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. Collins4
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