Chapter 20 The continental shelves of SE Australia

Abstract

Abstract The continental shelves of SE Australia from the eastern Great Australian Bight to southern New South Wales are situated within a tectonically stable, passive continental margin. The shelves began to develop during Late Cretaceous time and preserve a rich but punctuated record of shallow-marine sedimentation from the Late Paleocene to the present day. The southern margin is the world's largest modern, cool-water temperate carbonate sediment province, a function of the limited terrigenous-clastic sediment being delivered to the coast in response to continental-scale aridity, endorheic drainage and low topographical relief. With the exception of the South Australian gulfs and the protected portions of Bass Strait, the entire shelf sector of SE Australia is a high-energy, storm- and swell-dominated, microtidal province. Along the southern margin, the dominant sedimentary constituents in the modern shelf sediments are skeletal carbonate bioclasts derived from marine invertebrates that include molluscs, foraminifers, bryozoans, coralline algae and echinoids, variably intermixed with quartz sand and heavy minerals (Heterozoan Association). Sediments of Pleistocene and Holocene age are compositionally similar to their modern equivalents, and all show varying degrees of reworking and incorporation of older bioclasts to form the palimpsest sedimentary successions that characterize the entire region. The southern New South Wales sector of the Eastern Australian Shelf is dominated by siliciclastic sediments but preserves relict molluscan-rich carbonate sediments of glacial-age lowstand origin in the outer-shelf region in present water depths exceeding 100 m. The Quaternary stratigraphical record of the SE Australian continental shelves is dominated by successions that formed in the last glacial cycle, but older, inner-shelf successions have been documented from geographically restricted regions such as the South Australian gulfs, Roe and Coorong coastal plains, and the Bass Strait Islands. The outer-shelf successions are not in hydrodynamic equilibrium with modern shelf processes, as reflected in the paucity of mud, and are either largely relict (New South Wales) or dominated by complex modern bryozoan associations.