Facies and stratigraphic framework of a Khuff outcrop equivalent

Transcription

GeoArabia, v. 15, no. 2, 2010, p. 91-156
Permian Saiq and Triassic Mahil formations, Oman Mountains
Gulf PetroLink, Bahrain
Facies and stratigraphic framework of a Khuff outcrop equivalent:
Saiq and Mahil formations, Al Jabal al-Akhdar, Sultanate of Oman
Bastian Koehrer, Michael Zeller, Thomas Aigner, Michael Poeppelreiter,
Paul Milroy, Holger Forke and Suleiman Al-Kindi
ABSTRACT
The Middle Permian to Lower Triassic Khuff Formation is one of the most
important reservoir intervals in the Middle East. This study presents a sequence
stratigraphic analysis of the Khuff Formation of a well-exposed outcrop in
the Oman Mountains, which may provide a reference section for correlations
across the entire Middle East. On the Saiq Plateau of the Al Jabal al-Akhdar, the
Permian Upper Saiq Formation is time-equivalent to the Lower and Middle Khuff
Formation (K5–K3 reservoir units in Oman). The Permian section is dominated
by graded skeletal and peloidal packstones and cross-bedded grainstones with a
diverse marine fauna. The Lower Mahil Member (Induan Stage), time-equivalent
to the Upper Khuff Formation (K2–K1 reservoir units in Oman), is dominated
by grainstones composed of microbially-coated intra-clasts and ooids. In
general, the studied outcrop is characterized by a very high percentage of graindominated textures representing storm-dominated shoal to foreshoal deposits of
a paleogeographically more distal portion of the Khuff carbonate ramp.
A sequence-stratigraphic analysis was carried out by integrating lithostratigraphic
marker beds, facies cycles, bio- and chemostratigraphy. The investigated outcrop
section was subdivided into six third-order sequences, named KS 6 to KS 1. KS
6–KS 5 are interpreted to correspond to the Murgabian to Midian (ca. Wordian to
Capitanian) stages. KS 4–Lower KS 2 correspond to the Dzhulfian (Wuchiapingian)
to Dorashamian (Changhsingian) stages. Upper KS 2–KS 1 represent the Triassic
Induan stage. Each of the six sequences was further subdivided into fourthorder cycle sets and fifth-order cycles. The documentation of this outcrop may
contribute to a better regional understanding of the Khuff Formation on the
Arabian Platform.
INTRODUCTION
The Khuff Formation and its time-equivalents cover most of the Arabian Platform with hydrocarbon
production in Bahrain, Iran, Oman, Qatar, Saudi Arabia and the United Arab Emirates (UAE), and
exploration potential in Kuwait, Iraq and beyond (e.g. Sharland et al., 2001). It is a classical example
of a flat epeiric carbonate ramp (e.g. Aigner and Dott, 1990; Al-Jallal, 1995) extending for more than
2,500 km in SE-NW strike-direction and more than 1,500 km in SW-NE dip-direction. This setting
led to the formation of a layer-cake type platform with certain m-scale marker beds traceable for
hundreds of km across the Arabian Platform (Al-Jallal, 1995).
Deposition of the Khuff Formation on the Arabian Plate started in the Mid-Permian and accompanied
the detachment of the Cimmerian terranes from the Pangea Supercontinent (Konert et al., 2001). It was
mostly deposited as post-rift cover on a passive continental margin of the newly forming Neo-Tethys
Ocean during a period of relative tectonic quiescence and steady subsidence (Stampfli, 2000). The
climate during Khuff time is interpreted as transitional from icehouse to greenhouse with sea-level
oscillations of moderate wavelength and amplitude (Al-Jallal, 1995). The temperature regime was
probably similar to the arid conditions of the present day Arabian Gulf (Strohmenger et al., 2002).
The most important facies in the Khuff hydrocarbon reservoirs are grainstones, variably composed
of ooids, peloids or skeletal components. Their prediction and characterization is key to economic
production from Khuff reservoirs. To investigate the stratigraphic architecture and composition of
primary reservoir facies, an outcrop study was initiated at the Saiq Plateau, Al Jabal al-Akhdar in
91
Koehrer et al.
OUTCROP MAP, OMAN MOUNTAINS
57°E
57°30'
58°30'
58°
59°
Gulf of Oman
Q
Semail
Ophiolite
23°30'N
Rustaq
Wadi
Sahtan
Jabal
Bawshar
S
Wadi
Bani Awf
PTr
Nakhl
J
P
S
J
Pc
JK
JK
J
Saiq
Plateau
Study
Area
23°
23°30'
Hulw
Wadi
Mayh PTr
Jabal Abu Da’vd
Pc
Wadi
Mijlas Tt
CO
Ja
Semail
Ophiolite
Pc
25
km
Wadi
Aday
CO
Al Jabal
al-Akhdar
P
Muscat
Tt
Jabal
Tayin
N
0
Fanjah
ba
l AJ
by
a
Ja
ba
d
lA
J
sw
ad
23°
JK
Semail
Ophiolite
Tt
Tt
Hamrat ad Duru
Range
Ja
S
ba
57°
lH
am
m
ah
57°30'
(Q) Quaternary
(Tt) Tertiary
Q
Jabal
Safra
58°
Kawr Group (Triassic – Cretaceous)
Umar Group (Triassic – Cretaceous)
(JK) Kahmah Group
(end Jurassic – mid-Cretaceous)
Baid Formation
(Late Permian – Jurassic)
(PTr) Akhdar Group
(Late Permian – Triassic)
(CO) Haima Group
(Cambrian – Ordovician)
(P) upper Huqf Group
(Precambrian – Cambrian)
Hamrat Duru Group
(Late Permian – Late Cretaceous)
Wasia Group (mid-Cretaceous)
(S) Semail Ophiolite
(mid-Late Cretaceous)
Metamorphic sole
(mid-Late Cretaceous)
54°
BAHRAIN
Abu
Dhabi
UAE
N
26°
Gulf of Oman
Location
Muscat
OMAN
22°
0
59°
58°
QATAR
22°
SAUDI
ARABIA
200
km
Muti Formation (mid-Late Cretaceous)
Aruma Group (end Cretaceous)
Q
58°30'
Al Aridh Group
(Triassic – Late Cretaceous)
(J) Sahtan Group (Jurassic)
(Pc) lower Huqf Group (Precambrian)
S
Hawasina
Nappes
Arabian
Sea
18°
YEMEN
18°
54°
58°
Figure 1: Geological map of the Oman Mountains showing location of the study area on the Saiq
Plateau. The Saiq and Mahil formations of the Akhdar Group are shown together (PTr) (after
Bechennec et al., 1993).
the Sultanate of Oman (Figures 1 and 2). There, the Saiq and Mahil formations (Permian – Triassic
Akhdar Group) are exposed and form an accessible outcrop of Khuff Formation time-equivalent
strata (Glennie, 2006). During the Late Permian, the study area was most likely located some 150
km away from the interpreted Arabian Platform margin (Figure 3). Paleogeographic maps place the
study area just south of the Equator within or close to the unrestricted, open-marine carbonate shelf
(Ziegler, 2001).
The present paper provides an initial description of facies types and facies sequences. It proposes
a stratigraphic framework of Khuff Formation time-equivalent strata on the Saiq Plateau, Oman
Mountains, based on various stratigraphic methods. Currently, further work is underway on sections
in other parts of the Oman Mountains in a more regional perspective, on which will be reported
separately.
92
GENERAL STRATIGRAPHY
The studied section is located on the Saiq Plateau (Al Jabal al-Akhdar) some 150 km southwest of
Muscat. The entire succession in the mountain range exposes Proterozoic to Cretaceous strata (Figures
2 and 4). The Saiq and Mahil formations that are the focus of this study unconformably overlie
Proterozoic beds (Mistal, Hajir and Mu’aydin formations) variably composed of metasediments
and diamictites with granite boulders (Rabu et al., 1986). The Carboniferous – Lower Permian strata
represented by the Al Khlata, Saiwan, and Gharif formations, encountered in subsurface of Oman
(Osterloff et al., 2004), are not present in the Oman Mountains. The absence of the latter formations is
attributed to the uplift during the “Hercynian Orogeny”. The Permian – Triassic strata in the Oman
Mountains was subdivided into the Permian Saiq and Triassic Mahil formations by Glennie et al.
(1974).
The Permian Saiq Formation consists of two members (Montenat et al., 1977, Rabu et al., 1986; Baud
et al., 2001) (Figure 5):
(1) The Lower Saiq Member (Figure 6) is up to 17 m in thickness and mainly composed of reddish to
yellowish siltstones and shales that may contain ostracods (Rabu et al., 1986).
(2) The Upper Saiq Member (Figure 4) has a thickness of 624 m. Its lower part, 120 m thick, consists
of limestones while its upper part is completely dolomitized (Figure 5). The Upper Saiq Member
contains a very rich Permian invertebrate and microfossil fauna including brachiopods, corals,
bivalves, crinoids and gastropods. Stratigraphically important are large miliolid foraminifera.
The Triassic Mahil Formation conformably overlies the Permian strata. In contrast to adjacent sections
in the Wadis of the Oman Mountains, its base is well marked by a whitish colored step in the slope
profile on the Saiq Plateau (Figure 4). There, the Saiq/Mahil Boundary was placed by Rabu et al.
(1986) (Figure 2). In this paper, we follow this delineation of the Saiq-Mahil Boundary and propose to
further subdivide the Mahil Formation into three informal members (Figure 5):
(1) The Lower Mahil Member, 101 m in thickness, is completely dolomitized and dominated by abiotic
components, notably ooids, peloids and microbially-coated litho-clasts. Very few fossils are
present. This unit is capped by several layers with polymict breccia and soft-sediment deformation
features that are possibly associated with thrusting (J. Mattner, personal communication, 2009).
(2) The Middle Mahil Member, time-equivalent to the Sudair Formation, is 260 m in thickness and
completely dolomitized with few fossils. Colored claystones and argillaceous dolomites appear
at the base of the Middle Mahil. They constitute the only clastic deposits in the Triassic section.
The middle and upper part of the Middle Mahil Member consists of trough cross-bedded
oolitic-peloidal grainstones. These are interbedded with burrowed and graded mudstones and
wackestones. Top Middle Mahil is marked by a dissolution breccia and a paleo-karst horizon.
(3) The Upper Mahil Member, time-equivalent to the Jilh Formation, is 194 m thick and completely
dolomitized with very few fossils. It consists of stacked microbial laminites, burrowed mudstones
and intercalated trough cross-bedded oolitic-peloidal grainstones. Top Upper Mahil is marked by
a red-colored dissolution breccia.
The Triassic section is unconformably overlain by a several tens of m-thick sequence of Lower and
Middle Jurassic strata (Sahtan Group) (Figure 4). These beds are in turn unconformably overlain by
Lower Cretaceous deposits (Kahmah Group) (Glennie, 2006).
METHODOLOGY
Four individual sections on the Saiq Plateau were logged sedimentologically (scale 1:100) using
standardized logging sheets (Figure 2). They were tied together to one complete 725 m thick section,
time-equivalent to the Khuff Formation, by using prominent lithostratigraphic marker beds traceable
over the whole Saiq Plateau (Figure 7). Texture, lithology, sedimentary structures, components and
grain- sizes were recorded. Facies were classified in a facies scheme (Table 2). The rock character was
documented with numerous outcrop photographs.
93
Koehrer et al.
Sample
Level from
top (meter)
Table 1
Recognized foraminifera in the Saiq Formation and Lower Mahil Member (outcrop sections A-D).
Counted numbers of specimens refer to a single thin section (24 x 36 mm); The stratigraphic order
is respected from base (2) to top (218); Sample location in the section is indicated in Figure 7
2
716.3
Earlandia elegans (2 specimens), Eotuberitina reitlingerae (2 specimens), Climacammina sp. (7 specimens), Pseudolangella? sp. (3
specimens), Pachyphloia ovata (3 specimens), Nodosinelloides potievskayae (3 specimens), Geinitzina chapmani (1 specimen), Neodiscus
sp. 1 (12 specimens), Globivalvulina aff. bulloides (1 specimen), Eostaffella? sp. (1 specimen), Schubertella sp. (4 specimens), Chusenella
sp. (4 specimens).
4
714.8
Tubiphytes obscurus (3 specimens), Palaeotextulariid, genus indet. (2 specimens), Pseudolangella? sp. (2 specimens), Pachyphloia ovata (3
specimen), Miliolid cf. Baisalina? sp. (1 specimen), Miliolid cf. Hemigordius sp. (2 specimens), Globivalvulina aff. bulloides (1 specimen),
Nankinella minor (21 specimens), Staffella sp. (18 specimens).
5
713.3
Earlandia elegans (2 specimens), Eotuberitina reitlingerae (2 specimens), Climacammina sp. (3 specimens), Pseudolangella sp. (1
specimen), Nodosinelloides sp. (1 specimen), “Nodosaria” sp. cf. Frondinodosaria? (1 specimen), Geinitzina chapmani (3 specimens),
Pachyphloia ovata (7 specimens), Graecodiscus? sp.(1 specimen), Neodiscus sp. 1 (2 specimens), Hemigordius sp.(2 specimens),
Globivalvulina aff. bulloides (2 specimens), Nankinella minor (9 specimens), Schubertella sp. (1 specimen).
8
704.6
Earlandia elegans (2 specimens), Eotuberitina reitlingerae (2 specimens), Climacammina sp. (3 specimens), Pseudolangella sp. (2
specimens), Pachyphloia schwageri (1 specimen), Neodiscus sp. 1 (2 specimens), Globivalvulina aff. bulloides (2 specimens),
Pseudoendothyra? sp. (10 specimens), Sphaerulina? sp. (1 specimen), Schubertella sp. (1 specimen).
Recognized Foraminifera
9
701.8
Climacammina sp. (6 specimens), Geinitzina? sp. (2 specimens), Neodiscus sp. 1 (2 specimens), Staffellid (2 specimens).
10
698.8
Eotuberitina reitlingerae (1 specimen), Nodosinelloides potievskayae (2 specimens), “Nodosaria”? sp. (1 specimen), Globivalvulina sp. (1
specimen).
12
695.1
Earlandia elegans (2 specimens), Eotuberitina reitlingerae (1 specimen), Palaeotextulariid: genus indet., (2 specimens), Climacammina sp. (1
specimen), Pseudolangella sp. (2 specimens), Pachyphloia schwageri (6 specimens), Nodosinelloides potievskayae (1 specimen),
Hemigordiellina regularis (1 specimen), Miliolid cf. Neodiscus? (1 specimen), Nankinella sp. (1 specimen), Schubertella sp. (2 specimens).
15
686.1
Eotuberitina reitlingerae (1 specimen), Climacammina sp. (2 specimens), Nodosinelloides? sp. (4 specimens), Neodiscus sp. 1 (2
specimens), Nankinella minor (1 specimen).
16
682.8
Eotuberitina reitlingerae (2 specimens), Palaeotextulariid, genus indet. (1 specimen), Tetrataxis sp. (2 specimens), Climacammina sp. (8
specimens), Pseudolangella sp. (2 specimens), Pachyphloia schwageri (7 specimens), Globivalvulina aff. bulloides (2 specimens),
Nankinella? sp. (2 specimens).
17
666.8
Eotuberitina reitlingerae (2 specimens), Climacammina sp. (1 specimen), Pseudolangella sp. (2 specimens), Langella? sp. (1 specimen),
Pachyphloia schwageri (6 specimens), Nodosinelloides potievskayae? (2 specimens), cf. Geinitzina sp. (2 specimens), Neodiscus sp.1 (2
specimens), Hemigordiellina sp. (1 specimen), Nankinella minor (3 specimens), Sphaerulina? sp. (16 specimens), Schubertella? sp. (1
specimen), Schwagerinid, genus indet. (1 specimen).
18
659.9
Earlandia elegans (8 specimens), Eotuberitina reitlingerae (3 specimens), Langella? sp. (2 specimens), Geinitzina chapmani (5 specimens),
Hemigordiellina sp. (7 specimens), Globivalvulina aff. bulloides (3 specimen).
19
645.6
Calcivertella sp. (2 specimens), Eotuberitina reitlingerae (2 specimens), Nodosinelloides or Geinitzina (3 specimens), Miliolids cf.
Hemigordiellina sp. (3 specimens), Cornuspira sp. (1 specimen), Globivalvulina sp. (1 specimen), Nankinella minor (3 specimens).
20
636.3
Earlandia elegans (16 specimens), Pseudolangella sp. (1 specimen), Geinitzina chapmani (2 specimens), Pachyphloia schwageri (3
specimens), Nodosinelloides sp. (2 specimens), Hemigordius aff. permicus (22 specimens), Hemigordius sp. (2 specimens), Globivalvulina?
sp. (1 specimen), Nankinella minor (5 specimens).
21
620.3
Earlandia elegans (3 specimens), Eotuberitina reitlingerae (3 specimens), Nodosinelloides potievskayae (1 specimen), Pachyphloia
schwageri (2 specimens), Geinitzina? sp. (2 specimens), Hemigordiellina sp. (2 specimens), Globivalvulina aff. bulloides (1 specimen),
Nankinella minor (8 specimens).
23
615.6
Hemigordiellina sp. (1 specimen).
24
599.4
Earlandia? sp. (1 specimen).
25
598.5
28
561.8
Miliolid cf. Midiella? sp., staffellids? Remarks: staffellids and miliolids are probably quite common, but completely recrystallized.
30
550.5
33
529.3
Tubiphytes obscurus (2 specimens), Globivalvulina sp. (2 specimens).
36
507.1
Hemigordiellina sp. (3 specimens).
38
496.3
Abundant staffellid forams (undetermined, completely recrystallized).
39
495.8
Earlandia elegans (abundant), abundant staffellid forams (undetermined, completely recrystallized).
40
494.3
Pseudovermiporella nipponica (2 specimens), Earlandia elegans (19 specimens), Geinitzina sp. (3 specimens), Hemigordiellina regularis (2
specimens), Midiella? sp. (3 specimens), Globivalvulina cf. vonderschmitti (2 specimens), Sphaerulina ogbinensis (12 specimens), Nankinella
minor (13 specimens).
43
483.1
Agathammina? sp. (1 specimen), Hemigordiellina sp. (2 specimens).
48
466.3
Earlandia elegans (2 specimens), Neoendothyra cf. parva (3 specimens).
51
454.3
Parafusulina? sp. (3 specimens), Yangchienia? sp. (2 specimens).
54
449.2
Pachyphloia? (2 specimens).
61
429.7
Eotuberitina reitlingerae (2 specimens), Miliolids (probably abundant, recrystallized), Globivalvulina cf. vonderschmitti (5 specimens),
Globivalvulina aff. bulloides (4 specimens), Staffellids cf. Nankinella minor (common, recrystallized).
65
418.1
Hemigordiellina sp. (1 specimen), staffellids cf. Staffella? (15 specimens).
66
417.5
Eotuberitina reitlingerae (1 specimen), “Nodosaria” sp. (1 specimen), Midiella? aff. ovata (2 specimens), Staffella sp. (22 specimens),
Nankinella minor (2 specimens).
67
415.7
Earlandia elegans (17 specimens), Frondina permica (1 specimen), Nodosinelloides shikhanica (2 specimens), Cornuspira kinkelini (2
specimens), Hemigordiellina regularis? (16 specimens), Midiella? aff. ovata (22 specimens).
72
402.9
Earlandia elegans (3 specimens), Calcivertella sp. (2 specimens), Palaeonubecularia sp. (abundant), Hemigordiellina sp. (3 specimens),
Cornuspira sp. (2 specimens), Hoyenella? sp. (1 specimen), Hemigordius sp. (9 specimens), Midiella? sp.( 7 specimens), Eostaffella? sp. (1
specimen).
94
Sample
Level from
top (meter)
Table 1 (continued)
Recognized Foraminifera
74
397.3
Earlandia elegans (abundant, >20 specimens), Hemigordiellina regularis (2 specimens), Cornuspira kinkelini (2 specimens), Midiella? sp.1 (2
specimens), Nodosinelloides shikhanica (1 specimen), Nodosinelloides sp. (6 specimens), Geinitzina chapmani (4 specimens), Globivalvulina
cf. cyprica (2 specimens), Globivalvulina cf. vonderschmitti (2 specimens).
75
392.8
Hemigordiellina regularis (2 specimens), Hemigordiopsis sp., (9 specimens), Midiella? aff. ovata (3 specimens), Midiella? sp. 1 (2
specimens), “Nodosaria” sp. (1 specimen), Nodosinelloides sp. (2 specimens), Globivalvulina cf. cyprica (2 specimens), Globivalvulina cf.
vonderschmitti (2 specimens), Nankinella minor (5 specimens).
83
386.8
Cornuspira sp. (2 specimens), Hemigordiellina regularis (2 specimens), Shanita amosi (12 specimens), Neodiscus sp. 2 (2 specimens),
Midiella? aff. ovata (3 specimens), Globivalvulina sp.(2 specimens), Paraglobivalvulina mira (2 specimens), Sphaerulina zisonghengensis (10
specimens), Nankinella minor (8 specimens).
90
372.1
Miliolids cf. Hemigordiopsis? (2 specimens).
92
367.6
Earlandia? sp. (11 specimens), Miliolids cf. Midiella? aff. ovata (18 specimens, completely recrystallized), Globivalvulina cf. cyprica (2
specimens), Staffellid (1 specimen).
