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Late and Early to Middle radiolarians from the Hat Yai area, southern peninsular : Implications for the tectonic setting of the eastern margin of the Sibumasu Continental Block and closure timing of the Paleo-Tethys

Yoshihito Kamata, Akane Shirouzu, Katsumi Ueno, Apsorn Sardsud, Thasinee Chareontitirat, Punya Charusiri, Toshio Koike, Ken-ichiro Hisada

PII: S0377-8398(13)00101-1 DOI: doi: 10.1016/j.marmicro.2013.07.006 Reference: MARMIC 1489

To appear in: Marine Micropaleontology

Received date: 8 November 2012 Revised date: 8 July 2013 Accepted date: 18 July 2013

Please cite this article as: Kamata, Yoshihito, Shirouzu, Akane, Ueno, Katsumi, Sardsud, Apsorn, Chareontitirat, Thasinee, Charusiri, Punya, Koike, Toshio, Hisada, Ken-ichiro, Late Permian and Early to Middle Triassic radiolarians from the Hat Yai area, southern peninsular Thailand: Implications for the tectonic setting of the eastern margin of the Sibumasu Continental Block and closure timing of the Paleo-Tethys, Marine Micropale- ontology (2013), doi: 10.1016/j.marmicro.2013.07.006

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Late Permian and Early to Middle Triassic radiolarians from the Hat Yai area, southern peninsular Thailand: Implications for the tectonic setting of the eastern margin of the Sibumasu Continental Block and closure timing of the Paleo-Tethys.

Yoshihito Kamata 1, Akane Shirouzu 2, Katsumi Ueno 3, Apsorn Sardsud 4, Thasinee Chareontitirat 5, Punya Charusiri 5, Toshio Koike 6 and Ken-ichiro Hisada 1

1. Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8572, Japan 2. Department of Earth Sciences, Graduate School of Science & Engineering, Yamaguchi University, Yamaguchi, 753-8512, Japan 3. Department of Earth System Science, Fukuoka University, Fukuoka 814-0180, Japan 4. Bureau of Fossil Research and Geological Museum, Department of Mineral Resources, 10400, Thailand 5. Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand 6. Tokiwadai 36-6-606, Hodogaya-ku, Yokohama, Kanagawa Prefecture 240-0067, Japan.

Corresponding author Yoshihito Kamata Graduate School ofACCEPTED Life and Environmental Sciences,MANUSCRIPT University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8572, Japan TEL:+81-29-853-4302, FAX:+81-29-853-7887 E-mail:[email protected]

Keywords Radiolarian, Paleo-Tethys, Sibumasu, Indochina, Peninsular Thailand, Permian, Triassic.

Abstract ACCEPTED MANUSCRIPT

The fine clastic and siliceous sedimentary successions distributed in the Hat Yai area, southeastern peninsular Thailand, was examined using radiolarian biostratigraphy, lithology, and stratigraphy. The studied succession was essentially divided into two kinds of fine-grained sedimentary rock units: the lower unit and the upper chert unit. The lower shale unit and the upper chert unit yielded late Middle to early Late Permian and Early to Middle Triassic radiolarians, respectively. We clarified that the sedimentary rocks distributed in the study area are not entirely , but contain some sediments from the Permian and Triassic. This age determination suggests that the clastic-chert succession distributed in the study area should be distinguished from the Carboniferous Yaha Formation and correlated with the Permian to Triassic Semanggol Formation. Lithological change with increasing siliceous composition was observed in the uppermost part of the lower shale unit, and the geological age based on the radiolarians indicates a nearly conformable stratigraphic relationship for the lower shale unit and the upper chert unit. The stratigraphy and lithology of the Permian to Triassic succession in the study area, together with geological correlation around southernmost Thailand and northern , suggest that the Triassic chert should be interpreted as continental slope sediments overlying Permian clastic and/or calcareous facies, rather than typical pelagic deep-water sediments formed on an abyssal plain. The depositional environment of the chert in this area was likely restricted to the vicinity of a continental slope. Considering with the wide distribution of Triassic platform carbonates over southeastern peninsular Thailand, the continental margin of the Sibumasu along the Paleo-Tethys was represented by a stable passive margin during the Middle-early Late Triassic time. On the basis of the sedimentary setting and stratigraphy in the Permian and Triassic, it is suggested that the closure of the Paleo-Tethys between the Sibumasu and Indochina continental blocks took place at least after the Middle Triassic in southeastern peninsular Thailand. ACCEPTED MANUSCRIPT

1. Introduction In order to understand the geotectonic history of Thailand, several models of tectonic subdivisions have been proposed (e.g. Bunopas, 1981; Metcalfe, 1988, 1999; Barr and Macdonald, 1991; Ueno, 1999, 2002; Charusiri et al., 2002; Sone and Metcalfe, 2008; Ueno and Charoentitirat, 2011) based on and stratigraphy and micropaleontology, represented by foraminiferal and radiolarian biostratigraphy, paleobiogeography, the tectonic settings of granitoids, ultramafic rocks indicative of suture lines, and paleomagnetic data (Fig. 1A). These geotectonic interpretations and tectonic subdivisions in mainland Thailand have been refined by studies from northern Thailand,

