stylolitic. At QQ524848, an incompletely exposed sandstonedolostone contact zone has associated black chalcedonicmanganiferous crusts. This corridor of interfingering can be traced from Tarlton Range northeastward and thence northwestward into southwestern SANDOVER RIVER. Minor occurrences of Tomahawk-type quartz sandstone to the east of this corridor are mapped as Ninmaroo Formation on the Second Edition TOBERMOREY map, but are depicted in Tomahawk Formation colour. Westward, where dolostones range up to 30 m or more in thickness within the Tomahawk Formation, the converse applies. Component rock types in TOBERMOREY include grey to brown, more or less quartzic ooid, bioclast (especially echinoderm), peloid, lithoclast (quartzic, glauconitic) and Figure 28 Ninmaroo Formation. Columnar stromatolite boundstone (at bottom) in grainstone of bioclasts (calciate brachiopods, echinoderm ossicles, hyoliths, trilobites), generally with micrite envelopes or Nuia-type radial coatings, together with quartz, glauconite and muscovite in a coarse calcite spar mosaic. Scale bar = 2 mm. Photomicrograph C75147, crossed nicols; Marqua QQ138972, near Beenleigh Bore is considered to be disconformable on the Arrinthrunga Formation, although the contact is difficult to identify in the field (Kennard 1981: 5). In TOBERMOREY, a minimum 220 m (incomplete) has been intersected, but not cored, in drillhole Owen 2. Shorter intervals have been penetrated in cored drillholes BMR Grg 11 (Milligan 1963) and BMR 12 Cockroach (Smith 1967). The Ninmaroo Formation comprises a range of ooid, peloid, bioclast, intraclast (including flat pebble conglomerate), microbial and mixed-lithology limestone and dolostone, and minor quartz sandstone. These were deposited on a broad epeiric platform under normal open marine to restricted and emergent evaporitic conditions (Radke 1980, 1981, in Druce et al 1982). The shoaling cycles of Radke (1980) have not been recognised in TOBERMOREY. Ninmaroo Formation rocks have been subjected to burial and uplift diagenesis, including widespread dolomitisation (Radke 1982) The Ninmaroo Formation is extensive around the Toko Syncline in eastern and central TOBERMOREY, where it interfingers with the Tomahawk Formation to the west. The lateral facies change is well illustrated in hills around QR135298, about 6.5 km to the west-southwest of Desert Bore. There, the northern face of one hill is entirely of Tomahawk Formation, but the southern slope bears a lens Figure 29 Ninmaroo Formation. Arthropod track and horizontal burrows or trails in quartz sandstone bed. Hand lens is 4.5 cm long. Marqua QQ342906, Umberumbera Hills 25 Figure 30 Ninmaroo Formation. Synaeresis cracks in dolomudstone. Alkea QR137139, Atnetye Figure 31 Ninmaroo Formation. Decimetrescale domical stromatolites. M arqua QR120009, Tarlton Downs-Manners Creek boundary fence, 50 m south of Plenty Highway than 1 cm up to 14 cm in diameter. Some beds have erosive bases, and occur above or between dolomitic sandstone beds, from which the tabular intraclasts were probably derived. Good examples occur in the side of a gully at QR134295, 7 km to the west-southwest of Desert Bore; in cliff sections at QQ346906 in the Umberumbera Hills; and toward the base of a 0.5 m-thick bed on the western extremity of a prominent Ninmaroo Formation ridge at QQ392984. This suite of sedimentary structures indicates deposition under variable energy conditions, punctuated by recurring high-energy episodes, and is consistent with the depositional models of Radke (1980). Grey recrystallised sucrosic dolostones, some with ghost grainstone textures, are common. Such dolomitisation is due to burial diagenesis, producing dolosparstones, in which a moderate to coarse crystal mosaic obscures primary depositional textures. Some coarsely dolomitised lithotypes have experienced subsequent dedolomitisation (Figure 32). Two dominant joint trends typify the Ninmaroo Formation in TOBERMOREY; in the south-central map area a west- intraclast (dolo)grainstone and minor calci/dolomudstone and dolomitic quartz sandstone in medium to thick beds. These contain laminated and bioclast horizons, flat pebble conglomerate, stylobeds, low-flow regime parallel bedding, low- to high-angle cross-beds (including bidirectional examples), domical and columnar stromatolites (Figure 28), occasional ichnofossils (Figure 29) and characteristic synaeresis features (Figure 30). Symmetric ripples are rare. There are excellent exposures of decimetre-scale domical stromatolites along the Tarlton Downs-Manners Creek boundary fence, 50 m to the south of the Plenty Highway at QR120009 (Figure 31), and of columnar to clavate forms, to the south of Centenary Bore at QQ084958. These are generally enclosed by moderate- to high-energy grainstone. Flat pebble intraclast conglomerates are widespread, but minor. Typically of decimetre-scale thickness (2‑40+ cm), they contain flat pebbles variously of calcimudstone, peloid grainstone, quartzic peloid grainstone, calcareous sandstone and quartz sandstone, commonly in a calcite or dolomite spar mosaic. These usually rounded tabular clasts range from less 26 the formation is 168 m thick. Southeastward in MOUNT WHELAN, Gausden (1980) reported that the formation thickens to 290 m in drillhole AOD Mirrica 1. Druce (1976) and Shergold and Druce (1980) outlined a lithological succession within the formation: basal dolostone (30 m); medial fine dolomitic sandstone with coquinite interbeds (maximum 120 m); and upper, in part quartzic dolomitic limestone (30 m). However, the ‘basal’ dolostone is absent in the type section and, according to Radke (1981), this interval represents the uppermost Ninmaroo Formation. The Kelly Creek Formation is thus a resistant unit with a lower portion dominantly of quartz sandstone, dolomitic quartz sandstone and siltstone, and an upper portion primarily of dolostone and partially dolomitised limestone. This broad bipartite subdivision is evident in the Gaphole Creek type section. This section commences with thin to medium, planar bedded, fine quartz sandstone that contains streaming lineations, mud pebble and cobble mould horizons, ripples, low-angle cross-beds, bioclast-bearing granule beds and rare ichnofossils including Diplichnites. The upper portion of the section is of thick grey-brown, locally cross-bedded dolostone. The Kelly Creek Formation forms prominent hills and scarps within the Tarlton and Toko Ranges. This is well illustrated in the southeastern Tarlton Range, where the gently west-dipping Kelly Creek Formation forms a steepsided dissected plateau. A sharp topographic break marks the boundary with the Tomahawk Formation below. Throughout TOBERMOREY, outcrops of the Kelly Creek Formation are topographically more prominent and much less deformed than lithologically similar quartz sandstone of the Tomahawk Formation. According to Shergold and Druce (1980), the lower, dolomitic sandstone interval displays streaming and parting lineations, high-angle cross-beds, festoon cross-beds, shell banks and columnar stromatolites, which support an interpreted intertidal to supratidal depositional environment, although some shallow subtidal deposition cannot be excluded. Brachiopods and trilobites are rare in this unit; some trails are preserved in sandstone beds. A more diverse fauna of endoceratid nautiloids, gastropods and conodonts in the upper carbonate unit was taken as being suggestive of unrestricted, open marine environments. A 92 m-thick composite section of the Kelly Creek Formation was measured in the southern Tarlton Range plateau (Figure 33). Basal beds are of thin to medium, light grey to brown, locally micaceous, fine to coarse calcareous quartz sandstone, some becoming intraclastic along strike, and interbedded medium to thick yellow-brown, glauconitebearing intraclast-lithoclast-bioclast dolostone, some with flat pebble intraclast horizons and centimetre- to decimetre-scale high-angle or bidirectional to herringbone cross-beds. From about 25 m above the formation base, dolostone beds are few and thin- to medium-bedded quartz sandstone dominates. Here and elsewhere, this yellow-grey to maroon-brown sandstone bears lithic and glauconitic grains or flat pebbles, mud pebble mould horizons, and symmetric to asymmetric linear, linguoid and interference ripples. Lenticular granule quartz conglomerate is locally present in this sandstone interval (eg granule conglomerate with floating roundedsubrounded light grey quartzite and minor white quartz Figure 32 Ninmaroo Formation. Condensed ooid grainstone. Ooids are partially to completely dolomitised to coarse euhedral mosaic. Partially dolomitised ooids locally preserve precursor radial and, especially toward outer surface, concentric structure. Dolomite rhombs within ooids are zoned and some zones are replaced by finer calcite mosaic (dedolomite). Ooids are enclosed in hypidiomorphic calcite spar mosaic with admixed angular to rounded, medium silt- to fine sand-sized quartz. Scale bar = 2 mm. Photomicrograph C75145, plane polarised light; Marqua QR074010, near Centenary Bore northwesterly trend is dominant, whereas a southwesterly trend characterises the northeastern map area. Ordovician Kelly Creek Formation (Olk) In TOBERMOREY, the Kelly Creek Formation (Casey in Smith 1965a) outcrops around the Toko Range in the southeast, where it rests apparently conformably between the Ninmaroo Formation below and Coolibah Formation above (Shergold and Druce 1980: 163). It also occurs around the Tarlton Range in the southwest, where it is conformable on the Tomahawk Formation, but where the Coolibah Formation is absent. The Kelly Creek Formation is there succeeded conformably by the Nora Formation (Webby et al 1981: 29). Eastward, in MOUNT WHELAN (Reynolds 1968) and GLENORMISTON (Jones et al 1971), the basal contact is a disconformity on karstified Ninmaroo Formation and older Georgina Limestone. The formation is now also recognised in the upper part of the ‘Tomahawk beds’ interval in reference section X30 of Smith (1964: 48-49, 52), in the eastern Dulcie Range in HUCKITTA. Casey (in Smith 1965a) nominated a type section at Gaphole Creek in the Toko Range (TOBERMOREY), where 27 LEGEND flat-pebble intraclasts 10 invertebrate fossils yellow-brown dolostone light grey quartz sandstone: thinly bedded, planar bedded yellow-brown / grey interbeds of dolostone and fine quartz sandstone yellow-brown ?ooid dolograinstone cross-beds ichnofossils ripples 100 PQ876675 0 SANDSTONE 90 CARLO PQ855660 80 light yellow-brown fine quartz sandstone: friable, medium bedded, planar bedded SUMMIT light grey medium quartz sandstone: medium to thickly bedded, planar laminated to cm- to dm-scale crossbedded, asymmetric ripples NORA 50 40 30 PQ857659 KELLY CREEK FORMATION 60 maroon Carlo Sandstone scree: quartz sandstone with streaming lineation, flute casts, asymmetric ripples, mud pebble mould horizons, ichnofossils: Diplocraterion, horizontal traces FORMATION 70 20 quartz sandstone scree: asymmetric ripples, mud pebble moulds, ichnofossils including horizontal traces, vertical burrows Skolithos, Diplocraterion 40 30 KELLY CREEK FORMATION PLATEAU EDGE: yellow-grey quartz sandstone: medium bedded, parallel bedded, weathering to maroon; ichnofossils including sinuous horizontal traces, Diplocraterion quartz sandstone scree yellow-brown dolomitic quartz sandstone: lithic, glauconitic, poorly sorted 10 PQ887675 quartz sandstone scree light yellow-brown fine quartz sandstone: thinly to medium bedded 0 yellow-brown dolostone: mainly parallel bedded, with minor cm-scale cross-bed lenses PLATEAU TOP: dark brown ferruginous bioclast-lithoclast dolostone with gastropods yellow-brown quartzic dolostone: medium bedded white siliceous subplanar concretions 6-7 cm thick 10 quartz sandstone scree PQ890670 0 yellow-brown dolomitic-lithic -glauconitic fine-medium quartz sandstone: medium bedded light grey medium quartz sandstone: thickly bedded dark yellow-brown quartzic dolostone quartz sandstone: medium bedded quartzic dolostone yellow-brown fine quartz sandstone: thinly bedded 10 dark yellow-brown dolomitic quartz sandstone, dolostone 10 PQ901663 0 PQ854652 0 light grey fine-medium quartz sandstone: medium bedded, mainly parallel bedded very thick bed (1.6m) of light grey medium quartz sandstone: weathering to yellow-brown, cross-beds highlighted by brown dolostone grains, in cm- to dmscale sets yellow-brown dolomitic quartz sandstone 20 medium quartz sandstone: thickly bedded, planar bedded light yellow-grey medium quartz sandstone: medium bedded, friable; vertical burrows TOMAHAWK FORMATION scree light grey ooid-intraclast grainstone m02-041.dgn Figure 33 Composite measured section through Kelly Creek Formation, Nora Formation and Carlo Sandstone. Grid locations shown at left of each column. Tarlton PQ901663 to PQ855660, southern Tarlton Range 28 pebbles to boulders up to 30 cm in size at QQ421980; Figure 34). Ichnofossils include Diplocraterion, Diplichnites, Rusophycus (Figure 35), Cruziana and other arthropod scratch marks, sinuous horizontal trails including Planolites, Cochlichnus, pellet trails and bilobed trails, possible Skolithos, and a distinctive ?Gyrolithes (Figure 36), suggestive of intertidal to shallow subtidal depositional environments (Skolithos and Cruziana ichnofacies of Seilacher 1964). Horizontal trails are especially abundant at the summit of a flat-topped hill at QR469125 on the western margin of the Toko Range. Sandstone beds thicken and become more dolomitic toward the top of the formation in the southern Tarlton Range measured section and are associated with interbeds of commonly cross-bedded quartzic dolostone. Prominent outcrops in central TOBERMOREY, formerly mapped as the Triassic Tarlton Formation, are here remapped as Kelly Creek Formation, as confirmed by occurrences of the above ichnofossil fauna. These outcrops are now recognised