Ordovician-Silurian Boundary Graptolites of the Satun Area, Southern Peninsular Thailand

Paleontological Research, vol. 10, no. 3, pp. 207–214, September 30, 2006 6 by the Palaeontological Society of Japan

Ordovician-Silurian boundary graptolites of the Satun area, southern peninsular Thailand

SACHIKO AGEMATSU1, KATSUO SASHIDA1, SIROT SALYAPONGSE2 AND APSORN SARDSUD2

1Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki, 305-8572, Japan (e-mail: [email protected]) 2Geological Survey Division, Department of Mineral Resources, Rama, VI, Bangkok, 10400, Thailand

Received March 13, 2006; Revised manuscript accepted May 11, 2006

Abstract. Black shale exposed north of Satun, southern peninsular Thailand, lies upon Upper Ordovician limestone and includes abundant graptolites. This fauna contains Normalograptus pseudovenustus pseudovenustus (Legrand, 1986) and Normalograptus sp. N. pseudovenustus is a reliable index species of an interval containing the Ordovician-Silurian boundary. Based on the range of this species, the graptolite-bearing black shale is inferred to be of latest Ordovician age, with the study section including the O/S boundary. The taxonomy of these two graptolite species is discussed in this study.

Key words: biostratigraphy, graptolite, Ordovician-Silurian boundary, Thailand

Introduction Sukhothai and eastern Loei-Petchabun Fold Belts. In turn, the boundary between these two fold belts Hamada et al. (1975) reported Lower Silurian consists of two suture zones: the Nan-Uttradit to the graptolite-bearing shale from several areas of Thai- north and the Sra Kaeo-Chanthaburi to the south. land such as the Kanchanaburi area, western Thai- The Nan-Chanthaburi suture zone extends north to land, and the Ban Na area, southern peninsular the Changning-Menglian Belt in western Yunnan, Thailand, although precise stratigraphic studies linked China (Liu et al., 1991), and south to the Bentong- to the depositional environments of these shales have Raub Suture Zone in peninsular Malaysia (e.g., not been done. Hutchison, 1975). Bunopas (1992) recognized seven We undertook field work in the Satun area in longitudinal stratigraphic belts, BS-1 to BS-5, and BI- February 2001 and March 2003 and recovered poorly 6 to BI-7 (from west to east), the first five of which to moderately preserved graptolites from black shale cover the Shan-Thai Block in Thailand and the last that overlies Upper Ordovician limestone. This study two the Indochina Block in Thailand. The Satun area deals with the lithology and biostratigraphy of the probably lies within the BS-3 belt. According to Buno- graptolite-bearing shale and discusses the taxonomy pas (1992), the following lithostratigraphic units com- of two graptolite species. prise the Paleozoic strata in belt BS-3: the Tarutao Group (Cambrian), the Thung Song Group (Ordovi- cian), the Thong Pha Phum Group (Silurian to Per- Geological setting mian), and the Mae Hong Son Formation (Carbonif- erous to Permian) (Figure 1). It is generally agreed that modern Southeast Lower and Middle Paleozoic formations are present Asia is composed of several continental blocks that in and around the Satun area, southern peninsular rifted from the northern margin of Gondwanaland, Thailand, and they form north-south-trending hilly to- drifted northward across Tethys, then collided and pography that extends about 200 km. Wongwanich amalgamated with Asia (e.g., Metcalfe, 1999). Main- et al. (1990) described the following lithostratigraphy land Thailand consists of two principal continental in this area: the Cambrian Tarutao Group, the Ordo- blocks: the western Shan-Thai Block and eastern In- vician Thung Song Group, and limestone overlain by dochina Block (Bunopas, 1981). The boundary be- clastic rocks of Silurian age. Our study area is located tween them is marked by two fold belts, the western about 30 km north of Satun (Figures 1 and 2). 208 Sachiko Agematsu et al.

