A Carbonate-Incised Valley Filled by Shelf Sand Dunes – Model of Formation and Exploration Implications

Tobias H.D. Payenberg¹, Ron Boyd², Kevin Ruming², Jason J. Roberts³, and Simon C. Lang^4
1 Australian School of Petroleum, Univeristy of Adelaide, Adelaide, Australia
² University of Newcastle, Callaghan, Australia
³ Woodside Energy Ltd, Perth, Australia
⁴ University of Adelaide, South Australia, Australia

A partially-filled incised valley has been recognised on the outer eastern Australian continental shelf north of Fraser Island/Hervey Bay, using sparker seismic and multi-beam sonar data. The valley (50 m deep, 600 m wide and 4km minimum length) cuts into a carbonate platform of Neogene to Quaternary age, and approximately divides modern clastic sediments to the south from the Great Barrier Reef to the north.

The incision was cut by the Mary/Burnett Rivers currently entering Hervey Bay when sea level was at a lowstand around 15-20 kybp. However, the valley fill architecture is distinctly different to classic studies of incised valley fills. Multibeam backscatter data shows clastic, quartz rich subaqueous sands dunes with an exotic provenance driven by modern longshore sediment transport and shelf tidal processes migrating across the top of the carbonate platform at a right angle to the valley and fill it by cascading down the channel walls at current highstand conditions.

At the lowstand, sea level would have significantly dropped below the shelf break, which currently lies around 30-50 mbsl, and incised the lowstand valley. Arid climatic conditions during this glacial maxima favoured stream down-cutting of the carbonate substrate. The subsequent transgression occurred in a more humid climate that favours muddy rivers, producing an under-filled valley on a carbonate platform devoid of coastal sediments. Implications for existing incised valley systems facies models are significant, because the valley fill will comprise clean, shelf sandstone with little compartmentalisation typical of fluvial to estuarine incised valley fills.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005