The Chanthaburi terrane of southeastern Thailand: Stratigraphic

Transcription

Journal of Asian Earth Sciences 61 (2012) 16–32
Contents lists available at SciVerse ScienceDirect
Journal of Asian Earth Sciences
journal homepage: www.elsevier.com/locate/jseaes
The Chanthaburi terrane of southeastern Thailand: Stratigraphic
confirmation as a disrupted segment of the Sukhothai Arc
Masatoshi Sone a,⇑, Ian Metcalfe b,c, Pol Chaodumrong d
a
Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia
Earth Sciences, School of Environmental and Rural Science, University of New England, NSW 2351, Australia
c
GEMOC, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia
d
Bureau of Geological Survey, Department of Mineral Resources, Bangkok 10400, Thailand
b
a r t i c l e
i n f o
Article history:
Available online 10 September 2012
Keywords:
Thailand
Indochina
Sibumasu
Arc
Stratigraphy
Permian–Triassic
a b s t r a c t
A Permo-Triassic volcanic arc system, the Sukhothai Arc, is recognised between the Indochina and Sibumasu continental blocks. The Chanthaburi terrane is here interpreted as a fault-detached, highly disrupted southern segment of the Sukhothai Arc, occupying part of southeastern Thailand and extending
into Cambodia. The Klaeng tectonic line is defined as the boundary between the Chanthaburi terrane
and Sibumasu block. The stratigraphy of the Chanthaburi terrane is compared with that of the Sukhothai
terrane in Northern Thailand. The Late Palaeozoic–Mesozoic sequences of these two volcanic arc terranes
in the Sukhothai Zone share important similarities, but show marked contrasts to those of the Sibumasu
and Indochina blocks, where the Late Permian–Triassic is largely absent due to the Indosinian I unconformity (western Indochina) or is dominantly carbonates with little terrigenous clastic input (Sibumasu).
There is no clear evidence of pre-Carboniferous sedimentary rocks for either the Sukhothai or Chanthaburi terranes. Late Permian lyttoniid brachiopod shale near Klaeng in the Chanthaburi terrane was revisited. The brachiopod, previously reported as Leptodus, is re-identified to Oldhamina, the genus previously
known, elsewhere in Southeast Asia, only in the Huai Tak Formation of the Sukhothai terrane. Oldhamina
in Thailand is confined to the Sukhothai Arc. The marine stratigraphy of the Sukhothai Arc is represented
by a Permian–Triassic lithological succession of mixed carbonates and siliciclastics, with common volcanic material. The Late Permian and Triassic litho- and biostratigraphy of the Chanthaburi terrane are comparable with the upper Ngao and Lampang groups of the Sukhothai terrane; in particular, they share
similar successions from Oldhamina brachiopod bearing shale to Palaeofusulina–Colaniella foraminifer
bearing limestone in the latest Permian. Marine depositional conditions were terminated on the Sukhothai Arc by end-Triassic times, later than on the Indochina block (Late Permian) but earlier than on the
Sibumasu block (Jurassic/Cretaceous).
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
A Permo-Triassic volcanic arc system, the Sukhothai Arc, is
recognised between two major continental masses, the Indochina
block and the Sibumasu block; it is bounded by sub-parallel
sutures of contrasting tectonostratigraphic histories (Ueno and
Hisada, 1999; Ueno, 2002; Sone and Metcalfe, 2008a,b). These
sutures represent the Devonian–Triassic Palaeo-Tethys Ocean
(the Changning–Menglian and Inthanon/Chiang Mai sutures in
southwest China and Thailand and the Bentong-Raub Suture in
the Malay Peninsula) in the west and a Permian back-arc basin
(Jinghong, Nan, and Sa Kaeo sutures) in the east, respectively
(Fig. 1). The Sukhothai Arc and its bounding sutures resulted from
⇑ Corresponding author.
E-mail address: [email protected] (M. Sone).
1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jseaes.2012.08.021
a prolonged Carboniferous–Triassic subduction of the main PalaeoTethys beneath Indochina, back-arc opening and inversion, and
subsequent Triassic collision of Sibumasu to Indochina.
The Sukhothai Arc of mainland Southeast Asia comprises three
fault-detached terranes, from north to south, the Lincang terrane
(Yunnan), the Sukhothai terrane (Northern Thailand), and the
Chanthaburi terrane (southeastern Thailand to Cambodia)
(Fig. 1). This volcanic arc system extends southwards to the East
Malaya terrane of the Malay Peninsula (Sone and Metcalfe,
2008a). This is supported chiefly by the distribution pattern and
emplacement range of Permo-Triassic I-type dominated granitoids
(that is, the so-called Eastern Granite Province) through the arc
system (see discussion in Sone and Metcalfe, 2008b).
The Chanthaburi terrane was proposed by Sone and Metcalfe
(2008b) for the southern portion of the Sukhothai Arc (Fig. 1). The
Sukhothai terrane and the Chanthaburi terrane reveal comparable
M. Sone et al. / Journal of Asian Earth Sciences 61 (2012) 16–32
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Fig. 1. Tectonic subdivision of mainland Southeast Asia (revised from Ueno, 1999; Sone and Metcalfe, 2008b). Distribution of oceanic/pelagic elements in pink; non-oceanic/
hemipelagic elements in brown (partly adapted from Hara et al., 2009; Ueno et al., 2010). Ages of deep-sea sediments and oceanic seamount sequences are indicated with
blue (Palaeo-Tethys Suture Zone) and red (closed back-arc basins) letterings. CMSZ = Changning–Menglian Suture Zone. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
Late Palaeozoic–Mesozoic stratigraphies of mixed siliciclastics and
carbonates, and they have no proven pre-Carboniferous sedimentary rocks (apart from a possible Proterozoic basement indicated
by zircon inheritance, see 4.1). In particular, litho-and biostrati-
graphic patterns of the Chanthaburi terrane from Late Permian to
Early Triassic are notably similar to those of the Sukhothai terrane.
The Late Permian typically includes shale with lyttoniid brachiopods, which is then succeeded by Palaeofusulina–Colaniella
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Fig. 2. Geological map of southeastern Thailand, showing major rock units and suggested placements of the Klaeng tectonic line (suture between the Chanthaburi and
Sibumasu blocks) and the Sa Kaeo back-arc suture, bounding the Chanthaburi terrane (partly adapted from Chaodumrong, 1992; Salyapongse, 1997; Tansuwan, 1997a,b;
DMR, 1999; Vimuktanandana and Munchai, 2008).
foraminiferal limestone in the latest Permian (Changhsingian)
which then succeeds upwards into the Triassic. Permian–Triassic
sediments of both terranes are often volcaniclastic and tuffaceous.
In Khao Yai Phring of southeastern Thailand (Chanthaburi terrane), a lyttoniid brachiopod-bearing shale has been known for
more than 30 years, with the brachiopods reported as the genus
Leptodus (e.g., Bunopas, 1982; Bunopas et al., 1983; Fontaine
et al., 1999; Fontaine, 2002), but no detailed study has been conducted. Our current research reinterprets the lyttoniid brachiopods
as belonging to another genus Oldhamina, a typical Changhsingian
form of the tropical Tethys. Elsewhere in Southeast Asia, Oldhamina
was only known from the Huai Tak Formation of the Sukhothai terrane in Northern Thailand (Waterhouse, 1983; Koga et al., 1998).
Thus, in Southeast Asia, Oldhamina is confined to the Sukhothai
Arc (both Sukhothai and Chanthaburi terranes).