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where Paleozoic to Mesozoic basement rocks of the Sibumasu Continental Block and oceanic rocks, such as radiolarian bedded chert and seamount-type carbonate rocks, are widely distributed. The geological map produced by the Department of Mineral Resources (DMR) of Thailand indicates that peninsular Thailand, which is south of the Three Pagodas Fault Zone, is occupied by Pre- and Paleozoic to Mesozoic basement rocks. Furthermore, most of the peninsula lies within the Sibumasu Continental Block, except for the southeastern tip east of the Benton-Raub Suture that is the boundary between the Sibumasu and Indochina continental blocks (Fig. 1B). Recently, Paleozoic strata in peninsular Thailand were well documented and summarized by Hansen and Wemmer (2011), Ridd (2009, 2011), and Ueno and Charoentitirat (2011). According to these studies, Carboniferous-Permian strata are widely distributed in the upper peninsula, which is north and west of the Khlong Marui Fault, but are less widespread in the lower peninsula southeast of the Khlong Marui Fault (Fig. 1B). The stratigraphy, lithology, and sedimentary environments of the Carboniferous-Permian Khuan Klang Formation, Kaeng Krachan Group, and Permian Ratburi were documented and discussed in these studies. These Carboniferous-Permian successions possess gross tectono-stratigraphic and faunal similarities to the basins of NW Australia and strongly suggest that the origin of the Sibumasu block was off the northwestern Australian continental margin of Gondwanaland (e.g., Metcalfe, 1988; Ridd, 2009). They have also discussed about the sedimentary environments of the Paleozoic sediments. For example, the Kaeng Krachan Group represents rift-filling sediments under a developing extensional fault system during the separation of Sibumasu from Gondwana (Ridd, 2009, 2011), and the Ratburi Limestone was ubiquitously deposited on the Sibumasu block to form a wide carbonate platform in the late Early to Late Permian (Ueno and Charoentitirat, 2011). In contrast, geologicalACCEPTED information (e.g. stratigra MANUSCRIPTphy and lithology) is scarce regarding the relatively deeper fine clastic and siliceous rocks that are mainly distributed in the southeastern part of the lower peninsula from Hat Yai in Changwat to the Bentong-Raub Suture (Fig. 1B). Carboniferous, Permian, and Triassic radiolarians were reported from bedded chert and siliceous rocks (Sashida et al., 2000, 2002; Kamata et al., 2008, 2009); however, the stratigraphy and depositional environment have not been fully established. In particular, the Triassic succession distributed in this area is crucial for correlation with the Permo-Triassic Semanggol Formation, which is widely distributed in northern peninsular Malaysia, and to understand the tectonic setting of the eastern margin of the Sibumasu Continental Block and the collision and closure time of the Sibumasu and Indochina continental blocks.

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In the course of mapping the 1:50,000 Hat Yai Quadrangle (Sardsud and Saengsrichan, 2002), directed by the Geological Survey Division, Department of Mineral Resources (DMR), Thailand, late Middle Permian and Early to Middle Triassic radiolarians were detected from the fine clastic (shale) to chert facies distributed in the Hat Yai area, Changwat Songkhla, lower peninsular Thailand (Fig. 1C). This paper describes the lithology of radiolaria-bearing sedimentary rocks and age determination by radiolarian fauna. We also discusses the sedimentary environment of radiolaria-bearing facies, the tectonics of these facies, the tectonic setting of the eastern margin of the Sibumasu Continental Block, and the closure of the Paleo-Tethys in lower peninsular Thailand.

2. Geological outline of lower peninsular Thailand According to the DMR geological map, lower peninsular Thailand is widely covered by sediments. Paleozoic-Mesozoic basement rocks in this area are generally concealed by these younger sediments and crop out only in the form of scattered monadnocks. Carboniferous-Permian siliciclastic facies of the Yaha Formation, Khuan Klang Formation, and Kaeng Krachan Group and the Permian Ratburi limestone are broadly scattered in lower peninsular Thailand with Lower Paleozoic to sedimentary rocks (Fig. 1B). Among these, the Kaeng Krachan Group (first introduced as the Kaeng Krachan Formation by Javanaphet, 1969) represents a southeastern extension of the upper peninsula, which contains rift-filling sediments of a fault system from the separation of Sibumasu from Gondwana (Ridd, 2009, 2011). This group consists of various siliciclastics and is characterized by common intervals of diamictite (pebbly mudstone) with dropstones and dump structures in mudstone facies, indicating a glacially influenced environment. The Ratburi Formation, which Bunopas (1981) restricted to Permian carbonates in western and peninsular Thailand,ACCEPTED was originally introduced MANUSCRIPT by Brown et al. (1951) and consists mainly of massive to bedded limestone. Limestone in this group is mainly distributed in the northwestern part of lower peninsular Thailand around the Changwat Krabi and Surat Thai areas. In southeastern lower peninsular Thailand, equivalents of the Ratburi Limestone only have a small distribution in southern Changwat Yala (Fig. 1B). The Yaha Formation, introduced by Muenlek et al. (1985) for the Carboniferous succession distributed in the easternmost area of the lower peninsula, consists mainly of siliciclastic shale, medium- to coarse-grained sandstone, siliceous shale, chert, and conglomerates. The depositional age of the formation is substantiated by the occurrence of a late Tournaisian conodont from a chert and siliceous shale succession at Ko Yo in (Igo, 1973). Although Muenlek et al. (1985) correlated the Yaha Formation with the upper part of the Kaeng Krachan Group, the lithostratigraphy and geological age of the formation are very

