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.
Introduction
Hamada et al. (1975) reported Lower Silurian graptolite-bearing shale from several areas of Thailand such as the Kanchanaburi area, western Thailand, and the Ban Na area, southern peninsular Thailand, although precise stratigraphic studies linked to the depositional environments of these shales have not been done.
We undertook field work in the Satun area in February 2001 and March 2003 and recovered poorly to moderately preserved graptolites from black shale that overlies Upper Ordovician limestone. This study deals with the lithology and biostratigraphy of the graptolite-bearing shale and discusses the taxonomy of two graptolite species.
Geological setting
It is generally agreed that modern Southeast Asia is composed of several continental blocks that rifted from the northern margin of Gondwanaland, drifted northward across Tethys, then collided and amalgamated with Asia (e.g., Metcalfe, 1999). Mainland Thailand consists of two principal continental blocks: the western Shan-Thai Block and eastern Indochina Block (Bunopas, 1981). The boundary between them is marked by two fold belts, the western Sukhothai and eastern Loei-Petchabun Fold Belts. In turn, the boundary between these two fold belts consists of two suture zones: the Nan-Uttradit to the north and the Sra Kaeo-Chanthaburi to the south. The Nan-Chanthaburi suture zone extends north to the Changning-Menglian Belt in western Yunnan, China (Liu et al., 1991), and south to the Bentong-Raub Suture Zone in peninsular Malaysia (e.g., Hutchison, 1975). Bunopas (1992) recognized seven longitudinal stratigraphic belts, BS-1 to BS-5, and BI-6 to BI-7 (from west to east), the first five of which cover the Shan-Thai Block in Thailand and the last two the Indochina Block in Thailand. The Satun area probably lies within the BS-3 belt. According to Bunopas (1992), the following lithostratigraphic units comprise the Paleozoic strata in belt BS-3: the Tarutao Group (Cambrian), the Thung Song Group (Ordovician), the Thong Pha Phum Group (Silurian to Permian), and the Mae Hong Son Formation (Carboniferous to Permian) (Figure 1).
Figure 1.
1. Map showing the study area and the seven stratigraphic belts of Thailand. 2. Generalized stratigraphic nomenculture for Thailand (After Bunopas, 1992).
Lower and Middle Paleozoic formations are present in and around the Satun area, southern peninsular Thailand, and they form north-south-trending hilly topography that extends about 200 km. Wongwanich et al. (1990) described the following lithostratigraphy in this area: the Cambrian Tarutao Group, the Ordovician Thung Song Group, and limestone overlain by clastic rocks of Silurian age. Our study area is located about 30 km north of Satun (Figures 1 and 2).
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 N80°W 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, bryozoans, algae, trilobites, crinoids, bivalves, and cephalopods is present in the micritic limestone. The diameter of most bioclasts is less than 2 mm, rarely exceeding 4 mm (Agematsu et 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 thick overall. The graptolite-bearing shale is black and lies conformably upon tuffaceous sandstone. The 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.
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.
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, Wongwanich 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 shale is also thought to have been a lower slope or basin.
Graptolites and geologic age
Graptolite fossils in this study were collected from a 10 cm-thick interval which lies stratigraphically about 1 m above the boundary between the tuffaceous sandstone and graptolite-bearing shale. The graptolites are flattened, which makes it difficult to observe details, but poorly to moderately preserved specimens occur. Long nemata and virgellae are commonly preserved, some being longer than the rhabcodomes. We have identified two taxa: Normalograptus pseudovenustus pseudovenustus and Normalograptus sp. (Figures 4 and 5).
Figure 4.
Tracings from photographs of graptolites. 1–3. Normalograptus pseudovenustus pseudovenustus (Legrand, 1986): 1, IGUT-ag0006; 2, IGUT-ag0038; 3, IGUT-ag0028. 4. Normalograptus sp., IGUT-ag0021. Scale bars indicate 5 mm.
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.
Wongwanich et al. (1990) described a sequence consisting of siltstone, shale, and chert at a section close to our study section. They lithologically subdivided these sedimentary rocks into three members and reported two main graptolite faunas from them. The lower fauna from siltstone of the lower member of the section includes Normalograptus persculptus (Elles and Wood, 1907), and the upper fauna, from a thin black shale bed in the upper member of the section, contains several species such as Parakidograptus acuminatus (Nicholson, 1867), Normalograptus medius (Törnquist, 1897), Normalograptus normalis (Lapworth, 1877), and Pseudoclimacograptus sp. Three fossiliferous horizons of the middle member of the section yielded also the trilobite Dalmanitina sp. These fossils indicate that the sedimentary rocks are Upper Ordovician to Lower Silurian in age, and Wongwanich et al. (1990) considered that the Ordovician Silurian boundary lies in an interval between the Dalmanitina bed and the upper graptolite fauna. The sequence reported herein, comprising sandstone, graptolite-bearing shale, and black shale, lithostratigraphically correlates with the middle to upper members of the 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 extraordinatius 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).
Figure 6.
Upper Ordovician and Lower Silurian graptolite biostratigraphy in Britain and correlation among the graptolite faunas from the Langkawi Islands and Satun area.
