Genesis of Palimpsest Cool-Water Carbonate Sediment on the Continental Margin of Southern Australia

Abstract

Abstract The continental margin of southern Australia is a site of extensive Quaternary cool-water carbonate sedimentation. Seafloor sediments are a mixture of Holocene biofragments and late Pleistocene relict and stranded particles. Relict sediments, produced during Marine Isotope Stages 3 and 4, are generally preserved as iron-stained intraclasts. Such grains in the Great Australian Bight are photozoan in character. They include symbiont-bearing benthic foraminifers and coralline algal particles, 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 the region today. In contrast, east of the Great Australian Bight relict sediments 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 the sea-level rise associated with Marine Isotope Stage 2, are gray to buff in color and composed of both biofragments and intraclasts. These particles are mainly heterozoan in character throughout the region, with subtropical photozoan marine grassbank facies localized to portions of the inner Great Australian Bight. Stranded sediments are interpreted to have accumulated in an areally restricted photic zone and upon an overall narrow shelf covered in cool, nutrient-rich waters. Swell-driven seafloor sediment disturbance to depths of 100 m along the southern Australian margin in conjunction with reduced rates of sedimentation that characterize the cool-water carbonate realm, have resulted in mixing of sediment currently being produced with exhumed relict and stranded sediments generated during previous sea-level stands. The composition of these deposits and the nature of their pre-burial alteration are a function of sea-level history. Late Pleistocene relict sediments originally accumulated in low-energy, inboard environments but were modified in the surf zone during subsequent sea-level fluctuations. This resulted in the formation of intraclasts: abraded, infilled, and cemented biofragments of local origin. The widespread distribution of intraclasts and of composite grain assemblages derived from multiple sedimentary environments that display highly variable degrees of seafloor alteration may be signatures of modern cool-water carbonate assemblages worldwide. While such palimpsest sediments may complicate paleoenvironmental interpretation, their differentiation in the rock record can be utilized to discern subtleties in paleoceanography and sea-level history.