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controversial. According to the DMR map (1999), studied succession in this study is assigned to the Yaha Formation (Fig. 1C). As pointed out by Ridd (2009), the Carboniferous-Permian Kaeng Krachan Formation, partly represented by a glacially influenced facies, is not recognized east of Hat Yai (study area in this paper) in Songkhla Province. East of the Hat Yai area, some radiolarian occurrences were reported. An Early Carboniferous (Tournaisian) fauna was reported from Na Thawi in Songkhla Province (Sashida et al., 2000) and the Saba Yoi-Kabang area near the Songkhla-Yala provincial boundary (Sashida et al., 2002). The exposed succession in the latter study was assigned to the Yala Formation based on the DMR map (1999). Late Permian and Middle Triassic radiolarians have also been reported from two areas that were mapped as the Yaha Formation by the DMR (1999): the Chana area (Sashida et al. 2000) and the Rattaphum area (Sardsud and Saengsrichan, 2002; Kamata et al. 2008, 2009), both in Songkhla Province. It should be noted that the Permo-Triassic calcareous and siliciclastic-siliceous succession is distributed both in southeastern peninsular Thailand and northern peninsular Malaysia. The Permian to Triassic platform-type limestone successions include the Chuping and Kodiang (Perlis Province, northern Malaysia), the Khlong Kon Limestone (Saba Yoi, western Songkhla Province), and the Chaiburi Formation (Rattaphun, northwest Songkhla Province). The Chuping and Kodiang limestones have been studied using conodont biostratigraphy and found to range from the Late Permian to Late Triassic in age (Koike, 1973, 1982; Metcalfe, 1981, 1990, 1992). Late Triassic (late Carnian to middle ) radiolarians have also been obtained from the Kodiang limestone (Basir et al., 1995). The Khlong Kon limestone yielded Middle to Late Triassic foraminiferous fauna (Sashida et al., 1999). The Chaiburi Formation ranges from the Early to Late Triassic in age based on conodonts (Ampornmaha,ACCEPTED 1995) and radiolarians MANUSCRIPT (Sashida and Igo, 1992). The Permian to Triassic siliciclastic-siliceous succession is distributed across the border with the Semanggol Formation in Kedah Province in northern Malaysia, the Na Thawi Formation in Saba Yoi in western Songkhla Province, and a chert succession with fine clastics in Hat Yai in Songkhla Province (this study). The Semanggol Formation is subdivided into three members: lower chert, middle rhythmite, and upper conglomerate (Burton, 1973). Middle and Late Permian radiolarians were reported from the lower member (Sashida et al., 1995). The middle member consists of rhythmically intercalated beds of sandstone and shale and has been correlated with the Halobia-Daonella facies yielding Triassic pelecypods (Kobayashi, 1963). The occurrence of late Early Permian to Middle Triassic radiolarians from the lower chert member was also reported (Basir, 1997; Basir and Zaiton, 2007). In the Na Thawi and Saba Yoi areas in western Songkhla Province, Early Triassic and Middle

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Triassic radiolarians were reported from siliceous shale and bedded chert (Sashida et al., 2002). Sashida et al. (1999, 2000) summarized the sedimentary environments of the above-mentioned limestone-clastic-siliceous rock association, which resulted from a change from shallow to deep environments in the Sibumasu margin.

3. Lithology of the study sections The study sections are located about 20 km west of Hat Yai town in Changwat Songkhla, lower peninsular Thailand (Fig. 1C) and contain various kinds of clastic rocks consisting of sandstone, shale, siltstone, well-bedded siliceous shale, and bedded chert. These sedimentary rocks have broadly northward to northwestward strikes and dip to the east at various angles. The clastic and siliceous sediments were assigned to the Yaha Formation of Carboniferous age without the occurrence of good index fossils in previous geological maps (DMR, 1999). The rocks of the six studied sections (HY-01, HY-02, HY-03, HY-04, HY-05, and HY-21) were essentially divided into two fine-grained sedimentary rock units: the lower shale unit and upper bedded-chert unit (Fig. 2). The lower unit consists the alternation of yellowish-brown silty shale, brown shale, and black or brown shale with calcareous nodules. The yellowish-brown, silty shale is about 10 cm to 30 cm in thickness and shows several kinds of sedimentary structures, such as parallel and crossed lamination (Fig. 3A). Some layers of the silty shale contain many calcareous bioclasts, such as foraminiferal tests (Fig. 3B). The dark-colored and black shale with calcareous nodules is probably equivalent stratigraphically to the Permian ammonoid-bearing shale reported by Fujikawa et al. (2005) from the Phatthalung-Hat Yai area. It is important to note that the uppermost part of the shale succession becomes more siliceousACCEPTED and often contains intercala MANUSCRIPTted chert layers that are very similar in lithology to those in the upper bedded chert unit (Fig. 2). The bedded chert in the upper unit is composed of with generally 2–10-cm-thick chert beds alternating with claystone a few millimeters thick (Fig. 3C). It is brown or milky white and is less glassy and transparent than typical pelagic chert, such as the Paleo-Tethyan Fang Chert in northern Thailand (Kamata et al., 2009). The polished surface of a chert slab shows distinct laminations (Fig. 3D). Petrographically, the matrix consists of clay minerals with microcrystalline quartz and contains extremely fine, opaque organic matter (Figs. 3E and 3F). Radiolarians, which are scattered throughout the matrix (Fig. 3E), are considerably less densely distributed than in typical pelagic chert. The studied bedded chert contains not only siliceous organisms, such as radiolarians and sponge spicules, but also calcareous biota, such as foraminifers, ostracods, and thin-shelled bivalves

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(Figs. 3G and 3H). This fossil content is also quite different from that of typical pelagic chert.

