OLIGO-MIOCENE CANYONS IN THE GAMBIER SUB-BASIN, SOUTHERN AUSTRALIA—DEEPWATER ANALOGUES FOR PETROLEUM EXPLORATION

Abstract

The Gambier Sub-basin lies on the southern Australian passive continental margin that formed during continental breakup and seafloor spreading between the Australian and Antarctic plates. In addition to the numerous modern submarine canyons reported on the southern Australian margin, three palaeo-canyon systems have been identified within the Gambier Limestone of the South Australian Gambier Sub-basin. Favourable environmental conditions during the Oligocene and Early Miocene led to deposition of the Gambier Limestone, a widespread, prograding extra-tropical carbonate platform. A world-wide glacio-eustatic sea level fall in the Early Oligocene exposed the shelf in the Gambier Subbasin, causing widespread erosion and minor fluvial incision on the shelf and subsequent formation of nick points at the shelf edge. During the following marine transgression later in the Oligocene, the shelf was inundated and the nick points provided conduits for erosive turbidity currents to enlarge the canyons to the spectacular dimensions observed on seismic data. No less than 20 successive canyon cut and fill events ranging from Late Oligocene to Middle Miocene have been observed and mapped on seismic data across the shelf in the Gambier Sub-basin. The thick, dominantly fine-grained carbonate sheet logically represents a potential regional seal to underlying clastic reservoirs. However, the possibility exists for carbonate reservoir sands to be present within the palaeo-canyons, sealed by surrounding fine-grained carbonates. Although no hydrocarbons have yet been identified in the carbonates of the Gambier Sub-basin, the canyons provide an analogue useful for establishing the scale, internal architecture and geometry of canyon fill systems.