Lithostratigraphy of non-marine mesozoic rocks : Thung Yai-Khlong Thom areas in southern part of Thailand

Abstract

The non-marine mesozoic rocks in the Thung Yai-Khlong Thom area in peninsular Thailand have long been known as continental red beds. The study area of approximately 1,160 square kilometres is located in Nakhon Si Thammarat and Krabi provinces, with particular emphasis on Thung Yai and Khlong Thom area. In this study, the Trang group is proposed and lithostratigraphically subdivided into 4 formations, namely, Khlong Min, Lam Thap, Sam Chom and Phun Phin formations, respectively, in ascending order. The Trang group unconformably overlies the marine Triassic rocks (Sai Bon formation). The total thickness varies from 65 to 1,145 metres. The Khlong Min formation consists of 4 lithofacies; the mudstone intercalated with fossiliferous limestone, siltstone, calcareous sandstone and fossiliferous limestone with abundant vertebrate and invertebrate fossils. The total thickness varies from 58 to 116 metres. The lithology, sedimentary structures, geometry and fossil assemblages reflect transitional to fluviatile environment of deposition with occasional marine influxes of lagoonal environment during lower Middle Jurassic, with gradually change to fluviatile environment of deposition in the uppermost part. The fossiliferous limestone with abundant Modiolus sp. and wood fragments indicating marine incursion into the continent in Lower Cretaceous. The Lam Thap formation consists mainly of 2 lithofacies; the thick-bedded arkosic sandstone, and siltstone interbedded with shale with fern-like leaves, trace fossils, bivalves of Modiolus sp. and Unio sp. indicating Upper Jurassic to Lower Cartaceous. The total thickness varies from 30 to 197 metres. The rocks were deposited in floodplain and alluvial fan environments. The Sam Chom formation predominantly consists of conglomerate, coglomeratic sandstone and poorly cemented coarse-grained sandstone of alluvial fan origins. The total thickness varies from 8 to 100 metres. The Phun Phin formation consists of 2 lithofacies; the fine-grained sandstone and fanglomerate with trough and planar cross-beddings reflecting braided stream and debris flow origins with total thickness varies from 96 to 775 metres. Stratigraphically and paleontologically, the age of the Trang group should be assigned as lower Middle Jurassic to Upper Cretaceous.