as western outliers of the Toko Syncline succession. The lower, siliciclastic portion of the Kelly Creek Formation is dissected to the north of the Tarlton Range and in the Toko Range, but a measure of topographic prominence is nevertheless maintained. The lower formation boundary is drawn at the base of prominent red-brown to yellow-brown quartz sandstone hills in both regions. The dolomitic upper portion in the Toko Range was named by Radke and Duff (1980) as the Withillindarmna Dolostone Member, and mapped as a distinct unit by Simpson et al (1979). However, Shergold (1985) cited rapid lateral facies variation within the Kelly Creek Formation in discounting this unit, and it is not depicted by Simpson et al (1985). Thus, despite its distinctive joint pattern on aerial photographs of the Toko Range, the unit name is not used herein, although this and other dolostone intervals in the Kelly Creek Formation are distinguished on the present Second Edition map. In the Toko Range, this interval includes discrete medium to thick yellowbrown sucrosic dolostone (at least some after precursor dolograinstone), dolomudstone and dolomitic quartz sandstone beds, variously with intraformational pebble conglomerate, channels, microbial lamination and locally abundant horizontal and oblique burrows. These rock types are recognisable in the upper portion of the type section at Gaphole Creek. Equivalent dolostone on the northern flank of the Tarlton Range is well exposed between the Plenty Highway and the Tarlton Range proper. It includes medium to thick, mid-grey dolostone, some with centimetre-scale cross-beds or horizons of silicified flat pebbles; quartzic dolosparstone (Figure 37); medium-grained glauconitic quartz sandstone, locally with quartz sandstone flat pebbles; partially dolomitised limestone with centimetre-scale high-angle cross-beds and synaeresis features; laminated lithic limestone; quartz-lithic ooid dolograinstone; and interbedded recessive marl with horizontal trails. Jones et al (1971: 21) reported late Datsonian (now latest Cambrian) conodonts in the lower Kelly Creek Formation, immediately succeeded by an Arenig (Early Ordovician) conodont fauna, and therefore postulated a hiatus within the formation spanning the Warendian stage as then conceived (ie earliest Ordovician sensu Shergold and Nicoll 1992). This supposed intraformational hiatus is depicted in charts by Shergold et al (1976), Shergold and Druce (1980) and (questionably) Webby et al (1981), and is the basis of the intraformational Kelly Creek Movement of Webby (1978). However, Webby et al (1981) subsequently observed that no physical discontinuity has been demonstrated, and that while the relevant faunas remain undescribed, the nature and timing of the proposed break is in doubt. Indeed, Radke’s (1981) reassignment of the ‘basal dolostone’ interval of Shergold and Druce (1980) to the uppermost Ninmaroo Formation would appear to resolve this putative hiatus to the Ninmaroo-Kelly Creek boundary (Shergold 1985: 16). This was supported by Nicoll et al (1992), who identified the Kelly Creek Movement (their Kelly Creek Eustatic Event) as terminating deposition of the Ninmaroo Formation. Druce (in Shergold 1979a) listed four successive conodont faunas in the Kelly Creek Formation. The oldest was assigned to the mid-Warendian Cordylodus rotundatus‑C. angulatus Zone sensu Jones et al (1971); this was succeeded by an earliest Arenigian Scolopodus sexplicatus Zone fauna and thence by two additional early Arenigian faunas. In contrast, Nicoll et al (1992) correlated their Kelly Creek Eustatic Movement at the base of the formation with the terminal Warendian. Toko Group Casey (in Smith 1963), Smith (1972) and Draper (1980b) have charted the nomenclatural history of the Toko Group. The group name is derived from the Toko Series of Whitehouse (1936), intended to encompass all Ordovician rocks (at that time unnamed) to the west of Boulia, Queensland. Following subdivision and naming of the succession, Casey (in Smith 1963) took the Toko Group to embrace the Kelly Creek Formation to the Mithaka Formation, inclusive. Subsequently, Casey (in Smith 1965a) excluded the Kelly Creek Formation. Draper (1980b) further modified the group to exclude the Coolibah Formation and include the Ethabuka Sandstone. Webby et al (1981) rejected Draper’s (1980b) proposals and retained the conception of Casey (in Smith 1965a); their view is adopted herein. Coolibah Formation (Olc) The Coolibah Formation (Casey in Smith 1965a) outcrops around the Toko Syncline in TOBERMOREY, HAY RIVER, GLENORMISTON and MOUNT WHELAN. It is generally considered to be absent in the Tarlton Range, although a 2 m-thick calcareous sandstone bed in the southern part of the range, to date and herein treated as basal Nora Formation, has yielded the distinctive Coolibah Formation gastropod Teiichispira cornucopiae (Gilbert-Tomlinson in Hill et al 1969, Gilbert-Tomlinson 1973, Shergold and Druce 1980: 163). The formation comprises grey and white limestone, quartzic limestone, dolostone and marl, and local chert lenses. It is conformable between the Kelly Creek Formation below (Shergold 1985: 17) and the Nora Formation above. 29 Figure 34 Kelly Creek Formation. Intraformational granule conglomerate with floating rounded-subrounded light grey quartzite and minor white quartz pebbles to boulders. Scale bar = 2 mm. Photomicrograph C75133, crossed nicols; Marqua QQ421980, Umberumbera Hills Figure 35 Kelly Creek Formation. Cruziana and Rusophycus on sole of medium quartz sandstone bed. Hand lens is 4.5 cm long. Tarlton PQ886658, Umberumbera Hills Figure 36 Kelly Creek Formation. ?Gyrolithes on sole of medium quartz sandstone bed in ledge at 57 m level in measured section near Lucky Bore, southern Tarlton Range (see Figure 27). Hand lens is 4.5 cm long. Tarlton PQ920682 30 by stromatolitic limestone and bioturbated calcimudstone interbedded with siltstone (Shergold et al 1976: 25); fenestral limestone is also present. A section of similar thickness to the north at QQ765906 includes light grey peloid and nodular limestone and intraclast-bioclast grainstone (Figure 38) in recessive yellow-grey, thinly interbedded calcimudstone and marl. Elsewhere, Smith (1972: 127) listed thicknesses of 55 m at Gaphole Creek (TOBERMOREY), 36 m in drillhole Netting Fence 1 (GLENORMISTON) and 110 m in MOUNT WHELAN; Shergold (1985) reported 79 m in drillhole AOD Ethabuka 1 (MOUNT WHELAN) and 28 m in drillhole Mount Whelan 1 (MOUNT WHELAN; Green and Balfe 1980); and Gausden (1980) attributed 51 m in drillhole Mirrica 1 (MOUNT WHELAN). Citing drilling reports, Smith (1972: 127) and Shergold and Druce (1980: 163) additionally noted peloid grainstone, stylolitic limestone and micaceous, cherty and pyritic calcimudstone, together with basal sandstone, in drillholes Netting Fence 1 and Ethabuka 1. Green and Balfe (1980) reported the presence of bioclast grainstone to wackestone and a basal intraformational conglomerate in drillhole Mount Whelan 2 (MOUNT WHELAN). The Coolibah Formation contains a varied fauna of nautiloids, gastropods, sponges, trilobites, rostroconch molluscs, pelecypods, brachiopods and conodonts (GilbertTomlinson 1973, Pojeta and Gilbert-Tomlinson 1977, Wade 1977a). Conodonts have confirmed a middle to late Arenig age (Stait and Druce 1993). The Coolibah Formation was deposited in a shallow marine environment, under shallow subtidal to intertidal, fluctuating energy conditions (Shergold et al 1976). Figure 37 Kelly Creek Formation. Quartzic dolosparstone of abundant, admixed, subangular to subrounded, very fine to medium sand-sized quartz and minor glauconite in coarse hypidiomorphic to idiomorphic dolomite spar. Scale bar = 2 mm. Photomicrograph C75153, crossed nicols; Tarlton PR834009, northern Tarlton Range Nora Formation (Oln) Casey (in Smith 1965a) nominated a type section near Tobermorey No 8 Dam in southeastern TOBERMOREY, where he estimated a formation thickness of 53 m. However, Smith (1972) reported a thickness of 15 m in the type area, and Shergold et al (1976: 24) presented a type section of only 9 m thickness. The type section commences with silicified microbial mounds in conglomeratic limestone, overlain In the Toko and Tarlton Ranges of TOBERMOREY, the Nora Formation (Casey in Smith 1963) rests conformably on the Coolibah Formation and Kelly Creek Formation, respectively. In both areas the Nora Formation typically constitutes the scarp of the ranges, which are capped by the conformably overlying, resistant Carlo Sandstone. Westward, in Dulcie Range (HUCKITTA), the Nora Formation is eroded and is Figure 38 Coolibah Formation. Intraclast bioclast grainstone-rudstone of intraclasts, echinoderm ossicles and trilobites in coarse calcite spar. Intraclasts include reworked calcimudstones and bioclast grainstones with trilobites, echinoderm ossicles, bryozoans (top centre) and pelecypods or ostracodes. Scale bar = 2 mm. Photomicrograph C75154, plane polarised light; Toko QQ765906, Bloodwood Creek 31 unconformably overlain by the Dulcie Sandstone (Devonian). Southward and eastward, it extends onto HAY RIVER, MOUNT WHELAN and GLENORMISTON. Casey (in Smith 1963) coined the formation name for an exposed 60‑120 m-thick interval of recessive, thinly bedded olive, yellow, grey, brown and purple, micaceous and glauconitic siltstone, claystone, fine sandstone, minor dolostone and basal brown coquinite (Smith 1965a, 1972). A measured section of the formation in the southern Tarlton Range (Figure 33) is 42 m thick. Some clastic beds are rippled and cross-laminated (Shergold and Druce 1980). Casey (in Smith 1963) nominated a type area on the western scarp of the Toko Range near Halfway Dam, in southeastern TOBERMOREY. In the subsurface, the formation thickens southeastward to 190 m in drillhole Mirrica 1 (Gausden 1980), 235 m in Mount Whelan 2 (Green and Balfe 1980), and according to Shergold (1985: 18), 250 m in Ethabuka 1 in MOUNT WHELAN. The formation is generally masked by Carlo Sandstone scree, and so is exposed only in steep gullies incising the scarp (eg at PQ839963 in the northern Tarlton Range). The lower part of the formation is richly fossiliferous, reportedly yielding nautiloids, brachiopods, pelecypods, trilobites, ostracodes, gastropods, rostroconchs, echinoderms, sponges, bryozoans, corals, fish remains, questionable foraminifers and ichnofossils of mid-late Arenig age. Aspects of the nautiloids have been described by Beard (in Hill et al 1969) and Wade (1977a, 1977b), conodonts by Nieper (in Hill et al 1969), brachiopods by Telford (in Hill et al 1969), pelecypods by Pojeta and Gilbert-Tomlinson (1977), rostroconchs by Pojeta et al (1977) and trilobites by Fortey and Shergold (1984). The basal coquinite is a distinctive iron-enriched, dull mid-dark brown bioclast grainstone-rudstone bed, which is rich in echinoderm plates and gastropods (Figure 39). The upper portion of the formation has fewer body fossils, but does contain ichnofossils, including Diplocraterion, Diplichnites and horizontal trails, indicative of at least the Cruziana ichnofacies. The predominantly fine grainsize of the Nora Formation implies deposition under low-energy conditions, but higher-energy episodes are indicated by included bioclast grainstones. An intertidal to shallow subtidal marine depositional environment was deduced by Shergold et al (1976). More specifically, Shergold and Druce (1980) suggested an offshore bar environment for the basal coquinite, whereas Draper (1977) envisaged an offshore, below wavebase setting for the entire formation. Carlo Sandstone (Omc) The Carlo Sandstone (Casey in Smith 1963) is a unit of resistant red-brown, well sorted, fine to medium quartz sandstone and minor feldspathic sandstone. Its medium to thick beds cap the Toko and Tarlton Ranges in TOBERMOREY, HAY RIVER, GLENORMISTON and MOUNT WHELAN. A prominent clay-pellet bed denotes the base of the formation. Casey (op cit) nominated a type area near Carlo homestead in MOUNT WHELAN (location in Draper 1977: fig 2) and estimated an exposed thickness of 75 m (90 m according to Smith 1965b) in southwestern GLENORMISTON. A thickness of 22 m was measured in the southern Tarlton Range (Figure 33). The thickness increases southeastward to at least 150 m in MOUNT WHELAN (Smith 1972). In the subsurface, the formation attains 174 m in drillhole Ethabuka 1 (190 m according to Draper (1977), and 176 m in Mirrica 1 (Gausden 1980), both in MOUNT WHELAN. Lower and upper contacts are gradational and conformable, with the Nora Formation below and the Mithaka Formation above. Current features, including streaming lineation, flute casts, thick cross-bed sets and ripples, are characteristic. Measured current directions in the Tarlton Range (Smith et al 1961) are predominantly from the southeast (see Smith 1972: 129‑130). Measurements from the Toko Range (Draper 1977) gave similar results. Shergold et al (1976: 22) recognised a threefold subfacies division of the formation: the basal subfacies is characterised by a mixture of biogenic and sedimentary structures (thin to medium, fine quartzose sandstone bearing clay pellets); the medial subfacies is dominated by sedimentary structures (thinly to very thickly bedded, fine to medium sandstone with common ripples and cross-stratification); F i g u re 3 9 N o r a F o r m a t i o n . B a s a l ferruginous echinoderm grainstonerudstone bed with echinoderm plates and minor gastropods, nautiloids, trilobites and possible brachiopod fragments. Scale bar = 2 mm. Photomicrograph C75155, plane polarised light; Toko QQ765906, Bloodwood Creek 32 Figure 40 Carlo Sandstone. Arthropod scratch marks on sole of medium quartz sandstone bed. Pen is 13.5 cm long. Tarlton PQ839963, northern Tarlton Range and the topmost is dominated by biogenic structures (mottled bioturbated, very fine to fine sandstone). Draper (1977) has provided extended