Gondwana Research 19 (2011) 47–60

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Paleo-environments and tectonic setting of the Mesozoic Thung Yai Group inPeninsular Thailand, with a new record of Parvamussium donaiense Mansuy, 1914

Wirote Saengsrichan a,b,c,1, Thasinee Charoentitirat b,c, Assanee Meesook a,Ken-ichiro Hisada d, Punya Charusiri b,c,⁎a Department of Mineral Resources, Rama VI Road, Bangkok 10400, Thailandb Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailandc Earthquake and Tectonic Geology Research Unit, Chulalongkorn University, Bangkok 10330, Thailandd Division of Earth Evolution Sciences, Graduate School of Life and Environmental Science, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan

⁎ Corresponding author. Department of Geology, FacUniversity, Bangkok 10330, Thailand. Fax: +66 221854

E-mail address: cpunya@chula.ac.th (P. Charusiri).1 Deceased.

1342-937X/$ – see front matter © 2010 International Adoi:10.1016/j.gr.2010.05.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 August 2009Accepted 17 May 2010Available online 27 May 2010

Keywords:Jurassic–CretaceousPaleo-environmentsTectonic settingKrabiThung Yai GroupPeninsular ThailandParvamussium donaiense

The Thung Yai Group extends over a large area of peninsular Thailand, along the eastern margin of the ShanThai block. Bound by angular unconformities 300 m thick dominantly detritic brackish to non-marinedeposits with few intercalated limestone beds between Triassic marine and Tertiary non-marine sediments,represent the Thung Yai Group that comprises four formations: Khlong Min, Lam Thap, Sam Chom, and PhunPhin Formations. In the Ao Luk–Plai Phraya (ALPP) area, the Khlong Min and Lam Thap formations yieldmarine, brackish-water and non-marine fossil assemblages. These include trace fossils and for the first timein peninsular southern Thailand, the bivalve Parvamussium donaiense Mansuy, 1914. Based on fossildeterminations, the Thung Yai Group has a late Early Jurassic to Early Cretaceous age.Our new observations help unravel the tectonic history of Mesozoic Peninsular Thailand. After the completeclosure of the Paleotethys in the Late Triassic, renewed inundation, from the late Early Jurassic to the earlyMiddle Jurassic, brought a regime of shallow to open marine and lagoon sedimentation over northwestern,western and southern peninsular Thailand, in the eastern part of Sundaland bordering the Mesotethys tothe west.

© 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

Mainland Southeast Asia comprises three major terranes: WesternBurma, Shan–Thai and Indochina (Burrett, 1974; Stauffer, 1974;Hutchison, 1975; Gatinsky et al., 1978; Bunopas, 1981; Burrett et al.,1990; Barber and Crow, 2003; Metcalfe, 2006, 2010). Thailandconsists in the west of the eastern part of the Shan–Thai terrane andin the east of the western part of the Indochina terrane. Theintervening tectonic units (Fig. 1) are the Lampang–Chiang Rai unit,for the eastern part of Shan–Thai, and the Nakhon Thai unit, for thewestern part of Indochina (Charusiri et al., 2002, 2006; Hara et al.,2009). Our study area belongs to the southern part of eastern Shan–Thai terrane, west of the Pattani suture (Charusiri et al., 2002), whichis considered to connect southward with the Bentong–Raub suture, inMalaysia. The latter suture resulted from the collision of the Indochinaand Shan–Thai terranes (Bunopas, 1981; Hahn et al., 1986; Panjasa-watwong, 1991) mostly during Late Triassic times (Metcalfe, 1989;

ulty of Science, Chulalongkorn64.

ssociation for Gondwana Research.

Charusiri et al., 2002; Meesook et al., 2005) (Fig. 2). Our focus ofinterest here relates to extensive on-shore petroleum exploration.

The present study area is part of the Ao Luk and Plai Phrayadistricts (ALPP) that cover approximately 400 km2 within theSongkhla basin in the northern part of the Krabi Province, southernpeninsular Thailand (Fig. 1). Geomorphologically an intramontanesub-basin, the area of the ALPP is bound to the east by the PhanomBenja Range culminating at 1340 m above mean sea level (msl) andwest of it, the undulating terrains of the northwest–southeasttrending, about 500 m high Phanom Hills extend.

