The Lower Paleozoic Rocks of Thailand

The Symposium on Geology of Thailand 26-31 August 2002, Bangkok, Thailand

Thanis Wongwanich, Wattana Tansathien, Santi Leevongcharoen, Weerachai Paengkaew, Pitak Thiamwong, Jirasak Chaeroenmit, and Wirote Saengsrichan Department of Mineral Resources, Rama VI Rd., Bangkok 10400, Thailand

EXTENDED ABSTRACT The Lower Paleozoic rocks of Thailand are subdivided into the lower Lower Paleozoic rocks ranging in age from Cambrian to Ordovician and the upper Lower Paleozoic rocks ranging in age from the Silurian to Devonian, and up to the early Carboniferous in some rock units. In Shan-Thai Terrane, these rocks are widespread in western mountain area extending from Kanchanaburi Province in the west to Chiang Mai, Mae Hong Son and Uttaradit Province in the North; and to Chon Buri in the east (Fig.1) The rocks in the Peninsula are widely distributed in north- south direction alongside of Khao Luang mountain range, extending from Surat Thani Province through Nakhon Si Thammarat and Satun Province to North Malaysia. In Indochina Terrane, only the upper Lower Paleozoic rocks are exposed in northeastern part of Loei Province extending in north-south narrow belt across Thai-Laos border to Western Laos. In general, most of the lower Lower Paleozoic rocks of Thailand have been subjected to low grade regional metamorphism (greenschist facies) and contact metamorphism in the area close to granite. The rocks are also strongly folded and faulted. These rocks are closely associated with and lie unconformably above the Pre-Cambrian high-grade metamorphic rocks in Shan-Thai Terrane. Since the pioneer work of Lee (1923) and Brown et al. (1951), now it is clear that the lower Lower Paleozoic rocks of Thailand can be divided into two conformable rock units : a lower siliciclastic, the Tarutao Group, and an upper carbonate, the Thung Song Group (Fig.2). The Tarutao Group at the Tarutao Island, Satun Province, is a thick sequence of red sandstone, siltstone, shale and conglomerate with the Late Cambrian trilobite on the upper part of the sequence. However, most of the rocks on the mainland is quartzite. This group is a shallow shelf sequence periodically subjected to storm. The overlying Ordovician Carbonates, the Thung Song Group, is a thick sequence of tropical limestones, dolomites and calcareous shales. At least seven distinctive lithostratigraphic units have been recognized in the South as the Malaka Formation, Talo Dang Formation, La Nga Formation, Pa Nan Formation Lae Tong Formation, Rung Nok Formation, and Pa Kae Formation. The Pa Kae Formation is the red stromatolitic limestone proposed for the uppermost Formation of the Thung Song Group in the South (Fig.3) They were deposited in peritidal environment on a homoclinal ramp during a long marine transgression in Early Ordovician, followed by shoaling, reefs, and a deeper subtidal in a Middle, and a greater deepening during the Late Ordovician (Fig.4). Lithologically, the carbonate rock of the Thung Song Group is very similar throughout Thailand. However, the age of of the similar rock unit in the east is still in controversy. The Cambrian-Ordovician boundary lies within the top most part of the Tarutao Group (Stait et al., 1984) whereas the Ordovician–Silurian boundary is placed in the black graptolitic shale of the upper Lower Paleozoic rock, the Wang Tong Formation (Fig.5), about 25 meters above the Pa Kae Formation (Wongwanich et al., 1990). The upper Lower Paleozoic rocks of Thailand which lie conformably above the Early Paleozoic Rocks can be differentiated into 2 major north-south facial belts, the shelf-basin in the west and the Peninsula, and the magmatic arc facies in the Sukhothai Fold Belt and Eastern part of the Peninsula (Bunopas, 1990). The rocks in the west from Kanchanaburi to Mae Hong Son Province and the Peninsula are black graptolitic and tentaculitic shale, chert, sandstone, siltstone and varigated stromatolitic nodular limestone of shelf to back-arc basin facies. These rocks were formerly referred to as the Tanaosi Group and Kanchanaburi Group but now they are defined as the Thong Pha Phum Group. In the Peninsula from Surat Thani to Satun Province, this rock is represented by a continuous succession of those deeper water siliciclastic and carbonate rocks. At least three conformably lithostratigraphic units have been recognized in Satun Province. They are the Wang Tong Formation, Kuan Tung Formation and Pa Samed Formation (Fig.6). Studies on the graptolite, trilobite, brachiopod, ammonite and conodont by many paleontologists from Australia, England, Japan and USA. reveal that the upper Lower Paleozoic rocks of this shelf-basin facial belt in the Peninsula are ranging in age from Late Ordovician to Early Carboniferous. They seem to be conformably underlied the Upper Paleozoic rocks. However, recent study on deeper water fossils from the Pa Samed Formation of Prof. Boucot and his colleagues indicates the presence of a major disconformity between the Emsian dacryoconarid-rich beds and the overlying siliciclastics that have yielded Carboniferous faunas. The upper Lower Paleozoic rocks in the Sukhothai Fold belt of Shan Thai Terrane, in Eastern part of the Northern Highland, the Eastern Gulf and Eastern part of the south can be saparated into three facial belts including 1) back-arc-basin facies in the west, 2) volcanic arc facies in the middle, and 3) inner trench stope facies of fore-arc in the east (Bunopas, 1992). 17

