DEPOSITIONAL MOTIFS OF PALEOZOIC COLD-WATER CARBONATES (PERMIAN, AUSTRALIA)

Slow development of depositional models for cold-water carbonate systems has affected our ability to recognize and interpret such deposits in the rock record. The Lower-Middle Permian System of Australia contains a spectrum of limestones that were deposited in interior basins, atop structural highs, and along open marine shelves in and around glaciated regions of Western Australia, Queensland, New South Wales, and Tasmania. The deposits are not extensive, but they are conspicuous because they are encompassed by fossil-poor and siliciclastic-dominated rocks that record the acme and waning stages of the Late Palaeozoic Ice Age. Examination of the distribution, context, facies, and biota of these limestones reveals several motifs: (1) close association with glacimarine deposits and, in open shelf settings, cold-climate indicators; (2) highly localized distribution; (3) high-abundance assemblages of exceptionally large, calcitic benthos that may be dominated by a single group of organisms (i.e., bryozoans or Eurydesma); (4) absence of unambiguous photozoans; (5) clastic depositional textures with micrite-poor matrices; (6) a predominance of grain-supported fabrics with grains in sutured contact; and (7) extensive post-mortem bioerosion of grains. These motifs reflect very low sedimentation rates under conditions that inhibited early lithification. Low-diversity populations of robust benthos speak to cold and otherwise stressed environments. In interior basins, a similar assemblage persists across a broad range of lithologies. By contrast, assemblages in shelf settings exhibit strong partitioning with mud content, reflecting greater stratification in depositional energy, nutrient availability, and/or temperature. The isolated nature and relative scarcity of the limestones overall indicates that carbonate deposition was easily overwhelmed by terrigenous influx. The motifs described here provide a well-defined set of criteria that can be used to distinguish heterozoan carbonates that formed in cold settings from those that formed in warmer environments. Across Australia, regional differences in oceanographic and tectonic context imparted variations on these motifs that allow refinement of depositional models for Paleozoic cold-water carbonate systems.