Quantification and implications of the grain-size segregation and similar volumes between submarine slope and basin-floor deposits

Abstract:

Degradation and aggradation of deep-water seascapes result in a complicated stratigraphic record. Datasets rarely permit the inversion of this archive at a resolution high enough to understand process controls on sediment mass balance and grain-size segregation. An exceptional exhumed basin margin, exposed over 2500 km², has permitted 4 deep-water systems to be mapped from slope valley-fills, through leveed channel complexes, to the distal fringe of basin-floor fans in the Permian Fort Brown Formation in the Laingsburg-Karoo depocenter, South Africa. Edge-to-edge reconstructions of the sand-prone units supported by >1000 measured sections permits quantitative constraints to be placed on the dynamic distribution and volumetric partitioning of sediment by depositional environment and grain-size. Grain-size is well segregated, with channel-levee deposits being silt-rich and lobe deposits sand-rich, although system progradation results in a common levee-over-lobe stratigraphic motif. Strikingly similar volumes of deposits are recorded in channel-levee systems as in stratigraphically related basin-floor lobe complexes in all the systems (10’s km³). The results point to the conduits feeding sediment gravity flows that were density stratified and became increasing sand-rich as they passed into the basin due to flow-stripping and overbanking processes. These patterns also suggest a persistent goodness to fit between the flow magnitude and the scale of confinement such that the channel-levee systems propagated into the basin under steady boundary conditions, despite the presence of a stepped slope profile. This comprehensive field study is a unique example where quantification of the volumes of sediment in the transport and depositional zones have been calculated, and highlight the importance of external levee deposits as a sediment store when attempting to estimate the mass balance of sediment in deep-water systems.