Sediment Dispersal Pattern off an Eroding Delta on the West Coast of Taiwan
James T. Liu, Ray T. Hsu, Jeong-Shang Huang, Shenn-Yu Chao, 2003. "Sediment Dispersal Pattern off an Eroding Delta on the West Coast of Taiwan", 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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The Tseng-wen River is a small monsoon-regulated mountainous river on the wave-dominated west coast of Taiwan. Historical maps reveal a trend of shoreline accretion through delta progradation during the past two and a half centuries until the mid 1970s. Two decades after the completion of a reservoir in the upper reaches of the river in 1974, the depositional setting at the river mouth gradually changed from a progradational deltaic system to an estuarine system. Presently, during the flood season, most of the river-borne sediment is in the mud-size fraction. On the time scale of a tidal cycle, the area of initial river sediment discharge is restricted to the nearshore region in the immediate vicinity of the river mouth. Because of tidal movements in the Taiwan Strait, dispersal patterns of river-borne sediment are dominated by longshore tidal currents that display bilateral swings in the course of a tidal cycle. Yet, on a long-term basis the river discharge is the most important sediment source for the coast and nearshore region. One other important sediment source for the area is the reworked offshore palimpsest sediments. After initial deposition, wave agitation is the most important mechanism affecting sediment entrainment. Several lines of evidence, including grain-size distributions on the sea floor (on the scales of years), measurements of in situ flow and turbidity (on the scale of days), and numerical model simulations of the plume (on the scale of tidal cycles) indicate net northward (flood-oriented) sediment dispersal. The Tseng-wen River represents the type of river that has dwindling influence on the surrounding coast in terms of sediment supply and textural imprint.
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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.