Variations in Depositional Systems on Unstable Continental Slopes

A comparison of Mio-Pliocene depositional systems on two salt-based continental continental slopes reveals significant differences in depositional processes and dynamic evolution on the slope. These differences are the result of spatially and temporally variable sediment supply from shelf to slope, as well as the amount of accommodation available on the slope.

In the first locality, the central Gulf of Mexico, the Mississippi River system has provided an enormous flux of sediment since the early Miocene. At the same time, the withdrawal and down-slope displacement of autochthonous salt created large amounts of depositionally-contemporaneous accommodation. Deepwater gravity flow systems were captured in a series of confined minibasins, resulting in the well-documented process of fill and spill. Early sedimentation in the basins is dominated by deposition of laterally continuous, ponded sheet sands. Later sedimentation is characterized by bypass and deposition in discrete channelized systems. The transition from sheets to channels is directly related to shifts in the up-dip location of the shelf-edge Mississippi delta system.

The second locality is Mauritania, western Africa. There, sediment flux was relatively limited during the Miocene, despite the existence of globally-significant delta systems north and south of the study area during the Miocene. Sediment from these deltas was supplied by longshore drift, and crossed the shelf break via a system of submarine canyons. The dominant processes observed are the entrenchment and subsequent infill of numerous canyon systems, which provide most of the resulting accommodation on the slope. Early sedimentation in the canyons is dominated by mass-transport deposits, followed by sinuous channel systems that filled and eventually spilled over the confining banks of the canyons. Once the channels were no longer confined by the canyon margins, they developed into a loosely amalgamated fan lobe with a characteristic elongate wedge shape, consistent with unconfined deposition.