Geosynclinal theory and the organizational pattern of the North-west Borneo Geosyncline

Abstract

The North-west Borneo Geosyncline occupied most of Sarawak, Brunei and western Sabah, extending over a distance of about 800 km in a ne–sw direction. The very thick sequence of strata deposited in the geosyncline ranges in age from late Cretaceous to late Cainozoic; over thirty formations have been defined and these may be classified into four groups: (i) Rajang Group (late Cretaceous to early Miocene): thick intensely folded typical flysch with chert-ophiolite at or near the base. (ii) Baram Group (late Eocene to late Miocene): predominantly argillaceous, with some sandstones and thick limestones. (iii) Plateau Group (late Cretaceous to? Miocene): thick molasse-type continental deposits in the south. (iv) Brunei Group (Oligocene to Recent): estuarine and marine deposits with molasse affinities in the north. Previous syntheses have regarded western Borneo, lying to the south, as the probable foreland, but the author seeks to show that this region acted as a eugeanticlinal ridge and intermediate hinterland. The main eugeosynclinal furrow occupied the present outcrop of the Rajang Group, whereas the later miogeosynclinal furrow lay further north; a submerged miogeanticlinal ridge, not so clearly defined, separated the furrows. The Baram Group was deposited mainly in the miogeosynclinal furrow. A marked dynamic polarity is shown by migration of flysch deposition, orogeny, and molasse deposition, from south to north across the geosyncline; these processes also seem to have migrated along the geosyncline from west to east and north-east. The distribution of the main chert-ophiolite belt along the internal side of the eugeosynclinal furrow and of the late-geosynclinal and post-geosynclinal lavas and acid intrusions in western Sarawak and western Kalimantan follows the typical pattern, with some minor exceptions. A terminal volcanic phase is represented by late Cainozoic lavas on the eugeosynclinal deposits.