Abstract

The southwestern continental margin of Australia (19 degrees S-22 degrees S) in the western Indian Ocean is transitional between cool-water and warm-water carbonate realms. It comprises the incipiently rimmed, flat-topped, steep-fronted Rottnest Shelf in the south, the uniform subtropical starved Carnarvon Ramp off Shark Bay, and the Ningaloo fringing reef in the north. The margin is strongly influenced by the poleward-flowing, warm, nutrient-poor Leeuwin Current, which promotes overall downwelling and strong summer equatorward-blowing winds, which generate local seasonal upwelling. The structurally quiescent northern part of the Rottnest Shelf, with minimum accommodation, is characterized by luxuriant stands of sea-grasses and macrophytes growing on coralline-encrusted hardgrounds and rooted in sediments rich in coralline algae and larger, symbiont-bearing foraminifers together with abundant cool-water elements such as bryozoans, molluscs, and small foraminifers. Halimeda is poorly calcified and does not contribute to the sediment. Azooxanthellate corals, although present, are not common. Such a sediment pattern has many analogs in the geologic record, especially the early Cenozoic. The incipient rim is a morphologically complex linear ridge system interpreted to be a buried mid-Pleistocene barrier reef or beach-dune complex. The northern part of the ridge complex is capped by the Houtman Abrolhos reef platforms. Subphotic sediments on the deep, outer shelf and upper slope, affected by seasonal upwelling, are typical cool-water, poorly sorted, bryozoan-dominated deposits rich in small foraminifers and sponge spicules. The inner part of the more structurally active Carnarvon Ramp ranges from steep eolianite cliffs to hypersaline environments of Shark Bay to the Ningaloo fringing reef. Mid-ramp sediment, especially off Shark Bay, is mostly relict or stranded and foraminifer-dominated sand with abundant Mg-calcite-cemented intraclasts. These sediments, accumulating on a relatively barren seafloor, likely represent attenuated carbonate production brought about by downwelling and episodic incursions of saline, Shark Bay-derived waters onto the ramp. The outer ramp is either planktic foraminiferal sand, sorted by strong bottom currents, or spiculitic mud. Such sedimentation patterns should typify the western margins of large ocean basins during those periods in geologic history when circumglobal equatorial circulation was active, and equatorial gateways narrow, in contrast to eras of supercontinents, when cool-water carbonate sedimentation and upwelling prevailed in similar settings. Furthermore, because low-latitude trade winds relax during glacial epochs and the cool-water, upwelling belt moves equatorward into otherwise tropical realms, temperate and subtropical sedimentation patterns in these regions should oscillate at the 100 ky scale in the stratigraphic record.

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