ORIGIN OF MIXED CARBONATE AND SILICICLASTIC SEQUENCES AT THE MARGIN OF A "GIANT" PLATFORM DURING THE QUATERNARY (BONAPARTE BASIN, NWAUSTRALIA)

A 630-km-wide continental shelf characterized by mixed carbonate and siliciclastic sedimentation formed during the Quaternary across the Bonaparte Basin, NW Australia. During this time interval (similar to 2.6 million years), shelf-margin and slope deposits were disconnected from the inner shelf and hinterland by the 200-km-wide, low-gradient Malita intrashelf basin. In this study, two-dimensional (2D) and three-dimensional (3D) seismic, well log, and core data were used to determine the relative importance of allogenic and autogenic controls on the stratigraphic architecture of shelf-edge and slope deposits at multiple timescales. This work has determined that Quaternary sea-level variations (glacio-eustasy) provided a primary control on the stratigraphic evolution of shelf-margin and slope deposits. The early Quaternary period was marked by the aggradation and progradation of a carbonate margin under global sea-level highstand conditions. The onset of high-amplitude sea-level fluctuations at the Mid-Pleistocene Transition (ca. 0.9 Ma BP) enhanced the development of a mixed clastic-carbonate margin and slope system. During the late Quaternary, long-duration sea-level falls and lowstands and high rates of terrigenous sediment supply resulted in stacked fourth-and fifth-order systems tracts in the form of prograded shelf-margin and slope wedges. Conversely, rapid, high-amplitude, fourth-and fifth-order transgressions between these time intervals enhanced the aggradation of carbonate buildups at the shelf edge. Hence, high-frequency sea-level changes resulted in reciprocal sedimentation similar to many other mixed depositional systems of the late Quaternary. However, the main locus of carbonate and mixed deposition across the Bonaparte Basin shelf margin and slope varied spatially at longer times scales. Indeed, conventional seismic data have revealed that the third-order systems tracts at two separate locations in the Bonaparte Basin (eastern and northwestern shelf-margin) show stratigraphic asymmetry (rimmed carbonate margin vs. shelf-margin and slope progradation), which reversed during the late Quaternary. Our results suggest that this reversal in the locus of carbonate vs. mixed sedimentation was related to the shift of the detrital feeder system (the Malita tidal valley) during a major sea-level fall of the late Quaternary (tentatively ascribed to the ca. 0.6 Ma BP lowstand). This study illustrates the importance of both allogenic and autogenic parameters in controlling the stratigraphic architecture of shelf-margin and slope deposits at multiple timescales, in a very wide, mixed carbonate and clastic depositional setting.