| Summary: | Channel-lobe transition zones (CLTZs) represent critical areas between the submarine slope and basin floor systems, where sediment gravity flows transition from confined to unconfined. In modern systems, this area is characterised by a distinctive assemblage of erosional and depositional features. However, the transfer of the CLTZ into the stratigraphic record is not well constrained. By the detailed study of various well-constrained exhumed base-of-slope systems within the Tanqua and Laingsburg depocentres of the Karoo Basin, South Africa, the sedimentary and stratigraphic record of CLTZs is investigated. The first detailed study of key depositional and erosional bedforms that characterise ancient CLTZs, including sediment waves and giant scour-fills, are presented. Their process record suggests complicated flow-bedform interactions, where both the preservation and sedimentary characteristics are dependent on the evolution of the feeder channel and the lateral position to the channel-mouth. Within the base-of-slope environments, juxtaposition of lobes and channel-fills is common. The lobes in these proximal fan environments are sandstone-prone and differ in facies and architectural characteristics to lobes downdip on the basin floor. The sand-rich nature and juxtaposition of elements in ancient CLTZs means that they are considered attractive hydrocarbon reservoir targets in the subsurface. However, fine-scale reservoir modelling and streamline simulations suggests that the relatively limited sand-volume of channel-fills in comparison to lobe deposits in these environments can have a negative impact on reservoir performance due to its effect on reservoir connectivity. Overall, the stratigraphic record of CLTZs shows high diversity in architecture, facies characteristics and volume, which can be related to three primary controls: a) spatial variability, b) flow efficiency and c) channel evolution. These three controls infer that CTLZs are highly dynamic, migrating and changing in dimensions over time; this needs to be considered when assessing how CLTZs are recorded in the rock record.
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