descriptions of these. The basal subfacies caps the plateau tops of the Toko and Tarlton Ranges. Body fossils are not common and are mainly found toward the base and top of the formation (Shergold et al 1976: 25, Draper 1977). Sparse nautiloids, brachiopods, rostroconchs, gastropods, pelecypods, conodonts and fish are suggestive of a Middle Ordovician (latest Arenig to Llanvirn) age (Öpik and Gilbert-Tomlinson in Smith 1972, Shergold et al 1976). In contrast, ichnofossils are abundant and diverse, and include Skolithos, Diplocraterion, Rhizocorallium, Arenicolites, Arthrophycus, Diplichnites, Dimorphichnus, Merostomichnites, Rusophycus, Cruziana and arthropod scratch marks (Figure 40), among others (Shergold et al 1976, Draper 1977). These are collectively representative of the Skolithos and Cruziana ichnofacies of Seilacher (1964). The Carlo Sandstone denotes deposition in shallow littoral conditions in a shoaling or barrier island setting (Shergold et al 1976). Draper (1977) has outlined a highenergy barrier model, with component barrier island, flat and bay subfacies. maximum thickness of 156 m was intersected in drillhole Mirrica 1 in MOUNT WHELAN (Gausden 1980). The Mithaka Formation is conformably overlain by the Ethabuka Sandstone. The fauna of trilobites, ostracodes, nautiloids, gastropods, pelecypods, brachiopods, bryozoans, receptaculitaleans, sponges, conodonts, fish, chitinozoans, ichnofossils and probable echinoderms remains virtually undescribed, but is suggestive of a Middle Ordovician (Llanvirn) age (Shergold 1985). Pelecypods locally form shell banks in the lower part of the formation, and ichnofossils include Monocraterion, Arenicolites, Diplocraterion and Chondrites-like burrows (Draper 1977), as well as Cruziana (Seilacher 1970) and horizontal to oblique traces. These represent the Glossifungites, Skolithos and Cruziana ichnofacies of Seilacher (1964). Enormous Rusophycus, up to 30 cm in length (Figure 41), are associated with equally large asaphide trilobites (Draper 1980a). Draper (1977) envisaged a low-energy marine lagoonal setting with limited tidal range, inshore of a barrier of Carlo Sandstone, for the Mithaka Formation . Ungrouped Mithaka Formation (Omm) Ethabuka Sandstone (Ome) The Mithaka Formation (Casey in Smith 1963) comprises poorly exposed, thinly bedded, brown and grey gypsiferous siltstone and fine sandstone, white glauconitic and micaceous quartz sandstone, gypsiferous green shale, calcareous siltstone and minor coquinite and granule conglomerate. It gradationally and conformably overlies the Carlo Sandstone (Smith 1972: 130). The formation is largely confined to the Toko Syncline in HAY RIVER, MOUNT WHELAN, GLENORMISTON and southeastern TOBERMOREY where, according to Smith (1965a), a maximum 60 m is exposed. The type area of Casey (in Smith 1963) is in southwestern GLENORMISTON. Small outliers up to 20 m thick (Smith 1972) surmount the Tarlton Range in southwestern TOBERMOREY. A Draper (1980b) named the Ethabuka Sandstone for a thick sublabile to quartzose sandstone previously termed the Ethabuka beds (Mulready 1975) or attributed to the Devonian Cravens Peak beds (Reynolds 1968). The type section is in drillhole Ethabuka 1 (Mulready 1975) in MOUNT WHELAN, where according to Draper (1980b), the formation is 1147 m thick, resting conformably on the Mithaka Formation and unconformably overlain by the Jurassic Hooray Sandstone. In a fourfold subdivision of the formation, the two lower subunits (respectively 224 m and 200 m thick) are of very fine to fine sandstone with siltstone interbeds and rare pebbly layers; siltstone is more prevalent in the upper subunit. The two uppermost subunits (respectively 251 m and 472 m thick) are of fine 33 Figure 41 Mithaka Formation. Enormous Rusophycus from base of formation. Toko QQ983584, Gaphole Creek to medium sandstone; the upper 128 m contains additional interbedded claystone. Only the lowest subunit outcrops, as an escarpment around the junction of TOBERMOREY, GLENORMISTON and HAY RIVER within the axial Toko Syncline, with a maximum exposed thickness of 35 m (Shergold et al 1976). The thinly to medium-bedded sandstone bears clay pellets (locally as clay pellet conglomerate), crosslaminations, asymmetric linguoid and interference ripples, load casts, parting and streaming lineations, flute casts and other current structures, and minor phosphate pellet conglomerate and flaser bedding, together with the ichnofossils Rusophycus, Cruziana, burrows (including U-tubes) and trails (Draper 1980b). The undescribed fauna of pelecypods, trilobites, nautiloids, gastropods and brachiopods was assumed by Shergold (1985) to be Middle Ordovician (late Llanvirn). In outcrop, the Ethabuka Sandstone is unconformably overlain by the Cravens Peak beds, or where these are absent, by Mesozoic sandstone. Draper (1980b) drew attention to the similarity of the Ethabuka Sandstone with the lower Carlo Sandstone and invoked similar depositional conditions: a marine high-energy barrier, but with bioturbated intervals also indicating a lower-energy, subtidal environment. Haines et al (2001) viewed the Ethabuka Sandstone as representing an initial pulse (Rodingan Movement) of a protracted Alice Springs Orogeny. RQ0561. Revised, the Cravens Peak beds are 280 m thick and are of lower calcareous rocks (calcareous siltstone, calcareous sandstone, limestone and minor conglomerate) grading into upper sandstone and conglomerate. The suggested intervening hiatus of Gilbert-Tomlinson (1968) has not been corroborated. Turner et al (1981) documented scales of the thelodont fish Turinia, ostracodes and eridostracans from the lower calcareous unit, and the placoderm Wuttagoonaspis from the upper unit, confirming an Early-Middle Devonian (Emsian-Eifelian) age (see also Young 1996). Only the Wuttagoonaspis fauna is recorded from TOBERMOREY. These data enabled those authors to propose a shallowing depositional environment in the Cravens Peak beds from an initial shallow subtidal marine setting (stromatolites and marginal marine, stratigraphically lower fauna) upward via beach and offshore sandbar deposits (oncoids, phosphorite, lower fauna, high-angle tabular cross-beds) into a non-marine braided fluviatile environment (freshwater Wuttagoonaspis fauna) in upper beds. Haines et al (2001) portrayed the Cravens Peak beds as synorogenic deposits generated by a mid-Devonian pulse (Pertnjara-Brewer Movements) of the Alice Springs Orogeny. Devonian Mesa-forming quartz sandstone and minor mudstone and conglomerate overlying Arunta Province and Georgina Basin rock units in southern TOBERMOREY represent outliers of the Eromanga Basin. Outcrops range from leached white or pale orange-brown through to ferruginised dark maroon-brown, and are commonly variegated. Sandstone is planar to cross-bedded and locally contains lenticular granule conglomerate or scattered quartz and lithic pebbles and cobbles. Claystone may be ferruginised to maroon-purple or red-brown on outcrop crests, or leached white below. Mesozoic Undifferentiated Jurassic-Cretaceous (JK) Cravens Peak beds (Dc) In their original concept, the Cravens Peak beds (Reynolds in Smith 1965a) included rocks since discriminated as the Ethabuka Sandstone, Cravens Peak beds proper and the Cenozoic Poodyea Formation (Draper 1980b: 472). The principal outcrop area lies in HAY RIVER, MOUNT WHELAN and GLENORMISTON, but outliers are present in the southeastern corner of TOBERMOREY, centred on 34 Most of these rocks were assigned to the Tarlton Formation (Condon and Smith 1959) by Smith (1965a) and attributed a Triassic age on the basis of Triassic or Early Jurassic plant fossil determinations by White (1961). In addition, a few localities in southeastern TOBERMOREY were placed in the Cretaceous Longsight Sandstone. Mond and Harrison (in Senior et al 1978) revised the lithostratigraphic nomenclature of the basin, subsuming both of the above units into the Hooray Sandstone (Hill and Denmead 1960). Gould (in Hill and Denmead 1960) deduced a Jurassic to Early Cretaceous age from additional plant fossils collected from the Tarlton Range. All Mesozoic rocks in TOBERMOREY would thus appear to belong to the Hooray Sandstone. Nevertheless, pending specific studies, these rocks are mapped herein as undifferentiated Jurassic-Cretaceous. Prominent outcrops in central TOBERMOREY, formerly mapped as the Tarlton Formation, are here remapped as the Kelly Creek Formation, as confirmed by occurrences of ichnofossils of Ordovician aspect. These outcrops are now understood to be western outliers of the Toko Syncline succession. within the unit, whereas rare charophyte and other algal remains, plant tissue, ostracodes and foraminifers indicate brackish lacustrine conditions (Paten 1964). Lloyd (1968a) reported the foraminifer Ammonia beccarii from the Austral Downs Limestone, for which Lloyd (1968b) deduced a Miocene age. Grimes (1980) viewed the formation as having resulted from the damming of an ancestral Georgina River, regionally generating up to 35 m thickness of sediment. Smith (1965a) estimated a more modest thickness of 15 m in TOBERMOREY. The Austral Downs Limestone forms prominent, extensive plateaux overlying the Ninmaroo Formation in central-eastern TOBERMOREY and outliers extend into the central map area. Plateaux are best developed around Tobermorey homestead, where marginal cliffs are 2-3 m high. Outliers may also be steep-sided plateaux (eg QR292120 near No 6 Bore Manners Creek, where there are cliffs of silicified limestone at least 7.5 m high; Figure 42) or much more subdued (eg QR203428 in western alkea, where fragmental calcrete is capped by white to translucent chalcedony). One such outlier at QR521512 is a bioclast wackestone with gastropods. Palaeopedogenic features are common in the formation. These include rhizoliths, alveolar texture, and soil glaebules with circum- and intragranular cracks (see Esteban and Klappa 1983), collectively indicative of a subaerial calcrete origin. Both alpha fabrics (dominated by non-biogenic textures of dense micrite to microsparite) and beta fabrics (dominated by biogenic features) are known (Figures 43, 44; see Wright and Tucker 1991). A basal contact is preserved at QR296129, where the Austral Downs Limestone rests disconformably on Kelly Creek Formation. There, typical orange-brown medium quartz sandstone of the Kelly Creek Formation is patchily transformed to more ferruginous pisolitic ironstone upward, passing rapidly and unevenly into a 1.5 m-thick maroon-red ferruginous, fine to medium sandstone cap. This ferruginous sandstone bears coarse (1-2 cm), presumed vadose pisoids at its base, and sand-sized pisoids are scattered throughout, suggesting a palaeosol profile (cf Paten 1964). This profile is succeeded by interbedded, similar maroon ferruginous Palaeogene-Neogene Austral Downs Limestone (Cza) Terrestrial lacustrine limestone within the Georgina River catchment, including HAY RIVER, TOBERMOREY, SANDOVER RIVER, AVON DOWNS and adjacent areas of Queensland, is assigned to the Austral Downs Limestone (Noakes and Traves 1954). This limestone is pale grey, but especially toward the top, is silicified to resistant, redbrown and white chert and chalcedony. Western Queensland outcrops were described by Paten (1964), who noted a similarity with modern spring deposits in that area. He further recognised brecciation within the limestone, and a basal reworked ferruginous interval which he took to indicate that the Austral Downs Limestone postdated a widespread Cenozoic ferruginisation episode (part of Czf herein) in the region. Whitehouse (1940) recognised pedogenic limestone Figure 42 Austral Downs Limestone. Cliffs of silicified limestone. Alkea QR292120, near No 6 Bore Manners Creek 35 GLENORMISTON and MOUNT WHELAN are referred to the Poodyea Formation (Radke et al 1983). Component clasts in conglomerate include quartzose sandstone derived from the Carlo Sandstone, vein quartz and quartzite. Trough cross-strata occur in medium- to large-scale sets. The unit outcrops within existing valleys or as low ridges and is restricted to the central Toko Syncline, where it rests unconformably on the Nora Formation, Carlo Sandstone, Mithaka Formation, Ethabuka Sandstone and Cravens Peak beds. It is considered to represent fluviatile channel deposits. The type area is at RQ004666 in TOBERMOREY, where the thickness is probably less than 10 m. A pre-Quaternary Cenozoic age is likely, as the formation is post-Devonian, is undisturbed by structures which have influenced erosional patterns of Mesozoic rocks, and is locally overlain by Quaternary cover. Palaeogene-Neogene-Quaternary Ferricrete and manganocrete (Czf) Superficial, dark iron-rich (haematitic or goethitic) crusts up to several metres thick locally cap dolostone of the Ninmaroo Formation and especially, highly weathered sandstone of the Tomahawk Formation. Nodular and massive types are the most common, whereas encrustations and botryoidal textures are much less so. Where developed on Tomahawk Formation sandstone, relict textures and quartz sand grains may be preserved. Locally, it is partially silicified, or manganiferous up to the point of being more properly called manganocrete. It has a distinctive very dark photopattern, but not all such dark areas are ferricrete; some are ferruginous sandstone, and probably represent areas from which a former ferricrete cover has been stripped. Figure 43 Austral Downs Limestone. Pedogenic alpha fabric of soil glaebules with intra- and circumgranular cracks. Scale bar = 2 mm. Photomicrograph C75150, plane polarised light; Alkea QR334178, near Alkea Waterhole sandstone and white porous calcimudstone of the basal Austral Downs Limestone. Silcrete (Czz) Poodyea Formation (Czp) Silcrete forms local replacement cappings atop Ninmaroo Formation dolostone and Tomahawk Formation fine quartz arenite. Some retain relict textures of the parent