Anatomy of a Mixed-Process Shelf-Margin Delta, NW Australia: An Example of Process-Based Classification and Uncertainty Management from a Seismic Dataset

Bourget, Julien; Thompson, Sophie L.; Ainsworth, Bruce

Shelf-margin deltaic sediments accumulated at the edge of the wide (600 km) Bonaparte continental shelf (NW Australia) during the Late Quaternary. However, this shelf-margin delta lacks evidence of both major fluvial incision and significant turbidite system development basinward. The presence of an intra-shelf basin in the centre of the continental shelf enhanced the development of a brackish epeiric sea during periods of sea-level lowstand. This prevented fluvial valleys from fully incising the continental shelf during regressive periods and from reaching the shelf-margin at lowstands, since the incised fluvial systems debouched into the intra-shelf epeiric sea. Instead, the lowstand sedimentary transfer from the continent to the shelf-margin was achieved through tidal exchange in the 200 km-long Malita Valley, which connected the intra-shelf epeiric sea and the outer shelf areas.

Therefore, the Bonaparte shelf-margin delta was fed by a lowstand estuarine complex. Despite the lack of major river input, the shelf-margin delta developed over a vast area, comparable with the wave-dominated, fluvial-influenced (Wf) Nile and Niger lowstand deltas. It challenges many paradigms for shelf-margin deltaic sedimentation and has implications for our understanding of shelf-to-basin sediment transfer mechanisms.

Here we use a dataset combining very-high resolution 2-D seismic, conventional 3-D seismic and well data to unravel the stratigraphic evolution of the shelf-margin delta during the Late Quaternary. Near outcrop-scale, high resolution 2-D seismic and attribute analysis techniques on 3-D seismic are used to characterize the depositional element complexes of the shelf-margin delta. We apply a new process-based classification scheme to characterize the depositional architecture of this wave-dominated, tide-influenced, fluvial-affected (Wtf) shelf-margin delta. The delta can be subdivided into progradational episodes (Element Complex Assemblages; ECA). The ECA are constructed from fluvial, tidal and wave generated features (Elements and Element Complexes; EC) that can be observed directly from the seismic data. Process interpretations from each ECA can thus be made from its constituent EC components. The temporal and spatial evolution of processes on the delta can therefore be mapped. Poor seismic resolution in some areas leads to uncertainty in some interpretations, however, multiple potential process interpretations can then be carried forward in these cases.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013