Abstract

The Melawi and the Ketungau Basins of West Kalimantan 'are located south and north respectively of Cretaceous melange (Semitau subduction complex) which formed a structural high throughout most of Tertiary times. The southern boundary of the Melawi Basin is an unconformity between early Tertiary arkose of variable thickness and intermediate to acid Mesozoic plutonic and volcanic rocks which are probably a product of Early Cretaceous subduction. The northern basin developed separately between the inactive Semitau subduction complex and an active Tertiary subduction zone marked by the Lubok Antu melange on the Malaysian border. The Melawi Basin succession consists of an undated black shale overlain by a thick sequence of Late Cretaceous and Early Tertiary inter-bedded sandstone and mudstone, capped by a distinctive fluviatile quartz sandstone.

The base of the Ketungau Basin is not exposed, but a thick lower lithic arenite sequence is succeeded, in common with the Melawi succession, by sandstone, siltstone, and reddish mudstone with abundant bivalves, passing upwards into thick fluviatile sandstone with minor flood-plain mudstone.

Along the northern margin of the Melawi Basin, the Tertiary succession is capped by thick lacustrine black shale with turbidite sandstone beds. The youngest beds of the Ketungau Basin are again sands and shales with thin coal beds; the shales, however, commonly have a marine fauna.

Extensive acid tuffaceous beds recently discovered in the Melawi Basin. appear to define a unique stratigraphic interval in Late Cretaceous times. Radiometric age determinations of Late Oligocene to Early Miocene intrusive diorite plugs provide valuable evidence towards the age and correlation of the dominantly freshwater sedimentary rocks.

Gentle symmetrical folding in the Melawi Basin sequence is disharmonic with strongly asymmetrical folds on variable hinge direction, concentrated along the northern margin of the basin. An episode of such intense folding is not evident in the Ketungau Basin except perhaps in the steeply dipping sediments along its northern margin, adjacent to the Lubok Antu Melange. The more intense folding is due to the overriding of the younger sequences from the north which produces a fold-thrust belt 200 km long and 50 km wide.

Petroleum potential of the area appears to be upgraded by the recognition of marine Cretaceous sediments at depth and by the restriction of igneous intrusions and intense deformation to particular parts of the basins.