rock. In the region east of No 16 Bore Lucy Creek, silcrete is better developed than elsewhere, and distinctive yellow-brown, red, Sinuous outcrops of cobble and boulder conglomerate, cross-stratified conglomerate and pebbly sandstone in the southeastern corner of TOBERMOREY and in adjacent Figure 44 Austral Downs Limestone. Pedogenic beta fabric of soil glaebules with intra- and circumgranular cracks, bound by abundant rhizoliths. Scale bar = 2 mm. Photomicrograph C75134, plane polarised light; Tobermorey QR924297, near Indeear Bore 36 black and variegated varieties can co-occur in the same outcrops. Some of these have brecciated texture (eg hill beside Algamba Creek at QR103201). Locally, ferruginous and manganiferous types are present. At QR382272 in southern Alkea, laminated silcrete incorporates tabular ironstone rubble. Grimes (1980) recognised three episodes of ferricrete and silcrete development in adjacent western Queensland: Palaeocene, Oligocene and (post-Austral Downs Limestone) Mio-Pliocene. Along the southern Georgina Basin margin, pre‑800 Ma tectonism (Zhao et al 1994) created northwest-trending grabens (including the Keepera and Adam Troughs in southwestern and southeastern TOBERMOREY, respectively) in which thick Neoproterozoic successions accumulated (Walter 1980). Evidence for the Larapinta Event is from the Irindina Supracrustal Assemblage (ISA; Chan et al 1990, Mawby et al 1999) around Harts Range in ALICE SPRINGS and ILLOGWA CREEK, and the extent of its effect on the Narwietooma Package in TOBERMOREY is not yet clear. The ISA may represent latest Neoproterozoic to early Cambrian deposition within a deep extensional sub-basin (Irindina subbasin of Buick et al 2001a, 2001b) within the more extensive Centralian Superbasin, the former subsequently deformed and metamorphosed during the Larapinta Event. Alternatively the ISA may be an allochthonous thrust sheet emplaced over the basement. Undisturbed and unmetamorphosed Ordovician sedimentary rocks northeast of the Tarlton Fault in the Georgina Basin proper are located on the Altjawarra Domain. This stable basement is surmised to have allowed these sedimentary rocks to accumulate and remain preserved in pristine condition, even while dramatic Ordovician tectonism was operative in the adjacent Irindina sub-basin. The Alice Springs Orogeny, commencing around 450‑440 Ma (Late Ordovician; Scrimgeour and Raith 2001, Haines et al 2001) was a compressional intraplate event which exhumed the Arunta Province from beneath a formerly continuous intracratonic Centralian Superbasin, now represented by the Georgina Basin and other basins. As a consequence, Arunta Province rocks have been thrust against Georgina Basin rocks. In addition, synorogenic sedimentation associated with the Late Ordovician event (Rodingan Movement) may be represented by the Ethabuka Sandstone (Haines et al 2001), although the precise age of this unit remains uncertain. These authors also relate the Cravens Peak beds to the mid-Devonian Pertnjara-Brewer Movement. The Alice Springs Orogeny was the last major tectonic episode in central Australia. It rejuvenated older faults and produced the present major structures of the region. The southern Georgina Basin and adjacent Arunta Province have been subjected to extensive faulting with a dominant northwesterly trend. According to Smith (1972: 41), all the major northwesterly faults are normal faults, downthrown on the northeastern side, and locally with throws of more than 1000 m. For the Arunta Province of southwestern TOBERMOREY (Narwietooma Package of Pietsch (2001), this trend is evident in airborne magnetic data (see mapface). Major faults in TOBERMOREY are the Tarlton Fault in the west and Toomba Fault Zone in the east. These affect Devonian but not Mesozoic rocks and so were last reactivated within that interval, most likely during the mid-Carboniferous (340‑320 Ma) Mount Eclipse Movement (Haines et al 2001). The Tarlton Fault denotes the contact between the Arunta Province and Georgina Basin terranes in TOBERMOREY. According to Warren (1981: 5), the Tarlton Fault was already active by 1700 Ma. It is exposed along the southeastern Tarlton Range southward of the Keepera Ridges, and further to the southeast is intermittently discernible as far as northern Unconsolidated sand (Czs) Unconsolidated colluvial sand and minor silt blankets much of TOBERMOREY. In the northwestern map area, it develops as deep drifts over parent Tomahawk Formation rocks. Quaternary Regolith (Cz) This term embraces in situ weathering products and shallow skeletal soils, not otherwise categorised herein, which are directly derived from underlying shallow subcrop with little active transport. It is depicted on the mapface in the colour of the parent subcrop unit, with superimposed grey stipple. Alluvium (Qa) Alluvium of sand and minor gravel, silt and clay is deposited in the beds and banks of watercourses. Flat Qa floodplains are generally demarked from adjacent colluvial sand (Czs) by a break in slope. However in northwestern TOBERMOREY, alluvium consists of reddish quartz sand that is indistinguishable on the ground from wind-blown colluvial sand (Czs). In this area, one vertical exposure of alluvium in an eroded creek bank over 3 m high consists of essentially structureless sand and lamination is only locally discernible. Areas were mapped as alluvium where the photopattern indicated reworking of the sand by occasional flowing water. In floodout areas where floodwaters dissipate into a sheet, the boundary between Qa and Czs becomes arbitrary. Claypans (Qp) Clay accumulates in poorly drained depressions on floodplains and along relict drainage. Many are poorly vegetated, but a waterlogged minority supports denser vegetation. Structure and tectonics The eastern Arunta Province has been shaped by three major tectonic events: the 1780‑1730 Ma Strangways event, which resulted in high-grade metamorphism and granite intrusion; the 480‑460 Ma deformation and granulite-grade metamorphism of the Larapinta Event (Hand et al 1999); and the 400‑300 Ma Alice Springs Orogeny (Mawby et al 1999). In addition, an early Neoproterozoic extensional episode has probably influenced the adjacent Georgina Basin. 37 HAY RIVER. Georgina Basin rocks as low in the succession as the upper Tomahawk Formation are sharply upturned on the downthrown northeastern side. At PQ825699 near Canyon Bore, partially and completely dolomitised grey/brown and yellow-brown carbonate beds of the uppermost Tomahawk Formation dip vertically adjacent to the fault. Eastward toward the Tarlton Range, dips in overlying maroon sandstone of the Kelly Creek Formation become progressively less steep as they approach the flat-lying Nora Formation. The fault itself is concealed beneath alluvium adjacent to a fault-parallel creek; small outcrops of Arunta Province rocks are visible in the opposite bank. A similar situation obtains at PQ841669, but there the fault is marked by a prominent thick quartz vein. At PQ833685, a small anticline is developed in upper Tomahawk Formation limestone beds immediately adjacent to the fault. Warren (1981) interpreted tilted Mesozoic sandstone and an early Cenozoic ferruginous deep weathering profile near Junction Bore in southwestern TOBERMOREY as indicating a late movement on the Tarlton Fault during the Eocene-late Oligocene. The Toomba Fault is about 200 km long and extends from TOBERMOREY through northeastern HAY RIVER into MOUNT WHELAN. It has affected rocks as young as Devonian. Seismic interpretation in MOUNT WHELAN by Harrison (1979, 1980) shows that the Toomba Fault is a high-angle reverse fault (Reynolds 1968) dipping to the southwest at 40‑70° and with a vertical displacement of 6.5 km. Adjacent Palaeozoic rocks on the northeastern side are steeply upturned, overturned, or folded as monoclines and anticlines. In addition, right-lateral transcurrent displacement was up to 4 km. Harrison (1980) related these features to northerly compression in the Late Devonian or Early Carboniferous. The Marqua Monocline is a northwesterly extension of the Toomba Fault Zone. Its southern limb is vertical while its northern is almost horizontal (Shergold 1985). Rock units ranging from the Black Stump Arkose to Ninmaroo Formation are affected. The Toko Syncline, to the north of the Toomba Fault Zone in the southeastern map area, is a southeasterly plunging asymmetric syncline with steeper dips on its western limb. The syncline is traversed by an east-northeast fault set, complementary to the dominant northwesterly trend. These are normal faults, downthrown on their southeastern sides. Throws on these structures attain 60 m, but are generally less than 30 m (Smith 1965a, 1972). Apart from mesoscale deformation in the Tomahawk Formation (which see), folding and faulting are much less evident in central and northern TOBERMOREY, away from the basin margin. Second Edition mapping did not substantiate most north- and northeast-trending fold axes depicted in the Tomahawk Formation on the First Edition map in central-western TOBERMOREY. However, two north-northeast-trending lineaments, located 8 km and 10.5 km to the southeast of No 20 Bore Lucy Creek, are interpreted as faults associated with folds. The western occurrence (PR647107) has the better outcrop, and consists of a long, low ridge. The lineament along the eastern side of this ridge, presumably a fault, is not exposed, but parallel to it on the west is the axis of a tight anticline. Dips on the eastern and western limbs both range up to 50°, and dip reversal occurs within a distance of 30 m. The Ninmaroo Formation in central and northern TOBERMOREY is characteristically jointed. In the central map area, joint sets trend west-northwest and roughly parallel fault trends to the south. In the northeastern map area, joint sets parallel the complementary east-northeast fault set in the Toko Syncline. Geophysics NTGS airborne magnetic and radiometric surveys cover most of TOBERMOREY with north-south oriented flight lines spaced at 400 m (Elkedra 1999, Eromanga 2001) or 500 m (Huckitta East 1983). The Elkedra and Eromanga surveys were flown with a nominal terrain clearance of 60 m, whereas that of Huckitta East was 100 m. Semi-regional and radiometric data are not available for Tobermorey, but older BMR regional aeromagnetic data on 3200 m-spaced flight lines are available for this area. The two recent radiometric surveys have a nominal sampling interval of 70 m, using a 33 L crystal. Full details of NTGS surveys are accessible on the website http://www. dbird.nt.gov.au/ntgs/geophysics/airborne.html. National reconnaissance gravity data at nominal 11 km station spacing are available for the whole of TOBERMOREY. These were acquired in a series of ground traverses and helicopter surveys between 1957 and 1961 (Barlow 1966). This regional data set is supplemented by several company surveys, commencing with Alliance Petroleum Australia, who in 1963 performed a series of traverses on existing tracks at about 1.5 km spacing. Alliance also acquired more detailed data (0.8 km spacing) on semi-regular grids in the vicinity of the Tarlton and Toko Ranges (Alliance Petroleum Australia 1964). Subsequent gravity surveys were in generally northeastoriented traverses with varying interline and intraline station spacing. Plenty River Mining Company occupied over 500 stations, mainly on Toko, on lines 5 km apart and 1 km station spacing (Plenty River Mining Company 1984). Pacific Oil and Gas surveyed several widely spaced traverses across TOBERMOREY (Pacific Oil and Gas 1991). Station spacing on these lines was mainly 1 km, with some segments at 5 km and one traverse near the Queensland border at 200 m (Preston 1988). Early seismic work by BMR (Montecchi and Robertson 1966) was not generally successful in obtaining useful reflection data. However, velocity information was obtained from refraction work in central TOBERMOREY (Chenon 1966). The only modern seismic data acquired were four lines totalling 123 line km in southeastern TOBERMOREY, one of which (G89-303) passes close to drillhole Owen 2. This survey, conducted for Pacific Oil and Gas Ltd, was documented by Sweeney (1990). Gravity The gravity field of TOBERMOREY is relatively featureless in comparison with surrounding regions, with low-amplitude (less than 100 µm.s-2) anomalies and gentle gradients. As such, much of the map area has been 38 designated the Tobermorey Gravity Shelf (Barlow 1966). The total range of Bouguer anomalies is approximately 400 µ m.s -2. Higher amplitude, steeper anomalies are confined to the edges of the map area, where the flanks of the Hay River Gravity Low and Field River Gravity Spur (Barlow 1966) impinge respectively on the southwest and southeast of TOBERMOREY. These features are northwest- to north-northwest-trending regional structures. Otherwise, no pervasive structural trend is apparent in the gravity data. The gravity low in the southwestern corner of Tobermorey is the northern edge of a larger feature, itself superimposed on the more extensive Hay River Gravity Low, extending onto HAY RIVER and ILLOGWA CREEK. This feature is nowhere expressed in outcrop, but is interpreted to be a granite pluton at depth. A magnetically quiet area coincident with the gravity low in northwestern HAY RIVER indicates possible subcrop of this granite. There is a negative Bouguer gravity contrast of approximately 200 µm.s-2 between this region and the northwest-trending belt of Palaeoproterozoic migmatite and granite to the southwest of the Tarlton Fault. Bouguer gravity values diminish only slightly to the northeast of the Tarlton Fault, implying either little vertical displacement, or minimal overall density contrast between Georgina Basin sedimentary rocks and adjacent Arunta basement. Downhole density measurements suggest that the latter may be the case (Table 2). Such relative decrease in