The Mesozoic sequences in Thailand comprise marine, brackish andcontinental facies (Meesook and Grant-Mackie, 1996). Exclusivelymarine inwesternThailand, themost fully developed Jurassic sedimentsoccur in the Mae Hong Son–Kanchanaburi basin of northwest and westThailand. In peninsular Thailand, the Chumphon basin to the north andthe Songkhla basin, further south contain Jurassic marine sediments.With exception of the continental basins in northeastern Thailand,mostmarine and brackish Jurassic basins are elongated north trending,normal fault or strike-slip bound (Fig. 1). In the Songkhla basin, the AoLuk–Plai Phraya (ALPP) area comprises the Thung Yai Group thatencompasses four formations: Khlong Min, Lam Thap, Sam Chom, andPhun Phin Formations. The KhlongMin and Lam Thap Formations yield

Published by Elsevier B.V. All rights reserved.

Fig. 1. Map of Thailand showing major tectonic units, distribution of the Jurassic–Cretaceous sedimentary rocks, associated basins, and major tectonic features (modified afterPolachan and Sattayarak, 1989; Charusiri et al., 2002).

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bivalves and some other macrofossils that provide age constraints forstratigraphic correlation and the establishment of the continuity oflithologic units. Their relations with the better-known non-marineKhorat Group to the east are also considered.

Besides the first discovery of the Jurassic bivalve Parvamussiumdonaiense Mansuy, 1914 in the Songkhla basin, other Mesozoic

bivalves occur. Brackish-water heterodont bivalves among themshow ecological and taxonomical diversifications (Kondo et al.,2006). These play an important role in the evaluation of their paleo-environment and the delineation of their paleo-biogeography (Arias,2008). All fossils cited in the text are part of collections duringfieldwork by the authors, unless otherwise indicated.

Fig. 2. Map of southern peninsular Thailand showing regional distribution of mainly clastic sedimentary rocks belonging to the Thung Yai Group (modified after Raksaskulwong,1994; Teerarungsigul, 1999).

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The prime objective of this paper is to contribute a detailedlithostratigraphy and paleontology of the Mesozoic sediments in theAo Luk–Plai Phraya (ALPP) basin, using simultaneously tectonic,paleo-environmental, lithostratigraphic and paleontological data.

2. General stratigraphic setting

Mesozoic rocks in southern peninsular Thailand were firstreported by Kobayashi and Tokuyama (1959) with the discovery ofTriassic marine bivalve, Halobia sp. Hayami (1960) described somebivalves in argillaceous sandstone at the Hadd Sai Ri beach, Pak NamChumphon Bay, Chumphon Province, and proposed a probableJurassic age for his Eomiodon chumphonensis Hayami, 1960.

The systematic geological mapping in southern Thailand at a scaleof 1:250,000 commenced as a project of the Department of MineralResources in 1989 with special emphasis on Mesozoic sediments.

Previously, Sanansiang (1977) had mapped the Mesozoic rocksexposed mostly in the Chumphon–Nakhon Si Thammarat and Trangareas as a single unit (Fig. 1). Subsequently, Asama et al. (1981)described six species of fossil plants from clastic strata of the Trangarea and found these to be different from those of the Khorat Groupin northeastern Thailand (Kon'no and Asama, 1973) but closelysimilar to those of the Early Cretaceous Gagau Group in the Malayanpeninsula (western Malaysia).

Raksaskulwong et al. (1989) established theMesozoic stratigraphyin the Thung Yai–Klong Thom area, recognizing five formations: SaiBon, Lam Thap, Khlong Min, Sam Chom, and Phun Phin in ascendingorder.

The Khlong Min Formation that yields the Jurassic fossils reportedin this paper, was originally regarded as a member of the Tertiary SinPun Formation, containing the bivalve Corbula njalindungensis (Martin1919), which is indicative for the Miocene (Beets, 1981). In the Bang

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Hian area, Krabi and Surat Thani Provinces, Tantiwanit et al. (1989)subdivided the Mesozoic sequence into two formations: Sai Bon andLam Thap. The Sai Bon Formation consistsmainly of red brown clasticsand dolomitic limestone with abundant foraminifera and bivalves,while the Lam Thap Formation is chiefly composed of red beds, arkosic

Fig. 3. Geologic map and cross-sections showing the distribution, ages and simple

sandstone. Zuoqi (1993) studied the palynology of the red-beds ofpeninsular Thailand and found a spore-pollen assemblage in sand-stone and shale beds that consists predominantly of gymnospermpollens, of Late Jurassic age. Further to the north, ammonite andbivalve fossils indicative of the Early Bajocian (Middle Jurassic) were

structures of major rock units in the Ao Luk–Plai Phraya area, Krabi Province.

