Deepwater channel-lobe transition zone dynamics: processes and depositional

19 Submarine channel-lobe transition zones (CLTZs) form morphologically complicated areas, 20 commonly located at breaks-in-slope, and separate well-defined channels from well-defined 21 lobes. These areas play a vital role in the transfer of sediment through deepwater systems. 22 Extensive outcrop exposures in the Karoo Basin, South Africa, permit investigation of the 23 depositional architecture and evolution of entirely exhumed dip transects of a CLTZ for the 24 first time. Furthermore, the excellent paleogeographic constraint allows correlation to 25 genetically related updip channel-levee systems and downdip lobe deposits over 40 km, 26 with strike control over 20 km. Unlike the single time slice afforded by modern systems, the 27 Karoo example uniquely allows study of the temporal shifting of the CLTZ and transfer into 28 the stratigraphic record. 29 Key lateral changes along the base of slope include the variation from an inter-fingering 30 levee to lobe transition zone to a bypass dominated CLTZ over a width of 14 km. Key 31 recognition criteria for CLTZs in the ancient record include combinations of scours and 32 megaflutes, composite erosional surfaces, mudstone clast/coarse-grained sediment lags, 33 and remnants of depositional bedforms, such as sediment waves. Documented here in a 34 single CLTZ, these features are arranged in a zone of juxtaposed remnant erosional and 35 depositional features. The zone reaches 6 km in length, formed by at least four stages of 36 expansion/contraction or migration. Strike variations and changes in the dimensions of the 37 CLTZ through time are interpreted to be the result of physiographic changes and variations 38 in flow dynamics across the base of slope. The dynamic nature of CLTZs results in 39 complicated and composite stratigraphy, with preservation potential generally low but 40 increasing distally and laterally from the mouth of the feeder channel system. Here, we 41 present the first generic model to account for dynamic CLTZ development, encompassing 42 distinctive recognition criteria, fluctuations in the morphology and position of the zone, and 43 the complex transfer into the sedimentary record. 44

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