Bouguer gravity to the northeast of the Tarlton Fault as does exist is probably due merely to the more porous siliciclastic portions of the succession. The general similarity of density between basement and basin fill, together with the shallow dips of the sedimentary succession, suggest that much of the gravity signal is likely to be due to intrabasement lithological variation. Consequently, the curvilinear association of intense high and low Bouguer anomalies in northwestern TOBERMOREY is interpreted to result from a steeply dipping, thick mafic body concordantly within felsic igneous bodies and psammitic metasediments. However, the gravity high of the Field River Gravity Spur in southeastern TOBERMOREY is considered to arise at least in part from the thick carbonate succession in the Toko Syncline. An association between west-northwest-trending gravity highs and similar magnetic features in northern TOBERMOREY suggests that these represent west-northwest-striking basement with a mafic igneous component. The remaining gravity features on unit ninmaroo Formation arrinthrunga Formation hagen Member steamboat sandstone arthur creek Formation thorntonia limestone red heart dolostone Palaeoproterozoic felsic igneous rocks number of readings Porosity (%) 2900 1788 323 5454 636 68 118 5.0 8.5 6.7 8.9 3.9 7.3 2.5 3.8 TOBERMOREY are integrated responses from basement geology (particularly granitoid intrusions), basin shape and intrasedimentary compositional facies changes, and cannot be interpreted with confidence. Magnetics Most magnetic anomalies on TOBERMOREY range up to 500 nT in amplitude, both positive and negative, with isolated more intense features superimposed. Georgina Basin sedimentary rocks are almost totally non-magnetic; hence the observed magnetic signal is entirely from the basement interface and intrabasement units. The most pervasive magnetic trend is generally west-northwest and is exemplified by high-frequency anomalies in the southwest. Northwestern TOBERMOREY is exceptional in this regard, with indications of northeasterly-striking magnetic basement units. One of these corresponds to the intense gravity high discussed above. There are also some indications of deeper-seated north-south-trending magnetic sources. The southwestern high-frequency anomalies are clearly associated with Arunta Province subcrop, though in detail most Arunta outcrops occur in areas of little or no magnetic signal. The anomalies are due to mafic metamorphic rocks of the Narwietooma Package, which do not outcrop. There may also be a magnetic signal from outcropping migmatite and granite, if magnetite is a significant accessory. The Tarlton Fault is only expressed in the high frequency component of the magnetic data, with if anything a slight basinward rise in long-wavelength magnetic intensity across the fault. As with the gravity, this implies limited (less than 1 km) vertical displacement on the Tarlton Fault. There is some indication of continuity of magnetic source at depth across the fault. Indications from drilling and previous basement modelling (Wells et al 1966, Teasdale and Pryer 2002) are that the Georgina Basin undulates around depths of 1 km through much of TOBERMOREY. Intersections with felsic igneous rocks at 1221.9 m, 1158.8 m and 1104.6 m depth in Hacking 1, Owen 2 and Lucy Creek 1, respectively, are broadly consistent with this interpretation. The lack of a gravity low coincident with the magnetic low in southcentral TOBERMOREY suggests that the low may denote carbonate basin fill rather than a basement granitoid. Conversely, the magnetic high around QR500300 may represent elevated basement, as there is no correlatable sg bulk wet (tm-3) 2.63 2.6 2.69 2.62 2.67 2.7 2.77 2.66 sg standard deviation 0. 0.08 0. 0.04 0.05 0.03 0.03 Mean depth (m) 81.0 387.9 608.6 668.1 894.6 1101.8 1216.8 379.5 Mean thickness (m) 42.0 582.4 67.3 49.2 267.2 96.9 0.3 5.3 Table 2 Rock densities compiled from downhole logs and sample measurements of lithostratigraphic units intersected in drillholes (Hacking 1, Owen 2, BMR 12 Cockroach, Netting Fence 1) on or near TOBERMOREY; SG = specific gravity 39 gravity feature. Loss of high-frequency magnetic signal in northeastern TOBERMOREY is associated with decreasing Bouguer anomaly in that direction. This is also interpreted as an increase in basin thickness, with an increased proportion of siliciclastics in the subsurface relative to elsewhere in TOBERMOREY. other Cenozoic units in TOBERMOREY have variable, but low gamma signatures. In many areas these can be seen as subdued signatures of adjacent inliers, resulting from erosion and proximal dispersal. In northern and northeastern TOBERMOREY, these dispersal patterns have a distinct north-northwesterly trend, presumably imposed by prevailing wind directions. Another notable feature of the Cenozoic radiometric response in TOBERMOREY is the concentration of clays into watercourses on plains, resulting in clear delineation of Arthur Creek and the Hay and Marshall Rivers. Radiometrics Outcrops of Arunta Province rocks are generally too small for their radiometric response to be adequately resolved. The few larger outcrops appear to be of generally low total count in comparison to surrounding Cenozoic sediments, with some possible exceptions at PQ822680 and PQ558696. The various granitoids have variable total count signatures, with some evidence for at least two distinct types: one high, exemplified at PQ833668 (including L Pga); and one low, notably on the western margin of TOBERMOREY (including L Pgb). The Grant Bluff Formation has a distinctly low response. The only other radiometric features of Neoproterozoic sediments are elevated patches in the Marqua Monocline, mainly manifested in the Th channel. The radiometric response reverts to uniformly low throughout the outcropping lower portion of the Palaeozoic Georgina Basin succession up to and including the Arrinthrunga Formation. The gamma peak observed in downhole logging (see Downhole geophysics), which is associated with carbonaceous shale in the basal Arthur Creek Formation, is not apparent in the airborne surveys, presumably because it is too thin or too recessive. Areas of high total count on TOBERMOREY are associated with the Tomahawk, Ninmaroo and Kelly Creek Formations, but radiometric signatures within these units are highly variable. This reflects signal attenuation by overlying transported Cenozoic material. A proportion of the high K in the Tomahawk Formation is likely to be sourced from glauconite in sandstone, in addition to the contribution from K-bearing clay minerals in finer-grained rocks. The base of the Kelly Creek Formation is marked in many places by a band of high K. Patches of high K are also present elsewhere within the Kelly Creek Formation, but these are not clearly associated with stratigraphic horizons, except in one case on the northern limb of the Toko Syncline. Total count decreases toward the top of the Kelly Creek Formation. Radiogenic elements appear to be almost absent from the Coolibah Formation, with little response in any channel. The overlying Nora Formation presents a sharp contrast, with elevated counts due to mica, glauconite and clay minerals. Within the Nora Formation, there is a sharp decrease in all channels northwest of a fault near the northwestern end of the Toko Range. The radiometric response of the Carlo Sandstone is more subdued, but there are isolated patches of high Th. The Mithaka Formation reverts to higher radiometric levels, with strong contributions from Th and U, possibly associated with component carbonate. This signature is obscured in some elongate darker areas that are associated with watercourses. The radiometric response of the Austral Downs Limestone consists almost entirely of U, and is readily distinguished as such, although total count is not high. Most Downhole geophysics Downhole geophysical data are available for TOBERMOREY cored drillholes Hacking 1, NTGS99/1, BMR Tobermorey 14, Owen 2 and BMR 12 Cockroach. In addition, of eight MIM cored exploration drillholes in the Marqua Monocline area (McGeough and Shalley 1992), three (BHD4, 5 and 9) are of sufficient length to provide stratigraphic overview. Lucy Creek 1 has downhole gamma and resistivity logs, but was not continuously cored. These drillholes variously sample the latest Cambrian down to the basement. BMR Tobermorey 15 (Simpson et al 1985) sampled Neoproterozoic rocks and is not considered here. Stratigraphic drillholes BMR Grg 11 and Grg 12 (Milligan 1963) penetrated latest Cambrian to Ordovician rocks, but only a partial electric and gamma log record was obtained for Grg 11, and none at all for Grg 12. Drillhole locations in chronological order and their available downhole geophysical data are presented in Table 3. The Adam Shale is intersected in Tobermorey 14 (Figure 45; Gibson 1984) and possibly in BHD4 and 5; the disconformable contact with the overlying Red Heart Dolostone is clearly registered in the Tobermorey 14 gamma log. In this drillhole, other methods do not unequivocally distinguish the Adam Shale from a 4 m thickness of terrigenous pebble conglomerate and sandstone in the basal Red Heart Dolostone. This latter formation is intersected also in Hacking 1 (Weste 1989) and in the three BHD drillholes. The Thorntonia Limestone is recognised in Tobermorey 14, NTGS99/1, Owen 2 (Kress 1991) and the three BHD drillholes. Gamma values increase downsection toward the base of the Arthur Creek Formation in eight drillholes: Hacking 1, Owen 2, NTGS99/1, Tobermorey 14, BMR 12 Cockroach (Smith 1967) and the three BHD drillholes. This pattern provides a distinctive log signature enabling recognition of pyritic carbonaceous black shale in the basal Arthur Creek Formation, which is a potential petroleum source rock (see Petroleum). Geological history The oldest securely dated geological event in TOBERMOREY is the 1846 ± 6 Ma granite intersected at the bottom of cored drillhole NTGS99/1. This granite is attributed to the Mount Tietkens Granite Complex, which is intruded into an evidently older eastern Arunta Province. Elsewhere in the eastern Arunta Province, in what is now southeastern 40 Hole name Grg 11 Grg 12 Lucy Creek 1 BMR 12 Cockroach Tobermorey 14 Tobermorey 15 Hacking 1 Owen 2 BHD4 BHD5 BHD9 NTGS99/1 Easting (mE) Northing (mN) 744749 688513 669362 722299 751475 747875 705381 802898 772678 770077 768919 746996 7528511 7494225 7522444 7504233 7467466 7467040 7474502 7523560 7466471 7466176 7466820 7474345 Gamma Resist SP SPR Sonic + + + + + + + + + + + + + + + + + + + + + + + Density Neutron + + + + + + + + + + + + Table 3 TOBERMOREY drillholes in chronological order with locations and available downhole geophysical data (crosses). Resist = resistivity; SP = self potential; SPR = single point resistance (Southgate and Shergold 1991) commenced with marginal marine terrigenous siliciclastic deposition, which heralded the establishment of a broad carbonate platform facing open ocean to the present east, beyond the Tasman Line. The platform was subjected to brief episodes of anoxia, generating thin black shales among the dominant carbonate sediments of the Thorntonia Limestone. The platform was at least locally exposed at the conclusion of Thorntonia Limestone deposition, resulting in local karstification of the sediment pile. Renewed transgression marked the commencement of sequence 2 deposition (Southgate and Shergold 1991). In the southern Georgina Basin, this established a deeper, anoxic basinal regime (Arthur Creek Formation) from Templetonian time. Pyritic-carbonaceous black shale accumulated on the basin floor, in part due to slopegenerated turbidity currents. The influence of open ocean waters facilitated the proliferation of agnostine trilobites, among other organisms. Continued aggradation through the remainder of the Middle Cambrian led to shallowing above wave base into a normal open marine platform setting. Agnostines declined in importance as other groups flourished. As the platform shallowed, nodular anhydrite grew displacively within sediment under evaporitic conditions. Ensuing restricted marine to peritidal deposition on the platform (Arrinthrunga Formation) represents one or more distinct depositional sequences. An evaporitic, mixed carbonate-siliciclastic shoreline bordered the platform on the west and north (Kennard 1981), whereas the quartzic Steamboat Sandstone and ooid-peloid grainstone of the basal Arrinthrunga Formation formed a seaward highenergy barrier. Hypersalinity on the platform precipitated evaporites and permitted the expansion of microbial stromatolites and thrombolites before and during a postulated sea level rise (Kennard 1981). Renewed shoaling culminated in intermittent emergence and the reestablishment of hypersalinity (Eurowie Sandstone Member). This cycle was repeated before a final regression led to exposure and karstification of the platform in the mid-Late Cambrian (Iverian). Marine platformal conditions were widely reestablished by the Payntonian and continued through the Early and Middle Ordovician. A broad marine carbonate platform in HUCKITTA and southwestern TOBERMOREY, felsic and minor mafic protoliths destined to become the migmatite unit p–Cd were emplaced and deposited, notionally prior to 1820 Ma. These Narwietooma Package rocks were metamorphosed to amphibolite facies during the period 1780–1720 Ma (Pietsch 2001). Unnamed granites L Pga, L Pgb, L Pgc and L Pgd intruded thereafter. According to Warren (1981), the Tarlton Fault was already active by 1700 Ma. Tectonism prior to 800 Ma created northwest-trending grabens in which thick successions of the early Georgina Basin accumulated (Walter 1980, Zhao et al 1994). Initial deposition in TOBERMOREY was of glacigene diamictite and laminated siltstone of the Yardida Tillite. This unit is correlated with the 680–700 Ma Sturtian glaciation, the older of