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

The Otway Basin in southeast Australia contains a thick sequence of Cenozoi c shelfal carbonates and siliciclastics that preserve signals relating to t he progressive opening of the Southern Ocean since the Palaeogene. This mul tidisciplinary study integrates outcrop and subsurface well data from over 100 wells and bores throughout the Otway Basin with micropalaeontological a nalyses to constrain the age and palaeoenvironments of the Nirranda Group ( Late Eocene to Middle Oligocene) and the Heytesbury Group (Late Oligocene t o mid Miocene). These data were used to deduce the Late Eocene to Late Olig ocene palaeogeographical evolution of the area. During the Late Eocene para lic high energy siliciclastic shoreline to shelf facies dominated the regio n, deepening southwards where mid to outer shelf conditions preserved high energy sandy carbonate facies. Above the Eocene-Oligocene boundary low ener gy inner to mid shelfal silt and muddy sand persisted to the north, deepeni ng southward to carbonate-dominated low energy outer shelf to bathyal marls . The change from siliciclastics to carbonates at the Eocene-Oligocene boun dary in the Otway Basin may relate to regional tectonics. In the Early Olig ocene, high energy inner to outer shelf sand bodies formed in front of mari ne to inner shelf mudstone facies; the sand units are likely to have been i nfluenced by strong local longshore drift and ocean swells that increased a s 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 si liciclastic and carbonate facies were deposited passing to inner to mid she lf marl and mudstone and outer shelf to bathyal marls basinward. During thi s time, low to high energy shelfal calcarenite, chalk and marl dominated th e 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, environme ntal energy and ramp/shelf geometry. By Late Oligocene times (the Clifton F ormation) the Otway Basin was dominated by high energy carbonate facies dep osited in mid to outer shelf palaeoenvironments. The base of the Clifton Fo rmation preserves a shift in facies and foraminiferal faunas that correlate s to the major sea level fall at the Early-Late Oligocene boundary. This se a level fall is related to a major ice advance in Antarctica that correspon ds to mid-Oligocene unconformities globally. The switch from low to high en ergy facies across the Early-Late Oligocene boundary in the Otway Basin sug gests that the Southern Ocean su ells experienced by the modern Otway coast were well established by Middle Oligocene time, evidence of the strengthen ing 'proto' Antarctic swell regime. By Early Miocene times, with final deep ening of the Drake Passage, the Antarctic Circum-Polar Current formed and p redominantly inner to outer shelf marls and limestones were deposited in th e Otway Basin. (C) 2000 Elsevier Science B.V. All rights reserved.