Abstract

The intracratonic Gulf of Thailand Basin was established in late Mesozoic-early Tertiary times by block faulting along N-S and NNW-SSE trends. The main basinal areas in the centre of the Gulf are the N-S Pattani Trough and the NW-SE Malay Basin. To the west is a belt of narrow horsts and grabens, and to the east, the broad shallow basement area of the Khmer Shelf.

The sediments are mainly of Neogene age. Oligocene has been reported but older Tertiary strata are lacking, although no wells have fully penetrated the entire succession. The sediments are almost entirely of continental and paralic facies. Marine influences are present at times, but the lack of planktonic foraminifera suggests that this is restricted.

Three major sedimentary cycles are recognised, including two distinct regressive offlap sequences. The lowermost offlap sequence (Cycle I) is confined to the Pattani Trough and Malay Basin, and is (?) Oligocene-Lower Miocene in age. A later offlap sequence (Cycle II) of Middle-Upper Miocene age is more widespread. Both are characterised by deltaic progradation with paralic coal measures being laid down initially, followed by continental redbeds. Marine influences are more pronounced in the Southern Thai part of the Malay Basin.

A mid-Miocene unconformity is present locally, marking the end of delta progradation and the start of a broadly transgressive series, (Cycle III), which culminated in the establishement of widespread marine conditions during the Quaternary.