The Stratigraphic Evolution of the Sunda Shelf During the Past Fifty Thousand Years
Till J.J. Hanebuth, Karl Stattegger, 2003. "The Stratigraphic Evolution of the Sunda Shelf During the Past Fifty Thousand Years", 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 Sunda Shelf was extensively exposed during the last glaciation. As a result of the very low shelf gradient, this area is ideal for detailed studies of the sedimentary system and variations of coastal environments in response to sea-level changes. Thirty-six sediment cores from a transect inside the main paleo-valley of the North Sunda River on the middle Sunda Shelf were analyzed by sedimentological and geochemical methods to determine the facies associations. AMS 14C dating of plant material and benthic foraminifera allowed detailed stratigraphic correlation offshore Malaysia for the first time. The drastic environmental transformations from terrestrial to marine conditions and the fate of the paleo–river system on the shelf are also reflected in deep-marine sedimentary records of the southern South China Sea.
Large foresets of a delta system extended basinward following sea-level lowering before the Last Glacial Maximum. A widespread marshy soil formed immediately after exposure, and sediment bypassing dominated during the lowstand. The subsequent sea-level rise caused a stepwise submergence, controlled predominantly by the local morphological conditions. Drowning was restricted mainly to the North Sunda River valley up to 13.5 thousand calibrated years (ka). After 13.5 ka, flooding of the extended Sunda plain (≤ -70 m below modern sea surface) caused a loss of the upper course of the river system combined with a sudden interruption of the terrigenous supply. The transgressive deposits, which were formed during the sea-level rise, were closely related to the rapidly migrating paleo-shoreline. A fully marine regime of condensed muds was established in Holocene times, reflecting the completely new hydrographical conditions due to the final dissection of the remaining fluvial structures, and the opening of the Karimata Strait in the South.
Three stratigraphic units, which are reliably dated offshore for the first time, are connected to regressive (-lowstand), transgressive, and stable phases. They are separated by stratigraphically important discontinuities. These units are correlated to the established Malaysian stratigraphy onshore referring to the transitional unit, the alluvial complex, and to the younger sedimentary cover.
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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.