4. Radiolarians and age determination Radiolarians of the late Middle Permian were obtained from one horizon (HY05-04 in Fig. 2, and Table 1) of slightly silicified shale from the middle part of the lower shale unit. The upper chert unit contains moderately to well-preserved Early to Middle Triassic radiolarians and these are divided into five assemblages (Fig. 2, Plates 1–4, and Table 1). Permian radiolarian zonations have been established by biostratigraphic examination of the European Tethys (e.g. Kozur and Mostler, 1989), North America (Blome and Reed, 1992), China (Wang et al. 1994), and SW Japan (Ishiga, 1986, 1990; Kuwahara et al., 1998, 1999). Additionally, Triassic radiolarians had done the first systematic description by Nakaseko and Nishimura (1979), and their zonation have been proposed by Yao (1982), Yoshida (1986), Carter (1993), Bragin (1991), Kozur and Mostler (1994, 1996), and Sugiyama (1992, 1997). Recently, O’Dogherty et al. (2009) reviewed Triassic radiolarian genera and later cataloged their stratigraphic ranges (O’Dogherty et al., 2011). Early to Middle Triassic radiolarian biostratigraphy is well established by reports mainly from SW Japan (e.g. Sashida, 1983, 1991; Kamata, 1995, 1999; Sugiyama, 1992, 1997; Kusunoki and Imoto, 1996), Thailand (Sashida and Igo, 1992), Turkey (Kozur et al. 1996), and New Zealand in the Southern Hemisphere (e.g. Takemura et al. 2007; Kamata et al. 2007; Yamakita et al. 2007). The age determination of radiolarians from the study section referred to the above-mentioned biostratigraphic studies.

4-1. Permian radiolarians A monotonous radiolarianACCEPTED fauna was obtained fromMANUSCRIPT a horizon in the middle part of the lower shale unit. This fauna consists of several specimens of Follicucullus scholasticus Ormiston and Babcock. Follicucullus scholasticus was first described by Ormiston and Babcok (1979) from Guadalupian rocks in west Texas, USA, and is well known as a species with worldwide occurrence. This species is the representative species of the F. scholasticus zone (Ishiga, 1986, 1990) and ranges to the lower part of the Neoalbaillella ornithoformis zone (upper Capitanian to Wuchiapingian) (Ishiga, 1990; Kuwahara et al. 1998). In Thailand, the occurrence of F. scholasticus has been reported from eastern Thailand (Sashida and Igo, 1999), northern Thailand (e.g. Caridroit, 1993; Kamata et al., 2012), and northwestern Thailand (Wonganan and Caridroit, 2006). Sashida and Igo (1999) assigned the F. scholasticus assemblage to the late Middle to earliest Late Permian (Capitanian to Wuchiapingian). The present fauna is not directly correlated with the F. scholasticus zone (Ishiga, 1990) or F.

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scholasticus assemblage (Sashida and Igo, 1999) because of its monotonous faunal content. However, the biostratigraphic range of this species suggests that the present fauna could correspond to part of the late Middle to early Late Permian (Capitanian to Wuchiapingian).

4-2. Early to Middle Triassic radiolarians The upper chert unit yielded Early to Middle Triassic radiolarians from 49 horizons (Plates 1–4, Table 1). These radiolarians are divided into five assemblages (Fig. 2). Representative radiolarians of the assemblages are as follows: Assemblage 1 : This assemblage is characterized by the occurrence of Follicucullus scholasticus Ormiston and Babcock associated with poorly preserved bi-polar specimens of Spumellaria and spherical radiolarians. Follicucullus is a representative genus of the Middle to Late Permian; however, some occurrences in the Lower Triassic succession have been reported by Sugiyama (1992, 1997), Bragin (1991), and Takemura et al. (2007). Considering the coexistence of bi-polar Spumellaria, which are similar to those of the Lower Triassic, particularly the late Olenekian (Spathian) succession, as well as the absence of Permian-type Entactinarians and Latentifistularians this fauna resembles that of TR0 and TR1 of the late Olenekian (Spathian) (Sugiyama, 1997). The present fauna does not contain the common spicule-type genera Parentactinia and Archeosemantis of TR0 and TR1, but it is here tentatively and roughly correlated to TR0 and TR1. Assemblage 2 : This assemblage is characterized by the common occurrence of Entactinia cf. reticulata Sashida and Tonishi, Entactinosphaera chiakensis Sashida and Igo, Pantanellium sp., Acaeniospongus ? sp., Parasepsagon tetracanthus Dumitrica, Kozur and Mostler, Plafkerium sp., Tiborella sp., Eptingium nakasekoi Kozur and Mostler, and E. manfredi Dumitrica. This assemblage also contains smallACCEPTED amounts of the segmented MANUSCRIPT Nassellaria, Hozmadia gifuensis Sugiyama, H. reticulata Dumitrica, Kozur and Mostle r, Anisicyrtis sp., and Spongosilicarmiger sp. Species belonging to Entactinaria , Eptingium , Pantanellium , and Hozmadia are diagnostic of the upper part of the Hozmadia gifuensis assemblage (Sugiyama, 1992) and TR2A (Sugiyama, 1997). Entactinosphaera chakensis is reported from the latest Olenekian to early Anisian (latest Spathian to early Anisian) bedded limestone at Phattalung, southern peninsular Thailand, which is 70 km northwest of the study section (Sashida and Igo, 1992). The first occurrence of E. nakasekoi defines the base of TR2A, and the present fauna is similar to that of TR2A. Considering the lack of multisegmented nassellaria of the genus Triassocampe , this assemblage should be correlated to TR2A.