98
364.4
Hemigordiellina regularis (2 specimens), Midiella? sp. (8 specimens), Pachyphloia? sp. (1 specimen), probably large staffellids (cf.
Sphaerulina?) (common).
101
355.3
Hemigordius? (1 specimen), Midiella ex gr. reicheli (9 specimens), Shanita? sp. (2 specimens), Sphaerulina ogbinensis (14 specimens),
schwagerinid, genus indet. (1 specimen).
106
345.6
Miliolids cf. Midiella? (2 specimens), Septoglobivalvulina? sp. (10 specimens).
109
343.4
Large staffellids cf. Sphaerulina? (common, completely recrystallized).
111
340.7
Midiella? aff. ovata (14 specimens), Frondina? sp. (1 specimen), Geinitzina chapmani (2 specimens), Globivalvulina cf. cyprica (10
specimens).
115
330.5
Cornuspira sp. (1 specimen).
117
312.6
Miliolid cf. Midiella? (2 specimens), staffellid cf. Nankinella minor (4 specimens).
118
308.8
Earlandia elegans (abundant), Cornuspira kinkelini (4 specimens), Hemigordiellina regularis (7 specimens), Globivalvulina cf. cyprica? (6
specimens).
119
291.7
Palaeonubecularia sp. (8 specimens), Agathammina? sp. (1 specimen), Hemigordius sp. (1 specimen), Midiella ex gr. reicheli (7 specimens),
Neodiscopsis ambiguus (5 specimens), Retroseptellina decrouezae (16 specimens), Globivalvulina cf. vonderschmitti (1 specimen),
Dagmarita sp. (4 specimens), Rectostipulina n.sp. aff. syzranaeformis (3 specimens), Nodosinelloides potievskayae (2 specimens),
Pachyphloia sp. (1 specimen), “Nodosaria” sp. (2 specimens), “Endoteba” cf. controversa (2 specimens).
121
287.3
Earlandia elegans (18 specimens), Nodosinelloides? sp. (1 specimen), Miliolid cf. Midiella (sparite filled molds).
125
273.8
Earlandia elegans (2 specimens), Agathammina sp. (1 specimen), Milioilid cf. Neodiscopsis? sp. (2 specimens), Pachyphloia? sp. (1
specimen), Dagmarita? sp. (2 specimens).
126
272.5
Earlandia elegans (16 specimens), Hemigordiellina regularis (2 specimens), Miliolids (9 specimens, sparite filled molds).
132
264.6
Earlandia elegans (9 specimens), Globivalvulina cf. vonderschmitti (4 specimens).
133
261.3
Miliolid cf. Neodiscopsis? sp. (5 specimens, sparite filled molds), Globivalvulina? sp. (3 specimens), staffellid? (sparite filled molds).
134
252.8
Palaeonubecularia sp. (2 specimens), Earlandia elegans (3 specimens), Cornuspira kinkelini (3 specimens), Hemigordius aff. irregulariformis
(2 specimens), Midiella sp.1 (12 specimens), Glomomidiellopsis tieni? (1 specimen), Dagmarita sp. (3 specimens).
135
247.3
Staffellids cf. Nankinella minor (3 specimens).
137
230.9
?Pseudovermiporella nipponica (2 specimens), Eotuberitina reitlingerae (2 specimens), Earlandia elegans (3 specimens), Cornuspira sp. (2
specimens), Agathammina? sp. (1 specimen), Neodiscopsis sp. (2 specimens), Rectostipulina pentamerata (1 specimen), Frondina permica
(1 specimen), Dagmarita? shahrezaensis (4 specimens), Nankinella minor (4 specimen).
142
214.3
Miliolids cf. Neodiscopsis? (3 specimens, sparite filled molds).
152
186.6
Agathammina? sp. (2 specimens), Miliolid cf. Glomomidiellopsis uenoi (probably abundant, completely recrystallized), Sphaerulina? sp. (3
specimens).
153
184.9
Palaeonubecularia sp. (2 specimens), Earlandia elegans (3 specimens), Midiella ex gr. reicheli (4 specimens), Glomomidiellopsis uenoi (14
specimens), Rectostipulina quadrata (2 specimens), Ichthyofrondina sp. (1 specimen), Pachyphloia ovata (2 specimens), Globivalvulina cf.
vonderschmitti (4 specimens), Dagmarita sp. (2 specimens), Nankinella sp. (2 specimens).
154
181.8
Earlandia? sp. (abundant), Hemigordius sp. (2 specimens), Midiella sp. (abundant, sparite filled molds), Septoglobivalvulina? sp. (7
specimens).
155
179.9
Large miliolids cf. Glomomidiellopsis uenoi (abundant, sparite filled molds).
157
176.3
Earlandia? sp. (abundant), Miliolids (7 specimens, completely recrystallized).
158
174.8
Earlandia? sp. (8 specimens), Eotuberitina reitlingerae (2 specimens), Hemigordius aff. schlumbergeri (2 specimens), Nodosinelloides sp. (3
specimens), Septoglobivalvulina? sp. (2 specimens).
159
168.6
Hemigordiellina regularis (2 specimens), Nodosinelloides sp. cf. potievskayae (3 specimens), Nodosinelloides sagitta (4 specimens),
Septoglobivalvulina? sp. (5 specimens).
161
165.3
Miliolids cf. Neodiscopsis? (abundant, completely recrystallized).
163
162.8
Miliolids cf. Neodiscopsis? (12 specimens, completely recrystallized), Septoglobivalvulina? sp. (2 specimens), Staffellids? (8 specimens).
165
152.0
Earlandia? sp. (abundant).
170
130.0
Earlandia elegans (3 specimens), ?Nankinella sp. (2 specimens).
172
128.3
Earlandia elegans (1 specimen), Globivalvulina sp. (1 specimen), Staffellids? genus indet. (2 specimens).
175
106.3
Biseriamminid foraminifera cf Globivalvulina? sp. (2 specimens).
176
105.8
Nankinella sp. (3 specimens).
177
103.3
Globivalvulina sp. (1 specimen), staffellid cf. Staffella sp. (completely recrystallized), Nankinella sp. (2 specimens).
188
93.2
201
73.0
Earlandia? sp. (2 specimens).
206
62.9
Earlandia? sp. (3 specimens).
216
51.0
95
Koehrer et al.
57°38'E
57°40'
57°42'
57°44'
57°46'
D
23°6'N
23°6'N
C
B
1
A
2
23°4'
23°4'
0
23°2'
57°40'
57°42'
57°44'
57°38'E
57°40'
57°42'
57°44'
1.5
km
57°46'
57°46'
Mahil Fm
0
D
N
N
Mahil Fm
1.5
km
23°6'N
C
B
1
Saiq Fm
A
2
23°4'
Mistal Fm
Shams Fm
aru
s
Fm
Mahil
Fm
Sahtan
Gp
Natih
Fm
Kh
Mu'aydin Fm
Awabi and
Birkat Fms
23°2'
Sub-Recent to Recent: Broken limestone
fragments, slope colluvium (Quaternary)
Sahtan Gp: Russet quartz sandstone
and blue-black limestone (Jurassic)
Hajir Fm: Black foetid limestone
(Proterozoic – Paleozoic)
Natih Fm: Thick-bedded shallowmarine limestone (Cretaceous)
Mahil Fm: Grey-white and beige
bedded dolomite (Triassic)
Mistal Fm: Greywacke, siltstone, tawny
dolomite (Proterozoic – Paleozoic)
Nahr Umr Fm: Orbitolina marl, bioclastic
argillaceous limestone (Cretaceous)
Saiq Fm: Black limestone and
brownish dolomite (Permian)
Mistal Fm: Basaltic to andesitic
pillow-lava (Proterozoic – Paleozoic)
Shams Fm: Oolitic, bioclastic
thick-bedded limestone (Cretaceous)
Kharus Fm: Limestone and dolomite
with stromatolites (Cambrian)
Awabi/Birkat Fms: Micritic and beige
clayey limestone (Jurassic – Cretaceous)
Mu'aydin Fm: Siltstone and shale with
carbonate beds (Proterozoic – Paleozoic)
Mistal Fm: Diamictite with granite
boulders, siltstone, greywacke, quartz
sandstone (Proterozoic – Paleozoic)
Figure 2: See facing page for caption.
96
An outcrop spectral gamma-ray survey was run in the outcrop using a portable spectral GR
spectrometer (model GS-512, manufactured by Geofyzika, Czech Republic). The spectrometer is
equipped with a 3x3’’ NaI(TI) scintillation detector collecting natural gamma-radiation at the rock
surface. Total counts were measured within a time interval of 15 seconds with a sample point spacing
of 50 cm, producing separate logs for total bulk-GR, Uranium (U), Potassium (K) and Thorium (Th). To
detect overall GR trends usable for stratigraphic correlations and sequence interpretation, a sampling
time interval of 15 seconds was found to be sufficient after test measurements of 180, 90, 30 and 15
seconds. The concentrations of each of the elements are automatically calculated by the instrument
and displayed in ppm (U, Th) and % (K). Test measurements also showed that virtually no variations
are recorded in the Thorium-Log throughout the sections. Thus it was not plotted and analyzed.
Carbon and oxygen stable-isotope analyses were performed on 170 dolomitic samples at the University
of Bochum. Sample material was carefully removed with a dental driller from hand specimens. About
1 mg of untreated sample powder of each sample was reacted with 100% H3PO4 at 70°C for 2 hours
in an off-line vacuum line using a Finnigan Gasbench II. Carbon and oxygen isotope ratios of the
generated CO2 were measured on a Finnigan Delta S mass spectrometer at the University of Bochum.
For this reaction an acid fractionation factor of 1.00993 was used. Data was reported in the usual
δ-notation in permille (‰) relative to the known isotope reference standard Vienna Peedee Belemnite
Standard (V-PDB) (Coplen, 1994). The precision for the carbon (δ13C) and oxygen (δ18O) isotopic
composition of the dolomite is better than 0.08‰ and 0.14‰, respectively. Data was not corrected
for differential fractionation of calcite and dolomite during the dissolution by phosphoric acid (Land,
1980) as rock samples were only collected from the dolomitized part of the Saiq Plateau section.
A total of 236 thin sections were manufactured from rock samples collected in the field and analyzed
biostratigraphically. Facies types were analyzed in thin sections and interpreted in terms of vertical
facies successions.
Logged sections were digitized with WellCAD. Interpreted facies and sequence stratigraphic data
were compared to similar studies (Insalaco et al., 2006; Maurer et al., 2009) to tie the Saiq/Mahil
Formations into a regional framework.
FACIES ANALYSIS AND INTERPRETATION
Eight principal lithofacies types (LFT) and their sub-types were distinguished in the investigated
section (Table 2).
The Permian Upper Saiq Member is dominated by low-angle laminated to trough cross-bedded packto grainstones (Figures 10 and 11). These are mainly composed of peloids and bio-clasts of a diverse
marine fauna. The beds represent storm-dominated foreshoal to shoal deposits. Interbedded are
burrowed/rooted mud- to wackestones (Figure 8) and microbial laminites (Figure 9) representing a
more protected backshoal environment.
The Triassic Lower Mahil Member is dominated by cross-bedded peloidal-oolitic grainstones (Figure
11) and graded storm beds (Figure 10) with a dramatically reduced fossil content. These formed in a
foreshoal to shoal environment.
Figure 2 (facing page): (a) satellite image of Saiq Plateau (courtesy of GeoTech).
(b) Geological map of the Saiq Plateau (from Rabu et al., 1986) showing the traverse of the
sedimentologically logged sections (A−D; red lines), location of the photographs (yellow
points) shown in Figure 4 (Point 1) and Figure 6 (Point 2). Below are coordinates of the
logged sections:
Section A - Base: N23°04'17'', E57°41'53''; Top: N23°04'27'', E57°42'14'’.
Section B - Base: N23°05'07'', E57°42'25''; Top B: N23°05'20'', E57°43'11''.
Section C - Base: N23°05'33'', E57°41'18''; Top C: N23°05'02'', E57°41'07''.
Section D - Base: N23°05'53'', E57°39'49''; Top D: N23°06'27'', E57°39'42'’.
Coordinates of the two panorama pictures are listed in the captions of Figures 4 and 6.
97
Koehrer et al.
LATE PERMIAN: (256–248.2 Ma)
a
35°E
Deep-marine
clastics
40°
Continental deposits
45°
TURKEY
Med. Sea
55°
60°
Caspian Sea
Shallow-marine
clastics
Shallow-marine
carbonate platform
SYRIA
35°N
50°
35°
Amanous
LEBANON
Khuff
IRAN
Hudayb Group
Karmia
JORDAN
IRAQ
Evaporites
Arqov
30°
30°
Continental
deposits
KUWAIT
Marginal-marine/
deltaic deposits
25°
Kuh-i-Mand
Dalan
Anhydrite
>100m
SAUDI ARABIA
Abu Sa'fah
BAHRAIN
Awali
Red
Arabian
Shield
Anhydrite
<60m
Gulf of
Oman
Hail
Harmaliyah
Khurais
South
Pars
North Field
Abu Al Bukhoosh
Nasr
Zakum
QATAR
Ghawar
Riyadh
Deep-marine
clastics
Kangan
North Pars
Karan
Khursaniyah
Berri
Abqaiq
Anhydrite
60–75m
Open-marine
carbonate shelf
Arzanah
25°
A'
UAE Saiq Plateau
Umm
Shaif
Yibal
A
Sea
Khuff
Yibal
Cross-section in (b)
20°
20°
OMAN
Unayzah
YEMEN
Arabian Sea
15°
15°
N
0
500
Gulf of Aden
km
35°
40°
45°
50°
55°
Southwest
A
Northeast
Yibal
Saiq Plateau
Shallow-marine carbonate platform
Slope
b
0
Deep-sea sediment
Hawasina Basin
A'
Oceanic crust (Ophiolite)
km
100
Sea mount (exotic)
Continental crust
Mantle
Figure 3: (a) Paleogeographic map of the Arabian Platform during the Late Permian showing
assumed location of the study area within the open-marine carbonate platform (reproduced and
modified from Ziegler, 2001). (b) Schematic cross-section through the Neo-Tethyan margin of
Sultanate of Oman during Khuff deposition showing the interpreted location of the study area
(Saiq Plateau) and Yibal (modified from Pillevuit et al., 1997; Richoz, 2006).
98
Saiq
99
Wadi Firq
i an)
erm
n (P
ddy
Mu
rke
Ma
r
rke
r
r2
rke
Ma
Ma
r1
rke
Ma
ert
Ch
ial
r3
rke
Ma
ccia
Bre
ary
rke
r
Ma
und
l Bo
Top
)
ssic
Jura
up (
Gro
ral
Co
ial
rob
Mic
)
ahi
iq/M
Sa
ssic
tan
Sah
K
(Tria
rob
Mic
tion
ial
rob
Mic
rma
il Fo
Mah
hu
ff
E
qu
iva
l
72
5 m ent
Su
d
Eq air/Ji
uiv
l
ale hnt
Ka
hm
ah
Gro
(Cr
e
tac
e
ous
)
View from “Military Radar
Station” towards the East
up
South
Figure 4: Outcrop photograph of the Permian to Cretaceous strata on the Saiq Plateau, Oman Mountains (shown in Figure 1 as Point 1). Khuff
time-equivalent strata is represented by the Upper Saiq Formation and lower part of Mahil Formation. The picture was taken close to the local military
radar station (N23°04’55’’, E57°45’18’’).
atio
Form
Precambrian Formations
(Pre-Permian Basement)
North
Tethys
Carnian – Rhaetian
Carnian – Rhaetian
Member
Global
Formation
Stages
Group
Epoch
Upper
Period
Era
Koehrer et al.
Subsurface
Equivalent
Lithology
Non-deposition in Oman Mountains
Upper
Ladinian
Anisian
Lower
Olenekian
Mahil
245.0
Olenekian
Induan
Induan
Wuchiapingian
Dzhulfian
Saiq
Capitanian
Sudair
Shale
Akhdar
Dorashamian
253.8
260.4
Guadalupian
Permian
PALEOZOIC
Lopingian
251.0
Changhsingian
Dolomite
Lower
249.7
Jilh
?
Middle
237.0
Anisian
Upper
Ladinian
Khuff
Dolomite
Midian
265.8
266 ?
Wordian
Murgabian
Limestone
Lower
Middle
Triassic
MESOZOIC
228.0 Ma
Pre-Khuff
Clastics
Clastics
Figure 5: Generalized stratigraphic column of the Permian – Triassic strata in the Oman Mountains
(modified from Rabu, 1986). Not to scale. Radiometric ages according to Gradstein et al. (2008).
Correlations according to Menning et al. (2006).
Lithofacies Types
Burrowed to Vertically Rooted Mudstone to Wackestone
Description: This facies type consists of light gray, whitish weathered dolomite showing intense
bioturbation with normal grading in places (Figures 8a, b and d). Burrows cause a cloudy appearance
of the rock texture with particle- and mud-rich patches. In some instances, ichnofabrics such as spreiten
structures and burrows (e.g. Diplocraterion, Thalassinoides) can be identified. In some cases vertical
shafts occur. Grain-size ranges from siltite to very fine-grained arenite. The mudstone to wackestone
is poorly to moderately well sorted. Peloids are the dominant grain types with some intermixed intraclasts, gastropod shells and skeletal debris. The microfauna is dominated by foraminifera (staffellids,
small miliolids and few biseriamminids) as well as dasycladacean algae. Bed thickness of this facies
type varies between a few cm to several dm. Its main distribution is within the Permian part of the
section.
Interpretation: Burrowed to vertically rooted mudstones to wackestones are interpreted as deposits
of a low-energy shallow, subtidal setting of a restricted lagoon or backshoal environment. Shallow
and sheltered water possibly caused by the baffling action of sediment shoals is indicated by the
abundance of peloids and by burrowing, which points to an intense activity of sediment feeding
organism within a low-energy, oxidized setting. Bioturbation is not always discriminable from
rooting. Root traces, very low fossil content and diversity are important criteria to place this muddy
100
c. 8 m
101
Upper Saiq
Lower Saiq
Pre-Permian Basement
Lacustrine slates
Yellow siltstone with rootlets
Bioclastic limestone
Figure 6: Outcrop photograph showing conformity between lower and upper members of the Saiq Formation on the Saiq Plateau.
Picture taken in Saiq Village (N23°04’01’’, E57°38’32’’).
View towards NE
c. 8 m
Northwest
Southeast
Koehrer et al.
Top
Lithology
Mudstone
Wackestone
Packstone
Grainstone
Boundstone
Floatstone
Lithofacies Type
Rudstone
Remarks
Formation
Depth (meter)
Lithology
Mudstone
Wackestone
Packstone
Grainstone
Boundstone
Floatstone
0
Texture
Lithofacies Type
Rudstone
Formation
Depth (meter)
Texture
Remarks
Sample 137
N23°06’27‘‘
E57°39’42‘‘
"Top Breccia"
240
20
Sample 135
Sample 134
260
Figure 11e
Sample 133
Sample 132
40
Mahil
Sample 126
Sample 125
Sample 216
280
Figure 11f
60
Sample 121
Sample 206
Sample 119
Figure 10b
Sample 188
100
Sample 118
Sample 117
320
Sample 115
Sample 111
Sample 109
"Saiq-Mahil
Formation Boundary"
Sample 177
Sample 176
Sample 175
340
120
Sample 106
Figure 8d
Sample 172
Sample 170
"Coral Marker",
(Figure 10f, g)
Sample 101
360
140
Sample 165
Sample 163
Sample 161
Sample 159
"Microbial Marker 3"
Saiq
160
"Microbial
Marker 2",
Figure 9c
380
Sample 83
N23°05’53‘‘
E57°39’49‘‘
400
Sample 158
Sample 157
180
Sample 98,
Figure 8a
Figure 11a
Figure 9b
Sample 92
Sample 90
Figure 8b
Figure 9a
Sample 75
Sample 74
Sample 72
N23°05’02‘‘
E57°41’07‘‘
Sample 67
Sample 155
Sample 154
Sample 153
Sample 152
200
420
Figure 11c
Sample 66
Sample 65
Sample 61
440
Sample 142
Sample 54,
Figure 10d
220
Continues in next column
102
Outcrop Section C
80
300
Saiq
Figure 10c
Figure 8f
Figure 8e
Figure 10e
Outcrop Section D
Sample 201
Lithology
Mudstone
Wackestone
Packstone
Grainstone
Boundstone
Floatstone
Lithofacies Type
Rudstone
Remarks
Formation
Depth (meter)
Mudstone
Wackestone
Packstone
Grainstone
Boundstone
Floatstone
Lithology
Texture
Lithofacies Type
Rudstone
Formation
Depth (meter)
Texture
Remarks
Sample 51
N23°05’33‘‘
E57°41’18‘‘
460
"Chert Marker"
Sample 48
Sample 20
N23°05’20‘‘
E57°39’49‘‘
480
640
Sample 19
Sample 43
660
Sample 36
Outcrop Section B
680
520
Saiq
Sample 33
Figure 8c
Figure 10a
Sample 16
Sample 15
Sample 12
Sample 10
Sample 9
Sample 8
700
Sample 5
Sample 4
Sample 2
720
Sample 30
Sample 28
560
Outcrop Section A
Sample 17
Saiq
500
540
Sample 18
Sample 40
Sample 39
Sample 38
Lower Saiq Member
"Hercynian
Unconformity"
740
N23°04’17‘‘
E57°41’53‘‘
Precambrian
Basement
580
Outcrop Section (Figure 2)
2.4 km
Sample 25
Sample 24
"Muddy Marker"
Zone
N23°04’27‘‘
E57°42’14‘‘
Top Breccia
Microbial Marker 3
C
Chert Marker
B
Muddy Marker Zone
A
Sample 23
Sample 21
620
Marker Beds
1.2 km
D
N23°05’07‘‘
E57°42’25‘‘
600
3.2 km
Base
Lithofacies Type
Burrowed/vertically rooted
mudstone/wackestone
Bioturbated mudstone/
wackestone
Microbial laminites
Graded wackestone/
mudstone
Lithology
Graded packstone/
wackestone
Intra-clastic grainstone/
rudstone
Poorly sorted peloidal
packstone/grainstone
Poorly sorted bioclastic
Well sorted peloidal
grainstone
Well sorted oolitic
grainstone
Metamorphic
basement
Limestone
Dolomite
Sandstone
Skeletal floatstone
Siltstone
Figure 7: Simplified composite section of the Saiq Plateau summarizing texture, sedimentary
structures and marker beds. The location of each of the sections is shown in Figure 2. Individual
sections (A-D) were tied to one complete composite section using well-identifiable marker beds
traceable over the whole study area (small figure on the right bottom part of the log). Lithofacies
type and lithology color coding is applicable for all stratigraphic sections illustrated in this paper.