The Late Permian and the Triassic of the Chanthaburi terrane
are comparable with the Huai Tak Formation (upper Ngao Group,
Late Permian) and the Lampang Group (Triassic) of the Sukhothai
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Fig. 3. Lineaments (most are presumably faults) over southeastern Thailand to western Cambodia, showing highly disrupted nature of the Chanthaburi terrane. Highlighted
areas, between the Klaeng fault and the Tha Mai fault represent the blocks on which shallow-marine sediments of Carboniferous and Permian–Triassic ages, discussed in the
text, are located (revised after Ridd and Morley, 2011, fig. 11).
terrane. The Late Palaeozoic–Mesozoic stratigraphic histories of
these volcanic arc terranes as a whole show a marked contrast to
those of the bounding Sibumasu and Indochina blocks, where the
Late Permian–Triassic is largely absent due to unconformity (western Indochina) or is dominantly carbonates with little terrigenous
clastic input (Sibumasu).
The tectonic division of southeastern Thailand has been of
great dispute. The Sa Kaeo Suture has traditionally been considered the boundary between the Indochina and Sibumasu blocks
(e.g., Bunopas, 1982; Hada et al., 1997). However, Ueno and
Hisada (2001) questioned this in correlating the Sa Kaeo Suture
with the Nan Suture, and interpreting this as a closed back-arc
basin of the Sukhothai Arc. The latter view has been supported
by Sone and Metcalfe (2008b). We here identify the Klaeng
tectonic line (new name) as delineating the boundary (suture)
between the Sibumasu block and the Chanthaburi terrane
(Sukhothai Arc), as discussed below.
2. Chanthaburi terrane
The Chanthaburi terrane of Sone and Metcalfe (2008b) is defined to be the fault-detached southern portion of the Sukhothai
Arc (Fig. 1). It corresponds principally to the arc with the Eastern
Province granites delineated for southeastern Thailand by Cobbing
(2011, fig. 16.5). Its batholith consists exclusively of Triassic–earliest Jurassic I-type granitic plutons, such as the Chanthaburi, Khao
Soi Dao, Pliew, and Khao Chamao plutons (Plutons 1–4 in Fig. 2).
Late Triassic–earliest Jurassic (ca. 195–207 Ma) ages are indicated
for the first three plutons around Chanthaburi city, based on available 40Ar-39Ar, Rb–Sr and K–Ar radiometric ages (Charusiri et al.,
1992). A U–Pb LA-ICPMS Late Triassic (Norian) age of
218 ± 11 Ma is recently obtained from sphene (titanite) in the Khao
Chamao Pluton, 20 km NNE of Klaeng (Geard in Crow, 2011). Permian plutons are not known in the Chanthaburi terrane, although
some volcanics in the terrane have been considered to be late
Permian–Triassic in age (see DMR, 1999; Barr and Charusiri,
2011) (Fig. 2); for example, rhyolite from the Ko Chang volcanics
yields a Late Permian zircon U-Pb age of 258 ± 3 Ma (Khin Zaw
pers. comm. 2009 quoted in Barr and Charusiri, 2011, p. 426).
The eastern boundary of the Chanthaburi terrane with the
Indochina block is drawn by the Sa Kaeo Suture extending to
Cambodia (Sone and Metcalfe, 2008b) (Figs. 1 and 2). The Chanthaburi terrane is considered to occupy part of southwestern
Cambodia, judging from the distribution of the Sa Kaeo Suture
mélange, whose trend appears to extend into Cambodia (Figs. 2
and 3). This is concordant with the adjacent delineation of the
Middle Permian Khao Taa Ngog Formation) being equivalent to
the Cambodian Sisophon Limestone over the border, as can be
seen in the field, and marking the western limit of Permian platform carbonates in the Indochina block (e.g., Sone, 2010). Much of
the Palaeozoic to Triassic rock records of the Chanthaburi terrane
in southwestern Cambodia are covered by younger Mesozoic continental sediments forming the Cardamom Mountains (see Ridd
and Morley, 2011).
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Fig. 4. Simple geological map of the Khao Yai Phring area east of Klaeng city, southeastern Thailand, showing the Late Permian and Early Triassic fossil localities. Foraminifer
data (Fontaine and Vachard, 1981; Fontaine and Salyapongse, 1998, 1999; Fontaine et al., 1999).
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Fig. 5. Suggested stratigraphy of a Permian–Triassic siliciclastic to carbonate succession of the Sukpaiwan Formation in the Khao Yai Phring area east of Klaeng, southeastern
Thailand. Exact stratigraphic level of Khao Wong limestone not certain (see text). Foraminifer data (Fontaine and Vachard, 1981; Fontaine and Salyapongse, 1998, 1999;
Fontaine et al., 1999).
2.1. Klaeng tectonic line (new name)
The western boundary of the Chanthaburi terrane with the Sibumasu block is here identified as the Klaeng tectonic line (previously
termed the Klaeng fault in Sone and Metcalfe, 2008b). This tectonic
line is conceptually similar to the Chiang Rai tectonic line of Barr
and Macdonald (1991), a so-called ‘‘cryptic suture’’ between the
Sukhothai terrane and the Sibumasu block in Northern Thailand.
The concept of the Chiang Rai tectonic line has now been developed
as a Palaeo-Tethys Suture (Ueno and Hisada, 1999), termed the
Chiang Mai Suture in Metcalfe (2002) or the Inthanon Suture Zone
in Sone and Metcalfe (2008b) and Metcalfe (2011). The collisional
suture nature of the Chiang Rai–Chiang Mai zone was recognised
in the late 1970s (Mitchell, 1977). The Chiang Rai tectonic line is
drawn between the Permo-Triassic I-type granite belt and the Triassic S-type granite belt (Barr and Macdonald, 1991). The Klaeng
tectonic line in southeastern Thailand, which is also defined between the same two granite provinces (after Cobbing, 2011, fig.
16.5) (Fig. 2), represents a cryptic suture, formed by the collision
of the Sibumasu block and a southern Sukhothai Arc (Chanthaburi
terrane). This is contiguous with the Chiang Rai tectonic line.
There are numerous faults of NW–SE or N–S trend that dissect
the middle part of southeastern Thailand, most intensively in the
western half of the Chanthaburi terrane (Fig. 3). This probably resulted from convergence of splaying faults from the Three Pagodas
and Mae Ping fault zones (see Ridd and Morley, 2011). The Klaeng
fault is most likely a splay of the Three Pagodas Fault, not the Mae
Ping Fault, as interpreted by Morley (2002, p. 196). The Klaeng fault
corresponds more or less to the Klaeng tectonic line, although the
former is strictly for a Tertiary deformation structure while the latter is a tectonic boundary (Figs. 2 and 3).
Prominent mylonitic gneisses, migmatites, and low-grade
metamorphic rocks are widely distributed along the Klaeng fault
zone (Fig. 2); they include Khao Cao gneiss near Nong Yai, a phyllite along Route 344 (Stops 1 and 2 in Salyapongse et al., 1997), and
a Chonburi mylonite of Lacassin et al. (1997), where mylonite
shows sinistral shear. Complex igneous and metamorphic histories
in this fault zone were discussed by Morley et al. (2011, p. 285) and
Ridd (2012, p. 72). Ages of fault deformations are not well constrained, but most (related to sinistral deformation or a thermal
event) are supposed to be Tertiary (e.g., Morley, 2002; Morley
et al., 2011, p. 285). It is noteworthy that these faults and associated metamorphic rocks extensively occur along the Klaeng fault
zone between the S-type and I-type granitic batholiths, that is,
the boundary between the Sibumasu block and the Chanthaburi
terrane (southern Sukhothai Arc). Thus, it is interpreted that the
Triassic collisional boundary of the Palaeo-Tethys closure, the Klaeng tectonic line, was activated as a Tertiary strike-slip fault zone
(Klaeng fault zone) of dynamic metamorphism through the preexisting structural boundary.
