Seismic and Multibeam Expression of a Clastic Shelf to Deepwater Sediment Delivery System, Eastern Australian Continental Margin

Simon Lang¹, Ross Powell², Ron Boyd³, Kevin Ruming³, Ian Goodwin³, Tobias Payenberg⁴, and Marianne Sandstrom^4
1 University of Adelaide, Adelaide, Australia
² Northern Illinois University
³ University of Newcastle, Callaghan, Australia
⁴ Australian School of Petroleum, University of Adelaide, Adelaide, Australia

Seismic profiling and multibeam swath mapping covering >4000km² in 20m to 4300m offshore from Fraser Island, focussed on the terminus of a 2000km long, clastic littoral drift system meeting the shelf break on the eastern Australian continental margin. Sediment pathways were mapped showing highstand shedding of clean sandy clastics directly to deepwater via narrow canyons and gullies incised into a relict (Pleistocene and Miocene) carbonate platform. On the shelf, seismic reveals a small incised valley feeding a lowstand delta wedge covered by a broad sheet of tidal delta sand bodies. On the slope, seismic reveals canyon/gully clastic fills, inter-canyon/gully slope apron drapes, and a giant debrite field (20km x 15km). The lower slope shows channels and levees leading to the abyssal plain.

Of particular interest were deeply incised, shelf margin gullies funnelling shallow shelf sands directly into canyons cut deeply into interbedded slope sediments and underlying sediments. Grab samples from the canyons reveal clean quartz sands that feed an axial canyon parallel to the toe of the slope (>3900m) containing a channel and levee system that feeds a frontal splay complex (>4300m). On the slope, outside the gullies and canyons, is a drape of stratified hemipelagic clays, and very fine-grained quartz-foraminiferal sands, interbedded with pelagic carbonate foraminiferal ooze, glauconitic clays and fine sands. The muddy debrites show a chaotic seismic character.

This analogue may have useful deepwater exploration implications for canyon fills near wave-dominated margins lying along strike from major lowstand sediment accumulations (e.g. West Africa, NW Shelf Australia).