Architecture of a channel complex formed and filled during long-term degradation and entrenchment on the upper submarine slope, Unit F, Fort Brown Fm., SW Karoo Basin, South Africa

Abstract An exhumed upper slope channel complex composed of four channels that have a range of sedimentary fill styles has been investigated with the emphasis on understanding lithology distribution in space and time. Initiation of the channel complex, which forms part of a seismic-scale channel complex set, is marked by a basal erosion surface interpreted to have been carved by multi-event erosive sediment gravity flows, which bypassed sediment down dip farther into the basin. In this phase, deposits are restricted to slides/slumps from the conduit margins. Waning of this high energy phase is represented by limited deposition of scour-based sandstone and claystone clast conglomerates, which fine- and thin-upward into bedded siltstone prior to channel abandonment. This erosion-then-deposition cycle represents the record of the first channel-fill within the channel complex. The cycle is restarted with another phase of erosion and sediment bypass followed by deposition which makes up the second channel-fill. This process is repeated two more times to form the third and fourth channel-fills; however, the depth of erosion decreases upward in the successive channel-fills. The four sub-vertically offset stacked single channel-fills comprise a channel complex formed during a period of waxing-to-waning energy. As they are partially removed by a deeply entrenched (>100 m) slope valley, the channel-fills represent a remnant of an overall waxing period (degradational phase) at the scale of a depositional sequence. Comparison with published examples of slope channel complexes suggests that many features interpreted in 3D seismic data as single channels are in fact channel complexes, which has major implications for the appraisal of internal heterogeneities in these reservoir types.