The palaeogeographic and palaeoenvironmental evolution of a Palaeogene mixed carbonate–siliciclastic cool-water succession in the Otway Basin, Southeast Australia

Abstract

The Otway Basin in southeast Australia contains a thick sequence of Cenozoic shelfal carbonates and siliciclastics that preserve signals relating to the progressive opening of the Southern Ocean since the Paleogene. This multidisciplinary study integrates outcrop and subsurface well data from over 100 wells and bores throughout the Otway Basin with micropalaeontological analyses to constrain the age and palaeoenvironments of the Nirranda Group (Late Eocene to Middle Oligocene) and the Heytesbury Group (Late Oligocene to mid Miocene). These data were used to deduce the Late Eocene to Late Oligocene palaeogeographical evolution of the area. During the Late Eocene paralic high energy siliciclastic shoreline to shelf facies dominated the region, deepening southwards where mid to outer shelf conditions preserved high energy sandy carbonate facies. Above the Eocene–Oligocene boundary low energy inner to mid shelfal silt and muddy sand persisted to the north, deepening southward to carbonate-dominated low energy outer shelf to bathyal marls. The change from siliciclastics to carbonates at the Eocene–Oligocene boundary in the Otway Basin may relate to regional tectonics. In the Early Oligocene, high energy inner to outer shelf sand bodies formed in front of marine to inner shelf mudstone facies; the sand units are likely to have been influenced by strong local longshore drift and ocean swells that increased as the Southern Ocean widened to create a larger fetch. In the eastern half of the basin, later in the Early Oligocene, mixed paralic to inner shelf siliciclastic and carbonate facies were deposited passing to inner to mid shelf marl and mudstone and outer shelf to bathyal marls basinward. During this time, low to high energy shelfal calcarenite, chalk and marl dominated the westerly edge of the basin. The contrast in facies from west and east in the basin is inferred to be due to contrasting terrigenous input, environmental energy and ramp/shelf geometry. By Late Oligocene times (the Clifton Formation) the Otway Basin was dominated by high energy carbonate facies deposited in mid to outer shelf palaeoenvironments. The base of the Clifton Formation preserves a shift in facies and foraminiferal faunas that correlates to the major sea level fall at the Early–Late Oligocene boundary. This sea level fall is related to a major ice advance in Antarctica that corresponds to mid-Oligocene unconformities globally. The switch from low to high energy facies across the Early–Late Oligocene boundary in the Otway Basin suggests that the Southern Ocean swells experienced by the modern Otway coast were well established by Middle Oligocene time, evidence of the strengthening ‘proto‘ Antarctic swell regime. By Early Miocene times, with final deepening of the Drake Passage, the Antarctic Circum-Polar Current formed and predominantly inner to outer shelf marls and limestones were deposited in the Otway Basin.