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abundantly found in fine-grained clastic rocks of the Khao LakFormation, in the Chumphon Province, about 80 km north (Meesookand Grant-Mackie, 1994; Meesook et al., 2005, 2006a). The sameauthors also reported mudstone and sandstone of the Phra BatFormation in the Chian Yai and Hua Sai areas, Nakhon Si ThammaratProvince, were of Toarcian (Early Jurassic) age. Buffetaut et al. (1994)reported vertebrate fossils in Mesozoic non-marine clastic rocks atseveral localities in the peninsular Thailand and assigned these to theJurassic.

Raksaskulwong (1994) erected the Trang Group for the non-marine Mesozoic sequences exposed in the Chumphon to Trang areas(Fig. 2), subdividing it into the four formations of Chumphon redbed,Khlong Min, Khao Sam Chom and Phun Phin in ascending order.Teerarungsigul (1999)studying in detail the non-marine Mesozoicsediments in the Thung Yai–Klong Thom area, Nakhon Si Thammaratand Krabi Provinces, recognized within the Trang Group (Raksaskul-wong, 1994) the four formations of KhlongMin, Lam Thap, Sam Chom,and Phun Phin, in ascending order. The Khlong Min Formation

Fig. 4. Landsat TM5 image (A) and interpretation map (B) showing distribution of rock unKlong Marui Fault (KMF) is located in the upper left corner of the map.

overlies with an angular unconformity the marine Triassic Sai BonFormation that itself overlies unconformably the Paleozoic basement.However, Nakanart and Mantajit (1983) used the term “TrangFormation” for Middle Miocene laminated shale, changing laterallyto argillaceous limestone and grading to glauconitic siltstone, inpetroleum wells of the Andaman Sea.

In his revision of the lithostratigraphic nomenclature of non-marine Mesozoic rocks in peninsular Thailand, Raksaskulwong(2002), used the term “Thung Yai Group” that he subdivided intothe four formations Khlong Min, Lam Thap, Sam Chom, and Phun PhinFormations in ascending order.

3. Structure

A regional north–south strike of the Mesozoic and pre-Mesozoicstrata prevails between Chumphon and Trang (Fig. 2). However, northof Surat Thani, the northeast-trending reactivation of the Klong Maruistrike-slip fault (KMF in Figs. 1–4), with sinistral movement,may have

its, their major structures, faults and fractures. Note that the major northeast trending

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subsequently obscured this folding trend, as this fault also reaches thestudy area. Our regional reconnaissance survey also indicates thatother large lateral faults of similar trend, such as the Ranong Fault(RNF in Figs. 1 and 2) and the Surat Thani Fault (STF see Figs. 1 and 2),show left-lateral movements.

Kanjanapayont (2002) proposed a structural model of upperpeninsular Thailand, with special reference to the Krabi and SuratThain Provinces. For him the Mesozoic stratigraphic units form a largefirst-order syncline and four lower-order gentle synclines, transectedby strike-slip faults and fractures. These structures formed before thedeposition of the Tertiary sedimentary units. Since then, no majorfolding has taken place, except for the formation of fault-controlledTertiary basins.

Fig. 5. Composite stratigraphic column of Upper Paleozoic–Mesozoic rocks in the s

In the study area, the four gentle north-trending synclines ofKanjanapayont (2002) correspond to the Chumphon, Ta Pi, Krabi Eastand Trang synclines. The ALPP basin is located southwest of the Ta Pisyncline. Based on our regional field survey (Fig. 3) and stratigraphicinvestigation (Fig. 5) in conjunction with aerial photography andremote-sensing (Fig. 4), the major open syncline has beds dippingabout 20°–30° and extends from the Krabi Province in the south to theSurat Thani Province in the north. The general axial trend is north–southwith a gentle plunge (5°–15°) to the north. The result of remote-sensing reveals that most of the lineaments are northeast-southwest.

They mainly follow the northeast-trending Klong Marui Fault(see also Figs. 1–4). The minor joints are mainly north–south andnorthwest-southeast.

tudy area and its vicinity (modified after Poonpun, 2002) (U=unconformity).