103
Koehrer et al.
a
b
0
cm
10
0
cm
10
c
d
0
cm
10
e
0
mm
3
0
mm
3
f
0
cm
10
Figure 8: Mud-dominated facies group (in outcrop and thin-section). (a) Burrowed/vertically rooted
wackestone with intensely mottled surface (Saiq Plateau, 364.9 m-level, outcrop section C, sample
98); (b) Vertical, calcified burrows or roots in pale gray burrowed wackestone (Saiq Plateau, 376.3
m-level, outcrop section C); (c) Finely-grained beige bioturbated mudstone with Zoophycus traces
(Saiq Plateau, 676.1 m-level, outcrop section A); (d) Finely-grained dark gray burrowed
wackestone/mudstone with gastropod shells (Saiq Plateau, 347.4 m-level, outcrop section C); (e)
Dark, very bioturbated mudstone (possibly Chondrites) erosively overlain by graded packstone
(Saiq Plateau, 81.6 m-level, outcrop section D); (f) Very finely-grained bioturbated mudstone with
vertical burrows (possibly Chondrites) (Saiq Plateau, 80.3 m-level, outcrop section D).
104
Table 2
Upper Saiq and Lower Mahil members (Khuff) facies types, classification and
interpreted depositional environments
Group
MudDominated
BiogenicDominated
GrainDominated
Interpretation
Facies Types
LFA
Burrowed to vertically rooted mud- to wackestone
Deposits of a moderate-energy, shallow subtidal
lagoonal setting
2, 4
Bioturbated mud- to wackestone
Open subtidal deposits of the low-energy outer
ramp with strongly varying oxygenation, reduced
circulation and low sedimentation rates
8
Microbial laminites
Moderately to high-energy intertidal deposits
3
Graded pack- to mudstone
Sub-Type A: Graded wacke- to mudstone
Sub-Type B: Graded pack- to wackestone
Moderate to high-energy storm deposits above
storm wave base on the windward sides of shoals
7
Intra-clastic grainstone/rudstone
High-energy shallow subtidal storm deposits
7
Poorly sorted pack- to grainstone
Sub-Type A: Peloidal-rich pack- to grainstone
Sub-Type B: Bioclast-rich pack- to grainstone
Deposits of the moderate-energy,
deeper subtidal environment
4, 7
Well sorted grainstone
Sub-Type A: Well sorted peloidal grainstone
Sub-Type B: Well sorted oolitic grainstone
Proximal incipient to fully developed shoal
complexes within a high-energy,
shallow subtidal setting
5, 6
Skeletal floatstone
Deeper water, outer ramp deposits below SWB
8
LFA = Lithofacies-Association (seeTable 3)
facies in a landward position and distinguish it from muddy offshoal deposits. This facies type is
probably equivalent to facies type F5 of Insalaco et al. (2006).
Bioturbated Mudstone to Wackestone
Description: This facies type consists of gray-beige to dark-bluish gray weathered limestone or
dolomite with occasional wavy bedding (Figures 8c, e and f). These show a variety of intense undefined
bioturbation and a minor amount of preserved burrows, particularly feeding and spreiten structures
(Zoophycus, Thalassinoides and Chondrites). Mudstones and wackestones are mainly siltites to very
finely-grained arenites, moderately well to poorly sorted. The dm to a few dm thick beds contain
undefined skeletal debris, peloids and rarely bivalve or brachiopod shells. Green algae, particularly
gymnocodiacens, separates this facies type from protected lagoonal deposits. It mainly occurs within
the lowermost part of the Permian section.
Interpretation: Bioturbated mudstones and wackestones are interpreted as open-marine deposits of
a low-energy outer ramp setting (offshoal). This environment is characterized by strongly varying
oxygen levels, reduced circulation and low sedimentation rates. The dominance of lutitic components
indicates low-energy conditions with background sedimentation. Changing Eh-conditions are reflected
by changing color and ichnofabrics which highlight strong variations between well oxygenated outer
ramp environments (Zoophycus, Thalassinoides) and less well oxygenated conditions (Chondrites). This
facies type may correspond to facies type F11 of Insalaco et al. (2006).
Microbial Laminites
Description: This facies type is made up of light gray to whitish weathered dolo-mudstone with mm to
cm flat, crinkly to wavy laminations (Figure 9). Faint normal grading occurs together with desiccation
cracks and rare cm-scale tepee structures. Mat-like structures are interbedded with thin streaks of
peloidal packstones to grainstones and reworked clasts (flat pebble conglomerate). Bioturbation is
weak to absent. The main components are peloids, undefined skeletal debris, reworked laminite
clasts and rarely ooids. Biseriamminid foraminifera are characteristic constituents of this microbial
boundstone facies. The boundstone is poorly sorted with variable grain-sizes ranging from lutite to
medium-size arenites. The up to a few dm-thick beds are encountered in almost the entire outcrop
section.
105
Koehrer et al.
a
b
0
cm
10
0
mm
3
0
cm
10
c
d
0
cm
10
Figure 9: Biogenic-dominated facies group (in outcrop and thin-section). (a) Burrowed wackestone
(bottom) overlain by finely-laminated beige microbial boundstone (Saiq Plateau, 391.8 m-level,
outcrop section C); (b) Thin-section photograph of disrupted microbial laminite fabric with
fenestral pores (note geopetal fabric in some of the vugs) (Saiq Plateau, 366.9 m-level, outcrop
section C); (c) Microbial laminated pale gray mudstone-wackestone with tepee structures
(“Microbial Marker 2”) (Saiq Plateau, KS 5-4 Boundary, 345.0 m-level, outcrop section C); (d) Dark
beige boundstone with mm- to cm-scale wavy laminations and tepee structures (“Microbial
Marker 3”) (Saiq Plateau, KS 4-KS 3 Boundary, 175.0 m-level, outcrop section D).
Interpretation: Microbial laminites are interpreted as intertidal microbial mudflat deposits exposed
to periodical storm reworking. Bioturbation is generally weak or absent leading to the preservation
of laminated structures. This facies can be best understood as intercalations of thinly bedded detritus
stabilized by microbial mats (e.g. cyanobacterial filaments). The micro-graded laminae are the
product of episodically occurring turbulence due to major storms or spring tides. Such events lead
to reworking of partly lithified microbial laminites. This facies type is equivalent to facies type F6 of
Insalaco et al. (2006).
Graded Packstone to Mudstone
Description: This light gray to blackish colored weathered facies type shows a variety of physical
sedimentary features (Figures 10a, b, c and e). Mostly a thin interbedding of grainy and muddy
beds on a cm- to dm-scale is observed. Sedimentary structures include scoured bases, low-angle to
wavy lamination, hummocky cross-stratification (HCS), normal grading, bed amalgamation and
muddy (bioturbated) tops. Post-event bioturbation includes spreiten traces (Teichnichus), grazing and
crawling traces, vertical burrows (Skolithos) and escape structures. Grain-size varies between siltite to
fine rudite. Sorting is generally poor. Main components are peloids, intra-clasts, skeletal debris and
bivalve or brachiopod shells. The diversity of foraminifera is low with rare biseriamminids, miliolids
and staffellids. The cm to several dm-thick beds are present throughout the investigated section. The
thickest graded packstones to grainstones occur in the lowermost part of the Mahil Formation.
This facies type is further subdivided into two subtypes based on the amount of bioturbation and
texture. Graded wackestones to mudstones are intensely bioturbated and finely-grained (siltite).
Graded packstones to wackestones only show rare bioturbated tops and are coarsely-grained (arenite
to rudite).
106
a
d
0
0
cm
mm
3
10
e
b
0
mm
0
3
cm
20
f
c
0
cm
20
Figure 10: Grain-dominated facies group, fores0
5
hoal association (in outcrop and thin-section).
cm
(a) Graded, unbioturbated packstone to wackestone with abundant crinoidal columnar plates
g
(Saiq Plateau, 682.4 m-level, outcrop section A);
(b) Graded packstone to wackestone with
shell- and clast-rich base (thin-section photograph, Saiq Plateau, 68.9 m-level, outcrop
section D); (c) Graded packstone to wackestone
with internal hummocky cross-stratification
and wave-rippled top (Saiq Plateau, 76.2 mlevel, outcrop section D); (d) Thin-section
photograph of a bioclast-rich packstone with
skeletal fragments (mainly algal and foraminiferal debris) (Saiq Plateau, 449.1 m-level,
0
3
mm
outcrop section B); (e) Graded wackestone to
mudstone with post-event bioturbation (most
notably Teichnichus burrows) (Saiq Plateau, 85.1 m-level, outcrop section D); (f) Coral floatstone
(“Coral Marker”) with abundant solitary rugose horn coral heads (Saiq Plateau, KS 3 MFS, 133.8
m-level, outcrop section D); (g) Coral floatstone from (f) showing fasciculate rugose corals
(Waagenophyllum sp.) (thin-section photograph, Saiq Plateau, KS 3 MFS, 133.8 m-level outcrop
section D).
107
Koehrer et al.
Interpretation: This facies type shows diagnostic signatures of storm deposition. It is interpreted as
moderately low to locally high-energy storm beds, deposited above storm wave base (SWB). Storm
sheets represent deposits of an outer ramp, foreshoal environment. Reworked intra-clasts and sharp
erosive bases point to temporary high-energy storm events causing reworking of lithified sediment.
Rare oscillation rippled tops suggest the influence of wave activity. Bored and bioturbated tops are
indicators for sediment starvation and discontinuous sedimentation. There is no description of an
equivalent facies type in Insalaco et al. (2006).
Intra-clastic Grainstone/Rudstone
Description: The light gray to yellowish weathered dolo-grainstone/rudstone appear as massive
beds in outcrops (Figures 11e and f). Facies is low-angle cross-laminated and shows scoured bases and
normal grading. Rounded to elongated black intra-clasts, composed of grainy material, are aligned
along foresets or concentrated at the bases of cross-beds. Bioturbated tops occur occasionally. Grain
size usually ranges from fine arenite to coarse rudite. The poorly to moderately well sorted grainstone/
rudstone contains abundant micritized to microbially coated grainy lithoclasts, commonly peloids,
oncoidal flat pebbles and rarely skeletal debris and bivalve or brachiopod shells. Foraminifera are not
observed. Bed thickness ranges from few dm to several dm. This facies is common especially within
the Triassic part of section.
Interpretation: Intra-clastic grainstones/rudstones are interpreted as high- to moderate energy
proximal shoal deposits. High-energy storm events cause reworking of sediment and formation
of intra-clasts, resting upon sharp erosive bases. Microbial stabilization and micritization around
lithoclasts represent periods of sediment starvation and discontinuous sedimentation. Locally oncoids
and grapestone are developed. This facies type may be the equivalent of facies type F16 in Insalaco
et al. (2006).
Poorly-sorted Packstone to Grainstone
This facies is subdivided into 2 sub-types (a, b) based on different characteristic particles and
sedimentary structures:
(a) Peloidal Packstone to Grainstone
Description: This facies includes beige to blackish weathered dolomite with a packstone to grainstone
texture. The beige graded packstones to grainstones show common dm-scale low-angle lamination,
massive to faint normal grading, HCS, scoured bases as well as top down bioturbation. Dark to black
packstones are intensly mottled and faintly graded. The moderately well to poorly sorted rocks
are finely-grained arenites to fine rudites, commonly several dm thick. The main components are
peloids and bioclasts dominated by brachiopod and bivalve shells as well as rare corals, lithoclasts,
crinoids and gastropods. The foraminiferal assemblage is moderately diverse and dominated by large
miliolids, nodosariids, biseriamminids and staffellids. This facies type is very abundant in the Upper
Permian section.
Interpretation: Peloid-rich packstones to grainstones are interpreted as foreshoal to shoal margin
deposits of a moderate-energy, open-marine environment representing a transition between mid- to
outer ramp. Rapid sedimentation is due to high-energy storm events cause the development of erosive
bases and normal grading of rudite components. Common amalgamation and poor sorting point to
slightly reduced accommodation. The scarcity of ooids and micritic envelopes and the abundance
of crinoids and corals suggest open-marine steno-haline conditions. This facies type is most likely
equivalent to facies type F10 of Insalaco et al. (2006).
(b) Bioclastic Packstone to Grainstone
Description: The bluish to dark gray weathered dolomite or limestone facies consists of packstones,
less commonly grainstones (Figure 10d). Sedimentary structures include normal grading, erosive
bases, scour surfaces, low-angle lamination, umbrella structures and bed amalgamation. Packstones
are intensly mottled. The better sorted graded packstones to grainstones show top down bioturbation
in places. Burrowing is visible due to a ‘cloudy’ appearance of this facies with particle- and mud-rich
patches. Grain-size is fine- to coarse arenite. Packstones to grainstones are moderately-well to poorly
sorted. Main components are bioclasts dominated by undefined skeletal debris, corals, crinoids,
108
a
b
0
cm
0
mm
3
0
mm
3
0
mm
3
5
d
c
0
cm
10
f
e
0
cm
10
Figure 11: Grain-dominated facies group, shoal association (in outcrop and thin-section). (a)
High-angle cross-bedded, well sorted oolitic grainstone (Saiq Plateau, 366.1-m-level, outcrop
section C); (b) Thin-section photograph of the grainstone from (a) with abundant ooids (rounded)
and minor peloids; (c) Trough cross-bedded, well-sorted peloidal grainstone (Saiq Plateau, 200.8m-level, outcrop section C); (d) Thin-section photograph of the peloid-rich grainstone from (c); (e)
Poorly sorted coarsely-grained intra-clastic grainstone/rudstone (Saiq Plateau, 34.5-m-level,
outcrop section D); (f) Thin-section photograph of an intra-clastic grainstone/rudstone with
rounded to angular, partly micritized clasts (Saiq Plateau, 55.1-m-level, outcrop section D).
gastropods and fusulinid foraminifera. Foraminifera are common and dominated by paleotextulariids,
fusulinids and nodosariids. The several dm-thick beds exclusively occur within the Permian part of
the section.
Interpretation: Bioclastic packstones to grainstones are interpreted as moderate-energy deposits.
Grainy textures and cross-bedding indicate storm reworking. The differences in texture between
different sets point to variations in energy-levels. The diverse fossil assemblage including larger
benthic foraminifera and crinoids suggests fully marine conditions. Based on the relative abundance
109
Koehrer et al.
of normal marine fauna, this facies type represents a near-shoal subtidal setting, either foreshoal or
backshoal. This facies type is probably analogous to facies type F8 of Insalaco et al. (2006).
Well-sorted Grainstone
Based on compositional variations, this well-sorted, cross-bedded facies type is subdivided into either
peloidal (a) or oolitic (b) grainstone:
(a) Well-sorted Peloidal Grainstone
Description: Dolo-grainstones are light beige in color (Figures 11c and d). They are trough crossbedded with a sharp erosive base, overlain in places by intra-clasts. Locally sets of coarse skeletal
material such as coral rudstone layers occur along scour surfaces. Bioturbated tops are rare. The very
well sorted grainstones are fine to coarse arenitic. They contain abundant peloids, rarely ooids and
bioclasts such as corals, bivalve and brachiopod shells. Foraminifera are common and show a low to
moderate diversity with large miliolids, nodosariids, and biseriamminids. This facies type forms dmthick beds throughout the entire investigated section. Thick intervals of massive peloidal grainstones
are especially recognized within the Upper Permian section.
Interpretation: Cross-bedded, well-sorted peloidal grainstones are interpreted as deposits of proximal
amalgamated incipient shoal or bar complexes within the high- to moderate-energy mid ramp. The
diverse open-marine fauna suggests a position on the seaward fringe of the shoal. Well-sorted beds
and erosive features like scoured bases, common amalgamation and missing bioturbation indicate
extensive reworking in a low accommodation setting. Although there is no direct equivalent, this
facies type may correlate to facies type F10 of Insalaco et al. (2006).
(b) Well-sorted Oolitic Grainstone
Description: Oolitic dolo-grainstones are light gray to white in color and commonly show planar to
trough cross-bedding and erosive bases (Figures 11a and b). Microbial laminites are rarely observed
on top of the grainstone. Grain-size of the well to moderately well sorted deposit ranges from fine
to coarse arenite. This facies type comprises abundant ooids and coated grains, whereas peloids and
clasts are only rarely observed. It occurs as dm-thick laminae sets to beds mainly towards the top of
the Triassic section.
Interpretation: Cross-bedded, well-sorted oolitic grainstones are interpreted as shoal or bar complex
deposits. They represent high-energy mid ramp shoal bodies. Components and sedimentary
structures point to frequent high-energy conditions in a low accommodation setting. Grainstones
with similar features have been described from several modern environments and are produced in
shallow water (< 5m) or detached shoal settings, for instance in the Arabian Gulf (e.g. Purser and
Seibold, 1973). Thin patches of grainstones within lagoonal sediments might represent spillover lobes
or tidal channel sands induced by storm surges. This facies type is equivalent to facies type F9 of
Insalaco et al. (2006).
Skeletal Floatstone
Description: These dm-thick beds are dark gray to black dolomites or limestones containing abundant
corals and compound coral heads, partly in life position (Figures 10f and g). These are interbedded with
diverse fossil fragments in a muddy matrix. Common biota are allochthonous solitary rugose corals,
colonial rugose and tabulate corals, gastropods, brachiopods (e.g. Productus, Spiriferina, Pentamerus,
Richthofenia, Terebratula), bivalves and crinoids as well as rare undefined skeletal debris and peloids.
Foraminifera are very common. Particularly paleotextulariids, fusulinids and nodosariids were
recorded. Grain-size in floatstone ranges from fine to coarse rudite. Sorting is generally very poor.
The facies exclusively occurs in the Permian part of the section.
Interpretation: The poor sorting of this facies type, the muddy matrix and the high amount of
articulated open-marine fauna point to an allochthonous to parautochthonous origin of the skeletal
components and a relatively low-energy setting. Coral floatstones are interpreted as deposits of
open-marine coral patches originated as outer ramp deposits around or below SWB. Floatstones
with coarse shell and crinoid debris possibly represent storm-reworked deposits. There is no facies
equivalent interpreted by Insalaco et al. (2006).
110
Lithofacies Associations
Facies types were grouped into lithofacies associations (LFA) based on their interpreted depositional
environment (Table 3). The LFA-scheme is based on the Saiq Plateau outcrop and Khuff cores across
the Middle East. Eight lithofacies associations and their depositional environment are defined:
LFA 1: Sabkha / Salina (evaporitic supratidal setting);
LFA 2: Coastal marsh (intertidal setting);
LFA 3: Tidal flat (intertidal setting);
LFA 4: Backshoal (low-energy, shallow subtidal lagoon);
LFA 5: Shoal (high-energy, shallow subtidal setting above fair-weather wave base);
LFA 6: Beach / Barrier island (high-energy subaerial setting);
LFA 7: Foreshoal (moderate-energy, deeper subtidal setting between fair-weather wave base and
storm wave base);
LFA 8: Offshoal to basinal (low-energy, deep subtidal setting below storm wave base).
The lithofacies types observed on the Saiq Plateau were classified in terms of these lithofacies
associations and depositional environments. Within the section, mostly open-marine facies associations
occur (LFA’s 5, 7 and 8). LFA’s 3 and 4 were rarely interpreted. Although LFA’s 1, 2 and 6 are absent in
the studied section, they were included in the LFA scheme to link outcrop to subsurface sections.
Depositional Model
A conceptual 3-D depositional model of the Khuff carbonate ramp is presented in Figure 12. Most
lithofacies types found in the outcrop section represent the foreshoal, storm-dominated section of a
carbonate ramp. Fully open-marine conditions are present in large parts of the succession.
