Quaternary and modern environments of the Van Diemen Rise, Timor Sea, and potential effects of additional petroleum exploration activity

Environmental baseline data are required for Australias offshore areas before petroleum exploration. This has boosted demand for analysis and interpretation of the Quaternary evolution, modern geological processes and environmental features of the Australian continental shelf. One such area overlies the Van Diemen Rise on the Sahul Shelf in the eastern Timor Sea. Sea bottom sediments in the region are dominantly calcareous sand derived from skeletal carbonate material. A number of sinuous channel-like features cut through a series of terraces and banks which comprise the Van Diemen Rise. These features are the product of subaerial exposure and weathering of the underlying carbonate shelf during the last Quaternary glacial maximum. At about 18 000 BP sea level was ~120 m below the present shoreline, much of which was subjected to subaerial exposure and erosion. Only a narrow marine shelf existed close to the outermost edge of the present continental shelf. Shallow banks and shoals on this narrow shelf were the focus of significant coral reef growth. Calcrete concretions formed on the exposed land surface. Holocene transgression inundated the entire margin and shifted reef growth onto the shallowest parts of the flooded banks and terraces. Today sedimentation within the 50 m contour is entirely clastic and derived from wet-season river input. On the outer shelf foraminiferal calcarenites are being deposited. These become finer-grained and contain more planktonic components the greater the water depth. Silty clays and molluscan debris are accumulating in the sheltered channels between the banks and terraces of the Van Diemen Rise. Large foraminiferal and coralline algae dominate the shallow banks and rises. An assemblage dominated by Halimeda is growing on the outermost shelf edge banks. Seismic surveying and drilling are the main activities likely on the Van Diemen Rise. The short-term nature of these activities would cause no long-term impact on environmental conditions. They would cause minor short-term localised disturbance. Rock fragments from exploration drill sites would be dispersed by normal current and tidal action within a short period.