Lithostratigraphy and depositional environment

Graptolite-bearing shale and other sediments crop out in an area about 30 km north of Satun (Figures 1 and 2), and generally strike N80W and dip 45 degrees to the south. The lithostratigraphy in this area is as follows, in ascending order: nodular limestone, tuffaceous sandstone, graptolite-bearing shale, and bedded black shale without graptolites (Figure 3). The lower nodular limestone is gray-white to red in color and about 70 m thick. This limestone is characterized by thin, calcareous layers and limestone nodules. A rich fauna of conodonts, ostracods, brachiopods, bryozo- ans, algae, trilobites, crinoids, bivalves, and cephalo- pods is present in the micritic limestone. The diameter of most bioclasts is less than 2 mm, rarely exceeding 4mm(Agematsuet al., in press). The tuffaceous sandstone that overlies the micritic limestone is generally gray to pale gray in color. The boundary between these two lithologic units cannot be observed due to a lack of exposures. The tuffaceous sandstone is weakly stratified into beds about 10 cm thick and is about 2 m Figure 1. 1. Map showing the study area and the seven thick overall. The graptolite-bearing shale is black stratigraphic belts of Thailand. 2. Generalized stratigraphic no- and lies conformably upon tuffaceous sandstone. The menculture for Thailand (After Bunopas, 1992). graptolite-bearing shale shows marked fissility and a thickness of about 4 m, and graptolites are abundant where the fissility is the most pronounced. Under microscopic observation, this shale is composed of clay minerals and quartz grains whose maximum diameter attains 0.04 mm. This graptolite-bearing shale contains fragments of sponge spicules 0.02–0.04 mm long and lacks any other macro- and microfossils (Figure 3). The uppermost black shale is more than 10 m thick, comprising a succession of beds several centimeters thick, and bears no graptolites. Its lithologic characteristics seen under microscopic observation are quite similar to those of the lower graptolite-bearing shale. The fissility of this shale is less prominent than that of the lower graptolite-bearing shale. The lower limestone corresponds to the nodular limestone classified by Scholle et al. (1990) and is inferred to have been deposited on a hemipelagic sea floor (Agematsu et al., in press). Furthermore, Wong- wanich et al. (1990) reported pelagic trilobites from this limestone. These facts imply that the lower limestone was formed in a comparatively pelagic environment, such as lower slope or basin. The upper silici- clastic rocks including the graptolite-bearing shale do not contain coarse grains of quartz. Therefore, the depositional environment of the graptolite-bearing Figure 2. 1. Route map showing the study area. 2. Column shale is also thought to have been a lower slope or of study section and sample level of graptolite. basin. O/S boundary graptolites from Thailand 209

Figure 3. 1. Mode of occurrence of graptolite-bearing shale. Graptolites visible on bedding surface. 2. Photomicrograph of thin section of graptolite-bearing shale perpendicular to the bedding plane. 3. Photomicrograph of highly magniﬁed thin section of photo 2. Quartz grains and sponge spicules are contained in this shale. 4. Photomicrograph of thin section parallel to the bedding plane.

Graptolites and geologic age The lower fauna from siltstone of the lower member of the section includes Normalograptus persculptus Graptolite fossils in this study were collected from a (Elles and Wood, 1907), and the upper fauna, from a 10 cm-thick interval which lies stratigraphically about thin black shale bed in the upper member of the sec- 1 m above the boundary between the tuffaceous sand- tion, contains several species such as Parakidograptus stone and graptolite-bearing shale. The graptolites are acuminatus (Nicholson, 1867), Normalograptus medius flattened, which makes it difficult to observe details, (To¨ rnquist, 1897), Normalograptus normalis (Lap- but poorly to moderately preserved specimens occur. worth, 1877), and Pseudoclimacograptus sp. Three fos- Long nemata and virgellae are commonly preserved, siliferous horizons of the middle member of the sec- some being longer than the rhabcodomes. We have tion yielded also the trilobite Dalmanitina sp. These identified two taxa: Normalograptus pseudovenustus fossils indicate that the sedimentary rocks are Upper pseudovenustus and Normalograptus sp. (Figures 4 Ordovician to Lower Silurian in age, and Wongwanich and 5). et al. (1990) considered that the Ordovician-Silurian Wongwanich et al. (1990) described a sequence boundary lies in an interval between the Dalmanitina consisting of siltstone, shale,andchertatasection bed and the upper graptolite fauna. The sequence close to our study section. They lithologically sub- reported herein, comprising sandstone, graptolite- divided these sedimentary rocks into three members bearing shale, and black shale, lithostratigraphically and reported two main graptolite faunas from them. correlates with the middle to upper members of the 210 Sachiko Agematsu et al.

section of Wongwanich et al. (1990), and hence our graptolite bed is equivalent to the lower part of the upper member of their section. According to Legrand (1986), the stratigraphic range of N. pseudovenustus pseudovenustus is limited to the N. persculptus Zone or extends down to the Normalograptus extraordina- tius Zone. Consequently, the graptolite bed of this study is uppermost Ordovician and the Ordovician- Silurian boundary lies in an interval between this bed and the upper fauna of Wongwanich et al.(1990) (Figure 6). The graptolite-bearing shale in our study lacks other fossils, except for fragments of sponge spicules. However, the underlying limestone contains abundant conodonts that indicate a Middle Ordovician (Llanvir- nian) to Late Ordovician (Ashgillian) age (Agematsu et al., in press). This age is consistent with that inferred for graptolite shale.

