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

The Gippsland Basin in southeastern Australia preserves up to 2.5 km thick Tertiary to Recent marine carbonate sediments included in the Seaspray Grou p. During the Neogene, the Seaspray Group carbonates of the Mackerel and Fl ounder petroleum fields lay at the shelfal to bathyal palaeoenvironmental t ransition, 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 se diments 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 dur ing the earliest Miocene. During this time, low energy, upper slope Globigerina ooze (calcareous muds tone) was deposited in the oxygen minimum zone. During the Early to lower M iddle Miocene, upper slope marl was deposited in oligotrophic conditions. T he presence of neritic subtropical foraminifera and abundant shallow and de ep dwelling warm to cool-temperate plankton in the upper slope suggests tha t warm poorly stratified oceanic conditions prevailed in the region. This u pper Early Miocene warm phase and Mid Miocene Climatic Optimum in Gippsland corresponds to a time when global ocean conditions were much warmer and le ss vertically and latitudinally stratified than at present. These oceanic c onditions were associated with a reduced East Antarctic Ice Sheet (a higher sea level), a relatively weak Antarctic Circumpolar Current, and weak atmo spheric and wind driven circulation. During the upper Middle Miocene to Late Miocene cool phase (after c. 14.5 M a) strong upwelling and cooler oceanic conditions prevailed in the outer sh elf to upper slope canyon facies in Gippsland. The occurrence of common dee p dwelling plankton taxa in this interval suggests that regional oceanic co nditions 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 majo r Antarctic Ice Sheet expansion, when the world's oceans became more vertic ally and latitudinally stratified. From around 14.5 million years the trans ition from a warm 'Miocene Optimum' to cooler oceanic conditions was associ ated with a strengthened Antarctic Circumpolar Current and more intense win d driven atmospheric circulation in the Southern Ocean. (C) 2001 Elsevier S cience B.V. All rights reserved.