Deltas in the Gulf of Carpentaria, Australia: Forms, Processes, and Products
Brian G. Jones, Colin D. Woodroffe, Glenn R. Martin, 2003. "Deltas in the Gulf of Carpentaria, Australia: Forms, Processes, and Products", 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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Deltas in the shallow marine epicontinental Gulf of Carpentaria have developed in the slowly subsiding Karumba Basin and are influenced by monsoonal fluvial discharge and a diurnal tidal regime. The McArthur delta is protected from wave action by offshore islands, and the restricted width of the delta is a function of bedrock outcrops near the delta mouth. The upper delta plain is characterized by fluvial lateral-accretion point-bar deposits. In the lower delta plain, progradation has resulted in a buildup of shelly delta-front sands overlain by muddy intertidal and supratidal deposits. The latter dry mudflats are areas of wind deflation and may equate to emergent surfaces in older analogues. Fluvially active distributary channels have a relatively uniform width whereas abandoned channels adopt a tapering tide-dominated form. This delta shows progressive influence of riverine processes downstream onto the lower delta plain. Although the tide-dominated channels have a high sinuosity, their patterns reflect former fluvial channels.
The Gilbert River, by contrast, is not bedrock-controlled and contains a prograding coastal wedge of Holocene sediment extending laterally for a distance of 125 km along the coast. The subaerial portion of the delta has prograded 15–20 km during the past 6,500 years, and the subsurface facies show prodelta mud overlain successively by delta-front and subtidal sands, strandline beach and chenier ridges, and intertidal to supratidal mudflats. Thin floodplain deposits cover the inner portion of the delta. Changes in the locus of sedimentation result from fluvial avulsions and account for age variations in the local subtidal sand and beach-ridge accumulations.
Deltas in the northern Australian region are not adequately defined by simple morphological classifications. They drain from geologically mature landscapes and illustrate complex morphological patterns that develop in response to specific tidal and fluvial regimes. No major incision would have occurred around the Gulf of Carpentaria during the last low stand of sea level because the onshore and offshore gradients are equivalent, and the Holocene deposits have built out as temporary wedges of sediment (up to 30 km wide and 10 m thick) adjacent to the present shoreline.
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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.