Expected remnants of the Palaeo-Tethys ocean along the tectonic line are perhaps largely absent due to severe strike-slip displacement. Latest Permian radiolarian-bearing cherts with
siliceous shales and sandstones are exposed at Khao Wang Chik
along the Klaeng fault zone (Sashida et al., 2000). Ueno and
Charoentitirat (2011) noted this sequence to be unlikely true oceanic sediment but be more likely terrigenous sediment of the
Sukhothai Arc. Our field observation agrees with this in that cherts
of this unit are rather minor and it includes a considerable amount
of sandstone and other terrigenous siliciclastic sediment. On the
other hand, a large bedded red chert sequence is exposed in Khao
Phu Ta Luang near U-Tapao Airport (Stop 14 in Chaodumrong et al.,
2002). It is tightly folded with chevron folds. Considering its
lithology, this chert is interpreted as true oceanic sediment, a remnant of Palaeo-Tethys accretionary complex thrust over the Sibumasu block, now largely eroded away. It appears similar to the
22
Devonian-Triassic Fang Chert of the Inthanon Zone. Unfortunately,
its age is not known.
2.2. Carboniferous sediments (shallow marine)
In the middle of the Chanthaburi terrane, the Carboniferous is
known from three large adjacent limestone hills, Khao Kradat,
Khao Yai and Khao Yai Mo Noi, located around 61–64 km north
of Klaeng (Fig. 3). They are late Visean–Bashkirian, mainly Mississippian, in age (Ueno and Charoentitirat, 2011). They consist
mainly of highly fossiliferous, massive limestone, and the coral fauna shows affinities to those found in the Early Carboniferous of
central and northeastern Thailand and central Laos in the western
part of the Indochina block (Fontaine et al., 1996, 2002(1999);
Fontaine and Salyapongse, 1997b).
The Sukhothai Arc including the Chanthaburi terrane is interpreted to have been separated from the western margin of the
Indochina block, due to the opening of the Nan–Sa Kaeo back-arc
basin, in the latest Carboniferous to Early Permian (Sone and
Metcalfe, 2008b). This Early Carboniferous limestone in the
Chanthaburi terrane was originally deposited on a shallow marine
margin of Indochina, prior to the emergence/separation of the arc.
2.3. Late Permian–Triassic sediments (mixed shallow marine)
Post-Carboniferous rocks of the Chanthaburi terrane are Late
Permian (Lopingian) shale succeeding to latest Permian limestones
(Changhsingian) at Khao Yai Phring and Khao Hin area east of Klaeng, which are then overlain by an Early–Middle Triassic limestone
exposed in Khao Kong Din (Fontaine and Vachard, 1981) (Fig. 4).
Nevertheless, the basal Triassic (Induan) and the PT boundary are
not yet confirmed. This shale–limestone succession was termed
the Sukpaiwan Formation (Chaodumrong, 1992). A suggested stratigraphy of the Late Permian–Early Triassic rocks in the Khao Yai
Phring area is presented in Fig. 5.
In the Khao Yai Phring (Khao Iphring) hill area, the occurrence
of lyttoniid brachiopod deposit has been known from the Late
Permian shale for over 30 years (e.g., Bunopas, 1982; Bunopas
et al., 1983; Salyapongse et al., 1997; Fontaine et al., 1999;
Fontaine, 2002) (Fig. 6(1)). The lyttoniid was previously reported
as Leptodus but is reinterpreted to belong to another genus
Oldhamina in this study (see 3. Palaeontological note).
Fontaine et al. (1999) found some impure limestone lenses
nearby the Oldhamina shale locality; one at the southern part of
Khao Yai Phring hill bears the foraminifers Palaeofusulina ex gr. sinensis and Reichelina sp., and another is crinoidal limestone with the
algae Tubiphytes exposed on the eastern slope of a small hill
(Fig. 6(2) = Locality CS in Fontaine et al., 1999). They are indicated
collectively to be faunal assemblage 2 in Figs. 4 and 5. Moreover, at
the Khao Hin limestone hill located north of Khao Yai Phring
(Fig. 6(3)), a suite of typical Changhsingian (latest Permian) foraminifers Palaeofusulina, Reichelina, and Colaniella was confirmed
(Fontaine et al., 1999). The Palaeofusulina limestones in Khao Yai
Ipring and Khao Hin are considered to be stratigraphically higher
than the Oldhamina shale, following Fontaine et al. (1999); that
is, the latest Permian brachiopod shale is succeeded by Permian
foraminifer-bearing limestones (Fig. 5).
It is noteworthy that a large limestone hill Khao Wong west of
Khao Hin yields duostominid foraminifers and abundant oncolites
(Fontaine and Salyapongse, 1999) (Fig. 6(9)). The duostominids are
indicative of a Triassic age, as most common in the Anisian–Ladinian (Middle Triassic). The bedding trend of Khao Wong (340°/
40–65°W) is rather consistent with that of Khao Hin (340°/60°W)
(Fig. 4). These imply a possibility that the limestone sequences of
Khao Hin to Khao Wong are continuous from Permian to (Middle)
Triassic, younging eastwards as the beds are overturned. A further
investigation may clarify the presence of the Early Triassic or the
PT boundary.
A late Early Triassic foraminiferal assemblage is known from
oncolitic limestone at a large elongate hill Khao Kong Din (= Khao
Suk Phrai Wan by some authors) about 3 km south of Khao Yai
Phring (Figs. 4 and 6(4–7)). The foraminifers are Meandrospira pusilla (Ho), Glomospira tenuifistula Ho, and Glomospirella facilis Ho,
characteristic members of the M. pusilla zone of the upper Scythian
(Fontaine and Vachard, 1981; Vachard and Fontaine, 1988). The
Khao Kong Din limestone is underlain by reddish coarse sandstone
of unknown thickness at the base, which is well exposed at the
northwestern corner of the hill (Fig. 6(8)), and the limestone succession is often intercalated with layers of red1 argillaceous shales
(Fontaine and Vachard, 1981).
Consequently, it is here interpreted, after Fontaine et al. (1999),
that the latest Permian limestones in Khao Yai Phring and Khao
Hin, which overlie the Oldhamina shale, represent the basal part
of the limestone-dominant succession that extends to the late
Early Triassic sequence (Khao Kong Din), although the exact basal
Triassic (Induan) is not confirmed in this area (Fig. 5).
Two other limestone hills Khao Cha-ang On and Khao Tham Rat,
about 35 and 60 km north of Klaeng respectively, yield late Middle–early Late Triassic (late Ladinian–Carnian) foraminifers but
no oncolites (Vachard and Fontaine, 1988; Fontaine et al., 1996;
Fontaine and Salyapongse, 1999). They are located in faultbounded blocks in the middle of the Chanthaburi terrane (Localities 7 and 8 in Fig. 3). The Khao Cha-ang On limestone appears to
rest upon thick shale interbedded with sandstone (Salyapongse
et al., 1997). Thus, terrigenous clastic inputs are rather common
in the Triassic carbonate of the Chanthaburi terrane.
In addition, many other limestones around this region were also
reported to be oncolitic (Fontaine and Salyapongse, 1999). These
Triassic limestones are typically algal-rich oolitic to oncolitic
boundstones and bioclastic wackestone, with some stromatolites
and foraminifers (Vachard and Fontaine, 1988; Fontaine and
Salyapongse, 1999), who noted that the oncolitic facies, suggestive
of very shallow marine, reefal or lagoonal environments, appears
confined to the Scythian–Anisian limestones in the area.