The Saiq Plateau revealed a number of features that are unlike producing Khuff reservoirs but are
important for the regional understanding of the Khuff. For instance, the Saiq Plateau succession
contains a higher percentage of open-marine facies types than most other documented Khuff sections
(e.g. Al-Jallal, 1995; Strohmenger et al., 2002; Vaslet et al., 2005; Insalaco et al., 2006; Maurer et al.,
2009). These grainy textures are mainly composed of skeletal and peloidal components with limited
occurrence of ooids and indicate a high-energy, shallow-marine setting. Ubiquitously storm beds,
interpreted as amalgamated tempestites (Aigner, 1985) and abundant open-marine fossils suggest
deposition above the SWB. These are more abundant than trough cross-bedded grainstones. Coral
patches are common particularly at the base and top of the Permian section. These together with
incipient shoals and skeletal-peloidal bars occupy the foreshoal setting. There is a general scarcity of
subaerial exposure features (e.g. dissolution breccias). Striking feature of the section is the complete
absence of structures that would indicate the presence of evaporites (e.g. stratabound dissolution
vugs or cavities). This interpretation fits well with the assumed paleogeographic location within the
open-marine carbonate shelf near the edge of the Arabian platform (Figure 3) (Ziegler, 2001).
VERTICAL STACKING OF FACIES
Facies types are stacked into facies cycles of four hierarchies. The terminology to describe facies cycles
was adopted from Kerans and Tinker (1997). Accordingly, cycles (fifth-order) are stacked to form
cycle sets (fourth-order) that are arranged in sequences (third-order), which form the overall Khuff
supersequence (second-order).
Fifth-order Cycles
Facies types are stacked to fifth-order transgressive-regressive cycles or parasequences (van Wagoner
et al., 1990), approximately 2–8 m in thickness. These cycles represent the finest scale of cyclicity within
the studied section. A time estimation based on their number within the outcrop section may suggest
that they probably record a 100,000 year Milankovitch signal (after Vail et al., 1977) (Table 4).
111
Koehrer et al.
Yibal Field
(Southwest)
Low-energy intertidal Supratidal marsh
Sabkhas
and subtidal mudstone with paleosoils
and evaporitic
and karst
and wackestone
salinas
(<5 m water depth)
Intertidal
High-energy to subtidal
beach
channels
foreshore with
beachrock
Sout
hwes
t
100’s
Low-energy tidal flats
and intertidal deposits
(<1 m water depth)
Saiq Plateau Outcrop
(Northeast)
Storm washover
deposits into
lower-energy
subtidal settings
km
Late
North
Plate ral range
east
au o
utcro of Saiq
p fac
ies
Low-energy
deeper water/
embayment muds
Local coral
patch reefs
High-energy shoals
and banks with wellsorted grainstone
(<10 m water depth)
Moderate-energy foreshoal
graded storm sheets
(10–30 m water depth)
Figure 12: Conceptual 3-D depositional model for the Late Permian to Early Triassic shelf in the Al
Jabal al-Akhdar area. The model is generalized for all Khuff Sequences (KS 6 to KS 1). Colors
correspond to the lithofacies association color code of Table 3.
Table 3
Khuff lithofacies association (LFA) scheme. Saiq Plateau outcrop is dominated by
foreshoal (LFA 7) and shoal (LFA 5) facies associations
Lithofacies
Association
Depositional Environment
Sedimentary Features
LFA 1:
Sabkha
Salina
Coastal sabkhas developed during more arid climatic
conditions that promoted evaporite precipitation in
hypersaline waters and supratidal sediments
Coalescing nodular (chicken wire textures), isolated anhydrite
nodules within mudstone/ wackestone matrix, bedded/laminated
anhydrite with intercalated mudstone laminae
LFA 2:
Intertidal
Marsh
Coastal marsh populated with mangrove-like plants
developed during more humid phases which promoted
ponds, paleosols and karst dissolution
Irregular karst surfaces, in-situ brecciation textures, color
mottling reducing zones, root cast horizons, incipient paleosols
(root casts locally filled by anhydrite), massive calcrete/
dolocrete, locally bioturbated
LFA 3:
Tidal Flat
Low-energy tidal flats surrounded by higher-energy
channels, becoming locally evaporitic during more
arid phases
Structureless to laminated mudstones, wackestone and
packstones, discontinuity surfaces, mudcracks, tepees,
microbialites, fenestrae, rare ostracods and gastropods
LFA 4:
Backshoal
Low-energy, shallow water, with semi-restricted
circulation of marine waters associated with
backshoal to intershoal lagoons frequently influenced
by storm washovers
Moderate to poorly sorted mud/wacke/packstone, heterolithics,
peloids intra-clasts and oncoids, variably bioturbated, normal
grading, clay drapes, mudstone intra-clasts, abundant
gastropods, thin-shelled bivalves, ostracods and miliolids
LFA 5:
Shoal
Open circulation of fully marine waters, variable
hydrodynamic energies characterizing active shoals,
barriers and tidal bars with lower-energy backshoal
and intershoal areas
Moderately well sorted, peloidal and oolitic grainstones, well
developed trough and planar cross-stratified, high-diversity
fauna of gastropods, thick-shelled bivalves, bryozoans,
corals, shell debris highly abraded
LFA 6:
Beach/
Barrier
Island
High-energy exposed shoal/barrier islands,
dominated by wave and wind activity
Moderately well sorted grainstone, low-angle planar and trough
cross-stratification, intra-formational oolitic grainstone clasts,
ooids, peloids, keystone vugs and meniscus cements
LFA 7:
Foreshoal
Moderately low- to locally high-energy, deeper
subtidal storm dominated mid- to outer ramp with open
circulation of fully marine waters
Moderately to poorly sorted graded wackestones to pack/
grainstones, well developed low-angle lamination and HCS,
normal grading, post event bioturbation, locally hardground
development, abundant peloids, shells, intra-clasts, foraminifers,
crinoids and algae
LFA 8:
Offshoal/
Basinal
Moderate to low-energy, deep subtidal outer ramp
below storm wave base with strongly varying
oxygenation, reduced circulation and low
sedimentation rates
Poorly sorted skeletal floatstones/packstones and bioturbated
mud/wackestones, strong bioturbation including a minor amount
of preserved burrows and or feeding structures, rare biota
(foraminifers, corals, shells, peloids)
112
Table 4
Number and average duration of cycles (fifth-order) and cycle sets (fourth-order) interpreted within
each of the six Khuff third-order sequences (KS 1 to KS 6); Cycles possibly record a 100,000 year,
cycle sets a 400,000 year Milankovitch signal; Radiometric ages according to Gradstein et al. (2008)
Sequence
(3rd-order)
Time Interval (Ma)
(Gradstein
et al., 2008)
KS 1
KS 2
Number of Cycles
(5th-order)
Average Duration
of Cycles (Ka)
Number of Cycle
Sets (4th-order)
Average Duration
of Cycle Sets (Ka)
249.8–250.8
8
ca. 125
2
ca. 500
250.8–251.8
11
ca. 90
3
ca. 333
KS 3
251.8–253.8
19
ca. 105
4
ca. 500
KS 4
253.8–260.4
61
ca. 108
11
ca. 600
KS 5
260.4–266 (?)
57
ca. 98
12
ca. 467
KS 6
266–267.5 (?)
12
ca. 125
4
ca. 375
The most significant characteristics of these cycles are regular changes in texture, grain-size and fossil
content. Differences in color and weathering angle tend to mimic rock textures. They provide a useful
proxy for the identification of facies cycles in the outcrop. Brown-gray rocks with steep weathering
angles of 80–90° tend to represent peloidal-oolitic grainstones (LFA 5). In contrast, dark-gray to black
peloidal mudstones to packstones (LFA 7 and 8) have weathering angles of 60–80°. They contain the
most open-marine fauna. Light colored mudstones (LFA 3 and 4) and wackestones have weathering
angles of 30–60°.
Small-scale cycles can be subdivided into a transgressive and regressive hemicycle. They are separated
by turnarounds, i.e. zones of maximum and minimum accommodation (Cross and Lessenger, 1998). The
facies stacking pattern of small-scale cycles varies along the depositional gradient and bathymetry as
a function of lateral shifts in accommodation space. As individual cycle types represent end-members
related to a certain depositional environment, transitions between cycle types are possible. Similar to
the hierarchical subdivision of facies associations and facies types, we recognize four general cycle
motifs. These are built by a number of more specific cycle types. Examples of individual cycle types
are shown in Figures 13 to 17.
Foreshoal Cycle Motif
Description: This asymmetric cycle motif is 2–5 m thick. It is variably composed of stacked openmarine, normal graded mid- to outer-ramp facies types (LFA 7 and LFA 8) (Figure 13). The thinner
lower part, later interpreted as transgressive hemicycle, usually consists of bioturbated mudstone to
wackestone with various ichnofabrics or skeletal floatstone. The thicker upper part, later interpreted
as regressive hemicycle, consists of graded, commonly bioturbated packstone to mudstone and
bioclastic packstone showing erosive bases, scour surfaces and HCS. These may turn upwards into
massive, low-angle laminated peloidal packstone to grainstone.
Interpretation: Dark burrowed mudstone and coral-rich skeletal floatstone at the base of the cycle
motif indicate fully open-marine conditions and maximum relative water depth in a low-energy
depositional environment around SWB. The rise to fall turnaround (zone of maximum accommodation)
is defined at intervals with a maximum percentage of open-marine components and fossils. The facies
stacking pattern of the regressive hemicycle suggests a transition from the outer ramp to distal to
proximal foreshoal. This typical shallowing-upward trend is associated with an increase of energy
indicators, grain-size and sorting. Small-scale cycles of the foreshoal cycle motif are most common in
the transgressive to early regressive part of the composite sequences. They occur in the lower part of
the Upper Saiq Member as well as in the Triassic portion of the investigated section.
Shoal Margin Cycle Motif
Description: Cycles of the shoal margin motif are 3–5 m thick and asymmetric. They can be related
to a proximal shoal to shoal fringe setting (LFA 5–LFA 7) (Figure 14). The lower part usually consists
of bioturbated bioclastic packstone with erosive bases and locally interclasts at the bases of the beds.
Intercalated are graded storm beds with HCS or bioturbated mudstone to wackestone. The upper
113
114
0
0
0
mm
mm
mm
LFA 3
3
3
3
LFA 5
Thin-section
LFA 4
Landward
LFA 7 LFA 8
SWB
FWWB
This Figure
Outcrop
East
Thickness
(meter)
0
1
2
3
4
5
Cycles
Bioturbated
mudstone
Graded packstone
to wackestone
Peloidal packstone
to grainstone
Graded packstone
to wackestone
Lithofacies
Types
M W PG
Texture
Legend
Mudstone
Wackestone
Packstone
Grainstone
M
W
P
G
Shell/intraclast lag
Low-angle lamination
Hummocky crossstratification
Bioturbation
Peloids
Regressive
hemicycle
Transgressive
hemicycle
Figure 13: Outcrop and thin-section photographs of a typical small-scale cycle type of the foreshoal motif
(Upper KS 2, outcrop section D, Saiq Plateau outcrop).
West
FORESHOAL CYCLE
Koehrer et al.
LFA 3
115
0
0
0
mm
mm
mm
3
3
3
LFA 7 LFA 8
SWB
FWWB
This Figure
LFA 5
Thin-section
LFA 4
Landward
Outcrop
Southeast
Thickness
(meter)
0
1
2
3
4
Cycles
Intra-clastic
grainstone/rudstone
Graded packstone
to wackestone
Peoloidal packstone
to grainstone
Graded packstone
to wackestone
Intra-clastic
grainstone/rudstone
Graded packstone
to wackestone
Lithofacies
Types
M W PG
Texture
Legend
Mudstone
Wackestone
Packstone
Grainstone
M
W
P
G
Shell/intraclast lag
Bioturbation
Intra-clasts
Peloids
Regressive
hemicycle
Transgressive
hemicycle
Figure 14: Outcrop and thin-section photographs of a typical small-scale cycle type of the shoal margin
motif (Upper KS 2, outcrop section D, Saiq Plateau outcrop).
Northwest
SHOAL MARGIN CYCLE
Koehrer et al.
part mainly starts with amalgamated, graded bioclastic packstone to grainstone with erosive bases
and frequent scouring. These may be overlain by thicker, low-angle laminated peloidal packstone
to grainstone or coarsely-grained intra-clastic grainstone/rudstone generally poor in fossil content.
They show an increase in sorting compared to the lower, bioturbated bioclastic packstone.
Interpretation: This cycle motif represents the transition from a storm-dominated foreshoal
environment to a higher-energy, shallower shoal fringe setting. Common bed amalgamation
and scouring reflects lower accommodation, accumulations of peloids and low-angle lamination
suggest high-energy conditions and sediment input from the adjacent shoal complex. The fallto-rise turnaround or regressive maximum occurs at the top of the packstone to grainstone that
represent the time of maximum depositional energy and minimum accommodation. During the
transgressive hemicycle (lower part), deeper-water foreshoal conditions are indicated by a higher
degree of bioturbation and abundant open-marine bio-clasts. Storm influence is interpreted from the
hummocky-cross stratified graded beds. Abundant shoal margin cycles types are present in the upper
part of the Saiq Formation.
Shoal Cycle Motif
Description: Individual shoal cycles are commonly 1–5 m thick and strongly asymmetric. They are
mainly composed of mid ramp facies types (LFA 5 and LFA 6) (Figure 15). The very thin lower part
of the cycle motif is represented by sheets of open-marine facies types such as skeletal floatstone and
graded storm beds. The thicker upper part starts with thick beds of bioclastic packstone to grainstone,
thin layers of scoured graded beds or graded low-angle laminated peloidal packstone to grainstone.
Upward these sediments may pass into massive amalgamated intra-clastic grainstone/rudstone with
micritic envelopes and coated grains. In most cases, facies grade into well sorted and cross-bedded
peloidal or oolitic grainstone. In some cases the grainstone is overlain by microbial laminites (Figure
16).
Interpretation: The shallowing-upward trend is associated with an increase of energy, sorting and the
change from skeletal to peloidal or oolitic grains. The upward increase in non-skeletal grains depicts
an increase in depositional energy with its maximum towards the shoreline fringing shoal belt. This
cyclicity style is interpreted as a prograding shoal body (cf. Aigner, 1985). The fall-to-rise turnaround
or regressive maximum mainly occurs at the top of the peloidal-oolitic grainstone that represents
the time of maximum depositional energy. Cycle caps are rarely composed of microbial laminites
that represent further shoaling into a lower accommodation, possibly intertidal setting. During the
transgressive part, flooding is represented by outer ramp facies types (LFA 7).
The shoal cycle motif is most common during times of enhanced depositional energy and tends to
develop during early transgressive and middle to late regressive part of the composite sequences.
Specific cycle types of this motif are observed throughout the investigated section.
Shoal- to-Backshoal Cycle Motif
Description: This cycle motif is usually 2–4 m thick and consists of stacked mid- to inner ramp facies
types (LFA 3 – LFA 5) (Figure 17). The lower part of the cycle motif, up to several m-thick, starts with
a sharp erosive base, possibly interclast covered (flakestones) that pass into peloidal-bioclastic, poorly
sorted packstone to grainstone or high-angle cross-bedded peloidal-oolitic grainstone. The dm-thick
upper part of the cycle motif consists of burrowed to vertically rooted mudstone to wackestone.
Microbial laminites with occasional tepee structures are observed at the very top.
Interpretation: This stack of mid- to inner ramp facies types indicates restricted conditions and lower
accommodation. The basic theme of this cycle motif is the migration of low-energy lagoonal facies
over transgressive skeletal-peloidal shoal facies types. The top of the regressive hemicycle is marked
by extensive microbial mats. This facies indicates shallow water and calm sedimentation conditions.
Cycles of the shoal- to backshoal cycle motif typically occur around peak regression of composite
(third-order) sequences and were mainly documented in the lower part of the Permian section (Figure
17).
116
117
LFA 5
Thin-section
LFA 4
SWB
FWWB
0
0
mm
mm
3
3
LFA 7 LFA 8
This Figure
Figure 15: Outcrop and thin-section
photographs of a typical small-scale cycle
type of the shoal motif without muddy
cap (Upper KS 4, outcrop section C).
LFA 3
Landward
West
Outcrop
SHOAL CYCLE
East
Thickness
(meter)
0
1
2
3
4
Well-sorted oolitic
grainstone
Graded packstone
to wackestone
Well-sorted oolitic
grainstone
Bioturbated
mudstone
Lithofacies
Types
M W PG
Texture
Mudstone
Wackestone
Packstone
Grainstone
M
W
P
G
Shell/intra-clast lag
High-angle lamination
Bioturbation
Ooids
Bioclasts
Regressive
hemicycle
Transgressive
hemicycle
Legend
Cycles
118
0
0
0
mm
mm
mm
LFA 3
3
3
3
LFA 5
SWB
FWWB
LFA 7 LFA 8
This Figure
Thin-section
LFA 4
Landward
Outcrop
NE
Northeast
Thickness
(meter)
0
1
Cycles
Bioclastic packstone
to grainstone
Well sorted oolitic
grainstone
Microbial laminites
Peloidal packestone/
grainstone
Lithofacies
Types
M W PG
Texture
Legend
Mudstone
Wackestone
Packstone
Grainstone
M
W
P
G
Microbial lamination
Bioturbation
Ooids
Bioclasts
Peloids
Regressive
hemicycle
Transgressive
hemicycle
Figure 16: Outcrop and thin-section photographs of a typical small-scale cycle type of the shoal motif with
muddy cap (Lower KS 4, outcrop section C).
SW
Southwest
SHOAL CYCLE WITH MICROBIAL CAP
Koehrer et al.
LFA 3
This Figure
LFA 5
Thin-section
LFA 4
Landward
119
0
0
0
mm
mm
mm
3
3
3
LFA 7 LFA 8
SWB
FWWB
Outcrop
NE
East
Thickness
(meter)
0
1
2
3
4
5
6
Cycles
to grainstone
Burrowed to
vertically-rooted
mudstone to
wackestone
Microbial laminites
Well sorted oolitic
grainstone
to grainstone
Burrowed to
vertically-rooted
mudstone to
wackestone
Microbial laminites
Well sorted oolitic
grainstone
Lithofacies
Types
M W PG
Texture
Mudstone
Wackestone
Packstone
Grainstone
M
W
P
G
Microbial lamination
Bioturbation
Ooids
Bioclasts
Peloids
Regressive
hemicycle
Transgressive
hemicycle
Legend
Figure 17: Outcrop and thin-section photographs of a typical small-scale cycle type of the shoal- to
backshoal motif (Upper KS 5, outcrop section C).
SW
West
SHOAL- TO BACKSHOAL CYCLE
Koehrer et al.
Fourth-order Cycle Sets
Stacks of 3 to 10 cycles form transgressive-regressive cycle sets or parasquence sets (van Wagoner et
al., 1990), some 5–25 m in thickness. In large parts of the outcrop, these cycle sets are the most obvious
and easiest to recognize order of cyclicity. The estimated average duration of each of these cycle sets is
about 400,000 years, assuming an overall duration of the Khuff of around 17.7 My (Table 4). Thus they
are classified as fourth-order cycles, which may record a Milankovitch signal (after Vail et al., 1977).
They display the lateral movement of facies associations or belts according to Walther’s Law, most
apparent by the repeated retrogradation and progradation of shoal complexes (LFA 5). The studied
section is subdivided into 36 of those cycle sets, termed Khuff Cycle Sets (KCS) 1.1 to 6.4 from top to
bottom (Table 5).
Within the cycle sets, the shoal LFA shows the lowest average GR values (19.5 API) with a narrow
range of 7 API (Figure 18b). The tidal flat LFA shows the highest average GR values of 28.8 API. The
presence of these indicator facies associations was used to objectively calibrate the LFAs within each
cycle set motif to the measured GR logs. Offshoal (LFA 8), foreshoal (LFA 7) and shoal (LFA 5) facies
associations generally show lower average GR values compared to muddy backshoal (LFA 4) and
tidal flat (LFA 3) facies associations. However, it may be impossible to differentiate between offshoal
(LFA 8), foreshoal (LFA 7) and grainy backshoal (LFA 4) facies associations based on GR-log data
alone. Four principal cycle set motifs were identified (Figure 18a).
Cycle Set Motif 1: Offshoal to Foreshoal
Description: This motif shows a strongly serrated GR pattern of moderate absolute GR values ranging
between 17 to 27 API. There is a complete absence of grainstones in this cycle set motif (Figure 18a).
Interpretation: The serrated GR pattern is caused by thinly-interbedded grainy and muddy offshoal
and foreshoal sediments (LFAs 7 and 8). The absence of high and low GR values is due to low rates of
marly background sedimentation and the general lack of grainy shoal-associated deposits.
Cycle Set Motif 2: Offshoal to Shoal
Description: Cycle sets of this motif commonly show a moderate to highly variable GR log response
with values ranging from 16–27 API. A weakly developed ‘dirtying-upward’ trend during the
transgressive hemi-cycle set and a moderately to well-developed ‘cleaning-upward’ trend in the
regressive hemi-cycle set can be recognized (Figure 18a).
Interpretation: GR log response is due to interbedded wackestones to grainstones of the offshoal and
foreshoal setting. Upward-cleaning of GR values is caused by the presence of variably thick grainy
shoal deposits that commonly display low overall GR values.
Cycle Set Motif 3: Foreshoal to Backshoal
Description: This motif typically displays a threepart GR log pattern (Figure 18a). After a weakly
developed ‘dirtying-upward’ trend during the transgressive hemi-cycle set (17 to 27 API), GR values
decrease in the lower part of the regressive hemi-cycle set down to 16 API and strongly increase again
in the uppermost part (17–38 API).
Interpretation: The foreshoal LFA of the transgressive hemi-cycle set shows a large scatter in moderate
GR values due to the interbedding of grainy and muddy sediments. Muddy backshoal cycle set caps
above variable well-developed grainy shoal unit present in the regressive hemi-cycle induce the
distinctive cleaning-upward followed by a dirtying-upward trend in the regressive portion of the
motif.
