Subtropical carbonates in a temperate realm: Modern sediments on the southwest Australian shelf

The southwestern continental margin of Australia (19 degrees S-22 degrees S ) in the western Indian Ocean is transitional between cool-water and warm-w ater carbonate realms. It comprises the incipiently rimmed, flat-topped, st eep-fronted Rottnest Shelf in the south, the uniform subtropical starved Ca rnarvon Ramp off Shark Bay, and the Ningaloo fringing reef in the north. Th e margin is strongly influenced by the poleward-flowing, warm, nutrient-poo r Leeuwin Current, which promotes overall downwelling and strong summer equ atorward-blowing winds, which generate local seasonal upwelling. The structurally quiescent northern part of the Rottnest Shelf, with minimu m accommodation, is characterized by luxuriant stands of seagrasses and mac rophytes 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 f oraminifers, Halimeda is poorly calcified and does not contribute to the se diment, Azooxanthehate corals, although present, are not common, Such a sed iment pattern has many analogs in the geologic record, especially the early Cenozoic, The incipient rim is a morphologically complex linear ridge syst em interpreted to be a buried mid Pleistocene barrier reef or beach-dune co mplex. The northern part of the ridge complex is capped by the Houtman Abro lhos reef platforms. Subphotic sediments on the deep, outer shelf and upper slope, affected by seasonal upwelling, are typical cool-water, poorly sort ed, bryozoan dominated deposits rich in small foraminifers and sponge spicu les, The inner part of the more structurally active Carnarvon,Ramp ranges from s teep eolianite cliffs to hypersaline environments of Shark Bay to the Ninga loo fringing reef. Mid-ramp sediment, especially off Shark Bay, is mostly r elict or stranded and foraminifer-dominated sand with abundant Mg-calcite-c emented intraclasts. These sediments, accumulating on a relatively barren s eafloor, likely represent attenuated carbonate production brought about by downwelling and episodic incursions of saline, Shark Bay-derived waters ont o 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 targe ocea n basins during those periods in geologic history when circumglobal equator ial circulation was active, and equatorial gateways narrow, in contrast to eras of supercontinents, when cool water carbonate sedimentation and upwell ing 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 sedim entation patterns in these regions should oscillate at the 100 ky scale in the stratigraphic record.