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Assemblage 3 : This assemblage is characterized by the abundant occurrence of Triassocampe and collateral occurrence of Eptingium and Pseudostylosphaera . Triassocampe coronata Bragin, Triassocampe sp., Eptingium manfredi Dumitrica, E. nakasekoi Kozur and Mostler, Pseudostylosphaera japonica (Nakseko and Nishimura), Cryptostephanidium sp., Parasepsagon sp., and Pentactinocarpus sp. were also obtained and this faunal content resembles that of TR2B, of which the base is defined by the first occurrence of T. coronata and Pseudostylosphaera japonica . Assemblage 4 : Diagnostic species in this assemblage are Pseudostylosphaera japonica (Nakaseko and Nishimura), P. spinulosa (Nakaseko and Nishimura), Triassocampe deweveri (Nakaseko and Nishimura), T. coronata Bragin, and Cryptostephanidium cornigerum Dumitrica et al. This fauna is characterized by the occurrence of T. deweveri and the abundant and diverse genus Pseudostylosphaera . This fauna resembles TR2C, of which the base is defined by the first occurrence of T. deweveri . Assemblage 5 : This assemblage contains Triassocampe deweveri (Nakaseko and Nishimura), T. campanilis (Kozur and Mostler), Pseudostylosphaera japonica (Nakaseko and Nishimura), P. spinulosa (Nakaseko and Nishimura), Cryptostephanidium cornigerum Dumitrica, and Spine A2 of Sugiyama (1997). This fauna resembles that of Assemblage 4 but is distinguished by the occurrence of Spine A2. The first occurrence of Spine A2 defines the base of zone TR3A of Sugiyama (1997). On the basis of the faunal similarity and the occurrence of Spine A2, the present fauna can be correlated with TR3A. In summary, five assemblages (1 to 5) from the upper chert unit in this study correspond to TR0 or TR1, TR2A, TR2B, TR2C, and TR3 of Sugiyama (1997), respectively, indicating that the unit is Early to Middle Triassic, late Olenekian (late Spathian) to Anisian in age. ACCEPTED MANUSCRIPT 5. Discussion 5.1 Stratigraphy The sedimentary succession examined in this study had been assigned to the Carboniferous Yaha Formation (DMR, 1999). However, radiolarian biostratigraphy has clarified that the age of the fine clastic and siliceous sedimentary rocks distributed in the Hat Yai area are not entirely Carboniferous, but some part is Permian and Triassic age. The geological age of the Yaha Formation was substantiated by the occurrence of late Mississippian conodonts from a chert and siliceous succession at Ko Yo in Songkhla Province (Igo, 1973). The occurrence of Permian and Triassic radiolarians from study sections indicates that the siliceous succession in the Hat Yai area should be distinguished from the Carboniferous Yaha Formation.

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According to Igo (1973), conodont-bearing Carboniferous chert overlies fissile shale and underlies bedded coarse-grained quartzitic sandstone. In addition to conodonts, Carboniferous radiolarians were reported from a similar succession from Songkhla (Sashida et al., 2002). The radiolarian-bearing bedded chert conformably overlies thick-bedded sandstone layers in the Kabang area, Songkhla (Sashida et al., 2002). The stratigraphic units of the Yaha Formation should be restricted to the Carboniferous sequence consisting of chert (siliceous shale) and coarse-grained quartzitic sandstone because the lithic features of the study sections are very different from these Carboniferous successions. Sashida et al. (2000) reported the occurrence of Mississippian radiolarians from siliceous shale blocks within strongly sheared alternating beds of sandstone and shale in Na Thawi, Songkhla Province. As already pointed out by Sashida et al. (2000), the radiolaria-bearing siliceous shale blocks were thought to be derived from an equivalent of the Yaha Formation, and the texture of the blocks within the sheared beds could be due to sedimentary or tectonic mélange-forming processes related to Paleo-Tethyan subduction and the following collision of the continents. As mentioned above in the lithologic section, the uppermost part of the shale succession becomes more siliceous and often contains intercalated chert layers that are very similar in lithology to those in the bedded chert unit (Fig. 2). Although fossil evidence for the uppermost Permian and lowest Triassic ages has yet to be obtained from the study sections, this transitional lithological change in the upper part of the lower shale unit and the geological age determined by radiolarians indicates that the upper chert unit (Early to Middle Triassic) was conformably deposited on the lower shale unit (late Middle Permian).