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4. Lithostratigraphy

Sediments ranging from the Carboniferous to the Quaternary fillthe ALPP area (Fig.3). A composite lithologic columnar section showsthe rock units of the study area (Fig. 5).

The Upper Paleozoic consists of themore than 500 m greenish greypebbly mudstone and sandstone sequences of the Ko Yao NoiFormation that belongs to the Kaengkrachan Group. The group coversabout 30% of the total area mapped area, extends to the western,southern, and eastern parts of the study area. In the central partcharacteristic thinly bedded to massive fossiliferous limestones, rep-resent the Ratburi Group that yields fossil assemblages of predom-inantly corals, bivalves, brachiopods, bryozoans and foraminifers.

The Mesozoic succession includes the Sai Bon Formation andThung Yai Group.

Themore than 150 m-thick Sai Bon Formation consists of brown toreddish brown sandstone interbedded with siltstone, greyish mud-stone and greyish limestone with Triassic fossils, such as foraminifers,bivalves, brachiopods (Teerarungsigul, 1999).

The Thung Yai Group consists chiefly of about 700 m greyishbrown shales and reddish brown, fine-grained sandstones andconglomerates. The group is well exposed in the Wiang Sa area ofthe Surat Thani Province, the Thung Yai area of the Nakhon Si

Fig. 6. Stratigraphy of the Thung Yai Group. (A) Columnar section of the Khlong Min (lowKhanom). (B) Columnar section of the brackish Jurassic sedimentary rocks of the Khlong M

Thammarat Province, Klong Thom, Ao Luk and Plai Phraya areas of theKrabi Province andWang Vi Set area of the Trang Province. The age ofthe group is Late Early Jurassic to Cretaceous (Teerarungsigul, 1999;Raksaskulwong, 2002).

The marine sequence of the Khlong Min Formation, totalizes athickness of about 200 m. It is well exposed between km 8–25 of theKrabi–Khanom road (Highway no. 44), in particular at km 10+300 onthe western side of highway at Ban Khao Ngam, Ao Luk District, and atthe Huai Luk reservoir (see Figs. 3 and 5). The sequence includesgreenish grey, thin- to medium-bedded mudstone, intercalated withripple cross-laminated sandstone (Figs. 6, 7A, and B) and fossiliferouslimestone with common invertebrate and vertebrate fossils. Thestrata exposed at the Huai Luk reservoir (Fig. 6B), Plai Phraya District,are composed chiefly of greenish grey to reddish brown, thin- tothick-bedded mudstone intercalated with fossiliferous limestone,ripple cross-laminated sandstone (Fig. 7C) with abundant inverte-brate and plant remains.

The Lam Thap Formation is more than 100 m thick at Khao ChongMai Dam, east of Khao Hua Sing To and Huai Luk reservoir in the ALPParea of the Krabi and Surat Thani Provinces (Fig. 5). It comprisesmainly two lithofacies (Figs. 6A and 7D), the medium- to thick-bedded arkosic sandstones and intercalated siltstone with greyishmudstone. The mainly brown or reddish brown medium- to coarse-

er) and Lam Thap (upper) formations, at km 10+300 along Highway no. 44 (Krabi–in Formation at the Huai Luk reservoir, Plai Phraya District, Krabi Province.

Fig. 7. (A) Road-cut at km 10+300 on Highway no. 44 (Krabi–Khanom), Krabi Province showing west-dipping, dark grey mudstone (Md); specimen BH1 in sandstone (Ss) of theupper part of the Khlong Min Formation (KMF); unconformably overlain by sandstone of the Lam Thap Formation (LTF). (B) Close-up of a bedding plane of siltstone of the KhlongMin Formation that shows trace fossils and ripple marks with the paleocurrent to the west. (C) Open-pit outcrop of south-west dipping, thinly-bedded mudstone (Md) withspecimen BH2 interbedded with thickly bedded sandstone (Ss) and fossiliferous limestone (Ls) near the top of the KhlongMin Formation at Huai Luk reservoir. (D) Road-cut outcropof the Lam Thap Formation showing durable thickly-bedded arkosic sandstone (Ss) interbedded with less durable siltstone (St) at km 10+300 on Highway no. 44 (Krabi–Khanom),Ao-Luk District, Krabi Province.

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grained, subangular to subrounded sandstone of moderate sphericityconsists in general of 30% feldspar and 70% quartz grain with siliceouscements and iron oxide coating (Fig. 8).

