SEDIMENTOLOGICAL AND GEOCHEMICAL ATTRIBUTES OF RECENT NERITIC COOL-WATER CARBONATES: SOUTHERN AUSTRALIAN MARGIN

The continental margin of southern Australia is the site of extensive Quaternary cool-water carbonate sedimentation. Seafloor sediments are palimpsest; a mixture of Holocene biofragments and late Pleistocene relict and stranded particles. Relict sediments are interpreted to have been produced during Marine Isotope Stages 3-5 and are preserved as iron-stained intraclasts. In the Great Australian Bight such grains are photozoan in character and indicate that the wide shelves were bathed in shallow, relatively warm, oligotrophic waters and floored by marine grasses, similar to many modern shallow inboard embayments. In contrast, relict sediments on the continental shelf to the east are more heterozoan in character, with coralline algal and molluscan facies on the inner shelf and bryozoan sediments on the outer shelf. Stranded sediments, marooned during sea level rise associated with Marine Isotope Stage 2, are abraded and composed of both biofragments and intraclasts. These particles are mainly heterozoan in character throughout the region, except for photozoan marine grassbank facies localized to portions of the inner Great Australian Bight. Stranded sediments indicate deposition in cool, nutrient-rich waters on an overall narrow shelf with an areally restricted photic zone. Optical, mineralogical and chemical characterization of the Holocene and Pleistocene sediments reveal a progression of seafloor alteration that includes abrasion (rounding and size reduction), aragonite dissolution, and the coating of biogenic carbonate by Fe-oxides. Trace element analysis of Holocene grains suggests early onset of iron staining in an oceanographic setting. In spite of meteoric exposure of the relict sediments C and O isotopic signatures of relict molluscs are indicative of the marine environment. All told, seafloor diagenesis in this cool-water carbonate realm seems to be more effective than meteoric diagenesis in resetting the sedimentological and chemical signatures of the sediments before burial.