the two major glaciations in the Adelaide Rift (Preiss et al 1978, Walter and Veevers 1997). An intervening Rinkabeena Movement (Wells and Moss 1983) was followed by proximal glacial outwash deposits of the Black Stump Arkose, which are correlated with the younger (590– 610 Ma), Marinoan glaciation in the Adelaide Rift (Walter et al 1995, Walter and Veevers 1997). Ensuing peritidal and possibly deeper water conditions generated fenestral and oncoidal carbonate sediments of the Wonnadinna Dolostone. A modest Toomba Movement (Walter 1980) intervened before marine deposition of the Gnallan-a-Gea Arkose, Elyuah Formation and Grant Bluff Formation. Although deposition continued intermittently in the southwestern Georgina Basin during the terminal Neoproterozoic-earliest Cambrian transition, none is recorded in TOBERMOREY. The hiatus is due to the Huckitta Movement (Walter 1980), possibly representative of the more substantial Petermann Orogeny, which was centred on the Musgrave Block and southwestern Amadeus Basin. The first Cambrian marine transgression to affect the map area took place along the southern basin margin in the mid-Early Cambrian (Atdabanian stage in Siberian terms; Spizharskiy et al 1983). Marginal marine siliciclastic sediments with Diplocraterion preceded open marine carbonate shelf sedimentation, which included the development of calcimicrobial-archaeocyathan patch reefs (Walter et al 1979, Kruse and West 1980). The initial Middle Cambrian marine transgression was more widespread and extended into the central and northern Georgina Basin. Ordian deposits of sequence 1 G:\Geological Survey\Publishing\Production\NTGS_250k_explan_notes_update\Updated_NTGS_250k_explan_notes\Tobermory\Tobermory_Archive\Tob_PM\Tob_Table3.doc 41 HACKING 1 OWEN 2 SELF POTENTIAL GAMMA ohm m mV API units DEPTH (m) 0 0 5000 0 140 0 DEPTH (m) JUR-CRET TOMAHAWK FM 200 ARRINTHRUNGA FM 150 100 150 7000 140 GAMMA API units 40 0 200 200 250 250 300 mV 50 50 100 SONIC ohm m 20 0 250 RESISTIVITY NINMAROO FM RESISTIVITY EUROWIE SST MBR 300 400 450 500 550 ARRINTHRUNGA FM 400 450 ARRINTHRUNGA FM 350 350 500 550 600 600 650 STEAMBOAT SST 650 700 700 750 STEAMBOAT SST 750 850 850 900 900 ARTHUR CREEK FM 800 800 1000 1050 ARTHUR CREEK FM 950 950 1000 1150 1100 THORNTONIA LST 1100 1150 GRANITE 1150 1200 RED HEART DST GRANITE Figure 45 (this and page 43) Schematic lithologs and downhole geophysical logs of selected drillholes in TOBERMOREY 42 m02-40a.dgn the east (Ninmaroo Formation) extended into Queensland. Energy conditions were variable, with recurring high-energy episodes. Ooid, peloid and skeletal sands formed a seaward barrier system, again increasing salinity on the platform, with associated semi-emergent shoals and cyclic sedimentation (Radke 1980). Contemporaneously, terrigenous quartz sand (Tomahawk Formation) was being shed from a western landmass, possibly an exposed Arunta Province, into littoral and sublittoral environments above wave base on the platform. Abundant benthic soft-bodied organisms produced open marine Cruziana and littoral Skolithos ichnofacies traces. Storms deposited intraclast-bearing layers and hummocky cross-stratification. Peritidal to marine conditions persisted (Kelly Creek Formation) until a final shallow marine to peritidal carbonate platform phase (Coolibah Formation). Ordovician sedimentation thereafter was primarily siliciclastic. Offshore fine siliciclastic deposition below wave base (Nora Formation) was overtaken by a prograding high-energy barrier (Carlo Sandstone), which protected a low-energy marine lagoonal setting (Mithaka Formation) behind it (Draper 1977). During this Early-Middle Ordovician sedimentation phase, Neoproterozoic-Cambrian rocks in the Irindina Sub-basin to the west were subjected to deformation and granulite-facies metamorphism during the Larapinta Event of 480‑460 Ma, but without evident effect in TOBERMOREY. The 460‑450 Ma Rodingan Movement (Wells et al 1970), which was the first phase of the compressional Alice Springs Orogeny according to Haines et al (2001), again deposited coarse, high-energy barrier sands (Ethabuka Sandstone) on the platform and terminated Ordovician sedimentation in the Georgina Basin. Deposition in the Georgina Basin ceased until the Pertnjara-Brewer Movement in the mid-Devonian (390‑375 Ma; Haines et al 2001) generated the Cravens Peak beds. A third, mid-Carboniferous (340-320 Ma) phase left no depositional record, but may coincide with post-Devonian movements along major faults. The Alice Springs Orogeny produced the present major structures of the TOBERMOREY region. The region remained exposed until a widespread Eromanga Basin transgression in the Jurassic-Cretaceous deposited a marginal marine to marine quartz sand sheet. Continental conditions prevailed thereafter, with ferricrete and silcrete development in the Palaeocene, Oligocene and Mio-Pliocene (Grimes 1980). Lacustrine conditions in the Georgina River catchment generated the pedogenic Austral Downs Limestone. Quaternary deposition has been limited to widespread colluvial quartz sand and reworked equivalents in alluvial floodplains and claypans. NTGS99/1 SINGLE POINT RESISTANCE ohm m ARRINTHRUNGA FM DEPTH (m) 0 50 SELF POTENTIAL 0 GAMMA API units mV 1500 0 200 0 250 STEAMBOAT SST 100 150 200 ARTHUR CREEK FM 250 300 350 400 450 500 550 THORNTONIA LST 600 GRANITE BMR TOBERMOREY 14 RESISTIVITY DEPTH (m) ohm m 0 100 0 50 100 SELF POTENTIAL GAMMA mV 0 120 API units 0 100 ARTHUR CREEK FM THORNTONIA LST RED HEART DST ADAM SHALE DOMINANT LITHOLOGY dolomudstone, dolowackestone mottled dolostone intraclast-bioclast dolograinstone ooid dolograinstone microbial dololaminite quartzic dolostone, quartz sandstone Economic geology mixed and silty limestone and dolostone calci/dolomudstone-siliciclastic mudstone interbeds Georgina Basin coarsely recrystallised dolostone pyritic-carbonaceous shale, shale-dolostone laminite Petroleum siltstone, mudstone granite metavolcanic rock Petroleum exploration and prospectivity in the Georgina Basin is summarised by Questa (1994) and the petroleum system thoroughly revised by Ambrose et al (2001). As the m02-039a.dgn Figure 45 (continued) 43 system is of Cambrian-Ordovician age, it belongs to the Larapintine supersystem of Draper (2000). Initial reconnaissance of Georgina Basin petroleum prospectivity was undertaken by BMR in the 1950s. Petroleum companies, including Frome-Broken Hill, Geosurveys of Australia, Alliance Petroleum Australia (Wilson 1963), French Petroleum, Australian Aquitaine Petroleum and Shell Development, reviewed aspects of the region in the late 1950s to early 1960s (Smith 1965a). The first well drilled in the basin was Black Mountain 1 in Queensland, by Phillips-Sunray in 1962. Investigations continued throughout the eastern, southern and central regions during the early to mid-1960s. In TOBERMOREY, BMR drilled three stratigraphic holes [Grg 11, Grg 12 (Milligan 1963) and BMR 12 Cockroach (Smith 1967)] in the early 1960s. Lucy Creek 1 was drilled by Exoil in 1966 to evaluate a structural high in the west-central map area (Pemberton 1967). The impact of this early exploration phase was to downgrade the perceived prospectivity of the entire basin, and exploration virtually ceased until the early 1980s. However, BMR and latterly NTGS continued to drill stratigraphic holes and acquire geophysical data. One notable exception to this interim malaise was the drilling of Ethabuka 1 in the Toko Syncline on MOUNT WHELAN by Alliance Oil Development Australia NL in 1974. This well yielded the only sizeable flow of hydrocarbons to date from the Georgina Basin: an estimated 7080 m3/day of dry gas from the Kelly Creek Formation (Radke and Duff 1980). Gravity surveys were conducted over the northern Toko Syncline for Plenty River Mining in 1982-1983, but no drilling took place. A fresh phase of exploration in the southern Georgina Basin occurred during 1988-1991, as Pacific Oil and Gas (a subsidiary of CRA) took up permits over the Dulcie and Toko Synclines. During this period, the company acquired 675 line km of seismic data (four lines in TOBERMOREY) and drilled eight exploration/stratigraphic wells. Of these wells, Hacking 1 (1988; Weste 1989) and Owen 2 (1990; Kress 1991) are located in TOBERMOREY. Hacking 1 was sited on an interpreted structural high as a stratigraphic assessment of the source rock and reservoir potential of the Cambrian succession near the southern basin margin. Owen 2 was drilled for stratigraphic purposes and to test for potential early Cambrian siliciclastic reservoirs. Hydrocarbon shows were detected in both of these wells. Pyritic-carbonaceous black shale in the basal Arthur Creek Formation was identified as a world class source rock, attaining 40 m thickness in Hacking 1 (Questa 1994: 67). Total organic carbon (TOC) contents were found to be in the range 0.5‑16%, but more commonly up to 10%. Drillhole NTGS99/1 in south-central TOBERMOREY yielded TOC up to 2.9% from the Thorntonia Limestone and 3.4% from the lower Arthur Creek Formation. Middle Cambrian sediments were found to be marginally mature to overmature for oil generation, with maturity increasing southward. An estimated 40 billion tonnes of oil has been generated from Cambrian sedimentary rocks in the southern Georgina Basin (SIBGEO 1991, 1992). Potential reservoirs occur in the Coolibah Formation, Kelly Creek Formation, Arrinthrunga Formation, Chabalowe Formation (including Hagen Member), Steamboat Sandstone, upper Arthur Creek Formation, Thorntonia Limestone and Mount Baldwin Formation (Questa 1994). Porosity in the carbonate rocks is due to secondary dissolution and dolomitisation. Of greatest relevance to TOBERMOREY is the stratigraphic juxtaposition of reservoir and source-seal in the succession from black shale-bearing medial Thorntonia Limestone, to porous and vuggy upper Thorntonia Limestone, to basal Arthur Creek Formation black shale (Ambrose et al 2001). Phosphate Biogenic accumulation accounts for widespread, but economically insignificant phosphate in Cambrian carbonates in TOBERMOREY. Shergold (1985: 24) reported 1‑5% P2O5 from an accumulation of phosphatic-shelled lingulate brachiopods in the lower interval of the Thorntonia Limestone in cored drillhole Tobermorey 14. Biogenic phosphate grains (lingulate brachiopods and hyolith steinkerns) are also common in the medial and upper intervals in several TOBERMOREY drillholes. The Middle Cambrian is locally phosphatic near Ammaroo in ELKEDRA and the best intercept is 0.9 m at 30% P2O5 at 26 m depth, attributed to the Arthur Creek Formation (Morrison 1968, Howard 1990). On this basis, Morrison (1968) recommended that the Marqua area of TOBERMOREY be considered prospective, but no work has been undertaken. The Nora Formation may also be prospective, if a sample from Gaphole Creek yielding 13% P2O5 is indicative (Smith 1965a). Following a drilling campaign at Wonarah in the central Georgina Basin, which delineated a resource estimate of 72 Mt at 23 % P2O5 (report to Australian Stock Exchange by AKD Ltd, 24 January 2002), Rio Tinto has recently undertaken regional phosphate exploration throughout the Georgina Basin, but there are no active tenements in TOBERMOREY. Diamonds TOBERMOREY was included in a regional diamond exploration program by CRAE (Colliver and Bubner 1987a, 1987b). Reconnaissance stream sediment sampling, aeromagnetic and radiometric data were used to focus the search. Ten picroilmenite and chromite grains were found in southwestern TOBERMOREY and a single microdiamond was reported from Shank Creek in the northeast (Allnutt 1986). Follow-up sampling failed to provide conclusive results (Colliver and Bubner 1987a, 1987b). Recognition that basement similar to the eastern Siberian Platform (which hosts diamondiferous kimberlite) might underlie part of the Georgina Basin has led to a resurgence in diamond exploration. The model is based on the recognised existence of the Altjawarra Domain (sensu Myers et al 1996), a region of thickened, stable crust with relatively low heat flow and low crustal temperatures, which underlies much of TOBERMOREY, SANDOVER RIVER and eastern HUCKITTA. Any diamondiferous kimberlite pipes younger than 500 Ma would be preserved in the overlying Georgina Basin sedimentary cover. As of March 2002, Elkedra Diamonds NL have pegged all but the southwestern corner of TOBERMOREY, as part of a regional search (Figure 46). 