5. Age constrains

Age constraints are provided by the stratigraphic and paleonto-logical data (Asama et al., 1981; Zuoqi, 1993; Buffetaut et al., 1994;Meesook, 1994; Teerarungsigul, 1999; Meesook et al., 2005; Meesooket al., 2006a) that confer early Middle Jurassic to Late Cretaceous agesto the Thung Yai Group.

The Khlong Min Formation yields the Jurassic bivalves P. donaienseMansuy, Astarte sp., Protocardia sp., Modiolus sp., gastropods, conchos-traceans, the palynomorphs Classopollis sp. and Dicheiopollis sp., woodfragments (Zuoqi, 1993) and non-marine vertebrates, hybodont sharks,lepidotes-like actinopterigians, lungfishes, temnospondyle amphibians,mesosuchian crocodilians and cryptodiran turtles (Buffetaut et al.,1994). Plant fossils, are also found in this area. These fossils assignan Early to early Middle Jurassic age to the KhlongMin Formation. Nearthe townshipof BanSai Thong, approximately 6 kmnorthof Chumphon,along the Klong Hua Wang Phranang Tak canal, a faunal assemblageof the bivalves Actinostreon sp., Protocardia sp., Myrene sp., Eomiodonsp., with the trace fossils Skolithos sp. and Thalassinoides sp. is abundantand similar to that of Ban Map Ching and the ALPP study area.

The Lam Thap Formation contains fern-like leaves such as Filicales,Bennettitales and Coniferales indicative of Early Cretaceous age (Asamaet al., 1981). The non-marine bivalve Unio cf. heilongjiangensis Sha andFuersich (1993) occurs in thinly bedded conglomeratic limestones.

The Sam Chom and Phun Phin formations are barren. A Cretaceousage is probable on the base of their stratigraphic position.

Based on paleontological data the age range of the Thung YaiGroup is late Early Jurassic to Early Cretaceous (Table 2).

6. Paleo-environments

6.1. Khlong Min Formation

In southern peninsular Thailand, particularly the ALPP study area,marine to brackish bivalves occur abundantly in dark grey claystonesand grey limestones of the Khlong Min Formation. The marinebivalves P. donaiense, Protocardia sp., Modiolus sp., Hudsonella sp.,Juranomia sp., Phraemytilus sp., Eomiodon cf. securiformis andActinostreon sp. occur in association with ostracods, palynomorphsand some vertebrates, suggesting shallow to brackish environments.The record of P. donaiense for the first time in southern peninsularThailand is of major importance, since this bivalve occurs abundantlyin northwestern Thailand (Kemper, 1976; von Braun and Jordan,1976; Charoenprawat et al., 1985; Beauvais, 1988, 1989; Fontaine andSuteethorn, 1988; Meesook, 1994; Kozai et al., 2006). Moreover, asthe species P. donaiense occurs in Vietnam (Vu et al., 1991) and inTibet (Yin and Grant-Mackie, 2005) the significance of this species isstratigraphical as well as paleogeographical, indicating the Toarcian–Aalenian over a large area. Other fossils encompassmarine to brackishbivalves (Fig. 9) from locality BH2 of the Khlong Min Formation in theALPP basin (Fig. 6B), occur in mudstone intercalated with sandstoneand fossiliferous limestone. These areModiolus sp. (D in Fig. 9), Astarte

Fig. 8. Parvamussium donaiense (Mansuy, 1914), from the mudstone of the Khlong Min Formation at km 10+300, Highway 44 (Krabi–Khanom), Ao Luk District, Krabi Province:(A) RV, external mould; (B) LV, internal mould; (C) Paratype D, RV, internal mould; (D) Paratype A, LV, external mould; (E) RV, internal and external mould; (F) LV, external mould.

Table 1Dimensions of Parvamussium donaiense measurements.