Correlation and stratigraphic signiﬁcance

Graptolites described herein permit the Satun area succession to be correlated with sections in other areas, most immediately the Kanchanaburi area, western Thailand, the northern area of Satun, the Lang- kawi Islands and northwestern peninsular Malaysia, on the Shan-Thai Block. The lithological similarity of the Lower and Middle Paleozoic strata of the Lang- kawi Islands of Malaysia with those in southern peninsular Thailand has been observed by Hamada et al. (1975) and Cocks et al. (2005). Upper Ordovician strata of the Langkawi Islands consist of limestone and are covered by lowermost Silurian clastic depos- its, called the Lower Detrital Member (Band) (e.g., Kobayashi et al., 1964; Igo and Koike, 1967; Burton, 1967). The Lower Detrital Member, which yields abundant graptolite fossils, is lithologically divided into six units, Units 1 to 6 (Jones, 1978). He recognized four main graptolite faunas in these clastic units. The lowermost fauna, obtained from a carbonaceous siltstone of Unit 1, includes representatives of the Normalograptus persculptus Zone such as N. persculptus and Neodiplograptus aff. modestus (Lapworth, 1876). Unit 1 is overlain by mostly unfossiliferous beds which comprise siltstone, sandstone, and shale of units 2 to 5, but a shelly fauna predominantly including Dalmanitina malayensis Kobayashi and Ham- ada, 1964 has been reported from a siltstone bed of Unit 2. The uppermost well-bedded carbonaceous silt- Figure 4. Tracings from photographs of graptolites. 1–3. Normalograptus pseudovenustus pseudovenustus (Legrand, stone of Unit 6 contains three graptolite faunas that 1986): 1, IGUT-ag0006; 2, IGUT-ag0038; 3, IGUT-ag0028. 4. are components of the Orthograptus vesiculosus and Normalograptus sp., IGUT-ag0021. Scale bars indicate 5 mm. Monograptus cyphus zones, the Monograptus gregar- ius Zone, and the Monograptus convolutus and M. O/S boundary graptolites from Thailand 211

Figure 5. 212 Sachiko Agematsu et al.

Silurian boundary therein based on graptolite and trilobite fossils. The results of the present study support this assignation, and give further data to assist in the construction of the stratigraphy.

Systematic paleontology

The paleontological work was undertaken by S. Agematsu and K. Sashida. All specimens described in this paper are deposited in the Institute of Geosci- ence, University of Tsukuba with the preﬁx IGUT.

