Two main deltaic systems, Champion and Baram, filled in much of the Brunei shelf during the Late Miocene, Pliocene, and Pleistocene. “Fourth-order” deltaic sequences, commonly 50–100 m thick on the outer shelf, have been identified in seismic data, wireline well logs, and biostratigraphic data. Average duration of each Pliocene sequence is between 100,000 and 200,000 years. Tide- and river-dominated deltas were common in embayments formed during late stages of transgression and early highstand, but wave-dominated deltas were prevalent in middle to late highstand parts of sequences. During highstands of sea level, shales and siltstones dominated at shelf margins, whereas reservoir-quality sandstones occur 1–3 km or more landward of the contemporaneous shelf margin. From the Late Miocene through the Pleistocene, the Champion and Baram deltas prograded over thick mobile shales. The Champion delta (to the northeast) prograded approximately 40 km during the Late Miocene and 12 km in the Pliocene. The Baram delta (to the southwest) prograded approximately 40 km during the Pliocene. Delta progradation initiated movement on counter-regional growth faults on the upper slope that continued after the shelf margin prograded across those faults. The Frigate counter-regional fault system had 1–2 km of throw during the Pliocene and Pleistocene along the eastern part of the outer shelf. At the same time, much slower subsidence and even uplift occurred around diapiric shale ridges on inner shelf. Down-to-the-basin normal faults with 1–2 km of throw formed on the western part of the outer shelf (Baram delta) during the Pleistocene. Anticlinal folding and toe thrusts occurred on the middle and lower slope during times of major growth faulting (down-to-the-basin and counter-regional) on the outer shelf.
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It is the objective of this volume to bring to the fore a category of deltas with which many sedimentologists and stratigraphers are, at best, vaguely familiar. It is expected that this volume also will stimulate new research on tropical deltas by highlighting how their facies and stratigraphic architectures differ from mid- and high-latitude ones, by emphasizing their significance to the global sediment budget, and by stressing their uniqueness within a petroleum systems framework. This special publication emphasizes the need for models intrinsic to tropical deltas of Southeast Asia to supplement the more conventional general models currently in vogue, based on past studies of large and small mid-latitude deltas. The papers in this book explore how the combination of these complex factors has shaped deltas in this region. Sedimentological surprises such as distributary channels floored by thick accumulations of fluid mud lend a bit of “mystery” to tropical deltas. We hope that, rather than being merely a summary of tropical deltas, this book may open the door to a new and active phase of sedimentological and stratigraphic research in tropical environments across the globe.