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 (Llanvirnian) 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 Langkawi Islands and northwestern peninsular Malaysia, on the Shan-Thai Block. The lithological similarity of the Lower and Middle Paleozoic strata of the Langkawi 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 deposits, 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 Hamada, 1964 has been reported from a siltstone bed of Unit 2. The uppermost well-bedded carbonaceous silt-stone of Unit 6 contains three graptolite faunas that are components of the Orthograptus vesiculosus and Monograptus cyphus zones, the Monograptus gregarius Zone, and the Monograptus convolutus and M. sedgwicki zones, respectively. Our graptolite fauna thus correlates with the lowermost fauna in the Langkawi Islands (Figure 6).
The succession of depositional environments of the Langkawi Islands and the Satun area show a similar pattern. A sequence consisting of Upper Ordovician limestone with intercalated clastic layers, and overlying uppermost Ordovician, fine-grained, clastic layers overlain by lower Silurian graptolite-bearing black shale has also been reported from the Kanchanaburi area of western Thailand (Bunopas, 1981). Based on lithostratigraphic correlations, several sedimentary basins including the Satun area, the Langkawi Islands, and the Kanchanaburi area on the Shan-Thai Block were affected by similar environmental changes during the latest Ordovician to earliest Silurian interval.
Conclusion
An Ordovician to Silurian sequence consisting of sandstone, graptolite-bearing shale, and black shale is exposed in the Satun area. Two taxa, Normalograptus pseudovenustus pseudovenustus and Normalograptus sp. are identified from a graptolite fauna found in black shale. The former represents the Normalograptus persculptus Zone of the uppermost Ordovician. Wongwanich et al. (1990) described a clastic section containing shale, siltstone, and sandstone in the Satun area, and inferred the existence of the Ordovician 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 Geoscience, University of Tsukuba with the prefix IGUT.
Family Normalograptidae Štorch and Serpagli, 1993
Genus Normalograptus Legrand, 1987
Type species.—Normalograptus normalis (Lapworth, 1877)
Diagnosis.—Normalograptus species have astogenetic pattern H (Melchin, 1998) and climacograptid to glyptograptid thecae, which are rounded and alternating. Median septum straight, Th21 or some later thecae are dicalycal.
Normalograptus pseudovenustus pseudovenustus (Legrand, 1986)
Climacograptus venustus venustus Legrand, 1976, p. 158–162, figs. 3A–3E.
?Climacograptus venustus venustus Legrand, 1976, p. 160, 161, fig. 4A.
Climacograptus pseudovenustus Legrand, 1986, fig. 2.
Materials.—Thirty-two specimens were examined from the sample ST-14 in the Satun area (IGUT-ag0001–0007, 0009–0018, 0020, 0024–0026, 0028–0031, 0036–0038, 0043, 0044).
Description.—The rhabdosome, 18.4 mm in maximum length, has a long virgella and nema. The former is up to 10.9 mm and the latter attains 21.6 mm. The width is 0.6–0.8 mm at th11-th12 level, and reaches its maximum width 1.5–1.8 mm. Septum is almost straight. Gently sigmoid thecal walls are inclined proximally, and are vertical to weakly geniculate distally. The number of thecae is 10–12 proximally in 10 mm and 9.5–11.5 distally.
Remarks.—Although our specimens possess various-sized rhabdosomes, their morphological characteristics correspond to those of Climacograptus venustus Legrand, 1976, a preoccupied name subsequently changed by Legrand (1986) to Climacograptus pseudovenustus. Among the subspecies of pseudovenustus, the nominate subspecies Normalograptus pseudovenustus pseudovenustus (Legrand) has been distinguished from its other subspecies by a long virgella and relatively long rhabdosome, generally 20 mm. The specimens described herein are somewhat shorter than Legrand's type but all other measurements correspond with those given in Legrand (1976). One subspecies, Normalograptus pseudovenustus venustulus (Legrand), is also smaller than pseudovenustus pseudovenustus. However, this subspecies has a rhabdosome whose width is nearly constant as a whole, and differs from our specimens in this respect.
Occurrence.—From 0.9 to 1 m above the base of the black shale strata, Satun area, southern peninsular Thailand.
Normalograptus sp.
Materials.—One specimen is examined from the sample ST-14 in the Satun area (IGUT-ag 0021).
Description.—Rhabdosome, 9.6 mm in length, measures 0.8 mm in width at the level of th11-th12 and immediately reaches the maximum width, 1.2 mm. Walls of thecae are parallel to the rhabdosome axis. Virgella is 0.6 mm and nema is 6.8 mm in length.
Remarks.—This specimen is distinguished from others by thecal form, which is climacograptid, and a short virgella. However, it is impossible to determine a species name due to limited material and its poor preservation.
Occurrence.—From 0.9 to 1 m above the base of the black shale strata, Satun area, southern peninsular Thailand.
Acknowledgments
We are grateful to S. Henry Williams, Ph. Legrand, and P. Štorch for reading the early version of our manuscript and offering many useful comments and suggestions. We further acknowledge M. Melchin and one anonymous reviewer for reading our manuscript and offering useful suggestions. We thank the Department of Mineral Resources of Thailand for providing facilities for our research in Thailand. The field-work was funded by a part of a grant from the Mombusho International Scientific Research Program (no. 11440148 to K. S.).
