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Shelf-Margin Evolution in an Active Tectonic Setting: The Middle Miocene Western Baram-Balabac Basin, NW Borneo


Basin-scale stratigraphy of active tectonic margins records episodes of tectonism distinct from other controls, such as climate, eustatic sea level and sediment supply variations. Along the NW Borneo margin, Miocene–Present collisional tectonism has resulted in the accumulation of up to 12 km of Neogene strata in the Baram-Balabac foreland basin. The sedimentary infill of this richly petroliferous basin comprises shoreline–shelf to slope–basinal clastic sediments, which provide reservoirs for several billion barrel hydrocarbon accumulations. We integrate analysis of seismic-scale stratigraphy, outcrop facies, onshore structural geologic mapping and biostratigraphic data in the western part of the Baram-Balabac Basin (eastern Brunei and Labuan), and demonstrate (1) a link between coastal–deltaic and deepwater processes and sedimentation, and their reservoir characteristics, and (2) that significant changes in seismic-scale (10–1000 m) stratigraphic architecture caused by tectonic deformation are calibrated with shifts in outcrop-scale (1–100 m) stratigraphic stacking patterns and the variations in dominance of coastal depositional processes (i.e. rivers, waves, tides). A regional unconformity recognized both onshore and in offshore seismic data corresponds to a significant dislocation in the depocentre c. 15–20 km eastwards (paleolandwards) within the Berakas–Labuan Syncline. Below the unconformity, a strongly progradational to aggradational shelf-margin was controlled by active expansion across a basement-linked growth fault (Jerudong Fault). High sediment supply caused progradation of wave-dominated, fluvial- and tide-influenced deltas to the shelf edge, slope instability (forming mass transport complexes) and formation of slope–basinal turbidites. Above the unconformity, seismic horizons onlap a topographic high formed by inversion of the Jerudong Fault, which, coupled with increased subsidence in the adjacent Berakas–Labuan Syncline, resulted in retrogradational stratigraphic stacking, increased coastline rugosity and enhanced tidal power. This is manifested by increased tide-influenced facies and stratigraphic successions interstratified with relatively wave-influenced deltaic/shoreface deposits. Outcrop analysis is significantly improved by recourse to the modern depositional systems, and this aids better understanding and prediction of reservoir heterogeneities in nearby hydrocarbon fields.