Oceanographic Control of Photozoan and Heterozoan Carbonate Sedimentation During Transgression of a Modern, Tropical, Oceanic Ramp, North West Shelf, Australia

Noel P. James¹, T. Kurtis Kyser¹, Yvonne Bone², George R. Dix³, and Lindsay B. Collins^4
1 Queen's University, Kingston, ON
² University of Adelaide, Adelaide, Australia
³ Carleton University, Ottawa, ON
⁴ Curtin University, Perth, Australia

Surface sediments on this vast ramp are a mixture of particles formed during the last glacial maximum lowstand, subsequent transgression, and the recent Holocene highstand under conditions of evolving oceanography. The seafloor is strongly affected by cyclonic storms, long-period swells, and large internal tides. Circulation is dominated by the south-flowing, low-salinity Leeuwin Current, upwelling, seaward flowing saline bottom waters generated by seasonal evaporation, and fluvial flash-floods. Surficial outer ramp sediment is mainly pelagic, veneering lowstand inner ramp grass-flat sediments. Local phosphatic elements indicate periodic upwelling. A ridge of pelagic sand reflects enhanced near-surface productivity related to ocean current shear. Modern mid-ramp deposits, dominated by sedimentation during early transgression, are widespread stranded ooid-peloid sands that 14C dating shows formed between 15 and 12 Ka. Resumption of Leeuwin Current flow arrested ooid formation and Holocene deposits are principally biofragmental and heterozoan. Inner ramp sediment is a mixture of heterozoan and photozoan elements. Depositional facies reflect episodic environmental perturbation by riverine-derived sediments and nutrients, resulting in a mixed habitat of oligotrophic (coral reefs and large benthic foraminifers) and mesotrophic (macroalgae and bryozoans) indicators. This ramp system in a tropical arid climate has important applications for the geological record; mid-ramp sediments with bedforms created by internal tides can form in water depths exceeding 50m, saline outflow can arrest or dramatically slow mid-ramp sedimentation mimicking maximum flooding intervals. Changing oceanography during transgression can profoundly affect sediment composition, sedimentation rate, and packaging.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005