Specimen Valve Length (L) (mm) Height (H) (mm) L/H

Holotype Right valve 12.00 14.00 0.86Paratype A Left valve 6.00 8.50 0.71Paratype B Left valve 7.50 9.00 0.83Paratype C Right valve 7.00 8.50 0.82Paratype D Right valve 7.50 8.00 0.94Paratype E Left valve 6.50 7.50 0.87Paratype F Left valve 10.00 11.50 0.87Paratype G Left valve 7.00 8.50 0.82Paratype H Left valve 6.50 8.00 0.81Paratype I Right valve 7.50 9.00 0.83Paratype J Left valve 7.50 8.50 0.88

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sp. (C in Fig. 9), Protocardia sp. (A in Fig. 9), Hudsonella sp., Juranomiasp., Phraemytilus sp., Eomiodon cf. securiformis, Actinostreon sp.,Myrene sp., Trigonia sp. and a scleractinian coral (F in Fig. 9). Tracefossils are Skolithos sp. and Thalassinoides sp. Gastropods, ostracods,concostrachan, palynomorph, vertebrate hard parts (G in Fig. 9) andplant remains (H in Fig. 9), also occur in this area.

6.2. Lam Thap Formation

The Lam Thap Formation yields the non-marine bivalve Unio cf.heilongjiangensis, suggesting a Lower Cretaceous fluviatile or lacus-trine fresh water environment. This occurrence in southern Thailandenables the comparison of the Lam Thap Formation with the Sao KhuaFormation of the Khorat Group in northeastern Thailand (Meesooket al., 2006b). The same bivalve and associated non-marine mollusks

Table 2Bivalve biozonation of the Khlong Min and Lam Thap Formations, at the km 10+300along the Highway no. 44 (Krabi–Khanom) and Huai Luk reservoir, Ao Luk and PlaiPhraya area, Krabi Province.

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occur in South China (Li and Grant-Mackie, 1993), Korea (Chang et al.,2000; Kozai et al., 2005), and southwest Japan (Matsukawa et al.,2006). We therefore suggest that during Late Jurassic–Cretaceoustimes, Thailand, South China, Korea and Southwest Japan sharedsimilar paleoclimatic conditions.

For Teerarungsigul (1999) the Khlong Min and Lam Thapformations represent shallow marine to lagoonal environments,replaced later by meandering rivers, probably in semi-arid conditions.In western and northwestern Thailand shallow marine environmentsprevailed during the Early to early Middle Jurassic (Toarcian–EarlyBajocian) and non-marine fluvial to fluvio-lacustrine depositsdominated in northeast Thailand. The presence of similar faunal andfloral assemblages in the Khlong Min Formation, particularly in theALPP basin, indicates that the Early to early Middle Jurassic shallowseaway possibly extended to peninsular Thailand, eventually becom-ing non-marine in the Early Cretaceous. After the Early Cretaceous, thewhole of Thailand was covered with continental sediments as the redbeds of the Khorat Group in northeastern Thailand (Meesook et al.,2006a) and the Lam Thap, Sam Chom and Phun Phin formations inthe south (this study).

7. Tectonic setting

There is a wide consensus that both the Shan–Thai and Indochinaterranes were originally parts of Gondwana. After rifting, drifting andcollision, these terranes amalgamated to Eurasia before the end of theTriassic (Bunopas and Vella, 1978; Gatinsky et al., 1978; Macdonaldand Barr, 1978; Hahn, 1982; Chonglakmani, 1983; Sengor, 1984;Chonglakmani et al., 1985; Metcalfe, 1988, 1989; Charusiri, 1989;Hutchison, 1989; Burrett et al., 1990; Hada, 1990, and Panjasawat-wong, 1991; Hirsch et al., 2006). Eastward subduction of the oceaniclithosphere may perhaps have taken place in the easternmost part ofthe Shan–Thai terrane, and eastward oceanic subduction beneathIndochina may have taken place almost contemporaneously (Buno-

pas, 1981; Charusiri, 1989). Permian and Triassic radiolarian-bearingcherts in Malaysia support such collision tectonics, being part of theIndosinian orogeny (Metcalfe, 1988, 1989) and so does the contrast-ing I-type granite intrusion in Thailand (Charusiri, 1989) and thesinistral movement of major northeast-trending Ranong and KlongMarui strike-slip faults (Fig. 1) (Garson and Mitchell, 1970; Charusiriet al., 2002).

