Chumphon Basin Offshore Gr

Chumphon Basin (offshore); which is 50 km x 100 km long.

Synonym:

Unit I. This unit is poorly dated (probably of Eocene – Oligocene age), and comprises sandstones interbedded with brown-grey shales (Fig. 10.16). The sandstones are very coarse- to fine-grained, white with calcareous cement and notably are feldspar-poor (given the granites available for erosion in adjacent Peninsular Thailand). Clasts in the lithic sandstones are primarily sedimentary with no igneous-derived component. The sequence reaches thicknesses up to 1200 m, and expands towards the western boundary fault of the half-graben.

Unit II. This is the Miocene syn-rift stage. Unit IIa is of Lower Miocene age and in places unconformably overlies inverted and partially eroded half-grabens containing Unit I. The section is a uniform sequence of grey to dark-grey shales and siltstones with minor sandstones and lignite, at least 1000 m thick. Core analysis of the thin-bedded sandstones shows a high content of lithic fragments with a very low feldspar content. In the basin center the base of Unit IIb is marked by a lacustrine shale sequence. The Middle Miocene section is composed of varicolored claystones and fine- to coarse-grained sandstones up to 1500 m thick, which were deposited in an alluvial-floodplain environment.

Unit III. This is the post-rift sequence (i.e., Chao Phraya Gr), and is composed predominantly of alluvial plain and transgressive marine clays and shales with interbedded medium- to coarse- grained fluviatile sands. Total thickness is c. 500 m. It is assigned a Late Miocene–Recent age.

Marine and palynology