Late Quaternary Sedimentation and Peat Development in the Rajang River Delta, Sarawak, East Malaysia
James R. Staub, Robert A. Gastaldo, 2003. "Late Quaternary Sedimentation and Peat Development in the Rajang River Delta, Sarawak, East Malaysia", Tropical Deltas of Southeast Asia—Sedimentology, Stratigraphy, and Petroleum Geology, F. Hasan Sidi, Dag Nummedal, Patrice Imbert, Herman Darman, Henry W. Posamentier
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Thick, domed peat deposits dominate most of the surface of the mesotidal to macrotidal Rajang River delta, tidally influenced alluvial valley, and adjacent coastal plain. Northeast-striking shoreline terrace sands that crop out along the landward margin of the delta and coastal plain and gravel outcrops in the alluvial valley are the surface expression of the VIIa highstand surface of 125 ka (oxygen isotope stage 5e). The upper few meters of the VIIa surface have undergone podzolization, are leached white, and are easily mapped. Near the present coast a peat/lignite bed, at a depth of 80 m, represents the IIIb highstand surface (oxygen isotope stage 3), indicating that 40 m of subsidence has occurred in the last 40 ka. In the alluvial valley, gravel dominates the base of an incised-valley fill 10 km wide and 45 m thick and is overlain by a fining-upward succession, the upper part of which is tidally influenced. Eroded Pleistocene terraces, mantled with thick peat, flank the Recent incised-valley fill. Within interfluve areas in the landward one-half of the northeast delta plain and adjacent coastal plain, thick Recent peat deposits (> 10 m) rest directly on or within a few meters of the buried remains of Pleistocene sediments. These peat deposits began accumulating between 7.3 and 5.8 ka as the rate of sea-level rise slowed. Recent siliciclastic sediments laterally adjacent to these peat deposits are composed of tidally influenced sands, silts, and clays. The seaward one-half of the northeast delta plain, delta front, and prodelta are composed of a seaward-thickening wedge of siliciclastic sediment up to 40 m thick that has accumulated in the last 5 ka. The base of the wedge is marked by a gravel lag that immediately overlies a rooted, yellow-brown alluvial soil. Siliciclastic sediments in this wedge consist of delta-front and prodelta clays and silts, delta-front distributary-mouth sands, and shoreline sands. Young (< 5 ka), reduced-thickness (< 10 m) peat deposits lie conformably on top of this wedge in this part of the delta plain. In contrast, the southwest part of the delta plain is not underlain by shallow-depth Pleistocene sediments, and it started to prograde into the South China Sea prior to 7 ka; its surface is dominated by beach ridges and gley soils mantled by mangrove–Nipa vegetation.
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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.