A cartoon (Fig. 10) displays the post-Permian plate tectonicreconstruction of southern Thailand. It suggests the complete closureof Paleotethys in western peninsular Thailand by Late Triassic times.Particularly in the ALPP study area, a limestone conglomerate un-conformably underlying the Sai Bon Formation (A in Fig. 10A)supports a sedimentological response to the tectonic warpingresulting from Early to Middle Triassic compression of Shan–Thai.The ingression of the shelf deposition of the Nathawi Formation(Grant-Mackie et al., 1980) on the eastern Shan–Thai side, suggeststhat down warping may have opened passages for the Paleotethysinto the Songkhla and Yala–Pattani basins (B and C in Fig. 10A). InLate Triassic to Early Jurassic times, strong collision with westwardup-thrusting along the Pattani suture (C in Fig. 10B), accompaniedby uplift in Khao Nam Kang (B in Fig. 10B), contributed to a regionalwestward tilting of the Shan–Thai block and the closure ofPaleotethys in the Songkhla and Yala–Pattani basins. The Yala andPin Yoe Granites (Fig. 10B and C) of S- and I-types, respectively,intruded most basins.

However, as tectonic tilting developed, a Mesotethys Sea filled theALPP depression with the marine clastic sediments of the Sai BonFormation, triggering renewed subsidence in the basin (A in Fig. 10B).Subsequently, progressive Middle to Late Jurassic tectonic upliftdeveloped as the result of on-going compressive tectonics by theapproach of the Western Burma block. In the mean time, extensiveerosion and denudation of the uplifted highlands generated wide-spread non-marine siliciclasts (e.g. Sadao Red Beds and Lam ThapFormation, A in Fig. 10D) that spilled over the uplifted basins. This hitthe ALPP basin of western peninsular Thailand only later, as marinedeposition continued in the clastic sequences of the Khlong MinFormation (Thung Yai Group) (Fig. 10D). Further uplift, occurring inthe Late Jurassic to Early Cretaceous, can explain the absence bydenudation of Upper Jurassic sequences in the study and nearby areas.Once the subduction of the oceanic plate of the Western Burma blockbeneath western Shan–Thai took place, Late Cretaceous–EarlyTertiary, S-type dominated, felsic magmatism occurred in peninsularThailand (Charusiri et al., 2002). Contemporaneous with progressivetectonic stress, compression of the ALPP and nearby basins formedlarge synclines and reactivated the sinistral movement along theRanong, Klong Marui, and Krabi Surat Thani Faults.

8. Conclusion

The 300 m thick clastic sequence of the Thung Yai Group extendsto the eastern Shan Thai terrane in peninsular Thailand, consisting ofthe Khlong Min and Lam Thap formations. The older 200 m thickKhlong Min Formation includes greenish grey, thin- to medium-bedded mudstone intercalated with ripple, cross-laminated sand-stone and fossiliferous limestone with common invertebrate andvertebrate fossils. The younger, more than 100 m-thick Lam ThapFormation comprises two lithofacies including medium- to thick-bedded arkosic sandstones and siltstones intercalated with greyishmudstones with common bivalves. The Thung Yai Group overliesunconformably clastic Triassic and underlies unconformably Tertiarysemi-consolidated deposits. Based on faunal and floral assemblages,marine, brackish to non-marine Mesozoic strata of the Thung YaiGroup in southern peninsular Thailand has an age range from lateLower Jurassic to Upper Cretaceous. Lithofacies and fossil assemblagesof the Khlong Min Formation point to lagoonal environments duringthe brief marine ingression of the Mesotethys from late Early to early

Fig. 9. Jurassic fossil assemblages of the Khlong Min Formation. (A) Protocardia sp.; (B) Trigonia sp.; (C) Astarte sp., (D) Modiolus sp., (E) Gastropod, (F) Scleractinian Coral,(G) vertebrate bone, (H) Plant remains.

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Fig. 10. Tectonic model illustrating the meso-tectonic stage during Middle Triassic to Early Cretaceous for southern peninsular Thailand (modified after Charusiri et al., 2002). NoteA = Ao Luk–Plai Phraya Basin, B = Songkhla Basin, C = Yala–Pattani Basin, ST = Shan–Thai Block, IC = Indochina Block, WB = Western Burma Block, KMF = Klong Marui Fault,STF = Surat Thani Fault.

58 W. Saengsrichan et al. / Gondwana Research 19 (2011) 47–60

Middle Jurassic, the depositional environment gradually changing tothe meandering river system of the Lam Thap Formation. Character-istic for the brief marine incursion in the Jurassic is the presence ofthe bivalves P. donaiense, Modiolus sp., and Astarte sp. in siltstonebeds of the Khlong Min Formation that witness a Mesotethyaningression during the Early and Middle Jurassic. Changes in theenvironment were due to subsequent compressive interaction withthe Western Burma block, resulting in the uplift of the western Shan–Thai terrane and the disappearance of the Upper Jurassic sequencein this area.