Cycle Set Motif 4: Shoal to Tidal Flat
This motif was not observed on the Saiq Plateau, but is present in more landward sections of the
Khuff platform in Oman. It may be very useful for the overall understanding of lateral cycle set
variations on the Khuff platform in Oman.
120
3rd-order
Table 5
Summary of identified cycle sets (fourth-order), their corresponding motif,
facies and gamma-ray log trends
KCS
Cycle Set Motif
KS 1
1.1
Motif3: Foreshoal to
backshoal
Motif2: Offshoal to
shoal
Motif2: Offshoal to
shoal
Motif1: Offshoal to
foreshoal
Motif1: Offshoal to
foreshoal
Motif2: Offshoal to
shoal
Motif1: Offshoal to
foreshoal
backshoal
Motif2: Offshoal to
shoal
backshoal
Motif2: Offshoal to
shoal
Motif2: Offshoal to
shoal
Motif2: Offshoal to
shoal
Motif2: Offshoal to
shoal
Motif2: Offshoal to
shoal
backshoal
Motif2: Offshoal to
shoal
Motif2: Offshoal to
shoal
backshoal
backshoal
backshoal
backshoal
backshoal
backshoal
backshoal
Motif2: Offshoal to
shoal
backshoal
Motif1: Offshoal to
foreshoal
Motif2: Offshoal to
shoal
Motif2: Offshoal to
shoal
Motif2: Offshoal to
shoal
Motif2: Offshoal to
shoal
backshoal
Motif2: Offshoal to
shoal
Motif1: Offshoal to
foreshoal
Motif1: Offshoal to
foreshoal
1.2
KS 2
2.1
2.2
2.3
KS 3
3.1
3.2
3.3
3.4
KS 4
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
4.11
KS 5
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
5.10
5.11
5.12
KS 6
6.1
6.2
6.3
6.4
Facies and Gamma-Ray Trends
Massive stacks of oolitic and peloidal grainstone; gamma-ray log shows clear dirtying-upward trend.
Thin peloidal and oolitic grainstone layers within graded tempestites; smooth gamma-ray pattern, no trend.
Shoaling-upward of graded beds into peloidal-oolitic grainstone; slight cleaning-upward gamma-ray trend.
Thin mudstone layers intercalated with graded storm sheets; slight cleaning-upward gamma-ray trend.
Thinly-bedded tempestite sheets and mudstone; serrated gamma-ray pattern, no trend.
Bioclastic packstones grade into massive peloidal grainstone; clear cleaning-upward gamma-ray trend.
Main facies is bioclastic and peloidal pack- to grain stone; slight cleaning-upward gamma-ray trend.
Upward shallowing from bioclastic packstone into grainstone with muddy cap;
typical cleaning (= grainstone) and dirtying (= muddy cap) couplet in the gamma-ray log.
Cycle set boundary marked by thick oolitic grainstone bed; well-developed cleaning-upward trend.
Sequence boundary marked by muddy/microbial cap; gamma-ray log shows clear dirtying-upward trend.
Grainstone intercalated with peloidal packstone; cleaning-upward gamma-ray trend.
Coral floatstone grades upwards into oolitic grainstone; well-developed cleaning-upward gamma-ray trend.
Coral floatstone, bioclastic packstone and oolitic grainstone; cleaning-upward gamma-ray trend.
Upward-shoaling of tempestite sheets into layers of peloidal/oolitic grainstone;
well-developed cleaning-upward gamma-ray trend.
Massive piles of peloidal and oolitic grainstone; well-developed cleaning-upward gamma-ray trend.
Thin layers of foreshoal and shoal deposits overlain by muddy sediments;
serrated gamma-ray pattern, no trend.
Graded stormbeds shoal upwards into massive oolitic grainstone; well-developed cleaning-upward trend.
Graded tempestites overlain by massive oolitic grainstone beds; slight cleaning-upward gamma-ray trend.
Massive grainstone body with thick muddy/microbial laminite cap; clear dirtying-upward gamma-ray trend.
Intercalation of thin layers of foreshoal and backshoal sediments;
overall cleaning-upward gamma-ray trend.
Well-developed muddy cap on top of oolitic grainstone; typical gamma-ray cleaning and dirtying couplet.
Thin intercalations of grainstone, packstone and muddy sediments;clear dirtying-upward gamma-ray trend.
Stack of bioclastic packstone capped by muddy backshoal deposits;
clear dirtying-upward gamma-ray trend.
Bioclast-rich packstone topped by microbial laminite layers;
well-developed dirtying-upward gamma-ray trend.
Thinly-interbedded bioclast-rich packstone capped by muddy backshoal deposits;
Upward-shallowing of graded stormbeds into oolitic grainstone; serrated gamma-ray pattern, no trend.
Well-developed muddy cap on top of thinly interbedded bioclastic packstone;
Mudstone grades into m-thick bioclastic packstone/floatstone;
well-developed cleaning-upward gamma-ray trend.
Upward-shoaling of graded wacke- to packstone into grainstone; clear cleaning-upward gamma-ray trend.
Cycle set dominated by skeletal floatstone and bioclastic packstone;
overall cleaning-upward gamma-ray trend.
Thin beds of grainstone intercalate with bioclastic packstone and wackestone;
smooth gamma-ray pattern, no trend.
Thinly interbedded grainstone and bioclastic packstone; serrated gamma-ray pattern,
no trend.
Peloidal/bioclastic packstone and grainstone capped by microbial laminite unit;
slight dirtying-upward gamma-ray trend.
Massive structureless mudstone and storm sheets capped by grainstone layers;
clear cleaning-upward gamma-ray trend.
Intercalation of tempestite sheets and bioturbated mudstone; serrated gamma-ray pattern, no trend.
Lower Saiq clastics overlain by bioclastic packstone and floatstone; clear dirtying-upward gamma-ray trend.
Cycles not to scale.
121
Koehrer et al.
(a)
0
40 M W/P G
0
(API)
40 M W/P G
Motif 1:
Offshoal to Foreshoal
Ideal
Gamma- Texture
Ray
0
(API)
LFA
(API)
Ideal
Gamma- Texture
Ray
Motif 2:
Offshoal to Shoal
LFA
Ideal
Gamma- Texture
Ray
Motif 3: Foreshoal
to Backshoal
LFA
Motif 4:
Shoal to Tidal Flat
LFA
4th-order Cycle
Conceptual Khuff Cycle Gamma-Ray Set Motifs
40 M W/P G
Ideal
Gamma- Texture
Ray
0
(API)
40 M W/P G
Regression
SB
MFS
Transgression
5m
0
SB
Landward
Seaward
Fair-weather wave base
Motif 1
Motif 2
Storm wave base
Motif 3
Motif 4
LFA 3
Mean Total GR (API)
(b)
40
LFA 4
LFA 5
LFA 7
LFA 8
Gamma-Ray Versus Lithofacies-Association Crossplot of Saiq Plateau Section
35
30
25
28.8
n =18
28.3
n = 72
20
15
19.5
n = 344
21.3
n = 932
23.1
n = 84
Lithofacies Association (LFA)
Figure 18: (a) Conceptual Khuff cycle set motifs (fourth-order) and corresponding typical GR
patterns. Note that only motifs 1–3 are actually observed in the Saiq Plateau outcrop. However,
motif 4 is known from more landward sections of the Khuff Formation in various locations.
See Figure 7 for facies association color coding.
(b) Crossplot of measured GR values and facies associations (LFA) of the Saiq Plateau section.
Average GR values are higher in mud-dominated backshoal (LFA 4) and tidal flat (LFA 3)
sediments compared to foreshoal (LFA 7) and offshoal (LFA 8) deposits. The shoal-associated
grainstone (LFA 5) forms the purest carbonate and displays the lowest average GR values.
122
Description: This cycle set motif is characterized by a well-developed cleaning-upward trend during
the transgressive hemi-cycle set towards the MFS and a dirtying-upward trend throughout the
regressive part up to the cycle set boundary (Figure 18a).
Interpretation: The MFS of this cycle set motif is placed within the thickest-developed grainstone
units (LFA 5) marked by the lowest GR values. Towards the cycle set boundary, these high-energy
deposits are in turn overlain by muddy backshoal and tidal flat deposits showing moderate to high
GR values.
Third-order Sequences
Regional stratigraphic analyses by Strohmenger et al. (2002), Alsharhan (2006) and Insalaco et al.
(2006) suggest that the Khuff Formation can be subdivided into six or seven third-order sequences.
Six sequences, composed of a variable number of fourth-order cycle sets (KCS), were defined at
the Saiq outcrop bound by stratigraphically significant marker beds (Figures 19 to 23). They were
termed Khuff Sequence 6 (KS 6) to Khuff Sequence 1 (KS 1) from bottom to top. These are sequence
stratigraphic units not to be confused with Khuff reservoir units in Oman (K1 − K5) or elsewhere (e.g.
Sharland et al., 2004; Alsharhan, 2006).
Each of the six Khuff sequences (KS 1 to KS 6) is composed of one transgressive and one regressive
hemisequence bound by a zone of maximum flooding. Sequence boundaries are interpreted on top
of each regressive unit.
Khuff Sequence 6 (KS 6)
Sedimentological Description and Interpretation: The KS 6, 167 m in thickness (Figure 19), comprises
four fourth-order cycle sets (KCS 6.1 – KCS 6.4) and 12 cycles (fifth-order). It conformably overlies
reddish-white, rooted siltstones of the middle part of the Lower Saiq Member (Figure 6). The lower
122 m of the sequence consist of limestones while the upper 45 m are dolomitized. The onset of
dolomitization is sharp and bed-parallel.
The basal transgressive part of the KS 6 (uppermost part of the Lower Saiq Member) is dominated
by bioclast-rich limy storm beds interbedded with thinly laminated ostracod siltstones possibly
representing the transition from a continental-lacustrine to a shallow-marine environment (Rabu et
al., 1986). The mixed carbonate-siliciclastic basal unit is overlain by massive to low-angle laminated
bioclastic packstones to grainstones some 20 m in thickness with common scouring and erosive bases.
They contain a diverse marine fauna (e.g. fusulinids, bivalve, brachiopod and gastropod shells).
Bioturbation is visible due to ‘cloudy’ particle- and mud-rich patches. This part of the sequence
consists of coarsening-upward cycles of the foreshoal and shoal margin cycle motif (Figures 13 and
14). Further up in the section, thinly bedded mud-rich deposits become more frequent. These finelygrained burrowed mudstones and wackestones are pale gray to yellowish-weathered and contain
diverse ichnofabrics, most notably Zoophycus traces pointing to a low-energy deeper water setting.
Intercalated are graded storm sheets with abundant skeletal debris and crinoid ossicles. In some
cases, massive low-angle laminated bioclastic packstones to grainstones mark the top of individual
small-scale cycles of the foreshoal cycle motif. This part of the succession is interpreted as an overall
deepening-upward trend from shallow-marine to open-marine carbonates.
The zone of maximum flooding of the sequence is interpreted within the KCS 6.2, some 117 m from
the base in a 15 m thick unit that contains thick stacks of massive dark-blue, bioturbated mudstone
(“Muddy Marker”), in cases heavily stylolitized. It is the lowest energy zone mainly characterized
by suspension settling, starvation and background sedimentation below storm wave base. The dark
color suggests less oxygenated waters in a deeper water setting.
The dolomitic regressive part of KS 6 is 50 m thick and dominated by bioturbated peloidal packstone
and peloidal to oolitic grainstone. The grainstone is medium- to coarsely grained and shows welldeveloped low-angle lamination or trough cross-bedding. Bioclasts are reduced to some shell lags
at the base of individual graded storm sheets. This part of the KS 6 is interpreted as an overall
123
124
Grainstone
Boundstone
Floatstone
Rudstone
B
C
D
"Muddy
Marker"
(MFS)
"Microbial
Marker 1"
3rd-order
4th-order
5th-order
Lithofacies
Association
Lithology
Packstone
10
(API)
"M
u
Ma ddy
rk
(MF er"
S)
l
bia
cro "
Mi er 1
"
40
rk
Ma
Total
GammaRay
Outcrop
West
Base of Section A: Quarry in Hayl al Yaman
A
0
20 m
East
0
20 m
South
Base of Section B: Wadi 1.2 km north of Manakbir road
North
Figure 19: Saiq Plateau KS 6 composite section - texture and facies log, GR pattern and interpreted small-, mediumand large scale cycles (scale: 1:1000). See Figure 7 for coordinates of the sections and lithofacies type/lithology color
coding, Table 3 for lithofacies association.
720
700
680
660
640
620
600
580
?
Formation
Saiq
Depth (meter)
560
Wackestone
Lithofacies Type
Mudstone
Cyclicity
KS
KS 6
Stage
Murgabian
KCS
KCS 6.1
KCS 6.2
KCS 6.3
KCS 6.4
Outcrop Section B
Outcrop Section A
Texture
5 cm
A
B
C
1m
(A) Lower Saiq clastics
(B) Coral floatstone (KCS 6.4)
20 cm
(C) Crinoidal wackestone (KCS 6.3)
(D) Zoophycus-burrowed mudstone (KCS 6.3)
D
Main Facies
Koehrer et al.
Stage
Midian
Grainstone
Boundstone
Floatstone
"Chert
Marker"
A
B
C
"Microbial
Marker 2",
Figure 9c
10
(API)
40
Total
GammaRay
Outcrop
0
10 m
East
Base of Section C: 200 m north of Jebal Akhdar-Hotel
West
ert
"Ch ker"
r
a
M
"M
i
Ma crob
rke ial
r2
"
3rd-order
4th-order
5th-order
Lithofacies
Association
Lithology
Wackestone
Packstone
4 cm
A
B
(A) Chert nodules (”Chert Marker”)
(KCS 5.8, KS 5 MFS)
10 cm
(B) “Shanita amosi”-beds (KCS 5.3)
1 cm
(C) Microbial laminite (”Microbial Marker 2”)
(Top KCS 5.1)
C
Main Facies
Figure 20: Saiq Plateau KS 5 section - texture and facies log, GR pattern and interpreted small-, medium- and large scale cycles (scale: 1:1000). See Figure
7 for coordinates of the section and lithofacies type/lithology color coding, Table 3 for lithofacies asscociation.
560
540
520
500
480
460
440
420
400
380
360
Formation
Saiq
Depth (meter)
340
Mudstone
Lithofacies Type
KS
KS 5
Cyclicity
Outcrop Section C
Rudstone
KCS
KCS KCS KCS
5.3 5.2 5.1
KCS
5.4
KCS
5.5
KCS
5.6
KCS
5.7
KCS
5.8
KCS
5.9
KCS
5.10
KCS
5.11
KCS
5.12
125
Outcrop Section B
Texture
Koehrer et al.
shallowing-upward/coarsening-upward regressive hemi-sequence in which cross-bedded to lowangle laminated peloidal shoals prograded over open-marine bioturbated mudstones and graded
foreshoal facies types.
The KS 6/KS 5 sequence boundary is placed on top of a single 0.5 m thick microbial laminite (“Microbial
Marker 1”) showing crinkly lamination and possibly indicates restricted intertidal conditions.
Gamma-ray Pattern: The base of the Upper Saiq Member (equivalent to base Khuff) is marked by a
sharp decrease in total GR readings. The values drop from 50 API in the mixed clastic-carbonate unit
below to around 20 API in the massive bioclastic limestone of the KS 6 above (Figures 19 and 24). The
lower calcitic part of the KS 6 shows an upward increase in GR values towards the zone of maximum
flooding and subsequently a decrease in the upper, dolomitized part towards the KS 6 sequence
boundary.
Stable-isotope Pattern: Wthin the KS 6, δ13C values get progressively higher from around +2 ‰ at
the base to +5.1 ‰ at the KS 6 sequence boundary (Figure 24). The measured δ18O values within the
limestone section of the KS 6 average around -4‰. At the limestone-dolomite transition close to the
KS 6 MFS, a sudden major increase of δ18O values from -6.2‰ to as high as +2.7‰ is observed. Thus
the δ18O record seems to be strongly altered by the dolomitization of the Saiq Plateau section.
Sedimentological Description and Interpretation: KS 5 is 214 m thick (Figure 20) and consists of 12
cycle sets (KCS 5.1 – KCS 5.12) and 57 cycles (fifth-order). The sequence begins with a transgressive
surface reworking the microbial unit on top of KS 6. The 90 m thick transgressive hemicycle of the
sequence is characterized by dark gray, medium-grained bioclastic packstone to grainstone and skeletal
floatstone containing brachiopod shells (e.g. Productus, Spiriferina, Richthofenia). These facies types are
interbedded with bioturbated mudstone to wackestone and cross-bedded peloidal grainstone. Further
up in the section, beds become progressively muddier. Facies are mainly arranged in shallowingupward cycles of the foreshoal and shoal margin motif (Figures 13 and 14). The transgressive part of
the KS 5 forms an aggradational stack of open-marine to proximal foreshoal facies types representing
a low-energy setting periodically perturbed by storms. Locally incipient shoals developed, which are
represented by low-angle laminated to cross-bedded peloidal grainstone. Periodic shoaling is also
indicated by the presence of shoal- to backshoal cycles with microbial laminite caps (Figure 17).
The maximum flooding surface is placed in the KCS 5.8, some 90 m from the base, within a 10 m thick
zone containing burrowed mudstone to wackestone with abundant dm-sized, dark gray to reddish
chert nodules (“Chert Marker”), common hardground development and skeletal floatstone.
The regressive part of KS 5, immediately above the “Chert Marker” bed, starts with a prominent,
approximately 10-m-thick yellow-colored bed with a scoured/loaded base and several internal
concave erosional surfaces. The lower unit of the bed is a dark gray coarse-grained fusulinid floatstone.
It is erosively truncated by beige finely-grained bioclastic grainstone with corals and fewer fusulinids.
Relict channel structures are preserved on the top. Above this conspicuous yellow bed, the regressive
hemicycle consists of a bioclastic packstone to grainstone and coral floatstone. The dark packstone
contains an open-marine fauna including megalodon bivalves, corals, brachiopods and large miliolid
and staffellid foraminifera (e.g. Shanita amosi, Sphaerulina spp.). Packstones pass upward into crossbedded peloidal and oolitic grainstone. Cycles are of the shoal and shoal-to backshoal motif (Figures
15 to 17). The uppermost part of the regressive hemicycle is muddy consisting mainly of light gray
to white burrowed/rooted mudstone to wackestone interpreted as low-energy lagoonal/backshoal
deposits and intertidal microbial laminites with occasional tepee structures. Intercalated are higherenergy facies types such as amalgamated bioclastic storm beds with erosive bases and thin oolitic
grainstone sheets. Grainstone units are generally interpreted as spillover lobes deposited during
small-scale transgressive events.
The KS 5/KS 4 sequence boundary is placed within a microbial laminite unit (“Microbial Marker 2”).
This bed shows wavy and crinkly laminae and tepee structures indicating subaerial exposure with
various wetting and drying cycles. The microbial laminites are brecciated in part and re-cemented by
126
dolomite cement. Mudstone clasts within the dolomite breccia-matrix are angular and mm-cm sized.
The sequence boundary coincides with the final occurrence of a distinctive large foraminifer (Shanita
amosi).
Gamma-ray Pattern: The KS 5 can be subdivided into two parts based on the overall GR pattern
(Figures 20 and 24). The lower part, representing the transgressive hemisequence, shows constantly
a smooth GR curve with average values of around 20 API. Minimum GR values occur just above
the interpreted KS 5 MFS within a massive fusulinid floatstone. Above, the GR trend reverses and
values increase again towards the KS 5 sequence boundary in the upper regressive part. The highest
GR values of the entire carbonate section, averaging around 28 API, are reached around the KS 5
sequence boundary. This may be caused by the high amount of microbial laminites and burrowed to
rooted mudstones to wackestones that occur in this position.
Stable-isotope Pattern: The δ13C-curve of the KS 5 shows a rather straight pattern with values ranging
from +3.8‰ to + 5.5‰ with an average of around +4.9‰ (Figure 24). δ18O values of the KS 5 also
show a highly serrated pattern and range from +2.5‰ to -2‰ with average values around -0.5‰. A
major drop in δ18O from +1.5‰ to -3‰ is apparent in the uppermost part of the sequence towards the
KS 5/KS 4 transition.
Sedimentological Description and Interpretation: KS 4 is 170 m thick and the most grain-rich unit in
the outcrop (Figure 21). It is made up of 11 cycle sets (KCS 4.1 – KCS 4.11) and 66 cycles (fifth-order).
The lower 114 m thick transgressive hemisequence starts with a unit of burrowed/rooted wackestone
and microbial laminites (m-level 340–320), possibly representing the lateral equivalent of an anhydritic
interval (“Middle Anhydrite”) described in various wells in Bahrain, Oman, Qatar, Saudi Arabia, UAE,
and large parts of the Arabian Peninsula (e.g. Al-Jallal, 1995). Above this muddy interval, hummockycross stratified bioclastic beds grade upwards into thick stacks of massive, low-angle laminated to
trough cross-bedded peloidal grainstone. Cycles are of the shoal cycle motif (Figures 15 and 16).
Peloidal grainstones thicken-upward and bio-clastic storm beds become thinner and less frequent.
Muddy cycle caps are rarely preserved. The aggradation of grain-dominated peloidal grainstone with
occasional muddy caps represents a stack of shallow-water incipient shoals or sandwave complexes.
