Neogene Mixed Carbonate-Siliciclastic Systems from the Northwest Shelf of Australia: Influences of Siliciclastics Distribution from Shallow-Water Carbonate Sedimentation

Carla M. Sanchez¹, Craig S. Fulthorpe², and James A. Austin^2
1Department of Geological Sciences, Jackson School of Geosciences, The University of Texas, Austin, TX
²Institute for Geophysics, Jackson School of Geosciences, The University of Texas, Austin, TX

The Northern Carnarvon Basin (NCB) of the Australian Northwest Shelf (NWS) has been a site of mixed carbonate-siliciclastic sedimentation during the Cenozoic. Extensive 3-D and 2-D seismic coverage, wireline log data, and well completion reports are combined to study the relationships between siliciclastic sediment distribution and shallow-water carbonate platform development.

Successive deposition of heterozoan carbonates, quartz-rich siliciclastics, and shallow-water carbonate platforms (i.e. phototrophic production) occurred during the Neogene in the NCB. Cool-water carbonate deposition on a northwestward-prograding distally steepened ramp prevailed on this passive continental margin from early Miocene to late-middle Miocene. Quartz-rich siliciclastics of the Bare Formation then prograded northeastward over the heterozoan carbonates, perpendicular to the previous trend. Deposition of these siliciclastics coincides with a long-term eustatic fall and global cooling. The internal geometry of this siliciclastic interval, which is dominated by bidirectional offlap of mounded morphologies, suggests that it was deposited by a persistent deltaic system. The Bare Formation, therefore, represents a long-lived siliciclastic depositional interval within a predominantly carbonate margin. This northeastward-prograding siliciclastic interval is capped by seismic features characteristic of shallow-water carbonate platforms (flat-top, steep basinward/landward progradation, high amplitudes, and pull-up effect on underlying strata). An isochron map of the top of this carbonate platform system displays contours generally oblique to the morphology of this margin during the Neogene. We suggest that flat-topped carbonate platform deposition was favored by bathymetric highs created by the sandstone-rich barrier spits and/or delta lobes of the Bare Formation.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas