Abstract

New field evidence suggests that in north and west Borneo volcanic and epiclastic rocks are more widespread than recorded previously. Many terrigenous sediments of the “North West Borneo Geosyncline” are thought now to be largely volcanic; some turbidites are interpreted as tuffs, chaotic sediments as explosive breccias and some, if not all, cherts are derived probably from silicious ashes.

Structural control is dominantly by deep shearing aligned northeast with right lateral movement, and fold patterns can be related to this shearing in a classic example of basement/cover deformation. Volcanic centres are common at the intersection of shears or shear and anticline, but intervening synclines fill with mixed epiclastic and terrigenous material. Later movement on shears within narrow anticlinal ridges between basins may distort their original circular shape. Circular sedimentary basins could have formed also by cauldron subsidence inside circular or polygonal fractures.

Hydrocarbon accumulations in northwest Borneo are aligned in a similar direction to northeast shear zones visible on land. The Miri-Seria and Ampa-Champion fields are examples of this trend. Future prospecting should not ignore onshore circular sedimentary basins even though accumulations are more likely to be stratigraphic rather than structural traps.