2.3.1. Khao Yai Phring lyttoniid brachiopod shale
This brachiopod shale exposes in an excavation for an agricultural pond at 12°490 05.300 N, 101°480 10.100 E on the southern part
of the Khao Yai Phring (= Khao Ipring) hill (Fig. 6(1)). It would be
difficult to observe the bed rock unless the pond’s water level
was lowered; nevertheless, abundant lyttoniid-bearing blocks
quarried from the bed rock were dumped besides the pond. The
lithology of the brachiopod-bearing sediment is shale, which nevertheless has developed prominent phyllitic cleavage by regional
metamorphism. No other taxon was found in this fossil deposit,
but, from similar shales at nearby localities, moulds of some small,
although unidentifiable, bivalves have been noted.
The brachiopod has previously been considered a species of
Leptodus Kayser, yet this lyttoniid form has never been studied in
detail. Our current research confirms that the lyttoniid is not Leptodus, but belongs to another genus Oldhamina, indicative of a Late
Permian, most commonly Changhsingian age. It is here identified
as Oldhamina squamosa Huang, 1932 (see 3. Palaeontological note).
2.4. Other Triassic sediments (deeper marine)
In contrast to the shallow-marine carbonate-dominant Sukpaiwan Formation, deeper-water siliciclastic sediments of the Triassic
1
For interpretation of color in Figs. 6 and 7, the reader is referred to the web
version of this article.
23
Fig. 6. Shallow marine sedimentary units (Late Permian–Early/Middle Triassic) of the Chanthaburi terrane. (1) Khao Yai Phring lyttoniid brachiopod shale locality,
12°490 05.30 0 N, 101°480 10.10 0 E. (2) Khao Yai Phring crinoidal limestone lens (locality CS of Fontaine et al., 1999), 12°490 50.20 0 N, 101°480 21.80 0 E. (3) limestone layers exposed east
of Khao Hin, 12°520 35.50 0 N, 101°480 41.70 0 E, bedding 340°/60°W. (4) Khao Kong Din north quarry site, panoramic view from the northwest. (5 and 6) Thinly bedded limestone
with bedding 068°/50°E, lowest part of the Khao Kong Din limestone sequence, northwestern part of the whole Khao Kong Din area, 12°460 53.00 0 N, 101°470 47.30 0 E, (6), hand
specimen oncolites from the same locality as 6(5). (7) Bedded limestone (higher level) with bedding 085°/58°S in the eastern side of Khao Kong Din, 12°460 02.70 0 N,
101°470 54.80 0 E. (8) Coarse sandstone with thin mudstone intercalations at basal part of Khao Kong Din sequence (behind locality of 6(7)), bedding 065°/64°E, severely jointed
(joints 015°/80°W), 12°460 56.90 0 N, 101°470 47.20 0 E. (9) Khao Wong thick-bedded limestone (bedding 340°/40°W) at northern part of the hill, besides the Khao Wong temple,
12°530 52.50 0 N, 101°490 03.70 0 E.
are distributed over the eastern half of the Chanthaburi terrane
(Fig. 2). They are represented by the Pong Nam Ron Formation
and the Noen Po Formation. Despite their wide distributions, the
two are not sufficiently understood due to their limited exposures
and lack of age constraints.
The Pong Nam Ron Formation is a major siliciclastic unit that
extends over most eastern part of the Chanthaburi terrane and
over the Sa Kaeo Suture onto the margin of the Indochina block
(Fig. 2). It consists mainly of conglomerates and feldspathic to
lithic greywacke sandstones, interpreted as proximal turbidites
with classic Bouma sequences (Bunopas, 1982; Chaodumrong
et al., 2002; Chutakositkanon and Hisada, 2008).
No direct age evidence is available for the Pong Nam Ron Formation; nevertheless, the sequence is intruded by the Khao Soi
Dao I-type granite pluton (also known as the Krathing pluton,
see Cobbing et al., 1992) of latest Triassic–Early Jurassic age (ca.
195–207 Ma, Charusiri et al., 1992). This constrains the age of the
Pong Nam Rong Formation to be pre-Jurassic. The formation is generally regarded as Triassic sediment, based on lithostratigraphic
correlation with its lateral facies variant Noen Po Formation that
includes Middle–Late Triassic radiolarians. That is, the Pong Nam
Ron Formation is considered to comprise proximal turbidites,
while the Noen Po Formation represents deeper distal counterparts
(Chaodumrong et al., 2002; Chutakositkanon and Hisada, 2008).
Representative sequences of the Pong Nam Ron Formation include massive greywacke with abundant volcanics, conglomerates,
and interbedded sandstone and mudstone (Fig. 7(1–5)). A conglomerate to sandstone sequence is exposed along the Pong Nam
24
Fig. 7. Deep marine sedimentary units (Middle–Late Triassic) of the Chanthaburi terrane. (1–5) Pong Nam Ron Formation; (1) massively bedded greywacke outcrop near
Khao Soi Dao on Route 317, 12°500 50.20 0 N, 102°150 55.90 0 E; (2 and 3) conglomeratic sandstone exposed along the river in Nong Bon village (= Ban Nong Bon Unit of
Chutakositkanon and Hisada, 2008), 12°550 43.60 0 N, 102°260 21.20 0 E; (4 and 5) interbedded sandstone and mudstone of possible turbidites (= Noen Phuyai Yua Formation), west
of Chanthaburi city, 12°380 29.30 0 N, 102°030 36.30 0 E; (4) lower part of the section; (5) middle part showing coherent mud fragments caught up into sandy matrix by a turbidity
current while they were still soft. (6–9) Noen Po Formation; (6 and 7) originally pelitic sequence whose bedding S0 developed into phyllite foliation S1, quarry site south of
Chanthaburi city; (8 and 9) Middle Triassic chert at Ao Tan Ku beach west of Trat, 12°120 00.90 0 N, 102°160 54.10 0 E; (8) slumped chert beds; (9) slumped chert layers closely
associated with intraformational pillow basalt.
Ron river in Non Bon village (Fig. 7(2 and 3)). Note that this unit
was separated as part of the new Bang Non Bon unit by Chutakositkanon and Hisada (2008) who considered it part of the Sa Kaeo
mélange. The age of the Bang Non Bon unit is not clearly known,
although Chutakositkanon and Hisada (2008) supposed it to be
Late Permian–Early Triassic. Its mélange characters noted by
Chutakositkanon and Hisada (2008) were not sufficiently understood. Thus, the Bang Non Bon unit is still treated as part of the
Pong Nam Ron Formation here.
An excellent sequence is exposed in the quarry 7 km NW of
Chanthaburi city, which consists of medium to thin-bedded sandstone interbedded with mudstone and stratified conglomeratic
beds upwards (Fig. 7(4 and 5)). The mud layers often have soft sediment fragmentation caught up by turbidity currents. This sequence has been included in the Pong Nam Ron Formation Unit 2
in the official geological map of Chanthaburi Province (see Tansuwan, 1997b), but it is peculiar that this coarse-grained sequence is
located within the Noen Po Formation. Some authors (Chaodumrong et al., 2002; Chonglakmani, 2011) referred this unit to as another unit Noen Phuyai Yua Formation, which is supposed to
overlie the Pong Nam Ron and Noen Po formations but is poorly
studied.
The Noen Po Formation distributes widely through a rather elongate zone in the northwest and southeast of Chanthaburi (Fig. 2). It
consists typically of finer muddy sediments, interbedded with fine
sandstone, locally with radiolarian-bearing cherts. They are interpreted as distal turbidites (Chaodumrong, 1992). The Noen Po Formation is more or less equivalent to the Chanthaburi Chert-Clastic
sequence of Hada et al. (1997). Some sequences south of Chanthaburi city are metamorphosed into phyllite, with the initial foliation
parallel to the bedding (Fig. 7(6 and 7)). The cherts are dominantly
Late Triassic, with a few Middle Triassic cherts west of Trat (Hada
et al., 1997; Sashida et al., 1997). The Middle Triassic bedded chert
in Ao Tan Ku beach is intercalated directly with intraformational
pillow basalt (lower Noen Po Formation, or called the Laem Ngop
Formation); this is suggestive of concurrent submarine volcanism.