5.2 Geological correlationACCEPTED MANUSCRIPT In addition to the study (Hat Yai) area, the southeastern part of peninsular Thailand and northern Malaysia also contain Permian to Triassic radiolaria-bearing siliceous rocks. The Semanggol Formation is roughly divided into the lower chert, middle rhythmite, and upper conglomerate members and is widely distributed in the Perlis and Kedah Provinces of northern Malaysia (Figs. 1B and 4). The occurrence of Early Permian to Late Triassic radiolarians was reported from both bedded chert and siliceous limestone (e.g. Sashida et al., 1995; Basir, 1994, 1995; Basir et al., 1995). Basir (1995) reported the occurrence of Middle Triassic radiolarians from the bedded chert of the middle member. On the basis of geological age and lithology, the upper chert unit in the study sections can be correlated with the Middle Triassic chert of the middle member of the Semanggol Formation (Fig. 4). The middle rhythmite member is represented by bedded chert and intercalated

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sandstone and shale yielding pelecypods. This succession containing pelecypods is equivalent to the Daonella-Halobia facies (Kobayashi, 1963) and its extension is also recognized in Saba Yoi, Songkhla between Hat Yai (study area) and Kedah Province, Malaysia (Figs. 2 and 4). Recently, in Pattalung and Hat Yai northeast of the study area, some karst-forming carbonates were biostratigraphically examined. These carbonates had been assigned to the Permian Rutburi limestone (e.g. DMR, 1999). However, conodont, foraminifera, and radiolarian studies have clarified that the carbonate is not entirely Permian but also partially Early to Late Triassic (Ampornmaha, 1995; Sashida and Igo, 1992; Ueno et al., 2003). Lithofacies and foraminifera content of these sediments are correlated with the Khlong Kon Limestone in Saba Yoi and the upper part of the Chuping Limestone and Kodiang Limestone in peninsular Malaysia (Ueno et al., 2003). These limestone lithofacies indicate that a large-scale carbonate platform developed in the Permian to Triassic in this area (Fontaine et al., 1988, 1993; Sashida et al., 1999; Ueno et al., 2003). The above-mentioned correlation of the facies indicates that three geological units such as a carbonate platform of Permian to Triassic, a Daonella-Hallovia facies represented by intercalated fine clastics, and a radiolaria-bearing bedded chert in southernmost Thailand extend to northern Malaysia (Fig. 4).

6. Tectonic setting and closure time of the Paleo-Tethys in southern peninsular Thailand In addition to the siliceous fossils of radiolarians and sponge spicules, the Triassic chert in the study sections contains calcareous foraminiferal tests, ostracods, and planktonic bivalves (Figs. 3E-3H). It is nearly conformable with the underlying Permian shale unit. This stratigraphic relationship, together with its fossil contents, suggests that the Triassic chert is not pelagic deep-water sediment formed on an abyssalACCEPTED plain underlain by oceanic MANUSCRIPT crust consisting of basaltic rocks. Instead, it is better interpreted as continental slope sediments overlying of Permian clastic and/or calcareous facies (Fig. 5A). The depositional environment of the chert in this area was likely restricted to the vicinity of a continental slope and rise (Kamata et al., 2008, 2009). Taking the wide distributions of Triassic platform carbonates over southern peninsular Thailand (Ampornmaha 1995; Sashida et al. 1999; Ueno et al. 2003) into account, the continental margin of the Sibumasu Block along the Paleo-Tethys is represented by a stable passive margin without much tectonic activity, at least during the Middle-Early Late Triassic (until the Carnian) (Fig. 5A). The coeval Triassic chert and carbonates are now distributed rather closely in the same area of southern peninsular Thailand, but their original depositional sites would have been considerably separated horizontally (~several hundred kilometers). This fact further suggests that considerable crustal

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shortening took place after the deposition of these Triassic sediments (Fig. 5B). This event could be related to the collision of the Sibumasu and East Malaya (Indochina) blocks and the closure of the Paleo-Tethys. On the basis of the sedimentary setting of the chert lithology, faunal content, and stratigraphy with geological ages based on radiolarians, it is suggested that the closure of the Paleo-Tethys between the Sibumasu and Indochina blocks took place at least after the Middle Triassic in southern peninsular Thailand. This timing is very consistent with what has been concluded based on data from mainland Thailand (e.g. Metcalfe, 2000; Ueno and Hisada, 1999).