44 136˚30’ 22˚00 ’ Ma Y HWA HIG PLENTY Hay Marqua Figure 46 Locations of microdiamond and diamond indicator minerals in TOBERMOREY, overlaid on aeromagnetic image Carpentaria Exploration undertook regional exploration Cre eofk for MVT mineralisation over the present structural margins the Georgina Basin from 1976 to 1977 (Nenke 1977a, 1977b). Soil, stream sediment and rock chip samples were collected in the Dinner Creek-Marqua Creek area and from Centenary Bore to Christmas Bore (south-central TOBERMOREY). The Microdiamond occ 50 km highest Zn and Pb values in rock chips were attributed to Fe Diamond indicator and Mn scavenging in the weathering profile, as no surface mineralisation could be found. Ferruginous lag assayed up to 1.31% Pb and 834 ppm Zn (Nenke 1977a). Elevated Zn in subsurface and unweathered carbonate was thought to be due to cation substitution in the dolomite lattice. The greatest anomaly of 1.15% Zn over 2 m, which was from channel chip samples of fresh rock in the Marqua area, became the Boat Hill prospect. From 1981 to 1984, Agip Australia targeted MVT and stratabound base metal mineralisation (Agip 1982, 1983, 1984). Reconnaissance mapping, rock chip sampling and 15 drillholes demonstrated that Pb, Zn and Ag anomalism could be traced over a 10 km strike length in the vicinity of the Boat Hill prospect. The highest base metal values occur in close proximity to faults and the best drilling intercept was 0.5 m at 1.9% Zn. A geophysical programme was planned, but Agip relinquished the area in 1984 when the company ceased all base metal exploration in Australia. In 1991, MIM targeted low-angle reverse faults on the margin of the Georgina Basin for Pb and Zn (McGeough er Riv Copper-lead-zinc The structurally complex southwestern margin of the 23˚00 ’ Georgina Basin in southern Tobermorey contains several 136˚30’ Cambrian formations that are anomalous in Zn and Pb over kilometres of strike length. These have been the subjects of 0 systematic company exploration for over 30 years. Galena was first recognised in the Tomahawk Formation during government mapping. Smith and Vine (1960) and Smith (1965a) reported small quantities east of the southern end of the Tarlton Range, but no precise location was given. In the 1970s, BMR palaeontologist J Sheldon found an isolated grain near Canyon Bore, Tarlton Range (pers comm reported in Nenke 1977a). In the early 1970s, Fimiston Minerals established a Mississippi Valley-type (MVT) model for the Georgina Basin and targeted Upper Cambrian formations, particularly the Arrinthrunga Formation (Morris 1971, Keene and Morris 1971). CRAE targeted two areas in TOBERMOREY for MVT mineralisation, based on geological similarity with the Turkey Creek/Box Hole prospect in HUCKITTA. Interpretation of aeromagnetic, reconnaissance soil, stream, rock and borewater data from northeast of Tarlton Downs homestead and between McCrae Bore and No 6 Bore in the north-central map area did not detect any anomalies of significance (Tham 1971). 45 galena, sphalerite and pyrite in crystal clots up to 2 cm across were observed in the Thorntonia Limestone and Red Heart Dolostone in BHD4, 5 and 9 (Figure 47). Highest combined assays were 0.83% Pb, 2.24% Zn and 2 g/t Ag from 530.90‑531.36 m depth in the Red Heart Dolostone in BHD9 (McGeough 1992). BHD9 has galena and minor sphalerite infilling fractures and along stylolites over 514.0‑516.5 m depth in the medial Thorntonia Limestone and this interval averaged 941 ppm Pb and 0.54% Zn. Petrological descriptions and an assessment of paragenesis by Croxford (in McGeough 1992) were consistent with MVT mineralisation. Pyrite, sphalerite and hydrocarbons were the earliest phases and so are intimately interrelated (Figure 48). Galena was the last to form and filled the remaining space within vugs in saddle dolomite. Reconnaissance stream sediment and rock chip sampling along the Toomba-Craigie Fault by CRAE failed to locate any significant base metal anomalies (Stegman 1991). Cored stratigraphic drillhole NTGS99/1 intersected visible galena and sphalerite in the Thorntonia Limestone. Quartermetre composites of quarter core with visible galena returned assays of >2000 ppm Pb and the interval 585.75‑586.0 m averaged 1.02% Zn. This significantly increases the area of Thorntonia Limestone in TOBERMOREY known to be anomalous in base metals. Platinum group elements Figure 47 Thorntonia Limestone. Slabbed drillcore from lower light grey interval of formation showing galena and honey-coloured lowiron sphalerite as vug fill (centre). Scale bar = 1 cm. 526.3 m depth in drillhole MIM BHD9; Toko QQ689668 Shergold (1985) reported 1.5 ppm Pt and 0.8 ppm Pd in shale from the medial interval of the Thorntonia Limestone in cored drillhole Hay River 11A in HAY RIVER. Based on these assays, Geopeko briefly held ground to explore for platinum group elements (PGEs) in Georgina Basin shales. The company reviewed BMR magnetic data and reprocessed some in-house airborne magnetic and radiometric data from 1978. After discussions with BMR, they concluded that the elevated PGEs reported by Shergold (1985) were analytically incorrect (Sherrington 1987). Also encouraged by BMR assays and using a Zechstein model, Saracen Minerals targeted shales near the Boat Hill prospect. Nineteen shallow percussion holes totalling 550 m 1992, McGeough and Shalley 1992). Assays up to 1.8% Pb and 7090 ppm Zn in the Thorntonia Limestone in the Boat Hill area led MIM to conclude that mineralisation at the Boat Hill prospect is structurally controlled (McGeough 1992). MIM recorded 17 200 m loop 16‑channel SIROTEM traverses approximately 500 m apart across strike at the Boat Hill prospect. Of eight cored holes drilled, four tested conductive layers from the SIROTEM survey. Minor occurrences of Figure 48 Thorntonia Limestone. Sphalerite (1) and platy organic matter (2) intimately interrelated in vug fill; medial darker grey interval of formation. Scale bar = 0.1 mm. Polished thin section in reflected plane polarised light (from Croxford in McGeough 1992); 513.36 m depth in drillhole MIM BHD9; Toko QQ689668 46 2002). The occurrence appears to be hosted in dolomitic quartz sandstone and dolostone at or near the base of the Kelly Creek Formation and is very similar in style to the outcrops at Lucy Creek 2. Elevated levels of trace metals (eg 2460 ppm Pb and 60 ppm As) from the latter prospect, replacement ore textures and close proximity to basement structures suggest a geothermal or low temperature hydrothermal ore genesis model, similar to the manganese deposits in the Renner Springs district, northern Tennant Inlier (Ferenczi 2001). Surficial manganese occurrences are present in the Ninmaroo, Tomahawk and Kelly Creek Formations. Significant assay results have been reported from near Desert Bore (QR192326), where sampling by Eldekra Diamonds NL at CWN‑90 (in Tomahawk Formation) and Loc44 (in Ninmaroo Formation) returned up to 42% Mn and 28.7% Mn, respectively. Minor manganiferous mineralisation was also encountered in the Ninmaroo and Kelly Creek Formations in the vicinity of Treacle Hill Waterhole (QQ491831) during NTGS Second Edition mapping. were drilled and 491 samples analysed for Pt and Pd. All were below 10 ppb Pt and 10 ppb Pd. The area was relinquished in 1988 (Virtue 1988). MIM also analysed for PGEs in their BHD series of drillholes. None exceeded the 5 ppb Pt or 1 ppb Pd detection limits. This has downgraded prospectivity for PGEs in Georgina Basin shales within TOBERMOREY. Gold Shergold (1985) invoked a Carlin-style Au model for the southern Georgina Basin. Saracen and MIM included Au in their exploration of the Boat Hill area. In the former case, all assays were below the detection limit of 0.01 ppm Au (Virtue 1988) and in the latter, the highest value was 0.005 ppm (McGeough 1992). During the early 1990s, CRAE prospected the Toomba Fault Zone in HAY RIVER and TOBERMOREY for Au. Rock chip and stream sediment sampling were undertaken. Repeat bulk leach extractable gold (BLEG) sampling failed to return assays in excess of 0.6 ppb Au (Stegman 1991). Groundwater Uranium Community and pastoral water supply in TOBERMOREY is almost entirely reliant on groundwater. Altogether, 204 bores (including stratigraphic and exploration drillholes) have been drilled in the sheet, with standing water levels in the range 34‑71 m depth, total dissolved solids 417‑20 100 mgL-1 and yields up to 6.3 Ls-1 (Read 2002). Bore yields are generally low, with most producing in the range 0.5‑2 Ls-1; only a small minority (nine bores) exceed 4 Ls-1. Supplies suitable for stock (ie with total dissolved solids <5000 mgL-1) are widely available. However, only 21 bores yield potable water, ie with total dissolved solids <1000 mgL-1, NO3 <45 mgL-1 and F <1.5 mgL-1. About one third of bores meeting the first criterion have either excessive nitrate or fluoride. The most prospective formations are the Arrinthrunga Formation and particularly the Ninmaroo Formation, which is widespread, thick, and yields stock supplies from moderate depths. The Kelly Creek Formation is a poor to moderate aquifer for stock supplies. Highly saline water (total dissolved solids 15 100 mgL-1), from Tobermorey No 8 (RN 12836) in the Toko Syncline, suggests the local presence of evaporites in this unit. The Arthur Creek Formation has been little utilised, but does contain aquifers suitable for stock supplies, as also does the Carlo Sandstone in the Toko Syncline. Known aquifers in Jurassic-Cretaceous rocks are low yielding and limited to the Marshall River area in southwestern TOBERMOREY. A few bores draw moderate yields from unconsolidated Quaternary alluvium. Water from five bores in TOBERMOREY was analysed for U by CRAE. The highest level detected was 7 ppb U in Coles Bore (Tham 1971). Geopeko targeted the basal Georgina Basin succession as part of a search for Proterozoic sediment-hosted U. The company flew a radiometric survey over northern HAY RIVER in 1979 and extrapolated the results into southern TOBERMOREY. Cenozoic cover was a major problem and the only anomalies detected were attributed to weak surficial concentrations. No further work was undertaken (Eupene 1979). Roll-front uranium was considered to be a secondary target in exploration of the Marqua area by Agip, but no significant occurrences were identified (Agip 1982, 1983). Manganese In late 2002, Elkedra Diamonds NL discovered numerous manganese occurrences during a regional diamond exploration program within Els 22529, 22534 and 22536 over western TOBERMOREY (Elkedra Diamonds 2002). Most of these appear to be surficial in origin. However, two (Lucy Creek 2 and Halfway Dam prospects) are interpreted as stratabound. At Lucy Creek 2 prospect (PR703222), a 1‑2 m-thick Mn horizon outcrops over a strike length of 400 m (L Tompkins, Elkedra Diamonds, pers comm 2002) within dolomitic siltstone of the Tomahawk Formation. Initial rock chip samples gave encouraging assays containing high-grade manganese and low levels of iron, phosphorus, silica and alumina (eg D0066: 50.5% Mn, 0.44% Fe, 0.06% P, 6% SiO2 and 0.71% Al2O3; Elkedra Diamonds 2002). A follow-up RAB drilling program was undertaken in October 2002. Near Halfway Bore (QR020075) manganiferous material containing up to 42% Mn was excavated during construction of a small dam (L Tompkins, Elkedra Diamonds, pers comm Arunta Province Copper-lead-zinc Minor malachite (up to 1660 ppm Cu) and galena were observed in quartz veins in the Mount Dobbie Granite during a helicopter reconnaissance by MIM exploration (McGeough 1992). Normandy Poseidon explored for Jervois-style copper, lead and zinc along Hay River into southwestern 47 ACKNOWLEDGMENTS TOBERMOREY. Airborne and ground electromagnetic surveys and soil sampling were undertaken. Drilling of the most significant anomaly failed to locate mineralisation (Cozens 1995). Second Edition regional mapping by NTGS identified malachite and rare azurite in L Pgc granite. Mineralisation in outcrop in the bed of Camel Creek at PQ887602 is associated with fractures, shear zones, quartz and ironstone veins. A representative ironstone sample contains 2388 ppm Cu, 2 ppb Au, 3 ppm Pb and 20 ppm Zn. After heavy rains in 1999-2000, malachite was visible in Camel Creek downstream to the Hay River junction but was not detected by Normandy Poseidon’s earlier survey. Roof pendants of similar age granites would be prospective throughout TOBERMOREY. Ashcourt targeted the HAY RIVER-TOBERMOREY area for Au, considering it to be a similar geological setting to the Tanami region, but surrendered the licence due to an inability to raise joint venture capital (Griffiths 1998). Normandy Poseidon included Au analyses in their soil sampling over airborne magnetic and EM anomalies in the Hay River survey. Only one sample of 0.010 ppm exceeded the 0.001 ppm detection limit (Cozens 1995). The Arunta Province Stratigraphy text was prepared with reference to petrographic descriptions commissioned from Frank Radke (Amdel Limited, Adelaide). Ian Scrimgeour (NTGS, Darwin) commented on a draft version of the Arunta Province Stratigraphy. Bob Read (Natural Resources Division, Department of Infrastructure, Planning and Environment, Alice Springs) provided a contribution on groundwater. Phil Ferenczi (NTGS, Darwin) collated notes on manganese occurrences. Max Heggen (NTGS, Alice Springs) liaised with Aboriginal organisations and custodians regarding land access. The authors thank local landholders and Aboriginal custodians for access to their properties. Greg Ambrose (NTGS, Darwin) advised on aspects of petroleum geology and downhole geophysical logging, and Professor Barry Webby (Macquarie University, Sydney) on Ordovician correlations. Zhen Yongyi (Australian Museum, Sydney) identified a small conodont sample from the Tomahawk Formation. Peter Crispe, Chris Field, ‘Mac’ McClellan, Mick Crompton and Liam MacNally assisted in the field, with the able base support of Martin Cardona. Figures were prepared by Richard Jong and Gary Andrews. The manuscript was edited by Tim Munson and formatted by Stephen Cox. Tungsten References Local pastoralists have provided anecdotal accounts of minor scheelite occurrences in TOBERMOREY found in the 1960s by ultraviolet (UV) prospecting. One such reported occurrence in the vicinity of PQ938553 could not be relocated during Second Edition regional mapping. No significant W was detected during reconnaissance work along Hay River by Normandy Poseidon. Systematic soil sampling in the same area returned a maximum of 10 ppm W (Cozens 1995). Agip, 1982. Tobermory and Hay River EL3142. Annual report on exploration 1981‑82. Northern Territory Geological Survey, Open File Company Report CR1982‑0376. Gold Agip, 1983. Tobermory and Hay River, EL3142. Annual report 9.11.82-8.11.83, area to be retained. Northern Territory Geological Survey, Open File Company Report CR1983‑0328. Agip, 1984. Tobermory and Hay River, EL3142. Final report, 9/11/83 to 8/11/84. Northern Territory Geological Survey, Open File Company Report CR1984‑0191. Diamonds BHP targeted magnetic anomalies under cover between the Marshall River and the Plenty Highway as potential kimberlites. Unfortunately, drilling intersected magnetitebearing biotite schist and siltstone in the shallow subsurface (BHP 1984, 1985). Ahmad M, 2000. Geological map of the Northern Territory, 1:2 500 000. Northern Territory Geological Survey, Darwin. Ahmad M, 2002. Geological map of the Northern Territory, 1:2 500 000 (2002 Edition). Northern Territory Geological Survey, Darwin. Uranium Normandy Poseidon analysed for U in a reconnaissance surface geochemical survey along the Hay River from HAY RIVER into southwestern TOBERMOREY. Very few assays exceeded the 4 ppm detection limit (Cozens 1995). Alliance Petroleum Australia, 1964. Report for Tarlton Downs gravity survey. Northern Territory Geological Survey, Open File Petroleum Report PR1964‑0005. Other metals and non-metals Allnutt S, 1986. Final report for the period to 30th July, ELs 4612-4617, 4625-4627. CRA Exploration Pty Ltd. Northern Territory Geological Survey, Open File Company Report CR1986‑0288. Ba, Co, Mn, Ni and P were included in assays of reconnaissance surface samples and systematic soil sampling over airborne magnetic and EM anomalies along Hay River in southwestern TOBERMOREY. Most samples were sourced from Arunta Province rocks and none was considered anomalous (Cozens 1995). Ambrose GJ, Kruse PD and Putnam PE, 2001. Geology and hydrocarbon potential of the southern Georgina Basin, Australia. APPEA Journal 41, 139‑163. 