Two seismic-scale submarine channel–levee systems exposed in the Karoo Basin, South Africa provide insights into slope conduit evolution. Component channel fills in a levee-confined channel system (Unit C) and an entrenched channel system (Unit D) follow common stacking patterns; initial horizontal stacking (lateral migration) is followed by vertical stacking (aggradation). This architecture is a response to an equilibrium profile shift from low accommodation (slope degradation, composite erosion surface formation, external levee development, sediment bypass) through at-grade conditions (horizontal stacking and widening) to high accommodation (slope aggradation, vertical stacking, internal levee development). This architecture is likely common to other channel–levee systems. Supplementary material: A detailed correlation panel (presented schematically in Figure 2 ) is available at www.geolsoc.org.uk/SUP18456 .

This is a repository copy of Deep-water channel-lobe transition zone dynamics: Processes and depositional architecture, an example from the Karoo Basin, South Africa.

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The geomorphology and seismic stratigraphy of deep-water clastic systems from slope valleys through channel-levee systems to basin-fl oor fans have been observed and described in modern and ancient sub surface examples around the world. However, the distribution of sedimentary facies, grain size, and small-scale architectural elements remains poorly constrained. Extensive exposures (>2500 km 2 ) of four stacked deep-water composite sequences have been mapped from heterolithic channel-levee systems on the slope to sand-rich basin-fl oor deposits. The data set from Units C-F of the Fort Brown Formation in the Permian Laingsburg depocenter of South Africa permits a unique opportunity to document and compare their depositional architecture at a high resolution for tens of kilometers downdip.Isopach thickness maps indicate that compensational stacking across multiple stratigraphic scales occurs on the basin fl oor, whereas preferred axial pathways were present on the slope, leading to subvertical stacking patterns. Units C and D are sand-attached systems; slope valley systems are mapped to pass transitionally downslope through leveeconfi ned channels to lobe complexes over distances of >30 km. The slope valley fi lls of Units E and F, however, are separated from their downdip sand-rich lobe complexes by a thin, sand-poor tract several kilometers in length and are termed sand detached. Locally, this sand-poor tract is characterized by a distinctive facies association of thin-bedded turbidites with numerous scours mantled with rip-up clasts, and a top surface that includes megafl utes and remobilized sediments. This assemblage is interpreted to indicate a widespread area of sand bypass.This unique data set provides an exploration-scale insight and understanding of how different segments of a prograding slope evolved over time in terms of gradient, physiography, and hence the degree to which sand was stored or bypassed to the basin fl oor, and the evolution from sand-attached to sand-detached systems. The development of sand-detached systems suggests that a steeper gradient formed, possibly related to developing underlying structure, that led to the development of a stepped slope profi le. The study highlights that updip stratigraphic trapping at reservoir scale can occur with minor bathymetric changes.Gold Open Access: This paper is published under the terms of the CC-BY license.on December 9, 2014 geosphere.gsapubs.org Downloaded from Sand-attached and sand-detached slope to basin-fl oor systems on a stepped slope profi le Geosphere,

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