The Miocene palaeoenvironmental and palaeoceanographic evolution of the Gippsland Basin, Southeast Australia: a record of Southern Ocean change

Abstract

The Gippsland Basin in southeastern Australia preserves up to 2.5 km thick Tertiary to Recent marine carbonate sediments included in the Seaspray Group. During the Neogene, the Seaspray Group carbonates of the Mackerel and Flounder petroleum fields lay at the shelfal to bathyal palaeoenvironmental transition, an ideal palaeoceanographic setting to study the Neogene history of events in the evolving Southern Ocean. Seismic, foraminiferal and facies analyses of the Miocene Seaspray Group sediments at the Mackerel-1 and Flounder-1 wells have revealed the following palaeoenvironmental history of the area. Relatively cool to temperate upwelling conditions prevailed in the area during the earliest Miocene. During this time, low energy, upper slope Globigerina ooze (calcareous mudstone) was deposited in the oxygen minimum zone. During the Early to lower Middle Miocene, upper slope marl was deposited in oligotrophic conditions. The presence of neritic subtropical foraminifera and abundant shallow and deep dwelling warm to cool-temperate plankton in the upper slope suggests that warm poorly stratified oceanic conditions prevailed in the region. This upper Early Miocene warm phase and Mid Miocene Climatic Optimum in Gippsland corresponds to a time when global ocean conditions were much warmer and less vertically and latitudinally stratified than at present. These oceanic conditions were associated with a reduced East Antarctic Ice Sheet (a higher sea level), a relatively weak Antarctic Circumpolar Current, and weak atmospheric and wind driven circulation. During the upper Middle Miocene to Late Miocene cool phase (after c. 14.5 Ma) strong upwelling and cooler oceanic conditions prevailed in the outer shelf to upper slope canyon facies in Gippsland. The occurrence of common deep dwelling plankton taxa in this interval suggests that regional oceanic conditions became more vertically stratified during this time. From 14.5 and 5 million years ago, the Middle to Late Miocene palaeoenvironmental change recorded in Seaspray Group sediments corresponds to global cooling and major Antarctic Ice Sheet expansion, when the world's oceans became more vertically and latitudinally stratified. From around 14.5 million years the transition from a warm ‘Miocene Optimum’ to cooler oceanic conditions was associated with a strengthened Antarctic Circumpolar Current and more intense wind driven atmospheric circulation in the Southern Ocean.