Tectonic evolution of the NW Sabah continental margin since the Late Eocene

Abstract

The NW Sabah continental margin in the northern part ofNW Borneo, consists of 2 main elements: a Tertiary trench-associated sedimentary basin with up to 12 km of siliciclastic sediments and the Southern South China Sea Platform, separated by the deep and relatively narrow, NE-trending NW Sabah Trough. The Tertiary sedimentary sequences were 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). The sediments were subjected to strong compression to form an imbricate wedge related to the subduction of the South China plate. This was concomitant with the counter-clockwise rotation of the Borneo plate, and the relative motion of the two plates may have produced N-S wrench faults. 2. a post-early Middle Miocene phase of clastic shelf/slope deposition (Stage IV) which prograded northwestward over the underlying imbricate wedge and separated from the latter by a major regional unconformity. On the basis of differences in structural styles and sedimentation histories, the NW Sabah continental margin can be subdivided into 7 tectonostratigraphic provinces: (1) the CrockerAccretionary Prism, (2) the Inboard Belt, (3) the Outboard Belt, (4)the East Baram Delta, (5) the NW Sabah Margin, (6) the NW Sabah Trough and (7) the Southern South China Sea Platform. Some 90 exPloration and exploratory appraisal wells have been drilled in the NW Sabah offshore. Apart from minor oil and gas shows in the pre-early Middle Miocene deep­ marine sediments, all commercial accumulation~ discovered to date have been found in the Middle Miocene or younger clastic reservoirs in the Inboard Belt, Outboard Belt and East Baram Delta. The other tectonostratigraphic province have not been drilled to date. The tectonic evolution of the NW Sabah continental margin occurred in four stages: 1. The Late Eocene to early Middle Miocene subduction of the South China Sea oceanic crust beneath Borneo with deposition and subsequent 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 the regional uplift and erosion of the accretionary prism resulting in the Deep Regional Unconformity. This was followed by NW 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 strong compressional deforma­ tion, probably associated with deep-seated major N-S shear zones. The area was strongly folded, uplifted and eroded resulting in the Shallow Regional Unconformity.