Dark bioclastic packstones or skeletal floatstones represent significant marine flooding and openingup of the platform over the shoal to muddy inter- to backshoal environments.
The zone of maximum flooding of the KS 4 is interpreted within the KCS 4.5 within a 6 m thick unit
composed of thinly bedded, graded packstone to wackestone, interpreted as tempestite sheets. They
contain open-marine fauna mainly consisting of crinoids, rugose corals and undefined shell debris.
The regressive part of KS 4 is 56 m thick (Figure 21). Beds turn back into grainy textures with crossbedded and low-angle laminated peloidal packstone to grainstone. Rare bioclastic beds contain openmarine species (e.g. rugose horn corals, brachiopod shells and crinoids). Cycles are 3–5 m in thickness
and of the shoal motif (Figures 15 and 16). Within the thick grainstone piles, the Permian fauna is
progressively less abundant. The KS 4/KS 3 sequence boundary is placed on top of a 0.5 m thick
light gray weathered microbial laminite unit (“Microbial Marker 3”) interbedded with burrowed/
vertically rooted mudstone to wackestone.
Gamma-ray Pattern: The lower, transgressive part of the KS 4 shows a very serrated pattern in the
GR curve with values ranging from 17–35 API (Figures 21 and 24). Due to the high amount of stacked
peloidal-oolitic grainstones in the middle part of the KS 4, the area around the KS 4 MFS does not
show a well-developed GR signal. In the regressive part of the KS 4, average GR values are 22 API.
They gradually increase up to 28 API around the KS 4/KS 3 sequence boundary due to the renewed
occurrence of abundant microbial laminites.
Stable-isotope Pattern: A serrated pattern of the δ13C signature is observed within the KS 4 with highly
variable numbers between +2.9‰ to +6.1‰ (average: +5.1‰) (Figure 24). A first strong negative
δ13C-excursion appears around the interpreted KS 4-KS 3 sequence boundary. The δ18O-curve of the
KS 4 also shows a very serrated pattern. Values scatter from +1.5‰ to - 5‰. Around the KS 4/KS 3
sequence boundary, δ18O values rapidly increase up to a maximum of +0.5‰.
127
Depth (meter)
Grainstone
Boundstone
Floatstone
Rudstone
"Microbial
Marker 2"
A
B
C
"Microbial
Marker 3"
10
(API)
40
Total
GammaRay
"Microbial
Marker 2"
"M
ic
Ma robia
rke
l
r3
"
3rd-order
4th-order
5th-order
Lithofacies
Association
Lithology
Wackestone
Packstone
Formation
Saiq
Stage
Dzhulfian (Wuchiapingian)
West
Upper part of outcrop section C
Outcrop
0
20 m
East
10 cm
(C) Bioclastic packstone (KCS 4.3)
1 cm
A
(A) Microbial laminite (KCS 4.10)
(B) Cross-bedded oolitic grainstone (KCS 4.6)
B
C
Main Facies
Figure 21: Saiq Plateau KS 4 section - texture and facies log, GR pattern and interpreted small-, medium- and large scale cycles (scale: 1:1000).
See Figure 7 for coordinates of the section and lithofacies/lithology color coding, Table 3 for lithofacies association.
360
340
320
300
280
260
240
220
200
180
160
Mudstone
Lithofacies Type
KCS
KCS KCS KCS
4.3 4.2 4.1
KCS
4.4
KCS
4.5
KCS
4.6
KCS
4.7
KCS
4.8
KCS
4.9
KCS
4.10
128
KCS
4.11
KS
KS 4
Cyclicity
Outcrop Section C
Texture
Koehrer et al.
180
160
140
120
Stage
Changhsingian
Depth (meter)
100
Formation
Saiq
Grainstone
Boundstone
Floatstone
Rudstone
"Microbial
Marker 3"
A
B
"Coral
Marker"
C
"Saiq-Mahil
Formation
Boundary"
Mudstone
Wackestone
Packstone
Lithofacies Type
KS
KS 2
KS 3
KCS
KCS 2.3
KCS 3.1
KCS 3.2
KCS 3.3
KCS 3.4
KCS
4.1
129
KS 4
Cyclicity
10
(API)
40
Total
GammaRay
Outcrop Section D
3rd-order
4th-order
Texture
S3
Top
K
Outcrop
0
10 m
North
Figure 22: Saiq Plateau KS 3 section texture and facies log, GR pattern and
interpreted small-, medium- and large scale
cycles (scale: 1:500). See Figure 7 for
coordinates of the section and lithofacies
type/lithology color coding, Table 3 for
lithofacies association.
Lower part of Section D
r”
arke
ral M
“Co
South
A
B
C
10 cm
(A) Coral floatstone (KCS 6.4)
1 cm
(B) Coral floatstone (”Coral Marker”)
(KCS 3.2, KS 3 MFS)
2 cm
(C) Brecciated/disrupted bed (KCS 2.3)
Main Facies
5th-order
Lithofacies
Association
Lithology
Koehrer et al.
Sedimentological Description and Interpretation: KS 3 is 68 m thick (Figure 22), comprising four
cycle sets (KCS 3.1 and KCS 3.4) and 19 cycles (fifth-order). Its lower transgressive part (basal 40 m)
consists of dark coral floatstones and bioclastic packstones dominated by foraminifera, bryozoans,
crinoids and bivalves. These turn into beds of graded, low-angle laminated peloidal packstone to
grainstone and well sorted cross-bedded peloidal grainstone. Beds are mainly organized in cycles of
the shoal margin motif (Figures 15 and 16).
Towards the zone of maximum flooding, situated within the KCS 3.2, bioturbated bioclastic and peloidal
packstone with upwards increasing open-marine rugose horn corals, bivalve shell debris and rare
crinoids dominates. Gastropod shells occur in places. Maximum flooding of this sequence is picked
at a distinctive 1-m-thick coral floatstone with abundant rugose fasciculate corals (Waagenophyllum)
(Figures 10f and g). This bed, informally referred to as “Coral Marker”, is a marker bed traceable on
the Saiq Plateau for at least 10 km.
The regressive part of the sequence, 25 m thick, is characterized by beds of bioturbated and poorly
sorted peloidal and bioclastic packstone to grainstone (Figure 22). Sharp erosive bases and low-angle
cross-stratification are well developed within these beds. Muddy caps are absent.
The KS 3/KS 2 sequence boundary is placed on top of a 7 m massive, well sorted peloidal-oolitic
grainstone. It marks the return of well-developed shoal-associated carbonate sands.
Gamma-ray Pattern: Within the KS 3, the GR drops from 28 API at the base to 16 API at the KS 3/KS
2 sequence boundary, marked a thick peloidal grainstone unit (Figures 22 and 24).
Stable-isotope Pattern: The KS 3 shows a gradual decrease in δ13C from +6.1‰ at the base to +3.7‰
at the top (Figure 24). Average carbon-isotope values are +3.9‰. After a strong negative shift down
to -3.3‰ within the lowermost part of the KS 3, δ18O-values within the sequence show a straight
pattern with average values of around -2.2‰. Around the interpreted KS 3/KS 2 sequence boundary,
a strong negative shift in δ18O from -2‰ to a minimum of -4.5‰ is noted
Sedimentological Description and Interpretation: Three cycle sets (KCS 2.1 – KCS 2.3) composed of
11 cycles (fifth-order) stack to form the KS 2, 55 m in thickness (Figure 23). The lower transgressive
hemisequence, measuring some 23 m, is mainly composed of skeletal floatstone and peloidal
grainstone. Beds are stacked to 2–3 m thick cycles of the shoal margin motif (Figure 14). The interval
around the “Saiq/Mahil Formation Boundary” is marked by a disrupted/brecciated mudstone to
grainstone showing synsedimentary deformation fabrics. It is tentatively interpreted as seismite
deposit.
Maximum flooding is picked within pale gray to blackish, finely-grained dolomitic mudstones of the
KCS 2.2. These thinly bedded deposits are interpreted as distal open-marine graded storm beds. They
represent storm-influenced foreshoal deposits just above the SWB in an outer to mid ramp setting.
The thicker upper regressive part of the KS 2 is 32 m thick and shows a clear coarsening-up, thickeningup trend (Figure 23). Graded storm beds pass upwards into intra-clastic grainstone/rudstones with
microbial coated clasts and further into m-thick beds of cross-bedded peloidal grainstone. Facies
are arranged in cycles of the shoal motif. Shallowing proceeds during the hemisequence into the
development of thin intra-clastic-peloidal shoal complexes.
The KS 2/KS 1 sequence boundary is placed at the top of the thickest developed peloidal grainstone
that indicates the highest depositional energy.
Gamma-ray Pattern: This sequence shows a well-develop cleaning-upward GR pattern (Figures 23
and 24). Values constantly shift from 20 API in the lower part to 14 API around the KS 2 sequence
boundary. Average GR values within the KS 2 are 15 API.
130
120
100
80
60
40
20
Boundstone
Floatstone
Rudstone
Grainstone
"Saiq-Mahil
Formation
Boundary"
A
B
C
"Top
Breccia"
Wackestone
Packstone
Lithofacies Type
Mudstone
Cyclicity
10
(API)
40
Total
GammaRay
Outcrop Section D
Depth (meter)
0
Stage
Induan
Changhsingian
Formation
Mahil
Saiq
KS
KS 1
KS 2
KCS
KCS 1.1
KCS 1.2
KCS 2.1
KCS 2.2
131
KCS 2.3
"
op
"T ccia
e
Br
"S
a
i
q
Bou -Mahi
l Fm
nda
ry"
3rd-order
4th-order
Texture
Steeper upper part of outcrop section D
Sudair equivalent
Outcrop
0
10 m
East
Figure 23: Saiq Plateau KS 2 and KS 1
section - texture and facies log, GR pattern
and interpreted small-, medium- and large
scale cycles (scale: 1:500). See Figure 7 for
coordinates of the section and lithofacies
type/lithology color coding, Table 3 for
lithofacies association.
West
A
B
C
(A) Graded storm bed (KCS 2.3)
5 cm
(B) Intra-clastic rudstone (KCS 1.2)
1 cm
(C) Top Khuff Breccia (KCS 1.1)
10 cm
Main Facies
5th-order
Lithofacies
Association
Lithology
Koehrer et al.
Stable-isotope Pattern: A second strong negative δ13C excursion is apparent around the KS 2/KS
1 sequence boundary (Figure 24). Values drop from +3.7‰ at top KS 3 down to +0.7‰ within the
middle part of the KS 2. Generally, the KS 2 sequence is defined by a smooth δ13C curve with average
values around +2.2‰. δ18O-values show a straight pattern with average of around -1.9‰ throughout
the sequence.
Sedimentological Description and Interpretation: The 51 m thick sequence is the thinnest of all
Khuff sequences and is built by two cycle sets (KCS 1.1 and KCS 1.2) composed of 8 fifth-order cycles
(Figure 23). The sequence is mainly composed of high-energy facies types arranged in shallowingupward small-scale cycles of the shoal margin and shoal cycle motif (Figures 14 to 16). The lower
transgressive part of the KS 1, some 10 m thick, consists of beds of graded mudstone to packstone,
grading upwards into layers of cross bedded peloidal-oolitic grainstone.
The zone of maximum flooding is placed in thinly bedded, graded storm beds with interbedded dark
bioturbated mudstones and wackestones (KCS 1.2). They represent the lowest energy, most intense
burrowing and fully open-marine conditions.
The upper regressive 40-m-thick hemisequence (Figure 23) is dominated by up to 5-m-thick cycles
of the shoal motif that are build of graded packstone, intra-clastic grainstone/rudstone and crossbedded peloidal-oolitic grainstone intercalated only by thin transgressive storm sheets. Towards the
top of the sequence, the thickest ooid grainstone deposits of the Upper Khuff equivalent (KS 2 - KS
1) are observed. They are interpreted as high-energy shoal deposits. They are laterally persistent and
traceable over the whole study area (up to 10 km).
The KS 1 upper sequence boundary is marked by an up to 5 m thick polymict breccia (“Top Breccia”)
with a dolomitic grainstone matrix. Brecciation was most likely followed by rapid cementation of
the mud clasts. The origin of the breccia is not yet fully understood. Brittle thrusting together with
cohesive soft-sediment deformation features due to dewatering may hint at a structural origin (J.
Mattner, personal communication, 2009). Close proximity to thrust faults is indicated by isoclinal
folds in m-scale and boulders up to m-size (Figure 30b). The strong mechanical contrast between
the competent dolomites of the Lower Mahil Member below, and the incompetent basal shales of
the Middle Mahil Member above, could have caused bedding parallel displacement and sediment
brecciation due to thrusting during the Late Cretaceous.
The boundary between the Khuff-equivalent (Lower Mahil Member) and the overlying Middle
Mahil Member (Sudair Formation equivalent) is picked on top of this brecciated bed just below a
conspicuous thrombolite bed and the first occurrence of red and gray-green shales.
Gamma-ray Pattern: GR values generally show a very smooth pattern within the KS 1 with average
values of around 16 API (Figures 23 and 24). An important GR marker occurs at the top of the Lower
Mahil Member. It is characterized by the first appearance of shales in the entire investigated section.
These argillaceous beds cause a strong increase in the GR readings, reflecting a regionally important
GR marker at the base of the Middle Mahil Member (Sudair Formation time-equivalent) (Sharland et
al., 2004; Osterloff et al., 2004).
Stable-isotope Pattern: In the KS 1, δ13C again only show little variation and vary between +1.8‰ to
+3.1‰ (average: +2.3‰) (Figure 24). A positive δ13C shift from +2‰ to +3.5‰ appears in the first shale
beds of the Middle Mahil Member (Sudair Formation-equivalent). δ18O-values within this sequence
again scatter widely between -1‰ to -4.2‰ (average -3‰). At the base of the overlying shaley beds
of the Middle Mahil Member, the curve shows a positive shift of δ18O values from -2‰ to -0.5‰.
Second-order Supersequence
The Upper Saiq Member and Lower Mahil Member possibly comprise a single second-order
transgressive-regressive supersequence. During the second-order transgressive hemi-supersequence,
basal clastics (Lower Saiq Member) are overlain by open-marine limestones and dolomites. The
132
0
δ18O
7 -7
GammaRay
(API)
3 10
40 0
Uranium
(ppm)
Potassium
Cycles
Lithology
δ13C
(%)
35 0
Marker
Beds
0.7
KS 1
"Saiq/Mahil
Boundary"
"Coral Marker"
KS 3
100
KS 2
Lower Mahil
"Top Breccia"
TRIASSIC
50
Sequences
0
Formation
Age
(meter)
Depth
150
"Microbial
Marker 3"
200
KS 4
250
300
"Microbial
Marker 2"
KS 5
450
Upper Saiq
400
PERMIAN
350
"Chert Marker"
500
550
"Microbial
Marker 1"
600
KS 6
650
"Muddy
Marker"
700
"Pre-Khuff
clastics"
Figure 24: Data profiles and third-order sequences through 725 m outcrop section of Khuff
time-equivalent strata on the Saiq Plateau. The PTrB is interpreted within the transgressive part of
KS 2 and coincides with a major negative shift in carbon isotopes. Thus it is not synchronous with
the KS 3 sequence boundary. See Figure 7 for legend.
133
Koehrer et al.
Plate 1
1
250 µm
2
5
8
3
250 µm
6
500 µm
1 mm
11
500 µm
9
500 µm
12
1 mm
Plate 1: See facing page for caption.
134
4
250 µm
250 µm
7
500 µm
10
1 mm
13
500 µm
14
250 µm
1 mm
location of the 2nd-order MFS however is inconclusive based on the present dataset. In the subsurface,
the MFS of the entire Khuff Formation is commonly interpreted in the middle part of the KS 4 (e.g.
Alsharhan 2006; Insalaco et al., 2006). The Saiq Plateau outcrop however shows very little evidence
for a major transgression within this interval. Possible candidates to place the overall MFS include the
KS 6 MFS (“Muddy Marker”), the KS 5 MFS (“Chert marker”) as well as the KS 2 MFS. The different
interpretations of the second-order MFS might be due to differential tectonic movements in Oman
during the Dzhulfian (Wuchiapingian) (KS 4). The “Top Breccia” zone on top of the Lower Mahil
Member (top Khuff time-equivalent) is interpreted as second-order sequence boundary coinciding
with a major fall in relative sea-level.”
BIOSTRATIGRAPHY
Stratigraphic Distribution of Foraminifera and Regional
Biostratigraphic Correlation
A discussion of the biostratigraphic correlation of the studied outcrop sections with other surface
and subsurface units across the Arabian Platform necessitates a short summary of previous work on
biostratigraphy and stratigraphic subdivision. The biostratigraphy of the Khuff and its correlatable
formations is mainly based on brachiopods (Angiolini et al., 1998; 2003), ostracods (Crasquin-Soleau
et al., 1999, 2006), smaller foraminifera and paleoflora (including palynomorphs) (Stephenson,
2006; Berthelin et al., 2006). Vachard et al. (2005) and Gaillot and Vachard (2007) highlighted the
importance of smaller foraminifera as a potential tool for a sequence eco-biostratigraphic subdivision
of the Upper Khuff in the Middle East Gulf region, subsequently applied in Insalaco et al. (2006).
Data on the stratigraphic distribution of smaller foraminifera in the Lower Khuff are sparser and a
biostratigraphic subdivision has not been established so far.
Onset of sedimentation on the Arabian Platform above the pre-Khuff unconformity is generally
assumed to range diachronously from the Murgabian to the Midian (ca. Wordian – Capitanian). A
widespread transgression led to the deposition of Lower Khuff carbonates, followed by an evaporitic
interval (“Median Anhydrite”) over vast parts of the Arabian Platform. Carbonate deposition was reestablished in the Late Permian to Early Triassic forming the Upper Khuff strata.
The sections on the Saiq Plateau yield macro- and microfossil fauna throughout the Permian interval
(Plates 1 and 2). Whereas microfossils in the lower part of the Permian section (KS 6 – KS 5) are
fairly well preserved, the upper interval of the Permian part (KS 4 – lower KS 2) is affected by a
pervasive late diagenetic dolomitization. In many samples, the primary microfacies are destroyed,
and an unequivocal determination of fossils is often difficult. This hampers direct biostratigraphic
correlations with the usually rich and generally well preserved fauna from the same stratigraphic
interval in the subsurface of Oman and elsewhere on the Arabian Platform.
Plate 1 (facing page): Foraminifera of the Middle Permian (Guadalupian) interval of the Saiq
Formation (Sequences KS 6 and KS 5). Position of samples is indicated in Figure 7.
(1) Neoendothyra cf. parva (sample 48)
(2) Cornuspira kinkelini (sample 72)
(3) Hemigordius sp. (sample 20)
(4) Midiella? aff. ovata (sample 69)
(5) Palaeonubecularia “oncoid” (sample 72)
(6) Climacammina sp. (sample 2)
(7) Pachyphloia ovata (sample 5)
(8) Shanita amosi (sample 83)
(9) Paraglobivalvulina mira (sample 83)
(10) Yangchienia? sp. (sample 51)
(11) Chusenella sp. (sample 2)
(12) Sphaerulina zisongzhengensis (sample 83)
(13) Pseudolangella sp. (sample 18)
(14) Schubertella sp. (sample 5)
135
Koehrer et al.
Plate 2
1
100 µm
2
7
10
3
250 µm
500 µm
5
500 µm
8
6
250 µm
250 µm
11
500 µm
4
250 µm
9
12
250 µm
500 µm
250 µm
250 µm
b
c
a
13
14
250 µm
15
250 µm
17
d
250 µm
16
500 µm
Plate 2: See facing page for caption.
136
250 µm
Lower Triassic deposits are generally characterized by a low diversity fauna following the endPermian mass extinction. The pervasive dolomitization of the section prevents a biostratigraphic
interpretation of the presumed Triassic interval.
The basal part of the KS 6 (samples 2–17 in Table 1) yields a fairly high-diversity fauna including common
fusulinid (Chusenella sp., Schubertella sp., Globivalvulina aff. bulloides, Climacammina sp., Tetrataxis sp.),
miliolid (Neodiscus sp.), and lagenid (Pseudolangella sp., Pachyphloia ovata, Nodosinelloides potievskayae,
Geinitzina chapmani) foraminifera. In spite of the diverse open-marine fauna, neoschwagerinid and
verbeekinid species, mentioned in previous studies from the Oman Mountains (Montenat et al., 1977;
Weidlich and Bernecker, 2007), have not been encountered in this section.
The upper part of the KS 6 (samples 18–43 in Table 1) displays a low biodiversity dominated by
recrystallized thalli of gymnocodiacean algae (Permocalculus spp.) in a wacke-/packstones matrix.
The impoverished foraminiferal fauna consists mainly of staffellid and small miliolid forms
(Hemigordiellina regularis, Midiella? sp.), generally with completely recrystallized shells.
A narrow zone within KS 5 (samples 48–51 in Table 1) yields Neoendothyra cf. parva, Parafusulina sp. and
Yangchienia? sp., together with the enigmatic Sphairionia sikuoides, indicating the presence of Midian
(Late Wordian – Capitanian) deposits. A similar faunal interval with Parafusulina (Monodiexodina?) sp.
and Dunbarula sp. have also been encountered in the subsurface of Oman.
A burst of new faunal elements, including Shanita amosi and Paraglobivalvulina mira appears in the
uppermost KS 5 (samples 83–106 in Table 1), related to the development of extensive backshoal
environments. The beds with Shanita amosi are a well traceable marker just below the Median
Anhydrite in Oman wells and are also reported from equivalents of the Nar Member of the Dalan
Formation in Iran (Insalaco et al., 2006).