Figure 1 Map showing the distribution of the Lower Paleozoic rocks of Thailand. 18

Figure 2 Stratigraphic subdivisions of the Lower Paleozoic of Thailand (DMR,1992).

Figure 3 Detail lithostratigraphic column of the Thung Song group divided into 6 Formations at Tarutao Island: 1) Malaka Fm. deposited in intertidal, 2) Talo Dang Fm. Deposited as lagoon, 3) La Nga Fm. deposited as sand spit, 4) Pa Nan Fm. deposited as subtidal stromatolite, 5) Lae Tong Fm. deposited in lagoon, 6) Rung Nok Fm. deposited as shoaling and reef (Wongwanich, 1990). 19

Figure 4 The diachronous depositional model for transgressive development of the Thung Song ramp carbonates showing environmental setting of each Formation in Satun Province, Southern Thailand: 1) Malaka Fm. deposited in intertidal, 2) Talo Dang Fm. deposited in lagoon, 3) La Nga Fm. deposited as sand spit, 4) Pa Nan Fm. deposited in subtidal Stomatolite, 5) Lae Tong Fm. deposited in lagoon, 6) Rung Nok Fm. deposited in local build-up and reef; these 6 rock units deposited in lower-middle Ordovician, 7) Pa Kae Fm. deposited as deep water reefs of stomatolite (Wongwanich, 1990).

Figure 5 General stratigraphic column of the Pa Kae Formation and Wang Tong Formation showing important faunal elements. A posible position of the Ordovician-Silurian boundary is shown (Wongwanich, 1990). 20

Figure 6 Composite stratigraphic column of the Upper Ordovician-Devonian-Carboniferous rocks at Km 10-11, La Nga to Thung wa road, La Nga District, Satun Province. Showing the stratigraphic relation of the Pa Kae Fm. of Thung Song Gr.; and the Wang Tong Fm. and Pa Samed Fm. of the Thong Pha Phum Gr.(Wongwanich et al., 1990)1. Thin bedded limestone, 2. Limestone with chert nodule, 3. Argillaceous limestone, 4. Massive-thick bedded limestone, 5. Black shale interbedded chert, 6. Black shale, 7. Shale, 8. Siltstone, 9. Sandstone, 10. Sandstone with rounded pebbles. 21

The back arc-basin facies are black graptolitic and tentaculitic shale, chert, and occasional limestone extending from Fang to Chiengmai, Lampang, along eastern flank of the Western Mountain to Tak, Kanchanaburi, to eastern Gulf, and eastern part of southern Peninsula to Yala Pravinces. Further east of those belt is volcanic arc facies represented by limited outcoups of amphibolite, mica schist, agglomerate, and fine grained metatuffs. This belt extends from Chiang Rai to Lampang, Kamphaengphet, Nakorn Sawan, Suphanburi province in the west, to the east and the south at Nararthiwat province. The inner trench slope of fore -arc facies are represented by low grade metasediments, marble, and bedded chert, extending from Sukhothai to Nakhorn Sawan in the north, to Rayong and Chanthaburi in the east. However, the thin bedded sandstone interbedded with shale of trench facies are also exposed at Nan Province. The upper Lower Paleozoic rocks of the Indochina Terrane form as another N-S belt in the area between Loei and Udon Thani Province. The rocks are thin bedded chert interbedded with dark gray siliceous shale, quartzite, tuff conglomerate, and limestone with Silurian-Devonian-Lower Carboniferous fossils. These rocks were formerly known as Nong Dok Bua Formation but now are renamed to be Pak Chom Formation (Charoenprawat and Wongwanich, 1976; Bunopas et al., 1988). They lie unconformably above Silurian quaitzite, phyllite and schist of the Namo Formation. This Formation is thought to be the oldest rocks in the western margin of the Indochina Terrane in Thailand. Based on stratigraphical and paleontological studies on brachiopod, trilobite, conodont, microvertebrate and gastropods of many paleontologists from Australia, England, Japan and USA., it is clear that Thai fauna can be correlated with those of Burma, China, Australia and South America. This suggests that Thailand was a part of the greater Gondwana during the Lower Paleozoic and these terranes were still close to Australia during the Late Lower Paleozoic (Burrett et al., 1990).

Key words : Lower Paleozoic rocks, Tarutao Group, Thung Song Group

REFERENCES

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