7. Conclusion (1) Fine clastic and siliceous sedimentary rocks distributed in the Hat Yai area, southeastern peninsular Thailand, have been examined by radiolarian biostratigraphy, lithology, and stratigraphy. They are divided into two kinds of fine-grained sedimentary rock units: the lower shale unit and the upper chert unit. (2) Late Middle to early Late Permian and Early to Middle Triassic radiolarians were identified from the lower shale unit and the upper chert unit, respectively. This age determination suggests that the clastic-chert succession in the Hat Yai area should be distinguished from the Carboniferous Yaha Formation and correlated with the Permo-Triassic Semanggol Formation. (3) Lithological changes of increasing siliceous composition were observed in the uppermost part of the lower shale unit, and geological age based on radiolarians indicates a nearly conformable stratigraphic relationship between the lower shale unit and the upper chert unit. (4) The lithology and stratigraphy of the Permo-Triassic in the study area, together with geological correlation around southernmostACCEPTED Thailand and MANUSCRIPTnorthern Malaysia, suggest that the Triassic chert is not typical pelagic deep-water sediment formed on an abyssal plain. Instead, it is better interpreted as continental slope sediments overlying Permian clastic and/or calcareous facies. The depositional environment of the chert in this area was likely restricted to the vicinity of a continental slope and rise. (5) Considering the wide distribution of the Triassic platform carbonates over southern peninsular Thailand, the continental margin of the Sibumasu Continental Block along the Paleo-Tethys was represented by a stable passive margin without much tectonic activity, at least during the Middle to early Late Triassic. (6) The sedimentary setting and stratigraphy of the Permo-Triassic suggest that the closure of the Paleo-Tethys between the Sibumasu and Indochina blocks took place at least after the Middle

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Triassic in southern peninsular Thailand.

Acknowledgements The first author (Y.K.) expresses thanks to the guest editor (Dr. Cortese) and two anonymous reviewers for their critical review of the manuscript. This work was supported in part by Grants-in-aid (No. 15403013 and 16740293) from the Japanese Ministry of Education, Culture, Sports, Science and Technology. This research is part of the IGCP516 and 589 Projects.

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Captions of figures Fig. 1. (A) Outline map showing the tectonic subdivision of mainland and peninsular Thailand, (B) outline geological map of peninsular Thailand indicating the location of the study area, and (C) index map of Hat Yai, Songkhla Province, showing the distribution of the Yaha and Kaeng Krachan formations and the location of the study sections.

Fig. 2. Stratigraphic columns of the study sections in the Hat Yai area, Songkhla, peninsular Thailand. Composite columns show the lithologic composition of the lower shale unit and the upper chert unit and the horizons of radiolaria-bearing samples. The right-hand column presents a summarized stratigraphy of the study sections. Radiolarian zonations are based on Ishiga, (1986, 1990), Kuwahara (1999), Kuwahara and Sakamoto (1992), Kuwahara et al. (1998), Yao (1982), and Sugiyama (1992, 1997).

Fig. 3. Photographs showing the lithology of studied clastic and siliceous rocks. (A) Permian clastics of the lower shale unit at section HY05 showing cross-lamination and parallel lamination. (B) Photomicrograph of the Permian clastics of the lower shale unit at section HY05 (Note: silty shale consists mainly of calcareous bioclasts, and a foraminiferal test is present in the center of the image). (C) Photograph of the outcrops at section HY01 showing the occurrence of radiolaria-bearing bedded chert. (D) Polished slab surface of radiolaria-bearing chert showing distinct lamination and a lithology that is less glassy and transparent than typical pelagic chert. (E) Photomicrograph of the chert showing scattered radiolarian tests within a clay-rich matrix. (F) Enlargement of the chert matrix showing tiny opaque fragments (organic fragments) and radiolarian tests. (G) Photomicrograph ofACCEPTED the radiolaria-bearing chert MANUSCRIPT shows the occurrence of thin bivalve shells and (H) a foraminiferal test.

Fig. 4. Stratigraphic columns showing the ranges of depositional ages of the three Permo-Triassic facies (platform carbonate, Daonella-Halobia facies of intercalated sandstone and shale and radiolaria-bearing bedded chert) distributed in southernmost peninsular Thailand and northernmost peninsular Malaysia.

Fig. 5. (A) Schematic reconstruction of the sedimentary environment showing the lateral lithological distribution of platform carbonate, fine clastics, and chert or siliceous sediments in the eastern Sibumasu Continental Block in the Middle Triassic. (B) Present coeval distribution of Triassic chert

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and carbonates suggests that crustal shortening took place after deposition and the event could be related to the collision of the Sibumasu and East Malaya (Indochina) blocks. Table 1. List of radiolarians from study sections in the Hat Yai area, Songkhla, peninsular Thailand.

Plate I. 1-6. Follicucullus schlasticus Ormiston and Babcock, HY05-04. 7. Follicucullus scholasticus Ormiston and Babcock, HY02-102. 8-11. Spumellaria gen. et sp. indet. 8. HY02-103: 9. HY02-103: 10. HY02-103: 11. HY02-103. 12-13 ，15-16 Entactinosphaera ? sp. 12. HY02-103: 13: HY02-103: 15. HY02-103: 16. HY02-106 14. Eptingiidae, HY02-103. 17. Copicyntra ? sp. HY02-106 18. Triassospongosphaera ? sp. HY02-106 19-20. Acaeniospongus ? sp. 19. HY02-113: 20. HY02-130. 21-22. Copiellintra cf. akikawaensis Sashida and Tonishi, 20. HY02-125: 21. HY02-126. 23-24. Copiellintra sp. A, 23. HY02-130: 24. HY02-100. 25. Copiellintra sp. B, HY02-100. 26-27. Entactinia ? sp. 26. HY02+100: 27. HY02-100. 28-29. Entactinia cf. reticulata Sadhida and Tonishi, 28. HY02-100: 29. HY02-130. 30-31. Entactinosphaera chiakensis Sashida and Igo, 30. HY02-118: 31. HY02-123. 32-33. Entactinosphaera sp. B. 32. HY02-118: 33. HY-02-127.

