Marine Jurassic stratigraphy and bivalve paleontology of Thailand
Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. Marine Jurassic rocks of Thailand are well exposed in the Mae Sot and Umphang areas of Tak province, and less extensively near Mae Hong Son in the north, Kanchanaburi in the west, and Chumphon and Nakhon Si Thammarat in the south. They are underlain unconformably by Triassic and overlain by Quaternary strata.
This study is based mainly on 5 measured Toarcian-Early Bajocian sections. Twelve new lithostratigraphic units have been established: (in ascending order) Pa Lan, Mai Hung and Kong Mu Formations of the Huai Pong Group in the Mae Hong Son area; Khun Huai, Doi Yot and Pha De Formations of the Hua Fai Group in the Mae Sot area; Klo Tho , Ta Sue Kho, Pu Khloe Khi and Lu Khoc Tu Formations of the Umphang Group in the Umphang area; Khao Lak Formation in the Chumphon area; and Pra Bat Formation in the Nakhon Si Thammarat area.
Mudstone, siltstone, sandstone, limestone, and marl are the dominant lithologies. Mudstones, siltstones, and sandstones are widespread; limestones are found in the Mae Sot, Umphang, and Mae Hong Son areas, and marls are dominant only around Mae Sot. The sediments are about 900 m thick in Mae Sot and 450 m near Umphang and are rather thinner in the other areas, particularly so in the south.
Bivalves and ammonites are the dominant macrofossils in Jurassic sequences, particularly in Mae Hong Son, Mae Sot, and Umphang. Brachiopods, corals and gastropods are minor; foraminifera and algae are dominant in limestones and marls. From 5 measured sections, 39 bivalve species within 24 genera are described, including 32 new species.
Age determinations are based on ammonites, with some bivalves and foraminifera providing useful additional data. These fossils show that marine sequences are largely Toarcian-Aalenian plus some Bajocian (i.e. late Early Jurassic to early Middle Jurassic). Previous Late Jurassic ages given for strata in the Mae Sot and Umphang areas based on corals have not been confirmed in the present study.
The Umphang Jurassic is similar to that of Mae Hong Son by its bivalve content, but differs significantly from that of the Mae Sot area where bivalves are less diverse. Based on foraminifera and algae, the Jurassic of Umphang and Mae Sot can be correlated with that of Kanchanaburi. Regionally, the Thai Jurassic is eastern Tethyan and similar to that of Vietnam, Myanmar, Tanzania and Kenya. The marine Jurassic rocks can also be correlated with the non-marine Jurassic Khorat Group of northeastern Thailand, and specifically with the Phu Kradung and Phra Wihan Formations.
Ecologically, five faunal associations of paleoecologic significance can be distinguished in the Mae Hong Son-Kanchanaburi and Chumphon Basins ( i.e. Bositra, Benthic Bivalve, Mixed. Ammonite and Brachiopod Facies). The Bositra Facies occurs only in the upper part of the Khun Huai and the lower part of the Doi Yot Formations. The Benthic Bivalve Facies dominated by infaunal suspension-feeding bivalves, represents the Pa Lan, Klo Tho and Khao Lak Formations; epifaunal suspension-feeding bivalves are less abundant with only micocarnivores being found. Other facies are represented by ammonites and other faunas in most areas except for brachiopods which are confined to the Klo Tho Formation.
Shallow marine conditions (inner to middle shelf) existed over most of western and southern Thailand in the Toarcian-Bajocian, although the Mae Sot area was slightly deeper (outer shelf, perhaps upper slope). These conditions are indicated by abundance of faunas, the presence of oncolitic and oolitic limestones, and plant remains in sandstones. With the low Tethys sea levels of the Toarcian-Bajocian, the sea invaded only western Thailand. Trangression in the Toarcian is represented by mudstones and high faunal diversity and regression in the Aalenian-Early Bajocian is indicated by sandstones, and limestone conglomerates in most sequences.
The Thai Jurassic is less affected by tectonism than the Triassic. A very mild orogeny during Middle Jurassic has been proposed before the Himalayan orogeny.