ABSTRACT: Tectonic Evolution of the NORTHWEST Sabah Continental Margin Since Late Eocene

HAZEBROEK, H. P., DENIS N. K. TAN, and J. M. LAMY, Sabah Shell Petroleum Company Ltd.

The northwest Sabah continental margin, located in the northern part of northwest Borneo, consists of two main elements: a Tertiary sedimentary basin (with up to 12 km of siliciclastic sediments) and the Southern South China Sea platform, separated by the deep and relatively narrow, northeast-trending northwest Sabah Trough.

The Tertiary sedimentary sequence was deposited during two main phases of basin development: (1) A pre-early middle Miocene phase of generally deep-marine clastic sedimentation (Stages I, II, and III). (2) A post-early middle Miocene phase of clastic shelf/slope deposition (Stage IV) which prograded northwestward over the underlying sediment wedge and separated from the latter by a major regional unconformity.

On the basis of differences in structural styles and sedimentation histories, the northwest Sabah continental margin can be subdivided into 7 tectonostratigraphic provinces: (1) Crocker Province, (2) Inboard Belt, (3) Outboard Belt, (4) East Baram Delta, (5) northwest Sabah Shelf/Slope, (6) northwest Sabah Trough, and (7) Southern South China Sea Platform.

Some 95 exploration and exploratory appraisal wells have been drilled in the northwest Sabah offshore. Apart from minor oil and gas shows in the pre-early Middle Miocene deep-marine sediments, all commercial accumulations discovered to date are in the Middle Miocene or younger clastic reservoirs in the Inboard Belt, Outboard Belt, and East Baram Delta. The other tectonostratigraphic units have not been drilled to date.

The following four-stage model for the tectonic evolution of the northwest Sabah shelf is critically reviewed: (1) The late Eocene to early middle Miocene subduction of the South China Sea oceanic crust beneath Borneo with deposition and subsequent imbrication of deep-marine sediments into an accretionary prism. (2) The collision and subduction of the South China Sea attenuated continental crust with Borneo in early middle Miocene which led to regional uplift and erosion of the accretionary prism resulting in the deep regional unconformity. This was followed by NORTHWEST progradation over the Inboard Belt from middle Miocene to early late Miocene. (3) Cessation of active subduction in middle late Miocene was accompanied by major tectonic activities. The Inboard Belt was subjected to s rong compressional deformation, probably associated with deep-seated major north-south shear zones. The area was strongly folded, uplifted and eroded resulting in the Shallow Regional Unconformity. Transtensional tectonics at the western margin of the Belt resulted in the formation of two major depocenters, i.e., the Outboard Belt and the East Baram Delta. (4) From the late Miocene to Holocene, the Inboard Belt remained a shallow stable area, which was continuously eroded till Stage IVF times. In the Outboard Belt and East Baram Delta, a thick prograding wedge built out toward the northwest from Late Miocene to Holocene. A late Pliocene phase of deformation affected mainly the Outboard Belt and East Baram Delta and gentle anticlinal features with numerous crestal faults were formed.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)