MARINE JURASSIC LITHOSTRATIGRAPHY OF THAILAND

Marine Jurassic rocks of Thailand are well-exposed in the Mae Sot and Umphang areas and less extensively near Mae Hong Son, Kanchanaburi, Ch umphon and Nakhon Si Thammarat, in the north, west, and south respecti vely. They are generally underlain unconformably by Triassic and overl ain by Quaternary strata. Based mainly on five measured sections, four teen new lithostratigraphic units are established: (in ascending order ) Pa Lan, Mai Hung and Kong Mu Formations of the Huai Pong Group in th e Mae Hong Son area; Khun Huai, Doi Yot and Pha De Formations of the H ua Fai Group in the Mae Sot area; Klo The, Ta Sue Kho, Pu Khloe Khi an d Lu Kloc Tu Formations of the Umphang Group in the Umphang area; and the Khao Lak Formation in the Chumphon area. Mudstone, siltstone, sand stone, limestone and marl are the dominant lithologies. Mudstones, sil tstones and sandstones are widespread; limestones are confined to the Mae Set, Umphang, Kanchanaburi and Mae Hong Son areas; marls are found only in Mae Set. The sequences are approximately 900 m thick in Mae S ot and 450 m thick in Umphang and are rather thinner in the other area s, particularly in the south. Based on ammonites, with additional data from bivalves and foraminifera, the marine Jurassic is largely Toarci an-Aalenian plus some Bajocian. Late Jurassic ages given previously fo r strata in the Mae Sot and Umphang areas have not been confirmed. (C) 1997 Elsevier Science Ltd. All rights reserved.