The chert often show slumped structures (Fig. 7(8 and 9)).
2.5. Younger Mesozoic sediments (continental)
The Triassic marine strata of the Chanthaburi terrane are overlain unconformably by continental red beds of younger Mesozoic
ages, which often constitute outstanding landforms in southeastern Thailand. For example, the Khao Thalai red beds of fluvial facies
form a prominent N–S trending ridge along the Tha Mai fault zone
(Figs. 2 and 3). The sediment was previously interpreted as prePermian in age (Bunopas, 1982, p. 378). However, it includes Early
Jurassic detrital zircons dated by U-Pb LA-ICPMS to 196max ± 8 Ma
(Geard in Crow, 2011), indicating post-Triassic deposition related
to the Jurassic–Cretaceous Khorat Group. The Khao Thalai red beds
have been included in the Laem Sing Formation of the continental
Mesozoic by Chaodumrong (1992) and this is followed herein. Another portion of the Khorat Group is present at the southeastern
margin of Thailand, east of Trat, and forms abrupt skirts of a hilly
area along the Thai-Cambodia border (Fig. 2); this extends into
Cambodia as the foothills of the Cardamom Mountains.
3. Palaeontological note
Genus Oldhamina Waagen, 1883
Remarks: Oldhamina is a highly specialised, large form of the
Lyttoniidae. Its occurrence is well confined to the Lopingian (Late
Permian), most commonly to the Changhsingian Stage, and it was
endemic to the Tethys Sea.
Oldhamina squamosa Huang, 1932 (Fig. 8.1–8.7).
Synonymy: (selected for Southeast Asian occurrence, except for
the original designation OD).
Oldhamina squamosa Huang, 1932, p. 74, pl. 6, figs. 1–5; pl. 7, fig.
11 (OD, China).
Leptodus tenuis Waagen; Pitakpaivan et al., 1969, 23, pl. 19, fig.
17 only.
Oldhamina squamosa Huang; Waterhouse, 1983, p. 130, pl. 5,
figs. 1–10.
Leptodus sp.; Bunopas, 1982, p. 381; Bunopas et al., 1983, p. 19,
fig. 1., Salyapongse et al., 1997, p. 14., Fontaine and Salyapongse, 1997a, p. 74., Fontaine et al., 1999, p. 15., Fontaine,
2002, p. 573.
Oldhamina cf. anshunensis Huang; Koga et al., 1998.
Occurrence: Khao Yai Phring (Khao Ipring) shale, east of Klaeng,
southeastern Thailand; Changhsingian.
Material: Some 30 specimens of dorsal-side concavity moulds
(that is, moulds of dorsal internal plates and/or ventral valves)
were collected. Of them, seven specimens UM10544–10550 are
herein illustrated (Fig. 8). Repository at the Department of Geology,
University of Malaya. All are tectonised in beds of foliated shale,
thus accurate dimensions are no longer measurable; nevertheless,
the average linear length can be estimated at least 70 mm.
Remarks: The present form has dominantly large elongate convex ventral shells and numerous lateral septa (about 23 in number
counted for UM10547, Fig. 8.4), suggestive of Oldhamina not Leptodus. It is a large species attaining more than 70 mm long and
60 mm wide, as typically longer than wide, and the lateral septa
are inclined to the mid-line; it is most closely referable to Oldhamina squamosa Huang, 1932 from the Changhsingian of South China.
The type species of Oldhamina, Bellerophon decipiens de Koninck,
1863 from the Chhidru Formation (Changhsingian) of the Salt
Range, Pakistan, is a relatively under-sized form with fewer septa.
New topotypic material was well studied in Waagen (1883, p. 406,
pl. 31, figs. 1–9) and Noetling (1905, p. 134, pl. 15, figs 1–9, pl. 16,
figs 1–6).
Oldhamina grandis Huang, 1932 (p. 84, pl. 8, figs. 3a–c) is based
originally on a single shell. The species is now known to occur in
the upper Lungtan Formation (late Wuchiapingian) of Guizhou
(see Liao, 1980, p. 245). It is close to O. squamosa in having a large
shell and numerous lateral septa. As originally defined, O. grandis
may be differentiated from O. squamosa only by its subcircular
shell with a nearly equidimensional outline, while O. squamosa
tends to be longer than wide. Thus, there is a good possibility that
O. grandis is a synonym of O. squamosa.
25
Oldhamina anshunensis (Huang, 1932) [originally a variety of
O. squamosa] is separated from O. squamosa along several lines
of differences, as listed by Huang (1932, p. 78). The most obvious
differences are perhaps that O. anshunensis has a less convex profile and lateral septa less inclined towards the mid-line than O.
squamosa. The former is also a little smaller in size than the
latter.
The present discovery of Oldhamina marks the second unequivocal occurrence of the genus not only in Thailand but all over
Southeast Asia. Elsewhere in Thailand, Oldhamina was previously
reported only from the Huai Tak Formation (Changhsingian, upper
Ngao Group) of the Sukhothai terrane, where Waterhouse (1983)
described large shells as O. squamosa. Additional Huai Tak material
of Oldhamina was collected and illustrated by Ishibashi et al. (1998)
and Koga et al. (1998). In addition, Koga et al. (1998) described another small form as Oldhamina cf. anshunensis Huang from a nearby
locality, a bed approximately 60 m lower than the Waterhouse
horizon; it may merely be a smaller, less mature, equivalent of
the Huai Tak O. squamosa.
Yanagida (1967, p. 88, pl. 21, figs. 4a,b and 6) described three
lyttoniid specimens as Oldhamina aff. decipiens (de Koninck) from
the Nam Maholan Formation (Asselian, earliest Permian) of the
Indochina block. However, this form is most likely a species of another genus, either Palaeoldhamina Liang (Koga et al., 1998) or Keyserlingina Tschernyschew (Shi and Archbold, 1993, p. 31).
Elsewhere in Southeast Asia, the occurrence of true Oldhamina
is not confirmed. Oldhamina was once reported from the Gua Musang Formation of central Peninsular Malaysia by Leman (1993).
However, Campi et al. (2002), despite intensive study, failed to
confirm the presence of the genus in the formation; the Malaysian
form likely belongs to Leptodus, a common lyttoniid genus.
4. Stratigraphic overview of the Sukhothai terrane
The Sukhothai terrane forms the core part of the Sukhothai Arc.
It is delineated by the Chiang Rai tectonic line (the eastern fringe of
the Inthanon Zone) in the west and by the Nan suture in the east
(Fig. 1). It is dominated by intensely folded and faulted sequences
of Permian–Triassic marine siliciclastic and carbonate sediments
associated with Permo-Triassic igneous rocks of I-type granite
affinity. It has been interpreted to be of magmatic arc origin (e.g.,
Mitchell, 1977; Bunopas, 1982; Barr and Macdonald, 1991; Ueno
and Hisada, 1999; Barr et al., 2000; Sone and Metcalfe, 2008b).
The Carboniferous–Permian stratigraphy of the terrane is exclusively documented in Ueno and Charoentitirat (2011).
