泰國東南部Rayong省Klaeng地區的中?晚泥盆世放射蟲

泰國東南部Rayong省Klaeng地區的中?晚泥盆世放射蟲

* Corresponding author. E-mail: yoshi_kamata@geol.tsukuba.ac.jp.

Tectonic subdivision of mainland Thailand has been discussed by means of Paleozoic and Mesozoic stratigraphy, micropaleontology represented by foraminiferal and radiolarian biostratigraphy and paleobiogeography, tectonic settings of granitoids, and ultramafic rocks that are distinctive of suture lines. Recently, Ueno (2002) and Ueno and Charoentitilat (2011)proposed a new tectonic scheme of mainland Thailand and divided it into three geotectonic units: the Sibumasu Block, Sukhothai Zone, and Indochina Block from west to east, which are separated by the Mae Yuan Fault, the Chiang Rai Tectonic Line, and the Nan–Uttaradit Suture, respectively. In Northern Thailand, moreover, the peculiar Inthanon Zone is recognized in the eastern part of the Sibumasu Block, where Paleo-Tethyan oceanic rocks are widely distributed as tectonic slices. These geotectonic subdivisions in Northern Thailand are relatively clear. However, the tectonic subdivisions of Central and Southeast Thailand have not been well documented and are still controversial because there is short of information about basement rocks in those areas. In this circumstance,we have examined Paleozoic to Mesozoic stratigraphy,lithology, and radiolarian ages of siliceous rocks distributed in Southeast Thailand to clarify the geotectonic belongings of this area and the southern extensions of geotectonic units consisting of Northern Thailand. In the course of this research, we could obtain Middle to Late Devonian radiolarians from well-bedded siliceous rocks distributed in southwest of Klaeng, Rayong Province. In this study, we report lithology and age of the radiolaria-bearing rocks and discuss their stratigraphy and geological correlation nearby the Klaeng region.

Paleozoic and Mesozoic sedimentary rocks and mainly Permo-Triassic granitoids are distributed from the Sattahip area of Chonburi Province to the Klaeng area of Rayomg Province. Based on the geological map of Rayong Province by the DMR (Nakinbodee et al., 1984) and the 1:1, 000,000 geological map of Thailand (DMR, 1999), Pre-Cambrian gneiss,Silurian-Carboniferous Kaeng Krachang and Kanchanaburi formations, Carboniferous-Permian siliciclastics and carbonates, and Permo-Triassic volcanogenic clastics are mainly distributed in this region.Due to very little research of detailed local geology having been undertaken in this region, stratigraphic relationship and structural contact of these geological units have not been fully documented. Recently, geological age and stratigraphic position of some of these lithological units were reinterpreted (Geard, 2008;Ridd, 2011, Ueno and Charoentitirat, 2011).

Radiolaria-bearing bedded siliceous shale are exposed along the coast at Laem Krabang Phet(12?37’55” N, 101?33’55” E), approximately 20 km southwest of Klaeng. In this locality, thinly bedded siliceous shale is exposed and is partly intercalated with black carbonaceous shale and fine- to coarse-grained sandstone. Coarse-grained sandstone blocks, showing boudinage structure are contained in this sequence. The sandstone is quartz arenite and is composed mainly of quartz with muscovite grains.Quartz grains are angular and amalgamated each other.The siliceous shale has various colors such as gray to black, greenich gray, and reddish brown, and partly intercalated with light to pale greenish shale (tuff).Most of the siliceous shale consists both of clay rich layers of a few mm thickness and cherty layers of microcrystalline matrix.

The strata in this succession is tightly folded and the lithologies are repeated by layer-parallel fault.Moreover, pinch and swell structure and boudins of sandstone are common. Due to these deformation structures, it is difficult to estimate original depositional order and thickness. This sequence exposed at Laem Krabang Phet was previously attributed to the Silurian-Devonian Kanchaburi Formation (Nakinbodee et al., 1984), and a Middle Silurian Cephalopoda(Michelinoceras michelini) was reported from this site(Fontaine and Salyapongse, 1997).