Plate II. 1-3. EntactinosphaeraACCEPTED sp. C. 1. HY02-123: 2. HY02-125:MANUSCRIPT 3. HY02-125. 4-5. Entactinosphaera ? crassispinosa Sashida and Tonishi. 4. HY02-133: 5. HY02-135. 6-8. Entactinosphaera sp. A. 6. HY02-133: 7. HY02-126: 8. HY02-133. 9-10. Pantanellium sp. A. 9. HY02-128: 10. HY02-130. 11-12. Parasepsagon sp. 11. HY02-118: 12. HY02-129. 13-14. Parasepsagon tetracanthus Dumitrica, Kozur and Mostler, 13. HY02-133: 14. HY02-123. 15, 20. Plafkerium sp. 15. HY02-119: 20. HY02-129. 16-17. Triassospongosphaera sp. 16. HY02-118: 17. HY02-125. 18-19. Tiborella sp. 18. HY02-123: 19. HY02-126. 21-23. Eptingium nakasekoi Kozur and Mostler, 21. HY02-129: 22. HY02-121: 23. HY02-125.

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24-25. Eptingium manfredi Dumitrica, 124. HY02-120: 25. HY02-119. 26. Hozmadia gifuensis Sugiyama, HY02-113. 27. Hozmadia reticulata , Dumitrica, Kozur and Mostler. HY02-130. 28. Hozmadia sp. HY02-130. 29. Anisicyrtis sp. HY02-129. 30. Spongosilicarmiger sp. HY02-127.

Plate III. 1-6, 13. Triassocampe coronata Bragin, 1. HY01-03: 2. HY01-03: 3. HY01-05: 4. HY21-01: 5. HY01-05: 6. HY01-05: 13. HY01-05. 7-12, 14-16. Triassocampe sp. 7. HY21-01: 8. HY01-05: 9. HY01-05: 10. HY01-05: 11. HY01-03: 12. HY01-05: 14. HY01-01: 15. HY21-01: 16. HY01-03. 17. Pseudostylosphaera spinulosa (Nakaseko and Nishimura). HY21-01. 18, 20, 22. Eptingium sp. 18. HY21-01: 20. HY21-01, 22. HY21-01. 19, 21. Parasepsagon sp. 19. HY21-01: 21. HY01-05. 23. Eptingium nakasekoi Kozur and Mostler, HY21-01. 24. Pseudostylosphaera japonica , (Nakaseko and Nishimura) , 24. HY01-01. 25-28, 38. Pseudostylosphaera japonica (Nakaseko and Nishimura), 25. HY21-03: 26. HY21-03: 27. HY21-03: 28. HY04-03: 38. HY04-01. 29. Pseudostylosphaera sp. HY21-03. 30. Pseudostylosphaera ? sp. HY21-03. 31. Archaeospongoprunum bispinosum Kozur and Mostler, HY21-03. 32. Spongoxystris ? ACCEPTEDsp. HY04-02 MANUSCRIPT 33. Triassocampe coronata Bragin, HY21-03. 34. Triassocampe deweveri (Nakaseko and Nishimura), HY04-04 35-37. Triassocampe sp. 35. HY04-04: 36. HY04-04: 37. HY04-04. 39. Hozmadia sp. HY04-03. 40. Parasepsagon sp. HY21-03. 41. Tiborella cf. anisica Kozur and Mostler, HY21-03. 42. Entactinia ? sp., HY21-03.

Plate IV. 1. Triassocampe coronata Bragin, HY04-11.

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2. Triassocampe deweveri (Nakaseko and Nishimura) , HY21-03. 3-4. Triassocampe sp. 3. HY04-06: 4. HY04-06. 5-7. Pseudostylosphaera japonica (Nakaseko and Nishimura), 5. HY04-07: 6. HY04-11: 7. HY04-07. 8. 16. Pseudostylosphaera sp. HY04-11. 9. 17 Pseudostylosphaera spinulosa (Nakaseko and Nishimura), HY04-06. 10-11. Spine A2 of Sugiyama (1997), 10. HY04-06: 11. HY04-06. 12. Spine A1 of Sugiyama (1997), 12. HY04-06. 13. Paroertlispongus sp. HY04-06. 14-15. Spongoxystris ? sp. 14. HY04-07: HY04-06. 18-20. Cryptostephnidium cornigerum 18. HY04-07: 19. HY04-11: 20. HY04-11. 21. Spongostylus ? sp. 22-23. Eptingium sp. HY04-07. 24. Hozmadia sp. HY04-06 25. Pentactinocarpus sp. HY04-11. 26-27. Triassospongosphaera ? sp. 26. HY04-06: 27. HY04-06. 28. Tiborella cf. anisica Kozur and Mostler, HY04-11 29. Parasepsagon sp. HY04-07.

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Figure 2

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Figure 4

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Figure 5

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Table 1

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Highlights 1. We revised depositional age a part of Paleozoic in Peninsular Thailand. 2. We proposed Paleo-environment of above re-interpreted sediments. 3. We clarify a tectonic setting of margin of the Sibumasu Block. 4. We provided constraints for oceanic closure time (collision time of continentals).

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