Family Normalograptidae Sˇ torch and Serpagli, 1993 Genus Normalograptus Legrand, 1987 Figure 6. Upper Ordovician and Lower Silurian graptolite biostratigraphy in Britain and correlation among the graptolite faunas from the Langkawi Islands and Satun area. Type species.—Normalograptus normalis (Lap- worth, 1877) Diagnosis.—Normalograptus species have astoge- sedgwicki zones, respectively. Our graptolite fauna netic pattern H (Melchin, 1998) and climacograptid thus correlates with the lowermost fauna in the Lang- to glyptograptid thecae, which are rounded and alter- kawi Islands (Figure 6). nating. Median septum straight, Th21 or some later The succession of depositional environments of the thecae are dicalycal. Langkawi Islands and the Satun area show a similar pattern. A sequence consisting of Upper Ordovician Normalograptus pseudovenustus pseudovenustus limestone with intercalated clastic layers, and overly- (Legrand, 1986) ing uppermost Ordovician, fine-grained, clastic layers Figures 5.1–5.10 overlain by lower Silurian graptolite-bearing black shale has also been reported from the Kanchanaburi Climacograptus venustus venustus Legrand, 1976, p. 158–162, figs. 3A–3E. area of western Thailand (Bunopas, 1981). Based on ?Climacograptus venustus venustus Legrand, 1976, p. 160, 161, fig. lithostratigraphic correlations, several sedimentary ba- 4A. sins including the Satun area, the Langkawi Islands, Climacograptus pseudovenustus Legrand, 1986, fig. 2. and the Kanchanaburi area on the Shan-Thai Block were affected by similar environmental changes during Materials.—Thirty-two specimens were examined the latest Ordovician to earliest Silurian interval. from the sample ST-14 in the Satun area (IGUT- ag0001–0007, 0009–0018, 0020, 0024–0026, 0028– Conclusion 0031, 0036–0038, 0043, 0044). Description.—The rhabdosome, 18.4 mm in maxi- An Ordovician to Silurian sequence consisting of mum length, has a long virgella and nema. The former sandstone, graptolite-bearing shale, and black shale is is up to 10.9 mm and the latter attains 21.6 mm. exposed in the Satun area. Two taxa, Normalograptus The width is 0.6–0.8 mm at th11-th12 level, and pseudovenustus pseudovenustus and Normalograptus reaches its maximum width 1.5–1.8 mm. Septum is al- sp. are identified from a graptolite fauna found in most straight. Gently sigmoid thecal walls are inclined black shale. The former represents the Normalograp- proximally, and are vertical to weakly geniculate dis- tus persculptus Zone of the uppermost Ordovician. tally. The number of thecae is 10–12 proximally in Wongwanich et al. (1990) described a clastic section 10 mm and 9.5–11.5 distally. containing shale, siltstone, and sandstone in the Satun Remarks.—Although our specimens possess area, and inferred the existence of the Ordovician- various-sized rhabdosomes, their morphological char-

U Figure 5. Photographs of graptolites. 1–8. Normalograptus pseudovenustus pseudovenustus (Legrand, 1986): 1, IGUT-ag0007; 2, IGUT-ag0006; 3, IGUT-ag0002; 4, IGUT-ag0001; 5, IGUT-ag0038; 6, IGUT-ag0015; 7, IGUT-ag0011; 8, IGUT-ag0028. 9. Normalograptus sp., IGUT-ag0021. Scale bar indicates 5 mm. O/S boundary graptolites from Thailand 213 acteristics correspond to those of Climacograptus References venustus Legrand, 1976, a preoccupied name subse- quently changed by Legrand (1986) to Climacograptus Agematsu, S., Sashida, K., Salyapongse, S. and Sardsud, A., in pseudovenustus. Among the subspecies of pseudove- press: Ordovician conodonts from the Satun area, south- nustus, the nominate subspecies Normalograptus pseu- ern peninsular Thailand. Journal of Paleontology. Bunopas, S., 1981: Paleogeographic history of western Thai- dovenustus pseudovenustus (Legrand) has been distin- land and adjacent parts of Southeast Asia: A plate tec- guished from its other subspecies by a long virgella tonics interpretation. Geological Survey Paper,vol.5,801 and relatively long rhabdosome, generally 20 mm. p. Department of Mineral Resources of Thailand, The specimens described herein are somewhat shorter Bangkok. than Legrand’s type but all other measurements corre- Bunopas, S., 1992: Regional stratigraphic correlation in Thai- land, In, Piancharoen, C. ed., Proceedings of the National spond with those given in Legrand (1976). One sub- Conference on Geological Resources of Thailand: Potential species, Normalograptus pseudovenustus venustulus for Future Development, p. 189–208. (Legrand), is also smaller than pseudovenustus pseu- Burton, C. K., 1967: Graptolite and tentaculite correlations and dovenustus. However, this subspecies has a rhabdo- palaeogeography of the Silurian and Devonian in the some whose width is nearly constant as a whole, and Yunnan-Malaya geosyncline. Transactions and Proceed- ings of the Palaeontological Society of Japan, New Series, differs from our specimens in this respect. no. 65, p. 27–46. Occurrence.—From 0.9 to 1 m above the base of the Cocks, L. R. M., Fortey, R. A. and Lee, C. P., 2005: A review of black shale strata, Satun area, southern peninsular Lower and Middle Palaeozoic biostratigraphy in west pen- Thailand. insular Malaysia and southern Thailand in its context within the Sibumasu Terrane. Journal of Asian Earth Sciences, vol. 24, p. 703–717. Normalograptus sp. Elles, G. L. and Wood, E. M. R., 1907: A monograph of British graptolites, 539 p. Monograph of the Palaeontological So- Figure 5.11 ciety, London. 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