Structural and Sedimentological Evolution of Tertiary basins of northern Thailand
Wutti Uttamo, completed PhD project
Forty-two Tertiary intra-cratonic depositional basins in northern Thailand ranging from 30 to 2000 km2 in area and 36 major faults have been recognised and delineated from Landsat TM images. The small Tertiary basins are dominantly half graben structures and have distinct normal border faults. The large basins have S-shape or curved margins, and are commonly complex in structure. The basins that have sedimentary fills ranging in thickness from 500 to 3000 m, are rich in coal and oil shale deposits with one basin currently producing crude oil.
This study aims to understand the structural evolution, sediment provenance, lithofacies distribution and depositional environments of these basins. More than 70 % of the basins in northern Thailand are related to strike-slip tectonics and their formation was initiated by the movement of NW-trending dextral faults and NE-trending sinistral faults associated with N-S compression and E-W extension.
Sedimentological field work was carried out in five coal fields in northern Thailand and indicate that the Tertiary basins show a sedimentological evolution which developed from an initial half graben fill of alluvial fan, well-drained floodplain, small river, and small lake deposits to larger lake, poorly drained floodplain and swamp facies when the basins underwent rapid subsidence. The latest stage is dominated by the deposits of large braided river systems and associated well-drained floodplain.
Two major sediment provenance areas have been identified using heavy mineral assemblages and zircon varieties: the Palaeozoic metamorphic-sedimentary rocks and Triassic granitic rocks of the continental block in the west and the Mesozoic sedimentary rocks of the recycled orogen in the east. The change of climate from tropical and humid during Oligocene-Middle Miocene to temperate and semi-arid in Late Miocene-Pliocene accompanied by the increasing of the relief in the hinterland (the rapid uplift of the metamorphic core complex), resulted in an increase of unstable grains and heavy minerals in the sandstones.