Late Neogene strandlines of southern Victoria: a unique record of eustasy and tectonics in southeast Australia

Abstract

Digital elevation and magnetic data from the southern Victorian basins (Otway, Port Phillip, Gippsland Basins) have enabled the recognition of a vast Late Miocene – Pliocene strandplain succession that is a correlative of the Murray Basin Loxton – Parilla strandplain. A combination of ferricrete formation and erosional dissection has made the strandline geometries visible on geophysical images and allowed the mapping of strandlines across most of the onshore Otway (often under continuous basalt cover), Port Phillip and Gippsland Basins. Strontium isotope ages from well-preserved molluscan assemblages of the exposed strandplain successions indicates the earliest sediments were deposited above the Upper Miocene unconformity at around 5.8 ± 0.2 Ma (latest Miocene, Beaumaris). The youngest exposed strandplain sediments (from the Jemmys Point Formation, Gippsland) are Late Pliocene (ca 3.0 – 2.5 Ma), although younger strandlines occur offshore. Elevation differences across the strandplain of the Victorian basins give a quantitative measure of cumulative Plio-Pleistocene uplift. A broad east – west axis of regional uplift is present along the Western Highlands – Dundas Tablelands, with maximum uplift in the range of 250 m being indicated. The western extension of this axis affects the Mt Gambier coastal plain (Gambier Axis). The Padthaway High forms another uplift trending northwest from the Dundas Tablelands into the Murray Basin. The unconformable relationship between the Pliocene and Quaternary strandline systems across Victoria may be caused by intensified latest Pliocene Quaternary uplift and/or eustatic changes associated with the development of the Quaternary glacial episode. The strandplain successions of southern Victoria have probably developed during the latest Miocene and Pliocene in response to a period of relatively stable base-level (little uplift or eustatic fluctuation). Recognition of the southern Victorian strandline successions may provide potential new targets for heavy-mineral exploration.