The Sukhothai terrane, together with the Chanthaburi terrane,
was developed as a Permo-Triassic volcanic arc (Sukhothai Arc)
off the Indochina block by opening of the Nan–Sa Kaeo back-arc
basin. This volcanic arc system was induced by the east-dipping
Palaeo-Tethyan subduction beneath the margin of the Indochina
block (Ueno and Hisada, 1999; Sone and Metcalfe, 2008b). The exact geographic connection in central Thailand between the Sukhothai terrane and the Chanthaburi terrane is not clearly traceable
due to the major sinistral strike-slip offset of the Mae Ping Fault
system, forming the Chainat Duplex, and due to a Cenozoic fluvial
cover of the Central Plain in the Menam Delta. The recent study of
Ueno et al. (2012), nevertheless, clarifies the southern extension of
the Sukhothai Zone in the Central Plain.
4.1. Pre-Permian rocks
Pre-Carboniferous rock is not well understood in the Sukhothai
terrane, although an early Proterozoic crustal basement is indicated by an inheritance U-Pb zircon age of 2074 Ma from the
Lampang Volcanics (Barr et al., 2000). It is supposedly from a
26
Fig. 8. Oldhamina squamosa Huang, 1932 from the Khao Yai Phring shale near Klaeng, southeastern Thailand. All underwent tectonic deformation and are moulds of concave
side (dorsal) of conjoined shells (that is, impressions of dorsal exterior covered by ventral valves). (1) UM10544. (2) UM10545 with remain of another shell overlapping at
anterior. (3) UM10546. (4) UM10547. (5) UM10548. (6) UM10549. (7) UM10550 with cast of dorsal internal plate remaining.
xenocryst derived from a possible basement of the Sukhothai terrane; in such case, the Sukhothai Arc was an ‘ensialic island arc’
(underlain by continental crust) (see Frisch et al., 2011).
A poorly understood volcano-sedimentary unit, the Thung Saliam Group, in the southern part of the Sukhothai terrane was once
considered Siluro-Devonian in age, based on the local geological
setting (Bunopas, 1982); it consists of the Khao Khieo Tuff, Thung
Saliam Limestone, and the Khanu Chert in ascending order. However, the Khanu Chert has now been dated to be Sakmarian (Early
Permian) with radiolarians (Sashida and Nakornsri, 1997), implying that the whole group might be as young as the Early Permian.
No evidence of Silurian or Devonian age is present.
Another volcaniclastic unit, the Dan Lan Hoi Group, occurs in
the west of Sukhothai city. Detailed examination on the Dan Lan
Hoi Group was provided by Ueno and Charoentitirat (2011,
p. 82). It has long been considered of possible Carboniferous age
(Bunopas, 1982), but this age assignment is here interpreted as
highly questionable. The group consists of the Khao Khi Ma
Pyroclastics, Lan Hoi Formation, and Khao Luang Pyroclastics in
ascending order. Its Carboniferous age was originally concluded
based on the fact that the Khao Khi Ma Pyroclastics lies upon the
Khao Khieo Tuff of the Thung Saliam Group (then considered Siluro-Devonian), with a major angular unconformity (Bunopas,
1982, p. 164). However, as noted above, the Thung Saliam Group
is more likely Early Permian; hence, the Dan Lan Hoi Group above
the unconformity may even be younger. On balance, the Dan Lan
Hoi Group cannot be established as Carboniferous. The Dan Lan
Hoi Group and/or the Thung Saliam Group might be local volcaniclastic equivalence to some part of the Permian Ngao Group in the
north; this requires confirmation. In conclusion, there is no
unequivocal pre-Permian sediment in the Sukhothai terrane.
4.2. Permian sediments and PT boundary (mixed marine)
Permian sediment of the Sukhothai terrane is represented most
importantly by the Ngao Group, which is divided into the three
27
formations, namely, the Kiu Lom, Pha Huat, and Huai Tak formations in ascending order, and they are traditionally correlated
roughly to the Early, Middle and Late Permian series.
The Kiu Lom Formation consists of shales, tuffaceous sandstones and pyroclastics with thin intercalations of limestone,
which are well exposed near the Kiu Lom Dam north of Lampang.
The limestones yield the fusulinids indicative of an Asselian–Sakmarian age (Fontaine and Vachard, 1988). This formation was previously reported to ‘‘conformably’’ overlie the Dan Lan Hoi Group
(Bunopas, 1982); however, as discussed in Section 4.1, the true
Dan Lan Hoi Group to the south may even be younger than the Carboniferous. It is uncertain which rock unit lies beneath the Kiu Lom
Formation in the Lampang region.
The Pha Huat Formation is not well studied, despite its wide
distribution within the Lampang area. Some sequences of the formation can be well exposed within the Kiu Lom Lake if the dam
water level is lower (Chonglakmani, pers. comm., 2002). Its lithology consists dominantly of limestone with a volcaniclastic member
of thinly bedded tuff, and the Middle Permian fusulinids Verbeekina
and gigantic bivalves Alatoconcha are present (Chonglakmani and
Intarawijitr, 1994). A Middle Permian conodont fauna, reported
to be from the Pha Huat Formation (Carey et al., 1995), was revised
to late Wuchiapingian in age (Mei and Henderson, 2002, p. 604).
Ueno and Charoentitirat (2011) then considered the formation to
extend up to the Late Permian. However, the conodont locality
(Locality B of Carey et al., 1995) is situated within the known distribution of the Late Permian Huai Tak Formation. It would therefore be better considered to be part of the Huai Tak Formation.
The Huai Tak Formation is widely distributed to the north of the
Lampang area, and its bio- and lithostratigraphy have been well
established (Ishibashi et al., 1998 and references therein). Most
fossil occurrences are from its Changhsingian (latest Permian) part,
as carbonate strata overlie sandstone and shale beds (Fig. 9). Since
the Phra That Formation (Early Triassic) of the Lampang Group is
also common in the area, the discovery of the PT boundary has
been expected. However, known sections of the PT succession are
unfortunately found fault-bounded (see Ishibashi et al., 1994).
The exact PT boundary is not yet confirmed, accordingly.
A latest Permian Changhsingian is also present in the Phrae basin to the east, Bedded sponge-rich bedded limestones include the
typical Late Permian foraminifer Colaniella ex. gr. parva, and it is
intercalated with sandstones, shales and tuffs (Senowbari-Daryan
and Ingavat-Helmcke, 1993). This mixed lithostratigraphy appears
similar to the coeval Huai Tak Formation in the Lampang area. This
unit is now included within the new Permian unit Uttaradit Group
of Ueno and Charoentitirat (2011, p. 87).
4.3. Triassic Lampang Group (mixed marine)
The Ngao Group conformably graded upwards into the Triassic
Lampang Group. Marine depositional environment in the Sukhothai terrane persisted through Late Permian to early Late Triassic,
comparable to that of the Chanthaburi terrane. Triassic deposits
consist of both shallow-marine shelf carbonates and deeper-water
siliciclastic turbidites, collectively called the Lampang Group. The
sediments of the Lampang Group (the Lampang Basin) are known
to have been deposited in two adjacent sub-basins in the middle
of the terrane, namely, the Lampang sub-basin to the east and
the Phrae sub-basin to the west (Chonglakmani and Helmcke,
1989; Chaodumrong and Rao, 1992).
The Lampang Group consists of seven formations, the Phra That,
Pha Kan, Hong Hoi, Doi Long, Pha Daeng, Kang Pla and Wang Chin
formations in ascending order. Three of them, the Pha Kan, Doi
Long and Kang Pla formations are shallow marine carbonatedominant units, often including algal-rich oncoidal/peloidal limestone facies of shoal environments (Chonglakmani and Helmcke,
Fig. 9. Stratigraphy of the Late Permian to basal Triassic in the Doi Pha Phlung area
of the Sukhothai terrane in Northern Thailand, with biostratigraphic data (modified
after Ishibashi et al., 1998, fig. 2).