Sixteen samples were collected from this site to determine depositional age of the siliceous sequence,among which one sample contained radiolarian remains one sample.Palaeoscenidium cladophorumDeflandre,Stigmosphaerostyluscf.pusilla(Hinde),and Entactiniidae gen. et sp. indet. were obtained.Among these species,P. cladophorumis characterized by short apical and long basal spines with small spinules, and reported worldwide from the Devonian to Early Carboniferous (ex.: Wang et al., 2003). The second form is very similar toStigmosphaerostylus pusilla, which was reported from the late Middle to Late Devonian (Givetian to Famennian) in New South Wales, Australia, in southern China (Wang et al.,2003), and in Ohio, U.S.A. (Foreman, 1963). Although our materials are rather poor in preservation, this fauna is provisionally referable to a Middle-Late Devonian based on their reported biostratigraphy.

As mentioned above, a Middle Silurian Cephalopoda was reported from chert of this site and sandstone-siliceous sediments were attributed to the Silurian-Devonian Kanchanaburi Formation (Nakinbodee et al., 1984). The most recent geological map of the Klaeng region (Vimuktanandana and Munchai, 2008)correlated the sandstone-siliceous sequence to the uppermost Permian deep-facies exposed at the Khao Wang Chik, about 6 km NW of Klaeng, where Sashida et al. (2000) recovered a rich radiolarian fauna of the Latest Permian. But the occurrence of Devonian radiolarians in this study supports the previous assignment that the sandstone-siliceous sequence at the Laem Krabang Phet to the Silurian-Devonian Kanchanaburi Formation by Nakinbodee et al. (1984).The youngest zircon age (430±4 Ma) by U-Pb measurement is consistent with the Lower Paleozoic assignment (Geard, 2008). Due to the difficulty of reconstruction of detailed depositional order of the sandstone and siliceous sequence, stratigraphic position both of a Middle Silurian Cephalopoda and Devonian radiolarians in this study is unknown. But occurrence of the Devonian radiolarians provides a concrete evidence for sedimentary age of the Kanchanaburi Formation.

As mentioned above, well-bedded siliceous rocks exposed at the Laem Krabang Phet is intercalated with carbonaceous shale and accompanied by sandstone layers consisting of quartz arenaite. Quartz rich composition with muscovite grains requires input of material derived from felsic plutonic rocks. Zircon age of the sandstone by the U-Pb measurements shows perks mainly of 600 Ma, 800～1000 Ma, and 1200 Ma (Geard,2008). Although the Kanchanaburi Formation has a member of radiolaria-bearing siliceous layers, the depositional site of this unit is not necessarily a pelagic environment of the Paleo-Tethys, where radiolarian chert continuously and broadly deposited on the basaltic basements, but nearby a continental margin or continental slope. These lines of evidence suggest that the Kanchanaburi Formation should be a member of the Sibumasu continental block. Although stratigtraphic relationship is unknown, the Laem Khet Formation(Ordovician), the Carboniferous-Permian strata which has contend that most of these are Lower Paleozoic by Ridd (2011) and Ueno and Charoentitirat (2011), and the Kanchanaburi Formation should be interpreted as the Sibumasu Domain.

Middle Devonian to Middle Triassic bedded chert(=Fang chert) in the Chiang Dao area, Northern Thailand has been interpreted as Paleo-Tethyan oceanic rocks and characteristic unit of the Inthanon Zone as well as thedoiChiang Dao Limestone of Carboniferous-Permian seamaount-type carbonates (Ueno and Charoentitirat, 2011). The Devonian radiolaria-bearing chert at Klaeng is chronologically comparable to part of the pelagic Fang chert, but its lithology is essentially different from the latter in having strong influence from continental detritus. In Southeast Thailand,pelagic Paleo-Tethyan rocks such as Devonian-Triassic chert and Carboniferous-Permian seamount-type carbonates, which characterize the Inthanon Zone in Northern Thailand, are probably absent.

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Middle to Late Devonian Radiolarians from Klaeng of Rayong Province, Southeast Thailand

Yoshihito KAMATA1)*, Miyako KATO2), Katsumi UENO3), Akira MIYAHIGASHI3),Thasinee CHAROENTITIRAT4), Apsorn SARDSUD5)
1)Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki, Japan;
2)K.a.a.Japan Co.,Ltd. Asa-minami, Hiroshima city, Hiroshima, Japan;
3)Department of Earth System Science, Faculty of Science, Fukuoka University, Fukuoka814-0180,Japan;
4)Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok10330,Thailand;
5)Bureau of Geological Survey, DMR, Bangkok10400,Thailand

Radiolarian; Klaeng; Southeast Thailand; Devonian

10.3975/cagsb.2012.s1.17