Deep-water channel-lobe transition zone dynamics: Processes and depositional architecture, an example from the Karoo Basin, South Africa

Abstract

Submarine channel-lobe transition zones separate well-defined channels from well-defined lobes and form morphologically complicated areas, commonly located at breaks in slope. These areas play a vital role in the transfer of sediment through deep-water systems. Extensive outcrop exposures in the Karoo Basin, South Africa, permit investigation of the depositional architecture and evolution of entirely exhumed dip transects of a channel-lobe transition zone for the first time. Furthermore, the excellent paleogeographic constraint allows correlation to genetically related updip channel-levee systems and downdip lobe deposits over 40 km, with strike control over 20 km. Unlike the single time slice afforded by modern systems, the Karoo example uniquely allows study of the temporal shifting of the channel-lobe transition zone and transfer into the stratigraphic record. Key lateral changes along the base of slope include the variation from an interfingering levee-lobe transition zone to a bypass-dominated channel-lobe transition zone over a width of 14 km. Key recognition criteria for channel-lobe transition zones in the ancient record include combinations of scours and megaflutes, composite erosional surfaces, mudstone clast/coarse-grained sediment lags, and remnants of depositional bed forms, such as sediment waves. Documented here in a single channel-lobe transition zone, these features are arranged in a zone of juxtaposed remnant erosional and depositional features. The zone reaches 6 km in length, formed by at least four stages of expansion/contraction or migration. Strike variations and changes in the dimensions of the channel-lobe transition zone through time are interpreted to be the result of physiographic changes and variations in flow dynamics across the base of slope. The dynamic nature of channel-lobe transition zones results in complicated and composite stratigraphy, with preservation potential generally low but increasing distally and laterally away from the mouth of the feeder channel system. Here, we present the first generic model to account for dynamic channel-lobe transition zone development, encompassing distinctive recognition criteria, fluctuations in the morphology and position of the zone, and the complex transfer into the sedimentary record.