Abstract

A major unconformity at the base of an Early Tertiary sedimentary sequence has been mapped across Kalimantan from the Mahakam River for about 400 km westwards, where it is overlapped by younger Early Tertiary sediments, and continues to within 100 km of the west coast.

The basal sandstone overlying this unconformity broadly follows the contact between continental basement to the south and the Late Cretaceous to Middle Eocene orogenic complex to the north. The dominantly terrestrial sandstone is dated as Late Eocene by foraminifera, pollen and K/Ar dating of volcanics. It is overlain by mudstone succeeded by a sandstone/mudstone unit. A second unconformity truncates this sequence in West Kalimantan and is succeeded by overlapping terrestrial sandstone and mudstone which are equated with marine Oligocene mudstone and limestone in East and Southeast Kalimantan. A third unconformity confined to East Kalimantan is overlain by Miocene deltaic sediments.

An elongate, west-trending basin filled by the Early Tertiary sediments is folded and overthrust along its northern contact with the orogenic complex by north-dipping thrusts rooted in a detachment zone between the orogenic complex and the underlying continental basement.

Petroleum source rock such as lacustrine mudstone is present in the Early Tertiary sequence in West Kalimantan but the overall prospectivity is limited by deformation along its northern margin, tight sandstones and minor intrusives. The upper Mahakam River is also prospective. Gold is hosted by several rock associations from Early Cretaceous to Early Miocene and coal occurs in some fluviatile sandstones.