48 Abstracts, Geological Society of America Annual Meeting, Boston, Massachusetts, November 1‑10, 2001. Session T23, Sediment-hosted lead-zinc deposits: roles of basin evolution, tectonics and geochemistry in ore genesis 1 (http://gsa.confex.com/gsa/2001AM/ finalprogram/abstract_13273.htm). AUSLIG (Australian Surveying and Land Information Group), 1990. Atlas of Australian resources. Third series. Volume 6 Vegetation. Commonwealth Government Printer, Canberra. Barlow BC, 1965. Georgina Basin reconnaissance gravity survey, NT and Qld 1959. Bureau of Mineral Resources, Australia, Record 1965/96. Chenon C, 1966. Cockroach Waterhole area experimental seismic survey, Northern Territory 1964. Bureau of Mineral Resources, Australia, Record 1966/75. Barlow BC, 1966. Georgina Basin reconnaissance gravity surveys using helicopters, Northern Territory and Queensland 1960‑1961. Bureau of Mineral Resources, Australia, Record 1966/147. Collins WJ and Shaw RD, 1995. Geochronological constraints on orogenic events in the Arunta Inlier: a review. Precambrian Research 71, 69‑89. BHP 1984. Exploration licence 4198, Marshall River, NT, final report June 1983-December 1984. BHP Exploration Department. Northern Territory Geological Survey, Open File Company Report CR1984‑0121. Colliver IC and Bubner GJ, 1987a. EL4628 Warka, NT. Final report period ending 21st July 1987. CRA Exploration Pty Ltd. Northern Territory Geological Survey, Open File Company Report CR1987‑0206. BHP 1985. Exploration licence 4527, Tarlton Range, NT, final report May 1984-December 1984. BHP Exploration Department. Northern Territory Geological Survey, Open File Company Report CR1985‑0066. Colliver IC and Bubner GJ, 1987b. EL4629 Toko Range, NT. Final report period ending 21st July 1987. 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Bureau of Mineral Resources, Australia, Bulletin 100. Young GC, 1996. Devonian (Chart 4): in Young GC and Laurie JR (editors) An Australian Phanerozoic timescale. Oxford University Press, Melbourne, 96‑109. Wells AT and Moss FJ, 1983. The Ngalia Basin, Northern Territory: stratigraphy and structure. Bureau of Mineral Resources, Australia, Bulletin 212. Zhao Jianxin and Bennett VC, 1995. SHRIMP U‑Pb zircon geochronology of granites in the Arunta Inlier, central Australia: implications for Proterozoic crustal evolution. Precambrian Research 71, 17‑43. Wells R, Milsom JS and Tipper DB, 1966. Georgina Basin aeromagnetic survey, Queensland and Northern Territory 1963‑1964. Bureau of Mineral Resources, Australia, Record 1966/142. Zhao Jianxin, McCulloch MT and Korsch RJ, 1994. Characterisation of a plume-related ~800 Ma magmatic event and its implications for basin formation in central-southern Australia. Earth and Planetary Science Letters 121, 349‑367. Wentworth CK, 1922. A scale of grade and class terms for clastic sediments. Journal of Geology 30, 377‑392. Weste G, 1989. Hacking No 1 well completion report, EP 12, Georgina Basin, NT. Pacific Oil and Gas Pty Ltd. Northern Territory Geological Survey, Open File Petroleum Report PR1988‑0079. Appendix - revised stratigraphic units Arthur Creek Formation (emendation of existing definition) White ME, 1961. Mesozoic plant fossils from the Tarlton Range, Northern Territory. Bureau of Mineral Resources, Australia, Record 1961/32. Proposers: Freeman (1986: 57), after Smith (1964); amended by PD Kruse. Derivation of name: After Arthur Creek in eastern HUCKITTA and southwestern TOBERMOREY. Whitehouse FW, 1931. Palaeontologist: in Annual report of the Under Secretary for Mines to the Honourable EA Atherton, MLA, Secretary for Mines, including the reports of the Wardens, Inspectors of Mines, Government Geologists, and other officers, for the year 1930. Government Printer, Brisbane, 141‑142. Synonymy: Part of Sandover Beds of Öpik (1956); part of Arthur Creek Beds of Smith (1964); part of Marqua Beds of Smith (1972); Marqua Formation and upper Hay River Formation Member 2 of Shergold (1985); Upper Hay River Formation Member 2 of Donnelly et al (1988). Whitehouse FW, 1936. The Cambrian faunas of north-eastern Australia. Part 1: stratigraphical outline. Part 2: Trilobita (Miomera). Memoirs of the Queensland Museum 11, 59‑112. Distribution: Southern Georgina Basin, in HUCKITTA, TOBERMOREY and HAY RIVER; subsurface in SANDOVER RIVER and possibly western Queensland. Whitehouse FW, 1939. The Cambrian faunas of northeastern Australia. Part 3: the polymerid trilobites (with supplement No 1). Memoirs of the Queensland Museum 11, 179‑282. Geomorphic expression: Generally poorly exposed. Type section: Holostratotype section in interval 103.2‑554.7 m depth in cored drillhole NTGS99/1 (TOBERMOREY); core stored in NTGS Core Library, Alice Springs. Lower boundary at disconformable contact of foetid pyritic-carbonaceous black shale of basal Arthur Creek Formation with light grey bioclast dolograinstone of underlying Thorntonia Limestone. Upper boundary at sharp, apparently conformable transition between grey Whitehouse FW, 1940. Studies in the late geological history of Queensland. University of Queensland, Department of Geology, Paper 2 (1). Whitehouse FW, 1941. The Cambrian faunas of northeastern Australia. Part 4 – Early Cambrian echinoderms similar to the larval forms of Recent forms. Memoirs of the Queensland Museum 12, 1‑28. 56 Age and evidence: Middle Cambrian: Templetonian (pre-Triplagnostus gibbus Zone; Laurie 2000a) to Boomerangian Lejopyge laevigata Zone (Shergold 1985, 1995), based on agnostine trilobites. calci/dolomudstone-siliciclastic mudstone interbeds of uppermost Arthur Creek Formation and quartzic equivalents of overlying Steamboat Sandstone. Holostratotype replaces composite stratotype of Freeman (1986), due to doubts as to the latter’s completeness. Tomahawk Formation (formalisation of name) Reference sections: Composite section comprising cored drillholes NTGS HUC1 and NTGS HUC2 in eastern HUCKITTA [former lectostratotype of Freeman (1986)]. Complete and contiguous intersections in cored drillholes Baldwin 1 in HUCKITTA (470.4-889.3 m depth), Hacking 1 in TOBERMOREY (752.9-1209.9 m depth) and Owen 2 in TOBERMOREY (692.7-1053.3 m depth). Proposers: PD Kruse and AT Brakel, after Smith (1964). Derivation of name: After Tomahawk Yard on northern flank of Dulcie Range, central HUCKITTA (Smith 1964: 47). Synonymy: Lower part of Dulcie Sandstone of Joklik (1955); major portion of Tomahawk Beds of Smith (1964) excluding that above 400-500’ level in his reference section X30, now referred to Kelly Creek Formation. Lithology: Lower interval of foetid pyritic-carbonaceous black shale, passing upward gradationally into planar to undulose foetid shale/dolostone laminite. Passes upward, abruptly to gradationally, into upper interval of paler grey dolomudstone to dolograinstone and calci/dolomudstonesiliciclastic mudstone interbeds, minor quartzic dolostone and dolostone-clast breccia. Distribution: Flanks Dulcie Range in HUCKITTA, ELKEDRA, BARROW CREEK and ALCOOTA; extensive northward of Tarlton Range in eastern HUCKITTA, southeastern ELKEDRA, western TOBERMOREY and southwestern SANDOVER RIVER. Thickness: Maximum 457 m in Hacking 1; 452 m in NTGS99/1 holostratotype, 419 m in Baldwin 1, 361 m in Owen 2. Geomorphic expression: Contiguous tracts of hilly country around Dulcie Range and near Tarlton Range in southwestern TOBERMOREY; isolated low to moderate hills and some prominent ridges in northwestern TOBERMOREY, eastern HUCKITTA, southeastern ELKEDRA and southwestern SANDOVER RIVER. Structural attitude: Flat-lying to moderately dipping; subvertical adjacent to some faults. Relationships and boundary criteria: Disconformable on Thorntonia Limestone (Öpik 1956), or where this is absent, Red Heart Dolostone (Walter et al 1979); unconformable on older units. Lower boundary at sharp contact of light grey bioclast dolograinstone of underlying Thorntonia Limestone (or grey to orange-pink dolostone of Red Heart Dolostone) with foetid pyritic-carbonaceous black shale or paler dolostone of basal Arthur Creek Formation. Overlain with apparent conformity in eastern outcrop area by Steamboat Sandstone (Noakes et al 1959); boundary at sharp transition from grey calci/dolomudstonesiliciclastic mudstone interbeds of uppermost Arthur Creek Formation with quartzic equivalents of Steamboat Sandstone. Overlain with apparent conformity in western outcrop area by Chabalowe Formation (Stidolph et al 1988); boundary at transition from light grey, marine carbonate rocks of upper Arthur Creek Formation into peritidal, grey evaporitic carbonate of Hagen Member of Chabalowe Formation. Where these two units are absent, overlain with apparent conformity by Arrinthrunga Formation. Type section: Interval 0-148.9 m depth in cored drillhole NTGS ELK6 (ELKEDRA); core stored in NTGS Core Library, Alice Springs. Lower boundary stratotype is at 148.9 m depth, where light grey dolostone of Arrinthrunga Formation is disconformably overlain by thin (2 dm) basal mudstone and succeeding quartz-glauconite sandstone of Tomahawk Formation. Type section is dominantly of quartz ± glauconite sandstone with minor recurring thin mudstone interbeds and occasional conglomerates of tabular sandstone and siltstone lithoclasts. Reference section: Measured section on southeastern flank of Tarlton Range (TOBERMOREY): base at valley floor at PQ930670 commencing at base of lowest carbonate bed; top at summit of isolated hill at PQ920682 (Figure 27). Section 70 m thick, spanning upper carbonate interval of Tomahawk Formation to lower Kelly Creek Formation. Apparently conformable upper boundary stratotype of Tomahawk Formation is at 32 m from base of section and coincides with base of isolated hill. Lithology: Quartz ± glauconite sandstone with minor beds of dolostone, mudstone and conglomerate; upper part of formation dominantly of limestone with lesser interbedded sandstone and marl. Correlations: Beetle Creek Formation, Blazan Shale, Inca Formation, Age Creek Formation, Currant Bush Limestone, V-Creek Limestone and Mail Change Limestone in eastern Georgina Basin; Beetle Creek Formation, Inca Formation, Roaring Siltstone, lower Devoncourt Limestone in Burke River Structural Belt; Wonarah beds, Burton beds, Camooweal Dolostone, possibly Ranken Limestone in central Georgina Basin; Anthony Lagoon beds in western Georgina Basin (Shergold et al 1985). Thickness: 148.9 m in type section (incomplete). Maximum recorded thickness 225 m in measured section X54 (Smith 1972) in western Dulcie Range (HUCKITTA), but this section may include part of Kelly Creek Formation. True thickness estimated at 150-190 m. 57 Formation to steep face of Kelly Creek Formation. In Dulcie Range, upper boundary coincides with transition from deformed sandstone and minor carbonate beds (Tomahawk Formation) into undeformed, moderately dipping fine quartz sandstone of Kelly Creek Formation [eg at NQ871991 in reference section X30 of Smith (1964), southeastern Dulcie Range]. Structural attitude: Overall flat-lying in TOBERMOREY and adjacent areas; low to moderate dips in Dulcie Syncline. However, due to characteristic mesoscale deformation of sandstone, dip and strike may change significantly within metres. Freeman (1986) attributed this to subsurface cavernous solution of underlying carbonate rock (Arrinthrunga Formation) by groundwater and consequent progressive gravity subsidence. Conversely, carbonate beds are little affected; around Tarlton Range, limestone-dominant upper Tomahawk Formation is planar and gently dipping. Correlations: Lateral correlative of Ninmaroo Formation (Whitehouse 1936, Casey 1959) of eastern Georgina Basin. Relationships and boundary criteria: Disconformable on underlying Arrinthrunga Formation; lower boundary at abrupt change from dolostone to sandstone. Laterally interfingers with dolostone of Ninmaroo Formation along eastern margin in TOBERMOREY and SANDOVER RIVER. Conformable beneath Kelly Creek Formation; upper boundary at transition from limestone-dominant upper Tomahawk Formation to undeformed fine sandstone of Kelly Creek Formation. In Tarlton Range, this boundary coincides with clear topographic break from gently dipping bedding plane surface of uppermost Tomahawk Age and evidence: Late Cambrian (Iverian or Payntonian to probably late Datsonian Cordylodus prolindstromi Zone) based on trilobites and rostroconchs in basal beds (Casey and Gilbert-Tomlinson 1956: 65, Jones et al 1971: 21, Pojeta et al 1977) and conodonts in uppermost beds (Shergold and Druce 1980: 161, Jones et al 1971: 21, Shergold and Nicoll 1992). Possible Early Ordovician suggested by trilobite Lycophron sp, pelecypod Sthenodonta sp, indeterminate costate orthide brachiopods and indeterminate pseudoplanispiral gastropods (Laurie 2000b). 58
