Origin of the Late Neogene Roe Plains and their calcarenite veneer: implications for sedimentology and tectonics in the Great Australian Bight

Abstract

The Roe Calcarenite, a 2 – 3 m-thick, soft, quartzose molluscan sand of grainstone to rudstone texture, is a critical unit for deciphering the geodynamic and sea-level history of the southern Australian continental margin. Biostratigraphic and Sr-isotope analysis of molluscs and brachiopods confirms that the unit is Late Pliocene. Amino acid racemisation analyses indicate a minimum age of Early Pleistocene. The general depositional environment was a shallow illuminated shoreface to the inner shelf with the seafloor probably covered by seagrass, much like the modern seafloor offshore the Roe Plains today, but perhaps somewhat warmer. The calcarenite lies on an interpreted marine erosion surface cut into Upper Oligocene to Middle Miocene Eucla Group cool-water carbonates. Such planation, which affected all of the inner shelf, took place throughout the Early Pliocene due to the combination of uplift via basin inversion and eustatic highstand. The process, which led to *85 km of cliff retreat in *3 million years, is interpreted to be a variant on the shaved shelf process operating on the shelf today. The Roe Calcarenite is envisaged as the last of many calcarenites deposited during small-scale highstands that were eroded during subsequent transgressions. It is preserved because the Roe Plains were uplifted immediately after deposition, part of a widespread Plio-Pleistocene boundary tectonic event. It has been continuously exposed since uplift and subject to arid-zone pedogenic diagenesis. This succession is a relatively quiescent example of uplift, erosion and deposition related to basin inversion. It was much less intense than coeval events further east in the St Vincent, Otway and Gippsland Basins. Together, these Late Neogene tectonic-sedimentary packages illustrate the spectrum of stratigraphic successions that might be expected from basin inversion along an otherwise passive continental margin.