The lower and upper contacts of the Thung Yai Group with theunderlying clastic Triassic and the overlying semi-consolidatedTertiary deposits are unconformable.

Acknowledgements

We dedicate this paper to the memory of our beloved colleaguelate Wirote Saengsrichan. We owe grateful acknowledgements toChommada Srinuan for map preparation, report, and field investiga-tion, to Wattana Tansathian and Terapon Wongprayoon for fieldguidance and recommendations. We extend our sincere thanks toSurachet Poonpun, Pitak Thiamwong and Sakda Khundee for theirsupport with previous geological data and satellite image preparation.We also thank Jirapha Homhuek and Phanomrung Songchaeng forpreparing the manuscript. We thank Prasert Lekngam and ChamlongKetsathit for fieldwork. Last, but not least, we would like to thank theBureau of Geological Survey, Department of Mineral Resources and

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Department of Geology, Faculty of Science, Chulalongkorn Universityfor all nontechnical facilities. Parts of this research have beensupported by the Evolution of Mesozoic Biodiversity in Thailand(BRT R_245007) granted to AM and WS and by GISTDA, Thailandgranted to PC.

We are indebted to our reviewers Prof. J. Grant-Mackie (AucklandUniversity) and Ana Marquez-Aliaga (University of Valencia, Spain),for their constructive comments.

Appendix A. Parvamussium donaiense

The systematic classification and morphological terms used in thisstudy are those of Cox et al. (1969) for Bivalvia, modified after Waller(1978).

Phylum MOLLUSCAClass BIVALVIA Linne, 1758Subclass PTERIOMORPHA Beurlen, 1944Order OSTREOIDA Vaught, 1989Suborder PECTININA Vaught, 1989Superfamily PECTINACEA RafinesqueFamily PROPEAMUSSIIDAE Vaught, 1989Genus Parvamussium Sacco, 1987Subgenus Parvamussium s.s.Parvamussium donaiense Mansuy, 1914 (Fig. 8A–F)

Diagnosis

Small- to medium-size species of Parvamussium, characterizedby subequilateral outline, faint concentric plicae on right valve,weak radial riblets on left valve, and six to seven sharp straightinternal ribs which extend nearly to valve margin. Material: A totalof 11 specimens encompassing six complete, five incompleteinternal and external moulds and casts of the left and right valvesfrom locality BH1, from the Khlong Min Formation; locality BH1(Fig. 6A), grid reference 0476676E 0942352 N, Ban Mak quadrangle(4725I), along Highway no. 44 (km 10+300), Ao Luk District, KrabiProvince.

Description

Small- to medium-sized, rarely exceeding 14 mm in height,subcircular in outline, inequivalve, nearly equilateral, slightly convex,slightly higher than long. Test thin, apical angle exclusive of auricleabout 115°, beak not projecting above straight hinge line; auriclesunequal, anterior usually larger than posterior; byssal notch unde-veloped; surfaces of right valves generally smooth but with faintconcentric threads; left valves with weak spaced radial ribletsapproximately corresponding to internal ribs; interspaces of externalribs with numerous faint radial threads in some specimens but lackingany concentric ornament on left valves; 6–7 internal ribs very sharpand straight, extending from umbo nearly to valve margin, graduallystrengthening ventrally.

Dimensions

(Table 1, in mm): The average size is 7.7 mm length, 9.1 mmheight, and range of L/H ratio 0.85, but individual specimens mayreach 12 mm in length, 14 mm in height, with a L/H ratio of 0.85.

Occurrence

Common in dark grey mudstones, and rare in fossiliferouslimestone of the Khlong Min Formation at the Ban Khao Ngam,

western flank of road-cut outcrop on the Highway no. 44 (km 10+300), Ao Luk District, Krabi Province.

Age

Toarcian–Early Bajocian (late Lower–early Middle Jurassic).

Distribution

Parvamussium donaiense (Mansuy) occurs in the Pa Lan, Mae Sot,andUmphang areas of thewesternmost part of Thailand (Fontaine andSuteethorn, 1988; Kozai et al., 2006), Vietnam (Vu et al., 1991) andoutside Sundaland, the species was also identified in Tibet (Yinand Grant-Mackie, 2005).

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