THERMAL HISTORIES OF TERTIARY SEDIMENTS IN WESTERN CENTRAL SUMATRA, INDONESIA

Apatite fission-track and organic maturity data from the central regio n of Sumatra suggest that Tertiary sediments exposed in the Ombilin Ba sin, in the northern and western margins of the South Sumatra Basin, h ave low to medium thermal maturities (Re-average 0.39-0.50%). Apatite fission-track central ages and length data indicate that, at most, low levels of track annealing have occurred and the sediments have only e xperienced temperatures lower than 120 degrees C. Many samples show a strong component of inherited tracks. In the Ombilin Basin we suggest that the western Talawi sub-basin was not as deeply buried as the east ern Sinamar sub-basin, and probably never received any Miocene sedimen ts. The apatite fission-track age of one basement granite along an int ra-basement high, and forward modelling of fission-track length data f rom the basin, tentatively suggest that cooling (?and uplift) of the w estern Talawi sub-basin occurred in the late Oligocene to early Miocen e. The cause of the sub-division of the basin is ascribed to strike-sl ip tectonics. Late Eocene rocks exposed along the northern and western margins of the South Sumatra Basin, in the present-day back-are area, are undermature. Similarly aged and younger rocks in the subsurface o f the Central and South Sumatra Basins are reported to be mature/overm ature. This suggests that the areas of outcrop studied were not part o f the main Paleogene-Neogene graben system that was subsequently inver ted. Rather the studied areas have only experienced shallow burial dep ths and are likely to represent marginal, rift shoulder, areas of sedi mentation surrounding areas of deep rift sedimentation. (C) 1997 Elsev ier Science Ltd. All rights reserved.