Basal Sands to External Submarine Levees: Evidence for Progressive Slope Confinement?

Sand-prone successions at the base of external levees adjacent to erosional slope channel systems are widely reported from outcrop and subsurface investigations. These units form part of the characteristic fining- and thinning-upward motif that many levees ascribe to. Here, a progressive slope confinement model is presented that accounts for the occurrence of sand-prone bases to external levees. At the start of a sediment supply cycle (waxing and waning), slope confinement is weak, and sand-prone (intraslope) lobes will form at a point of the slope where there is slope accommodation. Through scouring of the lobe, finer-grained parts of flows will begin to spill, thus initiating the construction of levees. As the levees continue to build the energy of flows at a point on the depositional profile will increase due to constructional confinement, and will start to erode and degrade the slope further increasing flow confinement. The initial lobe and levee could be entirely eroded at this stage as the geomorphic surface deepens and widens. Ultimately, the gradient of the axial equilibrium profile will decrease through up-dip erosion and down-dip deposition, which will lead to disequilibria and decreased efficiency in sediment transport and the aggradation of channel. One implication of this model is the response to progressive slope confinement felt on the basin-floor. The response to increased sediment supply efficiency would be an initial basinward growth of a submarine fan that is linked to the geomorphic development of up-dip submarine slope confinement. The progressive confinement model contrasts with alternative instantaneous slope confinement models through large slope failure surface, where highest confinement and therefore highest efficiency is early in the sediment supply cycle.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California