Abstract

Borneo, located in equatoria 1 SE Asia, is the third largest island in the world, although it is topographically low. One of the unusual features of Borneo is the large amount of Cenozoic clastic sediments that have accumulated in several basins on and around the island. The ultimate source areas of the Cenozoic sequences have been suggested to be either mainland SE Asia/Indochina or Borneo itself. During the Paleogene until the earliest Miocene, deep marine turbidites were deposited in an accretionary wedge setting. Provenance studies on these Paleogene sediments based on detrital modes of sandstones and heavy mineral studies show that they have all been derived from a recycled orogenic source. Upper Cretaceous to Eocene sediments, the Sapulut and Trusmadi Formations, are compositionally mature (quartzose recycled), and may have been derived from mainland SE Asia/Indochina. During the Eocene there was an important change in sediment maturity and a provenance shift from quartzose to intermediate recycled sandstones. Heavy mineral studies show that the Eocene-Oligocene sediments of the Crocker Formation are mainly derived from granite, and that granite debris has been derived directly from its source or has been only slightly recycled/transported.

The ultimate source area for these sediments was probably the Schwaner Mountains of southern Borneo, although a minor component of ophiolite debris suggests derivation from nearby basement of northern Borneo. The relative immaturity and heavy mineral suites of the Crocker Formation indicate it was derived from Borneo itself rather than SE Asia/Indochina, but primarily from basement sources rather than by recycling of older sediments.