Shoreline evolution from the Late Cretaceous to the Miocene: A record of eustasy, tectonics and palaeoceanography in the Gippsland Basin

Well-developed clastic shoreline systems have been deposited in the Gippsland Basin over a 70 Ma period, from the latest Cretaceous to the present day. Twenty-three stacked coastal units have been mapped and described in the Traralgon and Balook formations of the Latrobe and Seaspray groups. These units are made up of prograding and backstepping shoreface deposits, which, in plan view, display well-developed strandline geometries at the terrestrial-marine interface. Shoreface deposits are interpreted to include prograding beach, barrier island and transgressive beach deposits. The lower coastal plain is characterised by persistent deposition of coals despite changes in shoreface type and significant palaeoclimate fluctuations. These coastal deposits display approximately 123 km of transgression from the Late Cretaceous shoreline at the base of the study interval to the mid Miocene Yallourn Formation near the top. The Late Cretaceous and Palaeocene shoreline deposits individually prograde and are separated by flooding surfaces reflecting eustatic changes. Overall they display long-term backstepping behaviour (retrogradation) as a result of basin subsidence. Though shorelines of the Eocene-Miocene are distinctly transgressional, probably reflecting basin subsidence, in the mid-Miocene they become individually and collectively progradational. A series of unconformities in the Oligocene, coupled with Miocene progradation, likely reflects a combination of a compressional tectonic regime and glacioeustatic fluctuations. Despite major changes in the tectonics and palaeoclimate, basin subsidence appears to be the dominant driver for changes in shoreline location.