The coastal geomorphology of Wilsons Promontory, Victoria : implications concerning quaternary sedimentation around the shores of Bass Strait.

The distribution of calcareous and siliceous sands around the southern and eastern coasts of Australia is of considerable importance to coastal studies in the region of Bass Strait. Bird has published a map of the southeast portion of the continent depicting the distribution of calcareous and non-calcareous sands from Adelaide as far east as Sydney (Bird, 1967, Fig. 16.1). Jennings & Bird (1964) indicate the dominance of calcareous sand from the area of Shark Bay in Western Australia south and eastwards around the mainland coastline as far as Wilsons Promontory. Calcareous sands are also present on the west coasts of a number of Bass Strait islands and have been described from Flinders Island (Dimmock, 1957) and King Island (Stephens & Hosking, 1932; Jennings, 1959a). Bird (1967) also shows the presence of carbonate sands extending down the west coast of Tasmania, although the extent of this occurrence appears to be limited (see Nicolls & Dimmock, 1965). The siliceous sands, which extend down the eastern coast of Australia, can be traced westwards as far as the southeastern corner of Wilsons Promontory where coarse siliceous beach sands occur in Waterloo Bay. Siliceous sands are present on the eastern and northeastern coasts of Tasmania and similar, low carbonate sands occur on the eastern sides of Flinders and King Islands. In many places the boundary between these sedimentary types is very sharp, beaches having a southwesterly aspect being composed of sand with a carbonate content of 20 - 60% whereas easterly facing beaches only a few miles distant have less than 5% of calcium carbonate, most of which consists of partly fragmented modern shell. The pattern of occurrence of this calcareous/siliceous dichotomy on the beaches bordering Bass Strait provokes a number of questions which require clarification: 1) Why does the carbonate sand so commonly found along the southern coastline of the continent apparently not extend eastwards through Bass Strait? 2) Why is the onshore boundary between these two sedimentary provinces so clearly defined? 3) How has the fluctuation of sea level throughout the Quaternary influenced this sedimentary dichotomy? 4) What part does the change in sediment play in the geomorphic pattern of erosion and deposition in the region? The answer to such questions as these lies in a better understanding of the pattern of past and present sedimentation throughout the region. The effects of subsequent modification and weathering of the exposed newly formed sediments must also be considered