THE ROLE OF GRAVITY-DRIVEN SEDIMENTATION IN THE EVOLUTION OF SALT WITHDRAWAL INTRASLOPE MINIBASINS: INSIGHTS FROM NORTH-CENTRAL GREEN CANYON, GULF OF MEXICO

Three-dimensional (3-D) seismic reflection data from Fuji basin, a salt-controlled intraslope minibasin in north-central Green Canyon, Gulf of Mexico, reveal a complex interaction between gravity- and suspension-driven sedimentation. Late Pleistocene (~470 ka) to Holocene fill within Fuji basin consists of approximately 45% mass transport complexes (MTCs), 5% channelized sandy turbidites, and 50% hemipelagites and muddy turbidites. At least ten MTCs within Fuji basin flowed radially towards its depocenter, either from basin flanks (i.e., intrabasinal) or as a result of larger scale salt motion (i.e., extrabasinal). Sediment transport directions are inferred on the basis of elongate basal erosional relief and smaller-scale scours, head-scarps, fold axes within the complexes, and stratigraphic thinning trends at the lateral and downdip margins of lenticular and wedge-shaped bodies. Channelized sandy turbidites in the basin are characterized by sinuosity and the development of channel complexes. An amalgamated set of three such complexes < 350 m thick and 3 km across, and located near the base of the volume, represents the main sand delivery pathway into Fuji basin. These deposits are thought to be due to shelf bypass, and possibly, proximity to the Pleistocene shoreline. Hemipelagites and muddy turbidites are relatively homogeneous, and their thickness is relatively consistent at basin scale. This third facies represents background sedimentation.

A process-driven model has been developed involving halokinetic autocyclicity as the primary control on sedimentation in Fuji basin. Passive salt motion accounts better for both directions of sediment transport and the frequency of late Pleistocene-Holocene MTCs than currently popular eustatic and steady-state bathymetric models. The conclusion is significant in casting doubt on the generally assumed importance of eustasy in controlling off-shelf "lowstand" sedimentation, and in implying marked variations in stratigraphic details at length scales of < 10 km.