KS 4 (samples 109–157 in Table 1) is characterized by the disappearance of schwagerinids and
presence of Neodiscopsis ambiguus and Rectostipulina quadrata, which have their first appearance in
Upper Khuff strata (Insalaco et al., 2006; Gaillot and Vachard, 2007). Rare occurrences of Neomillerella
mirabilis in a nearby section in the upper KS 4 may hint at similar assemblages in the Late Dzhulfian
(Wuchiapingian) of the Zagros-Fars area (Insalaco et al., 2006).
A Dorashamian (Changhsingian) age is based on the presence of a zone with large miliolids
(Glomomidiellopsis uenoi) accompanied by rare Nodosinelloides sagitta around the KS 4/KS 3
Plate 2 (facing page): Foraminifera of the Upper Permian (Lopingian) interval (except 16) of the
Saiq Formation (Sequences KS 4 to KS 2). Position of samples is indicated in Figure 7.
(1) Hemigordius aff. schlumbergeri (sample 158)
(2) Hemigordius aff. irregulariformis (sample 134)
(3) Midiella ex gr. reicheli (sample 119)
(4) and (5) Neodiscopsis ambiguus (sample 119)
(6) Neodiscopsis sp. (sample 137)
(7) Glomomidiellopsis uenoi (sample 153)
(8) Globivalvulina cf. vonderschmitti (sample 119)
(9) Dagmarita sp. (sample 119)
(10) Dagmarita? shahrezaensis (sample 137)
(11) Retroseptellina decrouezae (sample 119)
(12) Biseriamminid foraminifera (cf. Globivalvulina?) (sample 175)
(13) (a, b) Rectostipulina n. sp. aff. syzranaeformis (sample 119)
(c) R. pentamerata (sample 137)
(d) R. quadrata (sample 153)
(14) Ichtyofrondina sp. (sample 153)
(15) “Endoteba” cf. controversa (sample 119)
(16) Earlandia? sp. (sample 206, Mahil Formation, early Triassic)
(17) Nodosinelloides sagitta (sample 159)
137
Period
TRIASSIC
5
6
4
Glomomidiellopsis uenoi
Nodosinelloides sagitta
“Endoteba” cf. controversa
Rectostipulina quadrata
Neodiscopsis ambiguus
Dagmarita sp.
Paraglobivalvulina mira
(sample 83, 386.8 m-level)
Neomillerella mirabilis
Midiella ? aff. ovata
Globivalvulina cf. cyprica
Hemigordius spp. (sample 20, 636.3 m-level)
Sphairionia sikuoides
Parafusulina sp.
Globivalvulina spp.
Chusenella sp.
Nankinella/Stafella sp. indet.
Earlandia sp.
PERMIAN
Epoch
Lower
Lopingian
Guadalupian
3
Shanita amosi
Stage
Induan
Dorashamian
(Changhsingian)
Midian
2
Sphaerulina zisongzhengensis
Formation
Mahil
Saiq
1
Paraglobivalvulina mira
KS (3rd)
Murgabian
138
Schubertella sp.
Fossils
Chusenella sp. (sample 2, 716.3 m-level)
Shanita amosi (sample 83, 386.8 m-level)
Rectostipulina quadrata
(sample 153, 185 m-level)
Dagmarita sp. (sample 119, 291.7 m-level)
Earlandia sp. (sample 206, 62.9 m-level)
Sphaerulina zisongzhengensis
(sample 83, 386.6 m-level)
Neodiscopsis ambiguus
(sample 116, 117 m-level)
Glomomidiellopsis uenoi (sample 163,
162.8 m-level)
Thin Section Photographs
(foreshoal)
high-diversity fauna
Gymnocodiacean
wackestone
(backshoal)
low-diversity fauna
(backshoal)
high-diversity fauna
"Microbial marker 2"
End-Guadalupian
Faunal Extinction
Pelo-oolitic grainstone
(shoal)
poorly preserved
foraminiferal fauna
"Coral Marker"
End-Permian Faunal
Extinction (PFE)
Interval of strong,
dolomitization
Main Biotic
Events
Koehrer et al.
boundary (Vaslet et al., 2005; Insalaco et al., 2006; Gaillot and Vachard, 2007). Rare, indeterminable
biseriamminids and staffellids persist into the upper KS 3 (samples 158–172 in Table 1). Insalaco et al.
(2006) have described diverse assemblages including Paradagmarita and allied genera in uppermost
Permian Khuff equivalents. Conspicuously, these foraminiferal assemblages have not been found
in the studied sections and it is currently uncertain, whether the apparent absence is related to the
strong dolomitization, sampling bias, or to unfavorable ecologic conditions in the KS 3 interval.
The Permian Faunal Extinction Event (PFE) on the Saiq Plateau is marked by the sudden disappearance
of Permian fauna in the basal KS 2 (samples 175–177 in Table 1).
Biostratigraphic control is very poor for the Triassic interval due to the pervasive dolomitization
and low biodiversity following the end-Permian mass extinction. Only few Earlandia? sp. have been
encountered in the upper part of the KS 2 (samples 188–206 in Table 1). Several samples in KS 1
show microbially induced carbonate precipitation with oolites and aggregate grains constituting a
grapestone facies. However, the thrombolitic facies with earliest Triassic fauna (Rectocornuspira kalhori,
Spirorbis phylctaena), common in many Tethyan outcrop sections and subsurface wells right above the
PTrB boundary (Insalaco et al., 2006; Groves and Altiner, 2005), have not been found in the section of
the Saiq Plateau.
The reappearance of foraminiferal fauna with sporadic occurrences of Hoyenella sinensis and H.
tenuifistula occurs in the lower part of the Middle Mahil Member (Sudair Formation time-equivalent).
In the Musandam area, an association of Hoyenella sinensis together with Meandrospira pusilla is
interpreted to yield a Late Induan to Olenekian age (Maurer et al., 2008, 2009).
Discussion of Proposed Stage Boundaries
Fossil groups (conodonts, larger benthic foraminifera), which are preferably used for Tethys-wide or
global correlations, are largely absent on the Arabian Platform. The classical Tethyan Late Permian
index species are likewise rarely reported.
Paleobiogeographically, the Arabian Platform represents the southeastward prolongation of the
Southern Biofacies Belt (Altiner et al., 2000), which is characterized by the Late Midian (Capitanian)
Shanita fauna and Late Permian (Lopingian) Paradagmarita fauna. The absence of key index fossils
in the Middle – Late Permian and the paleobiogeogeographic differences in the faunal composition
(Kobayashi, 1999; Kobayashi and Ishii, 2003; Ueno, 2003; Gaillot and Vachard, 2007) hamper
straightforward correlations of the sections from the Peri-Gondwana margin with other Tethyan type
sections. Correlations are based on those sections, where smaller foraminifera have been reported to
co-occur with larger forams and are therefore afflicted with different degree of uncertainty.
Problems in correlating Khuff-equivalent strata with regional and global stratigraphic scales have
been repeatedly stressed in several publications. Some of the problems are rooted in the imprecise
definitions of stages (e.g. base of Midian) and the calibration of foraminiferal biostratigraphy with other
faunal groups (conodonts, ammonoids, brachiopods). According to the foraminiferal, brachiopod, and
sparse conodont data (Montenat et al., 1977; Lys, 1988; Rabu et al., 1990; Angiolini et al., 2003) from
the Saiq Plateau and Al Huqf - Haushi areas, the lower part of the Khuff has been correlated with the
Wordian (Murgabian) (“Wordian transgression” in Angiolini et al., 2003). In contrast, Vachard et al.
(2002) updated the earlier work of Montenat et al. (1977) on the Saiq Plateau and assumed a Midian
(Capitanian) age based on foraminiferal biostratigraphy (“Midian transgression”).
Figure 25 (facing page): Stratigraphic ranges of selected smaller foraminifera and main biotic
events in the sections on the Saiq Plateau. The first Triassic index fauna, reflected by the
occurrence of Hoyenella sinensis and H. tenuifistula, appear in the lowermost part of the Middle
Mahil Member (Sudair Formation time-equivalent) (Maurer et al., 2008). Fauna suggests an
Olenekian age. Note: Paraglobivalvulina mira extends into the Changhsingian in Musandam
(Maurer et al., 2008, 2009).
139
Koehrer et al.
Recent work in Tunisia (Angiolini et al., 2008) and data from Sicily (Kozur and Davydov, 1996) indicate
that part of the Midian might actually belong to the Wordian. However, the base of the Midian itself
is biostratigraphically not strictly defined (Leven, 2003) and parts of the late Murgabian overlap with
the early Midian. Due to the absence of Yabeina-Lepidolina assemblages in western Tethyan sections
(including the Midian type section), the FAD of Dunbarula and Kahlerina instead has been used to
characterize lower Midian strata. But Dunbarula nana has already been reported from the Afghanella
schencki Zone in Iran (Kobayashi and Ishii, 2003), which is located well in the Murgabian. Dunbarula
nana has been mentioned from the Saiq Plateau (Montenat et al., 1977), from the Lower Dalan
Formation in Iran (Insalaco et al., 2006) and might be present in subsurface wells of Oman.
Due to the absence of unequivocal age-diagnostic index fossils in the studied section, it is currently
difficult to precisely trace either the Murgabian/Midian, or the Wordian/Capitanian boundary. The
lower part is herein conventionally attributed to the Murgabian (Wordian) and the Murgabian/Midian
boundary most probably lies somewhere around the KS 6/KS 5 sequence boundary (Figure 25).
The end-Guadalupian faunal extinction selectively wiped out several fossil groups, which were
assumed to host photosymbionts including the larger benthic foraminifera (Schwagerinidae,
Verbeekinidae, Neoschwagerinidae) (Ota and Isozaki, 2006). This biotic event is associated with strong
perturbations of the carbon-isotope signal during the Capitanian (“Kamura event” sensu Isozaki, et
al., 2007) and a widespread regression in the latest Capitanian. A latest Midian (Capitanian) fauna is
indicated by the last occurrence of schwagerinids and the FO/LO of Shanita amosi in the upper KS 5,
which also corresponds to a widespread regression (KS 5/KS 4 boundary), that can be followed across
the entire Arabian Platform (Al-Jallal, 1995; Alsharhan, 2006). The Guadalupian/Lopingian boundary
is therefore assumed to approximately coincide with the top KS 5 sequence boundary (Figure 25).
Evidence for Lopingian deposits on the Arabian Platform generally relies on the presence of rare,
primitive Colaniella and abundant Paradagmarita and its allied genera (Gaillot and Vachard, 2007).
Deposits of Wuchiapingian age are herein assumed to enclose the KS 4 according to the above stated
faunal correspondance with data from Insalaco et al. (2006) (Figure 25).
The rare and poorly preserved biseriamminids in the upper KS 3 do not provide sufficient data to give
a specific assignment. The typical Late Changhsingian foraminiferal fauna including Paradagmarita
monodi is absent in the studied section. The absence of this species is most likely associated with the
general scarcity of leeward shoal facies in the outcrop. Strongly recrystallized Glomomidiellopsis uenoi
in the basal KS 3 has been selected alternatively to confirm the presence of Changhsingian deposits.
The Wuchiapingian/Changhsingian boundary is placed in accordance with Insalaco et al. (2006) at
the top KS 4 sequence boundary (Figure 25).
The Permian/Triassic Boundary (PTrB) in more landward settings like Yibal (Figure 3) is characterized
by bioclastic/oolitic grainstones with abundant latest Permian smaller foraminifera followed by a
widespread occurrence of microbial sediments (Masaferro et al., 2004; Insalaco et al., 2006; Ehrenberg
et al., 2008; Maurer et al., 2009). A stromatolitic interval contains the first occurrence of earliest Triassic
fauna indicated by Rectocornuspira kalhori and Spirorbis phylctaena (Insalaco et al., 2006). On the Saiq
Plateau however, the PTrB is indistinct as the classical thrombolite unit is absent. Biostratigraphically it
can only be approximated by the drastic decline of invertebrate fauna in the basal KS 2 (Figure 25).
TENTATIVE LOCATION OF THE PERMIAN/TRIASSIC BOUNDARY
The Permian/Triassic Boundary (PTrB) is one of the most important markers for regional correlation
of the Khuff. Its position was tentatively placed based on three independent stratigraphic methods:
biostratigraphy, chemostratigraphy and sequence stratigraphy.
Biostratigraphy
Biostratigraphically, the PTrB is defined by the first occurrence (FO) of the conodont Hindeodus parvus
(Krull et al., 2004). In this study, no conodont remains were detected in outcrop samples (D. Korn,
personal communication, 2009).
140
GammaRay (API)
Cycle Sets
Cycles
Lithology
Mudstone
Wackestone
Packstone
Grainstone
Boundstone
Floatstone
Formation
Facies Type
Rudstone
Stage
Depth (meter)
Texture
10
25
Uranium
(ppm)
5
20
Chemostratigraphic
Remarks
Marker
Fossils
Sequencestratigraphic
Remarks
δ13C
% (PDB)
0
4
70
End of 2nd
negative
δ13C shift
FO of Microbialite facies
with Earlandia
(Sample 201)
Begin of 2nd
negative
δ13C shift
Mahil
80
Induan
Top KCS 2.2
KS 2-MFS
Interval devoid
of fossil remains
(azoic?);
apparent facies
shift to mudstones and
wackestones;
strong
dolomitization
90
End of 1st
negative
δ13C shift
Boundary
between
Saiq and
Mahil Fm
(major facies
change)
PFE
Saiq
100
Changhsingian
PTrB ?
Begin of 1st
negative
δ13C shift
Top KCS 2.3
LO of
Nankinella/
Staffella sp.
indet. &
Globivalvulina
spp.
(Sample 177)
Top KS 3
110
Figure 26: Permian – Triassic Boundary (PTrB) – integration of lithology, sedimentary facies,
cycles, spectral gamma-ray and carbon isotopes (scale 1:250). See Figure 7 for color coding.
The extinction of the Permian fauna, commonly referred to as ‘Permian-Faunal Extinction’ (PFE) or
‘event horizon’, is located 6 m above the KS 3 sequence boundary (Figure 26). It occurs at the base
of a prominent disrupted/brecciated pack- to grainstone bed, just below the Saiq/Mahil Formation
Boundary (Figure 22, photo C). Above the PFE, there is an interval with only rare fossil remains
(Figure 26). On the Saiq Plateau, only rare echinoderm fragments, shell debris as well as indications
of burrowing occur.
The prime marker for the Triassic faunal recovery in many parts of the Arabian Platform is the
widespread occurrence of Triassic microbialite carbonate sheets and thrombolites (e.g. Baud et
al., 1997; Insalaco et al., 2006; Weidlich and Bernecker, 2007). This marker is not developed on the
Saiq Plateau. However, sample 201 (Figure 7) is reminiscent to this facies, showing alternations of
laminated dark, micritic to light, sparitic layers with possible fenestrae fabrics.
141
20
"Top Breccia"
Lithology
Age
0
Sudair
East
Subsurface
Equivalent
Saiq Plateau (23°05’53‘‘N, 57°39’49‘‘E)
(meter)
West
Depth
Koehrer et al.
δ13C
0
7
7
“Top Breccia”
60
TRIASSIC
40
6
80
5
100
Khuff
4
120
PTrB
PTrB
140
160
PERMIAN
3
2
180
1
200
Figure 27: Correlation of carbon isotope (δ13C) data measured on the Saiq Plateau (left) with the
data from Richoz (2006) in Wadi Sahtan (right) (scale: 1:1000). Note position of the interpreted
Permian − Triassic Boundary (PTrB) at the end of the first strong negative δ13C shift. The Triassic
section of the Khuff equivalent is characterized by more negative δ13C values compared to the
Permian part. Values become increasingly positive in the overlying Sudair equivalent. See Figure
7 for lithology color coding.
Chemostratigraphy
Carbon Isotopes
Numerous studies suggest that the PTrB can globally be recognized on the sharp negative shift in
carbon isotopes (δ13C). This is commonly defined as “Carbon Isotope Shift” (CIS) (e.g. Magaritz et al.,
142
7
(meter)
Depth
Age
Subsurface
Equivalent
7
0
TRIASSIC
40
6
North
Wadi Sahtan (23°20’14‘‘N, 57°18’27‘‘E)
South
20
ia"
recc
B
"Top
60
80
5
100
4
Khuff
120
3
2
1
140
PERMIAN
0
Sudair
δ13C
Lithology
rB
PT
160
180
200
1988; Baud et al., 1989; Wang et al., 1994; Septhon et al., 2005; Ehrenberg et al., 2008). In the investigated
section, the δ13C drops gradually from the KS 3 sequence boundary towards the end PFE and further
up to a minimum δ13C-value 5 m above. This point is subsequently interpreted as PTrB (Figure 26).
The δ13C-log from the Saiq Plateau was compared with the δ13C-log measured by Richoz (2006) in
Wadi Sahtan, some 25 km to the NW (Figure 27). The general trend of the δ13C pattern with a decrease
across the PTrB and more negative values in the Triassic part of the Khuff time-equivalent is apparent.
Less prominent intra-Triassic negative δ13C shifts above the PTrB are apparent and correlatable. Values
become increasingly positive in the overlying Sudair-equivalent, coinciding with the appearance of
the clastic shale beds.
143
Koehrer et al.
Spectral Gamma-ray
Coincident with the negative δ13C shift, the PTrB is regionally marked by a significant negative shift
in U (Uranium event) (e.g. Szabo and Kheradpir, 1978; Alsharhan, 1993, 2006; Al-Jallal, 1994; Sharland
et al., 2001; Bashari, 2005; Insalaco et al., 2006; Ehrenberg et al., 2008; Maurer et al., 2009). This drop in
U is possibly caused by a chemical oceanographic change in earliest Triassic seawater associated with
the abrupt onset of deep-ocean anoxia (Wignall and Twichett, 1996).
Our U curve shows higher values and a serrated pattern in the Permian part of the section. In contrast,
the Triassic section is generally characterized by a drop in U readings and a smoother U curve
(Figure 24). Unlike published data from the subsurface, the transition from higher to lower values on
the Saiq Plateau is not sharp but gradual occurring over an interval of c. 30 m within the KS 2.
The rather indistinctive general pattern of the spectral GR-curve around the Permian/Triassic
transition might be due to the overall lack of detrital and shaley material in the investigated outcrop
section or due to local diagenetic effects.
Sequence Stratigraphy
Regional studies suggest that the PTrB occurs globally throughout a transgression (Wignall and
Twitchett, 2002; Insalaco et al., 2006). Thus the Permian/Triassic transition should be associated
with an opening of the Khuff platform. This is confirmed by the facies and stratigraphic analysis of
the investigated outcrop. Opening of the platform and a deepening is inferred from a re-occurrence
of Late Permian open, normal-marine fauna (e.g. rugose horn corals, crinoids and brachiopod
shells) right above the KS 3/KS 2 sequence boundary within the transgressive KS 2-hemisequence
(Figure 26). Fauna indicates an agitated and open shallow shelf environment in a foreshoal setting.
The transgression is also marked by a drastic and abrupt facies change at the Saiq/Mahil Formation
boundary. Thickly-bedded packstones to grainstones (high-energy shoal) pass into thinly-interbedded
mudstones to wackestones (distal foreshoal) (Figures 23 and 26).
SEQUENCE STRATIGRAPHIC SYNTHESIS
Khuff Sequence Stratigraphic Framework
The integration of facies cycles, lithostratigraphic marker beds, bio- and chemostratigraphy was used
to establish a sequence stratigraphic subdivision of Khuff Formation time-equivalent strata in the
Oman Mountains. This analysis builds on work by Mabillard et al. (1985), Coy (1997), Osterloff et al.
(2004) and Insalaco et al. (2006). Accordingly, the Upper Saiq and Lower Mahil Member (Khuff timeequivalent) can be subdivided into six third-order sequences (KS 6–KS 1) (Figure 28):
Sequence KS 6: The KS 6 comprises a time-interval from the middle to the end of the Murgabian. The
MFS (“Muddy marker”) probably corresponds to the P20 MFS of Sharland et al. (2004). This sequence
is equivalent to the lower K 5 reservoir interval in the subsurface of Oman.
Sequence KS 5: Covering approximately the Midian stage, the top of the sequence is biostratigraphically
correlated with the end-Guadalupian mass extinction. It encompasses the upper part of the K5
reservoir interval in the subsurface of Oman. The KS 5 MFS (“Chert marker”) is also interpreted as
MFS of the second-order supersequence.
Sequence KS 4: This sequence coincides with the Dzhulfian (Wuchiapingian) stage and includes the
P30 MFS of Sharland et al. (2004). It corresponds to the K 4 reservoir interval in the subsurface of
Oman.
Sequence KS 3: Falling entirely within the Dorashamian (Changhsingian) stage, the KS 3 encompasses
the lower and middle part of the K 3 reservoir interval in the subsurface of Oman. The MFS (“Coral
marker”) of the KS 3 might correspond to the P40 MFS of Sharland et al. (2004).
144
Khuff
Sequence
Reservoir
(Oman)
3rd-order
Cycle
Chronostratigraphy
Formation
KS 1
K1
Arabian Plate
Sequence
Stratigraphy
Marker Beds
(Saiq Plateau)
Main Events
Lower
Induan
Mahil
Lower
TRIASSIC
"Top Breccia"
KS 2
KS 3
K2
K3
Tr10 MFS
"Saiq/Mahil
Boundary"
P40 MFS ?
