Holocene Coastal Sedimentary Environments of the Coorong Coastal Plain, Southern Australia

Abstract

The Holocene sedimentary environments and coastal landforms of Younghusband Peninsula and its back-barrier Coorong Lagoon are the present day equivalent of the relict barriers (dune ranges) that traverse the Coorong Coastal Plain. These modern landforms, together with their associated ephemeral carbonate precipitating lakes and distinctive marine, lagoonal and lacustrine shelly faunas, provide an interpretive model for the geomorphology, stratigraphy and palaeoenvironments of the Pleistocene coastal successions. Younghusband Peninsula progressively developed as a coastal beach-dune barrier complex following the culmination of the post-glacial marine transgression approximately 7000 years ago, so forming the narrow, shallow water lagoon in its lee that now extends some 150 km along the coast from the mouth of the River Murray to slightly northeast of Kingston SE. The present location of the peninsula relates to the shallow accommodation space associated with the emergence of the Late Pleistocene aeolianite barrier complexes of Robe and Woakwine Ranges. The Coorong Lagoon and its associated ephemeral lakes are the sites of distinctive mineral associations including the formation of dolomite. The beach and dune sands are predominantly of inner-continental shelf derived bioclastic calcium carbonate, comprising the comminuted remains of molluscs, bryozoans, coralline algae, echinoderms and foraminifers. The southern Australian continental margin has hosted such biota since the separation from Antarctica some 43 Ma ago. The region has thus been a major carbonate province since early Paleogene time. Within the corridor between the Late Pleistocene Robe and Woakwine Ranges, regional uplift, together with upward shoaling sedimentation has greatly modified the original environment and illustrates the sequential changes in coastal sedimentary environments during an interglacial highstand event, an analogue for the Pleistocene successions. To the south of Mount Gambier, in an area of faster neotectonic uplift and more restricted sediment supply, the coastal landscapes are dominated by rocky outcrops of Robe Range and cobble flint beaches.