Bulletin of the Geological Society of Malaysia, Volume 22, Dec. 1988; pp. 135 – 151
University of Malaya, 59100 Kuala Lumpur, Malaysia
Abstract: Eastern Borneo has been nucleated since Late Cretaceous time around the Miri Zone, whose basement appears to be a microcontinent rifted from the shelf of Vietnam and south China. The eastern margin of the Miri Zone is interpreted as an Atlantic-type margin, with down-faulted continental crust giving way eastwards to Late Cretaceous to Eocene oceanic lithosphere (‘Chert-Spilite Formation‘ and underlying ‘Crystalline Basement‘), of the same age as the ocean floor of the adjacent Celebes Sea.
The NE trending Rajang Group was deposited as a Late Cretaceous through Paleogene turbidite fan directly on the Chert-Spilite Formation. Eastwards subduction of this oceanic basement resulted in the western and northern Sulawesi volcanic arc causing narrowing of the marginal sea gap between the trench and the Miri Zone microcontinent, resulting in shoaling of the infilling Rajang Group flysch to locally give carbonate reefs through the Paleogene.
By Early Miocene time, the Rajang Group had been compressed between the Miri Zone microcontinent and the Sulawesi arc-trench system into a fold-thrust collision orogenic zone. Ophiolite was obducted and it shed blocks of itself into extensive olistostrome deposits of the Dent and Segama Valley areas.
As the Miri Zone collided with the Rajang Group, thrust tectonics resulted in Late Oligocene-Early Miocene granite intrusions in the Long Laai, and Late Miocene in the Mount Kinabalu area. The tin mineralization of the Long Laai plutons is interpreted as mobilized from the underthrust Miri Zone basement.
Aulacogen-like rift arms extended outwards from the opening Makassar Straits as the northern arm of Sulawesi rotated clockwise. The rift system was filled by fluvio-deltaic sediments of the Tanjong Formation, which is oil, gas and coal-bearing in the Tarakan Basin, but unexplored for oil and gas in Sabah, although its coal deposits are well known at Silimpopon.