"Coral Marker"
P30 MFS ?
Dzhulfian
(Wuchiapingian)
KS 4
PERMAIN
Peloid-/ooiddominated ramp
K4
"Microbial
Marker 2"
KS 5
Midian
P25 MFS ?
"Chert Marker"
K5
?
End-Guadalupian
mass extinction
Bioclastdominated
ramp
"Microbial
Marker 1"
Saiq
Guadalupian
End-Permian
mass extinction
"Microbial
Marker 3"
Upper
Lopingian
Dorashamian
(Changhsingian)
Intra-clast-/ooiddominated ramp
Tr20 MFS ?
P20 MFS ?
"Muddy Marker"
Azoic interval
KS 6
Lower
Murgabian
P17
Initial
transgression
clastic deposition
Figure 28: Chronostratigraphic and sequence stratigraphic synthesis of the study interval.
Not to scale.
Sequence KS 2: This sequence chronostratigraphically belongs to the uppermost Dorashamian
(Changhsingian) and lower Induan stages. Its MFS most likely corresponds to the Tr10 MFS of
Sharland et al. (2004). It represents the upper part of the K 3 as well as the entire K 2 reservoir interval
in the Omani subsurface.
Sequence KS 1: This sequence coincides with the upper Induan stage. The KS 1 MFS may be correlated
with the Tr20 MFS of Sharland et al. (2004). The KS 1 corresponds to the K 1 reservoir interval in the
subsurface of Oman.
Regional Correlation
Figure 29 illustrates a tentative correlation between the Middle to Upper Khuff section of offshore
Fars (figure 9 of Insalaco et al., 2006), the Musandam outcrop section (figure 14 of Maurer at al., 2009)
and the measured section of the Saiq Plateau (this study).
145
Induan
Kangan Formation
Agjar Shale
Mbr
Dorashamian (Changhsingian)
Upper Dalan Formation
Age
Formation/
Member
Lithology
0
20 m
KS 2
Texture
KS 3b
West
Khuff
Sequence
KS 1a-1c
Cycle I
3rd-order
Cycle
KS 3a
KS 4a-4c
Cycle II
Lithofacies Type
Claystone
Mudstone
Wackestone
Floatstone
Packstone
Rudstone
Grainstone
Boundstone
Cycle III
Cycle IV
(A) Offshore Fars, Iran (Insalaco et al., 2006)
146
~450 km
Age
Induan
0
20 m
Tectonic breccias
Lithofacies Type
Texture
East - Northwest
(B) Musandam Mountains, UAE (Maurer et al., 2009)
Dorashamian (Changhsingian)
Formation/
Member
Bih Formation
Lithology
Claystone
Mudstone
Wackestone
Floatstone
Packstone
Rudstone
Grainstone
Boundstone
3rd-order Cycle
Formation/
Member
Mahil Formation
Age
Induan
20 m
Lithofacies Type
3rd-order
Cycle
Cycle I
Khuff
Sequence
KS 1
KS 2
Texture
KS 3
KS 4
Cycle II
Southeast
0
(C) Saiq Plateau, Oman (This Study)
Figure 29: See facing page for caption and legend.
~250 km
Dorashamian
(Changhsingian)
Saiq Formation
Lithology
Mudstone
Wackestone
Packstone
Grainstone
Boundstone
Floatstone
Rudstone
Cycle III
Cycle IV
Koehrer et al.
Graded wackestone/mudstone
Dolopackstone-grainstone with staffelids
Oolitic dolograinstone
F5R: F5 and root traces
F5: Massive dolomudstone
147
Secondary Dolomitisation
Figure 29 (continued): Tentative outcrop to subsurface 3rd-order sequence
correlation of Middle and Upper Khuff reservoirs of offshore Fars (South
Pars, Iran) with the Bih Formation (Musandam Mountains, Oman) and the
Saiq Plateau (Oman Mountains, Oman) (modified and extended from
Maurer et al. 2009), flattended on the Permian – Triassic Boundary. Facies,
lithology and stacking patterns are shown.
Limestone
Dolomite
F15: Black band
Oolitic grainstone with flat
F16: pebbles (beach rock) microbial influence
Shale
Anhydrite
Legend for Lithology (A, B and C)
Unidentified (but with estimated texture)
Claystone
Mollusk dolorudstone
Lithoclastic dolopackstone-grainstone
Thrombolitic doloboundstone
F12: Thrombolitic boundstone
F9: Oolitic grainstone and
dolograinstone
F10: Fine-grained peloidal
dolopackstone to dolograinstone
F11: Very fine mudstone to
packstone often bioturbated
Medium- to coarse-grainstone
F8: and dolograinstone
with bioclasts or oolites
F6: Laminated dolomudstone to
dolowackestone
Very coarse-grainstone and
F7: dolograinstone with pebbles
(storm deposit)
F5H: F5 - hypersaline lagoon
Microbial laminites
18°
22°
26°
30°
IRAQ
50°
YEMEN
SAUDI
ARABIA
Ghawar
54°
OMAN
58°
km
N
C
18°
Location Map
Arabian
Sea
22°
26°
300
30°
Skeletal floatstone
Well sorted oolitic grainstone
Well sorted peloidal grainstone
Poorly sorted peloidal packstone/
grainstone
Poorly sorted bioclastic
Gulf of
Oman
0
Fahud
B
IRAN
Yibal
Lekhwair
UAE
54°
North Field
A
QATAR
BAHRAIN
Kangan
Kuh-I-Mand
Pars North
KUWAIT
50°
Intra-clastic grainstone/rudstone
Graded packstone/wackestone
Bioturbated mudstone/wackestone
Laminated to massive dolomudstone
Bioclastic dolopackstone-grainstone
Burrowed/vertically rooted mudstone/wackestone
Facies legend for Saiq Plateau (C)
Bioturbated dolomudstone-packstone
Pisolitic dolopackstone
Facies legend for Musandam Mountains (B)
F2: Dolomudstone with
anhydritic nodules
F3: Breccia (anhydrite and
dolomudclasts)
F4: Green shaly dolomudstone
F1: Massive to laminated anhydrite
Facies legend for Offshore Fars (A)
Koehrer et al.
“Cycle IV” of Insalaco et al. (2006) might correspond to the KS 4 of our scheme (DzhulfianWuchiapingian stage) (Figure 29). For this sequence, similar high-energy facies, most notably oolitic
grainstones, dominate in the offshore Fars section and on the Saiq Plateau. No data is avalaible from
Musandam from this stratigraphic interval.
“Cycle III” of Insalaco et al. (2006) probably corresponds to the lower part of KS 3 in our stratigraphic
nomenclature (lower Changhsingian stage) (Figure 29).
“Cycle II” of Insalaco et al. (2006) can possibly be correlated with the upper part of our KS 3 and KS 2.
For this interval, the Saiq Plateau section significantly differs in facies and depositional environment
compared to the other two data points. Whereas mixed lagoonal and shoal facies dominate in offshore
Fars and Musandam, the KS 3 on the Saiq Plateau is mainly made up of foreshoal to shoal deposits
indicating a more open-marine, distal position on the Khuff carbonate ramp (Figure 3). Within the
upper part of “Cycle II” (KS 2) in Musandam and offshore Fars, oolitic grainstones are common
whereas mud-dominated foreshoal to offshoal facies types dominate on the Saiq Plateau.
Probably equivalent to “Cycle I” of Insalaco et al. (2006), the KS 1 reflects one third-order depositional
sequence (Figure 29). This sequence varies significantly in thickness between the three sections,
possibly reflecting changes in accommodation space, differential subsidence histories and tectonic
activity. However, similar depositional facies characterized by intra-clastic, oolitic and peloidal
grainstones are present in all sections.
From the data set it can be concluded that the KS 4 and KS 1 on the Saiq Plateau possibly provides
an outcrop analog regarding the facies types to the gas-bearing Khuff Formation of the North Field
area.
TECTONIC AND DIAGENETIC OVERPRINT
Tectonic Overprint
The Oman Mountains are one of the most tectonized and structurally complex areas on the Arabian
Plate. We are aware of the fact that the logged outcrop sections on the Saiq Plateau and the resulting
sedimentological interpretations are affected by this strong tectonization. Tectonic distortion of
primary sedimentary fabrics in the study area mainly occurs on two different scales (Figure 30).
On a larger scale, brittle bed-parallel to low-angle thrust-faulting may have caused dm to several m
of lateral displacement of individual beds (J. Mattner, personal communication, 2009). Minor thrustlike ramps also cross-cut primary sedimentary bedding planes in places (Figure 30a). The larger fault
planes, mostly visible on satellite images and noted down in the geologial map of the study area (Rabu
et al., 1986), were avoided when selecting the exact location of the logged sections A-D (Figure 2).
However, less obvious low-angle thrust-planes along the section traces may be only unravelled by
detailed structural mapping.
The boundary between the competent Lower Mahil Member (KS 2 – KS 1) and the overlying
more shaley Middle Mahil Member (Sudair equivalent) is strongly affected by bedding-parallel
anastomosing thrust faults, recumbent isoclinal folds up to a m-scale and horses of brittle material of
up-to meter size (Figure 30b) (J. Mattner, personal communication, 2009).
On a smaller scale, stylolitization also modifies original sedimentary boundaries in the outcrop
sections (Figures 30c to f). In general, nearly every sedimentary bed boundary is overprinted by
(micro-) compaction stylolitization, leading to the development of columnar stylolites with low to
moderate amplitudes of up to 3 cm. Along some of these sedimentary bedding planes, mm- to cmthick reddish stylolitization seams with clay residues were observed. Altogether, stylolitization may
have removed about 10−30% of the pre-compacted rock volume of the investigated section (J. Mattner,
personal communication, 2009).
148
a
b
Bedding
0
cm
~20
d
c
f
e
0
cm
~1
Figure 30: (a) Low-angle minor thrust-like ramps (yellow arrows) in competent dolo-grainstone
cross-cutting primary sedimentary bedding plane (black line) (Saiq Plateau, base of outcrop
section C, N23°05'32'', E57°41'16'', photograph by J. Mattner); (b) Picture of the major detachment
plane associated with the Lower/Middle Mahil Formation boundary showing tectonically
mobilized m-sized boulder of competent strata (possibly Lower Mahil Member) within muddy
host-rock (Middle Mahil Member) (Saiq Plateau, road-side quarry NW of Al Jabal al-Akhdar
Hotel, c. N23°06’00’’, E57°39’00’’, photograph by J. Mattner; (c) Columnar microstylolites (yellow
arrows) are visible along nearly every sedimentary bedding plane of the investigated section (Saiq
Plateau, outcrop section D, 68.4 m-level); (d) Columnar stylolites in graded packstone to
grainstone (Saiq Plateau, outcrop section D, 63.1 m-level); (e) Example of compaction
stylolitization with calcified seams (yellow arrow) within coral floatstone (Saiq Plateau, road cut
c. 100 m NE of Al Jabal al-Akhdar Hotel, c. N23°04'50'', E57°42'20'', photograph by J. Mattner); (f)
Reddish to brown cm-thick stylolitization seam (yellow arrow) along irregular sedimentary
bedding plane on the Saiq Plateau (road cut c. 100 m NE of Al Jabal al-Akhdar Hotel, c. N23°04'00'',
E57°42'00'', photograph by J. Mattner, see hammer for scale).
149
Koehrer et al.
Diagenetic Modification
8.0
6.0
δ13C (V-PDB)
A detailed evaluation of the diagenesis of the
Saiq and Mahil Formation on the Saiq Plateau
is presented in Coy (1997). In this study, no
attempt was made to perform closer paragenetic
investigations as analyses on cement petrography and trace element content were not
carried out. Coy (1997) concluded that is likely
that the δ13C values of the Saiq and Mahil
dolomites reflect the initial marine isotopic
composition of the precursor whereas δ18O is
more readily modified by subsequent diagenetic
events. Thus oxygen stable isotope values of
the investigated dolomites are expected to be
significantly modified by diagenetic alteration.
4.0
2.0
0.0
Triassic sample
Permian sample
-2.0
-6.0
-4.0
-2.0
0.0
δ18O (V-PDB)
2.0
4.0
Carbon isotope values for the dolomites of the
Figure 31: Cross-plot of bulk rock stable
Permian part of the Saiq Plateau section range
isotope values of outcrop samples from the
from +6.4 to +2.1‰ with a median δ13C value
Saiq Plateau section. Permian and Triassic
of +4.5 ‰ V-PDB (Figures 24 and 31). Permian
samples are plotted with different symbol
oxygen isotope values range from -4.9 to +2.35‰,
shape.
with a median δ18O value of -1.5 ‰. The carbon
isotope values for Triassic rock samples range
from 0 to 4.5‰ (median +2.15‰ V-PDB), oxygen
values vary from -4.0 to -1.2‰ (median -2.4‰). The strong depletion of the measured oxygen isotope
values is consistent with the data presented in Coy (1997). No correlation was found between δ13C
and δ18O values within the investigated section. However, average δ13C and δ18O values are generally
higher for Permian than for Triassic samples (Figure 31).
CONCLUSIONS
The study of the Permian and Triassic carbonates on the Saiq Plateau, Al Jabal al-Akhdar, in the
Sultanate of Oman yielded the following results:
(1) The investigated section is interpreted to be time-equivalent to the subsurface Middle Permian to
Lower Triassic Khuff Formation.
(2) The outcrop is characterized by a very high percentage of grain-dominated textures. They
represent the storm-dominated shoal to foreshoal section of the Khuff carbonate ramp. Most
facies are open-marine and high-to moderate energy. There is a scarcity of peritidal deposits.
Indicators for subaerial exposure and evaporites are absent.
(3) The interpreted depositional setting is in line with the established late Permian and lower Triassic
paleogeographic location within the unrestricted marine carbonate shelf.
(4) Facies are stacked to transgressive-regressive cycles (fifth-order) of four general motifs: foreshoal,
shoal margin, shoal and shoal- to backshoal.
(5) Stacks of these cycles form 36 transgressive-regressive cycle sets (fourth-order) clearly reflected
in gamma-ray patterns. These are termed KCS 1.1 to 6.4 from top to bottom.
(6) The investigated section was subdivided into six transgressive-regressive sequences (thirdorder), termed KS 1 – KS 6. KS 6 – Lower KS 2 are interpreted to correspond to the Permian
Upper Saiq Member. The Triassic Lower Mahil Member comprises Upper KS 2 – KS 1.
150
ACKNOWLEDGMENTS
This study is part of an extra-mural research project sponsored by Shell (Qatar). We are also grateful
to Petroleum Development Oman (PDO, Muscat) for financial support and permission to publish this
paper. The authors would especially like to thank Jan Schreurs, Gordon Forbes and Joachim Amthor
(all PDO) for reviewing and improving earlier versions of this manuscript. We would also like to
thank Claus von Winterfeld, Aly Brandenburg and Gordon Coy (all PDO) for assistance in many
ways. We are grateful to Erwin Adams (Shell), Daniel Vachard (University of Lille), Joerg Mattner
(GeoTech, Bahrain), Sylvain Richoz (University of Vienna), Heiko Hillgaertner (PDO), Henk Droste
(Shell) and Deborah Bliefnick (Badley Ashton) for sharing their knowledge of the Khuff. Ulrike
Schulte (University of Bochum) and Peter Swart (University of Miami) are thanked for stable isotope
analysis. Per Jeisecke (University of Tuebingen) is thanked for the preparation of thin sections. We
thank Shuram Oil and Gas (Muscat) for logistics of our field work. The final version of this manuscript
greatly benefited from the comments by two anonymous reviewers and Moujahed Al-Husseini.
GeoArabia’s Arnold Egdane is thanked for designing the final version of the figures. Access to the
WellCAD software was kindly provided ALT (Luxembourg).
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ABOUT THE AUTHORS
Bastian Koehrer studied Geosciences at the Universities of Tuebingen, Germany, and
Bristol, UK, focusing on Sedimentary and Petroleum Geology. His MSc thesis (2007)
was on reservoir characterization and 3-D modeling of a dolomite body from the Triassic
Muschelkalk. Currently Bastian is a research and teaching associate at the Center for
Applied Geosciences (Petroleum Geoscience Lab) in Tuebingen. The main objective of
his PhD thesis, funded by Shell and Petroleum Development Oman (PDO), is a detailed
description and characterization of the Khuff platform in outcrop and subsurface of the
Sultanate of Oman. He aims to establish a regional valid sequence stratigraphic framework
and conceptual geological model of the Khuff Formation that highlights nature and
dimensions of potential reservoirs on exploration and production-scale. Among others
Bastian is a member of the AAPG, SEPM, IAS and DGMK.
[email protected]
Michael Zeller is currently enrolled as a PhD student at the Rosenstiel School of
Marine and Atmospheric Sciences, University of Miami, Florida. He obtained an MSc
in Sedimentary Geology from the University of Tuebingen (Germany) in July 2008.
The main focus of his MSc thesis has been digital outcrop modelling of an Upper Khuff
equivalent (Al Jabal al-Akhdar, Sultanate of Oman) and the resulting implications for
static modelling in KS3-KS1 reservoir units. His study has been sponsored by Shell
QSRTC, Shell EPI and Petroleum Development Oman.
[email protected]
Thomas Aigner studied Geology and Paleontology at the Universities of Stuttgart,
Tuebingen and Reading/England. His diploma thesis was on the Geology and
Geoarcheology of the Egyptian pyramides plateau in Giza (1982). For his PhD dissertation
on storm depositional systems (1985) he worked at the Senckenberg-Institute of Marine
Geology in Wilhelmshaven and spent one year at the University of Miami in Florida. He
then became an exploration geologist at Shell Research in Rijswijk/Holland and Houston/
Texas focussing on basin analysis and modelling (1985-1990). He worked as adjunct
lecturer for applied sedimentology at the University of Wuerzburg (1988-1990). Since
1991 Tom is a professor and head of the sedimentary geology group at the University of
Tuebingen. 1996 he was an ‘European Distinguished Lecturer’ for American Association
of Petroleum Geologists. In 2007/8 he spent a sabbatical with PDO and Shell Qatar.
His current projects focus is on sequence stratigraphy and reservoir characterisation/
modelling in outcrop and subsurface.
[email protected]
Michael Poeppelreiter studied at the Mining University of Freiberg, Germany, the
Postgraduate Research Institute of Sedimentology, United Kingdom, and the University
of Tubingen, Germany, where he earned a PhD in 1998. Since then, Michael has worked as
sedimentologist/3-D modeller with Shell in Holland, as carbonate geologist/3-D modeller
at Shell’s Bellaire Technology Center in Houston, USA and at present, he is senior
carbonate geologist at the Qatar Shell Research and Technology Centre in Doha, Qatar
where he is coordinating the Khuff/Sudair outcrop analogue study. Michael published
numerous papers on carbonate reservoirs, reservoir modelling and borehole image log
technology. He is guest lecturer at the University of Tuebingen, Germany. His research
interests include structural control on reservoir distribution in carbonate reservoirs.
[email protected]
155
Koehrer et al.
Paul Milroy has recently joined BG Group as Carbonate Technology Manager for BG
Exploration and Production, Reading, UK. Prior to joining BG, Paul was a Senior
Reservoir Geologist in the Shell’s Carbonate Research Team, Rijswijk, The Netherlands.
Paul obtained a PhD in Geology at University of Bristol in 1998 before completing
postdoctoral research at the University of Tokyo in 2001. He then worked as a reservoir
geologist with Badley Ashton & Associates, before joining Shell in 2006 to work on the
quantification of carbonate reservoir heterogeneity. He is a member of AAPG, SEPM and
BGRS and has research interests in sedimentology, diagenesis, reservoir characterisation
and modelling.”
[email protected]
Holger Forke studied Geology and Paleontology at the University of Erlangen. His
diploma thesis and PhD dissertation (2001) focused on the biostratigraphic correlation of
Carboniferous-Permian deposits from the Southern Alps (Austria) and Urals (Russia).
He has then worked at the Senckenberg Research Institute in Frankfurt/Main and at the
Institute of Geology (University of Erlangen) within the DFG Priority Programme 1054
‘Late Paleozoic sedimentary geochemistry’. In recent years, he participated in expeditions
and mapping campaigns to Svalbard and the Canadian Arctic in cooperation with the
Norwegian Polar Institute, University of Bremen, and BGR Hannover. His work mainly
deals with Late Paleozoic foraminifera and conodonts with emphasis on the application
for sequence biostratigraphy. He is currently a guest researcher at the Museum of Natural
History, Leibniz Institute for Research on Evolution and Biodiversity at the Humboldt
University Berlin, Germany.
[email protected]
Suleiman Al-Kindi has a degree in Geology/Geophysics from the University of Durham,
a PhD in Marine Geophysics from the University of Cambridge, and an MBA degree from
the University of Hull. Sulaiman has joined the exploration Department in the Petroleum
Development Oman (PDO) in 2002 as a hydrocarbon system analyst working on the deep
gas petroleum system of North Oman and then moved into the regional team working
on several gas exploration opportunities. He then joined the Qatar Shell Research and
Technology Carbonate team as regional geologist in April 2007. He is currently working
as regional geologist with PDO Exploration team.
[email protected]
Manuscript received May 5, 2009
Revised September 15, 2009
Accepted October 13, 2009
Press version proofread by authors January 16, 2010
156

Facies and stratigraphic framework of a Khuff outcrop equivalent

Transcription

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