1989; Chaodumrong and Rao, 1992). The other formations consist
of detrital siliciclastic and volcaniclastic sediments. Of them, the
Hong Hoi Formation (Middle Triassic) include conglomeratic pebbles immediately reworked from older Middle Triassic limestone
28
and rhyolitic-andesitic acid volcanic rocks; these are regarded as
intraformational clasts suggestive of local extensional basin setting
(Lüddecke et al., 1991). The Pha Daeng Formation (Carnian/early
Late Triassic) distributes over both of the Lampang and Phrae
sub-basins (Chaodumrong and Burrett, 1997). The Rong Kwang
conglomerate (= the upper Wang Chin Formation, Norian/Late Triassic) is overlain by deep marine turbidites bearing the Triassic bivalve Halobia (see Hahn, 1976).
4.4. Younger Mesozoic sediments (continental)
The Khorat Group of younger Mesozoic continental red beds extends from the Khorat Basin of the Indochina block to the Sukhothai terrane covering the Lampang Group. This has been called
the Nan–Phyao basin (Hahn, 1976, p. 157) or the Sayabouri–Nakhon Thai basin (Racey, 2009). The above-mentioned Halobiabearing turbidites are covered by possible continental (or deltaic)
sandstones and shales with basal conglomerates (Unit ms1 of
Hahn, 1976). The contact to the Wang Chin Formation is supposed
to be unconformable, although it is reported to be locally conformable (see Hahn, 1976). This clastic cover includes abundant limestone pebbles, presumably derived from the Ngao or Lampang
Group (Hahn, 1976). Thus, it is suggestive of Late Triassic (postCarnian) uplift. The non-marine condition in the Sukhothai terrane
possibly started by the latest Triassic.
5. Discussion
5.1. Deep-marine Triassic and tectonic delineation of the Chanthaburi
terrane
Details of Middle and Late Triassic deep marine turbidites and
chert–pillow basalt intercalations (Pong Nam Ron and Noen Po formations) in the eastern part of the Chanthaburi terrane was presented in Section 2.4. Ueno and Charoentitirat (2011, p. 109)
reconstructed a tectonic model in which the Nan–Sa Kaeo backarc basin subducted beneath Indochina during Permo-Triassic time
(that is, eastward subduction in present direction) and sediments
of the Pong Nam Ron (proximal) and Noen Po (distal) formations
filled a closing or relict back-arc basin where the latter unit was
placed in the depocentre. However, structural data from Middle
Permian schists of the Pha Som Complex in the Nan Suture suggests westwards tectonic vergence (Singharajwarapan and Berry,
1993). Moreover, palaeocurrent directions of the Pong Nam Ron
turbidites indicated by flute casts are north and northwestwards
(Chutakositkanon and Hisada, 2008), that is, opposite to the flow
direction for the Pong Nam Ron to Noen Po formations illustrated
by Ueno and Charoentitirat (2011). There is no evidence for Permo-Triassic eastwards subduction of the Nan–Sa Kaeo back-arc basin, nor for the consequent volcanic arc in western Indochina,
illustrated by Chutakositkanon and Hisada (2008), Ueno and
Charoentitirat (2011), and Ridd (2012).
The presence of local basalt intercalated with Middle Triassic
chert in the Noen Po Formation (Laem Ngop unit) of Ao Tan Ku is
perhaps better explained as submarine arc volcanism of the southern Sukhothai Arc itself (the Chanthaburi terrane), rather than by
Permo-Triassic eastwards back-arc subduction or underplating
oceanic crust assumed by those authors (Chutakositkanon and
Hisada, 2008; Ueno and Charoentitirat, 2011; Ridd, 2012). As
pointed out by Sone and Metcalfe (2008b), there is a stratigraphic
gap of chert sedimentation between Permian cherts in the Sa Kaeo
mélange and Middle–Late Triassic cherts in the Noen Po Formation,
and their depositional sites are geographically separated. Considering these uncertainties, the Middle–Late Triassic Noen Po and Pong
Nam Ron formations are not here considered to be the infill of the
Nan–Sa Kaeo back-arc basin, following the view of Sone and
Metcalfe (2008b).
Ueno and Charoentitirat (2011) and Ridd (2012) consider the
Sukhothai Arc/zone in southeastern Thailand to be delimited to
the east by the Tha Mai fault zone, as they called it the Klaeng Zone
or the Klaeng Terrane, respectively. Therefore, their southern Sukhothai Arc is unequal to the Chanthaburi terrane discussed here.
Their concepts of the arc zone or terrane are based chiefly on the
view that the shallow marine sediments of the Sukpaiwan Formation in the west and the deeper-marine Pong Nam Ron/Noen Po
formations in the east are separated along the Tha Mai fault. However, the Tha Mai fault is principally a Cenozoic structure, with no
evidence of reactivation for pre-existing crustal structure assumed
by Ridd (2012). In principle, a volcanic arc terrane should be defined based on the cluster of former arc batholith (Late Triassic–
early Jurassic I-type granites for the Chanthaburi terrane) as shown
by Sone and Metcalfe (2008b) and Cobbing (2011) and this study
(Fig. 2), and not based solely on the nature of surface sediments.
5.2. Correlation between the Chanthaburi and Sukhothai terranes
In this study, pre-Carboniferous rocks are not confirmed either
in the Chanthaburi or Sukhothai terranes. Carboniferous limestone
(Visean–Bashkirian) in the Chanthaburi terrane is the oldest
unequivocal sediment, and it bears some faunal affinity to the
Indochina block, as noted in Section 2.2. These facts support the
interpretation of Sone and Metcalfe (2008b) that the two terranes
developed as part of a volcanic arc (Sukhothai Arc) marginal to the
Indochina block, with opening of a Nan–Sa Kaeo back-arc basin in
the Early Permian.
As shown in Fig. 10, Permian and Triassic stratigraphy of both
Chanthaburi and Sukhothai terranes is characterised by mixed
lithology of siliciclastics and carbonates, with common inclusion
of volcanic clasts and tuff. Although the known Carboniferous–Triassic stratigraphy of the Chanthaburi terrane is rather incomplete,
the Late Permian–Middle Triassic succession appears comparable
to the correlated section of the upper Nagao Group to the lower
Lampang Group in the Sukhothai terrane. In particular, lithostratigraphic change from the Oldhamina-bearing shale to the Palaeofusulina-Colaniella bearing limestone in the uppermost Permian,
followed by Triassic limestone, the Triassic limestones of both terranes include algal-rich oolites/oncolites/peloids, which are often
intercalated with terrigenous sediments, suggesting very shallow-marine condition.
Overall, the Permian–Triassic marine stratigraphy of the Sukhothai and Chanthaburi terranes reveals lithologically changing, perhaps tectonically unstable, sedimentary environments; this is
typical of volcanic arc environments (see Orton, 1996). Marine sedimentation continued in both arc terranes up to the Late Triassic,
and this is overlain by Jurassic–Cretaceous continental sediments
of the Khorat Group equivalents (see Sections 2.5 and 4.4).
5.3. Stratigraphic dissimilarities with the Indochina and Sibumasu
blocks
5.3.1. Indochina block
For comparison with the Sukhothai Arc terranes, we here outline
Late Palaeozoic–Mesozoic stratigraphy of the western part of the
Indochina block, particularly for the Khorat region (Northeast Thailand) (Fig. 10). The Carboniferous to late Mesozoic subsurface stratigraphy of the Khorat region has been well constraint, based on
data from drilled wells and seismic profiles (e.g., Kozar et al.,
1992; Mouret, 1994; Booth and Sttayarak, 2011). A Mid-Carboniferous unconformity is evident, although its cause is not clearly understood (Booth and Sttayarak, 2011). This unconformity is not known
in the Sukhothai Arc terranes. Carbonate-dominant successions of
29
Fig. 10. Tectonostratigraphic comparison chart of the Sukhothai Arc terranes (Chanthaburi and Sukhothai terranes) to the bounding Indochina and Sibumasu blocks. Data
compiled from various sources (including Ueno and Charoentitirat, 2011). UC = unconformity. Age ranges of the Palaeo-Tethys and back-arc basin (after Sone and Metcalfe,
2008b).
