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Sand-Attached to Sand-Detached Deepwater Systems: Is there Predictability in their Stratigraphic Distribution?

Hodgson, David; Van Der Merwe, Willem C.; Brunt, Rufus L.; Flint, Stephen S.

The physical passage from the submarine slope systems to the basin-floor results in a complicated stratigraphic record of process change from confined channels to unconfined lobes. Predicting the degree of up-dip sand connectivity is critical when assessing stratigraphic trap potential in this transition zone. In a sand-detached system a widespread area dominated by erosional processes and coarse sediment bypass may lead to formation of a stratigraphic trap.

In the Laingsburg depocentre, South Africa, the extensive exposures (>4500 km2) of slope-to basin-floor systems in composite sequences C-to-F provide the opportunity to describe and differentiate sand-attached and sand-detached systems. Units C and D have been mapped from entrenched channel systems that pass down-slope through levee-confined channel systems to attached lobe complexes over distances of 50-100 km. A general increasing trend in the proportion of sandstone in the fill of channel systems down-dip suggests that physical connectivity of channels to lobes is likely, although multiple barriers and baffles (e.g. erosion surfaces, channel-margin thin-beds, channel lags) results in a complex. However, in overlying Units E and F, there are extensive sand-poor tracts (>15 km in length, and 20 km in width) that separate up-dip channel-levee systems from their related down-dip lobe complexes. The facies associations preserved in these sand-poor areas are thin -bedded, but commonly, their top surface includes megaflutes, rip-up clast surfaces, and soft-sediment deformed interval indicating a high energy erosional bypass environments.

The base-of-slope, as defined by the architecture change from channel-levee dominated to lobe-dominated deposits, does not move much through time. Furthermore, the mudstone between composite sequences thins down palaeoslope. The combination of these two parameters would lead to a more abrupt change in slope gradient in the base-of-slope area. The abrupt gradient change could lead to an increase in sediment bypass. Therefore, the stratigraphic transition from sand-attached to sand-detached suggests an intrinsic and/or inherited control that might be shared with other subsurface systems. In reflection seismic datasets a physical connection might be imaged, although the degree of reservoir connectivity, and stratigraphic traps, might be less clear.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013