Down-Dip Evolution and Change in Architectural Style in a Slope Channel-Levee System: Unit D of the Laingsburg Formation, Karoo Basin, South Africa

Claudio N. Di Celma, Rufus L. Brunt, David Hodgson, John Kavanagh, and Steve Flint
Department of Earth and Ocean Sciences, University of Liverpool, Liverpool, United Kingdom

High-resolution seafloor and subsurface imaging have provided high-resolution views of the submerged continental margins, furthering our understanding of the evolutionary stages of submarine valleys. At finer scales, however, their sedimentary fills exhibit a high level of architectural complexity that remains difficult to assess in detail with conventional seismic data. Outcrop analogues can help to improve characterisation of these architectural details, although multiple sections through the same stratigraphic unit are rare. An example of a submarine valley that can be documented in multiple cross-sections over 30 km down-dip is Unit D of the Permian Laingsburg Formation. This provides an opportunity to describe in detail the down-slope architectural changes from an entrenched to a mostly constructional channel-levee system at a similar scale to many modern slope systems. Five main architectural elements can be recognised: (i) the erosional fairway; (ii) wedge-shaped external levees; (iii) channel-axis deposits flanked by (iv) internal levees; (v) mass flow deposits. In proximal sections, the 120 m-deep erosional fairway provides confinement for the fill with asymmetric external levees indicating the conduit was underfilled when abandoned and significant sinuosity. The axial fill has a bipartite architecture comprising an early-stage set of laterally migrating channel complexes, and late-stage aggradational channels flanked by internal levees. 20 km downslope of this section, the amount of basal incision decreases progressively whereas the thickness of the external levees increases. As a result, the nature of the system becomes more constructive and levee confined, and evidence of channel avulsion more common.

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