Sedimentology and Oceanographic Controls on Temperate, Shallow-Water Carbonate Deposition: Spencer Gulf, South Australia
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Spencer Gulf is a large, shallow, metahaline embayment along the southern Australian continental margin (~22,000 km2, > 60 m water depth). The gulf experiences inverse estuarine circulation and seasonally fluctuating water temperatures, salinities, and nutrient levels. The gulf floor is covered with temperate carbonate sediments (sands, gravels, and muds) that are skeletal remains of heterozoans; bivalves, coralline algae (rhodoliths), bryozoans, benthic foraminifera, and echinoids are common constituents throughout. These carbonates are interpreted to be accumulating in five depositional environments: dense seagrass meadows, sand flats with sparse seagrass growth, rhodolith pavements, open gravel/sand barrens, and muddy seafloors. Detailed seasonal oceanographic data and hydrodynamic model outputs were used to delimit oceanographic influences on sedimentation patterns. The depth of the euphotic zone is interpreted as the most important control on benthic environment distributions within the gulf. Salinities, hydrodynamics, and nutrient concentrations also affect biological assemblages and facies patterns. Seagrasses and rhodoliths require sufficient light for growth and, thus, compete for space on euphotic seafloors. Seagrasses colonize the shallowest, best-illuminated areas. Rhodolith pavements occur in regions of the gulf where seagrasses are excluded: seafloors where surface-water nutrients promote growth of phytoplankton that decrease light penetration, and energetic channels with high bottom-water currents. Calm, subphotic seafloors are characterized by soft sediment environments (open gravels, sands, and muds). Benthic calcareous organisms are surprisingly diverse throughout the gulf, despite elevated seawater salinities (up to 47‰). Abundant carbonate muds set Spencer Gulf apart from other shallow temperate provinces. The origin of this mud is primarily from the breakdown of skeletal particles via maceration. Mineralogical composition is, therefore, a mixture of aragonite, intermediate-Mg calcite, and low-Mg calcite. Rhodolith pavement deposits were investigated in detail to determine how oceanographic conditions influence sedimentation in these environments. Rhodolith pavements provide habitats for diverse calcareous invertebrates, which can be used as environmental indicators. The rhodoliths themselves within Spencer Gulf, however, do not show morphological differences that can be used as indicators of water energy levels, as some authors have suggested. This study demonstrates that integration of detailed oceanographic data and facies analysis aids interpretations of benthic environments and sediment distribution patterns.