Middle Carboniferous to Middle Permian are extensively distributed in the western Indochina block (Fig. 10). They are called the
Saraburi Group, and includes the Wang Saphung Formation and
its subsurface correlative Si That Formation (Late Carboniferous),
the Nam Maholan Formation (Late Carboniferous–Early Permian),
and the platform carbonates of the Pha Nok Khao and Khao Khwang
formations (Early–Middle Permian); a detailed review is provided
by Ueno and Charoentitirat (2011). Limestones are often intercalated with siliciclastic sediments (e.g., Wielchowsky and Young,
1985; Chairangsee et al., 1990; Fontaine et al., 1995), or they locally
became deeper during Permian time (Chutakositkanon et al., 2000).
The latest record of marine sedimentation in the western Indochina
block is Capitanian (late Middle Permian), marked by the Lepidolina
multiseptata fusulinid zone in the upper Saraburi Limestone near
Loei (Fontaine et al., 2002) and the upper Sisophon Limestone of
western Cambodia (Ishii et al., 1969).
Marine Late Permian is not confirmed in the western Indochina
block so far. In turn, it is represented locally by a continental deposit in northern Laos, with the famous occurrence of the reptilian
Dicynodon fossil bed (Battail, 2009). This Late Permian deposit includes a reworked Middle Permian fossiliferous limestone (Battail
et al., 1995), indicating that the region underwent a major Late
Permian uplift. Thus, marine sedimentation was apparently terminated by a Late Permian uplift (Indosinian I orogeny), as pointed
out by Sone and Metcalfe (2008b). This is now evident with subsurface stratigraphy (Booth and Sttayarak, 2011).
30
The Triassic of the Indochina block is chiefly non-marine, with a
few exceptions of possible local transgressions in Laos (Fontaine
and Workman, 1978, p. 556) and in the south of Aranyaprathet
near the Cambodian border (Fontaine et al., 1996). In the Khorat region, the continental Huai Hin Lat Formation or the Kuchainarai
Group of the early Late Triassic (Carnian to Norian) overlies unconformably the Middle Permian marine sequences, as evident in the
subsurface geology (Kozar et al., 1992; Booth and Sttayarak, 2011).
On the whole, there is a long stratigraphic hiatus from Late Permian through to early Late Triassic, that is, the Indosinian I unconformity. Late Permian–Middle Triassic marine sediments are almost
absent in the western Indochina block, and this is contrastive to
the terranes of the Sukhothai Arc, where marine sedimentation
persisted through the PT transition to the Late Triassic (Fig. 10).
5.3.2. Sibumasu block
The Sibumasu block possesses near complete marine stratigraphic records from the Cambrian to the Triassic (e.g., Metcalfe,
1996). This contrasts markedly with the Chanthaburi or Sukhothai
terranes. Furthermore, there are clear dissimilarities in lithostratigraphic patterns and faunal affinities.
The Early Permian of Sibumasu is characterised most importantly by having glacio-marine sediments on a peri-Gondwana
margin (e.g., Stauffer and Mantajit, 1981; Stauffer and Lee, 1989),
accommodating cool-water peri-Gondwanan faunas (e.g., Waterhouse, 1982). In turn, the post-Early Permian of Sibumasu is represented by a predominantly limestone succession. None of these
Permo-Carboniferous stratigraphic features of Sibumasu is comparable to that of the Sukhothai Arc terranes. The Middle–Late Permian limestones of Sibumasu are known to have faunas of lower
diversity, compared with the Indochina or Sukhothai terranes,
reflecting its higher palaeolatitudes and/or lower water temperature upon its journey from peri-Gondwana (e.g., Fontaine et al.,
1994; Ueno, 2003).
Thick and prolonged carbonate successions from Middle Permian to Late Triassic cover large parts of the Sibumasu block, for
example, the Ratburi–Chaiburi limestones (Thailand), the Chuping
and Kodiang limestones (Malaysia) and the Thitsipin and Nwabangyi formations (Myanmar). These Permo-Triassic carbonates of
Sibumasu consist dominantly of bioclastic and micritic packstone
of platform carbonates, but terrigenous clastics are almost absent
(see Chinoroje, 1993; Dawson et al., 1993; Fontaine et al., 1994;
Ampornmaha, 1995), implying the fact that the terrane was largely
a submarine platform since its separation from Gondwana (here
termed the post-Gondwana sequence, Fig. 10). This lithostratigraphy is clearly distinct from the coeval sequences of the Sukhothai
Arc terranes, where a mixture of marine siliciclastics and carbonates, with abundant volcaniclastic inputs, are common, as outlined
earlier.
Whilst the Jurassic is entirely continental in the Sukhothai Arc
terranes and the Indochina block, Jurassic marine sediments are
substantial in some areas of the Sibumasu block (Kozai et al.,
2011; Meesok and Saengsrichan, 2011). This demonstrates major
stratigraphic differences between the Sibumasu block and the Sukhothai Arc (Fig. 10).
6. Conclusions
(a) The Klaeng tectonic line is defined for the boundary between
the Chanthaburi terrane and the Sibumasu block in southeastern Thailand. It reveals a former collisional suture that,
nevertheless, has largely been disrupted by Tertiary strikeslip movement.
(b) There is no clear evidence of pre-Carboniferous sedimentary
rocks either in the Sukhothai or Chanthaburi terranes. This is
consistent with the Permian–Triassic Sukhothai Arc model
(Ueno and Hisada, 1999; Sone and Metcalfe, 2008b).
(c) Lyttoniid brachiopods from Late Permian shale of Khao Yai
Ipring near Klaeng, previously reported as Leptodus, belong
to Oldhamina. It marks the second unequivocal occurrence
of the genus not only in Thailand but all over Southeast Asia.
The occurrence of Oldhamina in Thailand is confined to the
Sukhothai Arc.
(d) The Late Permian–Triassic marine stratigraphy and associated fauna of the Chanthaburi terrane is comparable to that
(upper Ngao Group–Lampang Group) of the Sukhothai terrane, in particularly, litho- and biostratigraphic changes
from Oldhamina shale to Palaeofusulina-Colaniella limestone
in the uppermost Permian that then continues to the Triassic. Volcaniclastic material is very common.
(e) This Late Permian–Triassic stratigraphy of the Chanthaburi
and Sukhothai terranes is clearly distinct from those of the
Indochina or Sibumasu blocks, where the Late Permian–Triassic is largely absent due to the Indosinian I unconformity
(western Indochina) or is dominantly calcareous with little
terrigenous clastic input (Sibumasu).
(f) Marine depositional conditions were terminated on the Sukhothai Arc by end-Triassic; this is later than on the Indochina block (Late Permian) but earlier than on the
Sibumasu block (Jurassic/Cretaceous).
Acknowledgements
This study was supported by a High Impact Research Grant
UM.C/625/1/HIR/140 and a UM Research Grant RG146-11AFR from
the University of Malaya (to Sone) and by an Australian Research
Council Discovery Grant (to Metcalfe). The Department of Mineral
Resources, Thailand, facilitated our fieldwork. Special thanks go to
S. Salyapongse who guided Sone to important localities. A. Treerotchananon, Mustaffa K. Shuib, and A. Chitnarin are also thanked for
helping fieldwork and analysis. K. Ueno and K. Hisada are thanked
for their helpful reviews of the paper. This is a contribution to
IGCP516.
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The Chanthaburi terrane of southeastern Thailand: Stratigraphic

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