--> Highly Resolved Pleistocene Surface Drainage Revealed by Seismic Survey Data in the Offshore Gippsland Basin

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Highly Resolved Pleistocene Surface Drainage Revealed by Seismic Survey Data in the Offshore Gippsland Basin

Abstract

Software tools are now able to tease compelling palaeo-landscape imagery from seismic survey datasets and this has led to a broadening of the seismic interpretation community. Exploration seismic survey data is now frequently used in marine archaeological, geomorphological and volcanology applications. Groups working in the former two themes have demonstrated over the last decade that legacy 3D seismic survey data often contain enough high frequency components in the top 2–3 milliseconds of stratigraphic signal to finely resolve surface drainage systems that etched shallow continental shelves during geologically recent lowstand exposures. These are systems that operated during maximal coastline retreat at the height of ice ages over the last few million years. This research follows that template to reveal the most recent systems of surface drainage across the shallow continental shelf of the Gippsland Basin. An integrated sedimentological, 2D seismic and aeromagnetic study in 2003 produced a map of Pleistocene barrier bars and drainage channels across the shallow shelf of the Gippsland Basin. Elements of this map are reproduced by the current approach though many of the more ambiguous landscape features are clarified. The general trend of drainage is shown to have a stronger eastward component perhaps suggesting these features are more recent than previously thought. One seismic survey in particular reveals a substantial discharge and sedimentation belt in the vicinity of the modern northern coastline. This is probably the palaeo-Snowy River system, which appears to have travelled with a strong westward component as sediment was shed before winding around to adopt the regional trend across flatter, less accommodating terrain, as it headed towards the coast. Evidence of greater drainage gradient to the north may relate to relative uplift of the shelf in this area postulated to have occurred ~0.2 Ma. However, westward river flow presumably had to pre-date uplift of the Baragwanath Anticline and consequent exhumation of foreland ridges to the west (~0.25 Ma). A seismic sequence stratigraphic model is presented to account for evolving dominance between these controls. However the sensitivity of the model to seismic time resolution at the scale of this investigation demonstrates the broad scope for interpretation when modelling palaeogeography at greater depths where the high frequency component of the seismic signal has been significantly attenuated.