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Spilling Into Confinement: Processes in Internal Levees to Submarine Channels


Submarine channel-levee systems are commonly simplified into two main depositional areas: axial (channelised areas) and marginal (external levees and overbank). However, a common observation from seismic sections is that in large (kms wide and 100's m deep) confined slope systems there is a significant component (>50%) of an opaque seismic facies. Interpretations of this component of the fill include terrace deposits, mass transport deposits, abandonment and internal levee deposits, however, outcrop analogues to these systems are rare. One well-documented example is preserved within Unit D, of the Fort Brown Fm. Karoo Basin, South Africa. Unit D crops out on the south limb of the Baviaans syncline as a deeply entrenched slope valley >100 m deep and 2 km wide (outcrop width) with a dominantly heterogeneous fill (70%). Detailed correlation at outcrop has shown that the heterogeneous fill is a record of the latest stage in the evolution of slope valley and these deposits act to confine a vertically aggradational phase of channelised deposition within the slope valley. Six behind outcrop research boreholes fully cored through Units C and D along the slope valley allow detailed characterisation of sedimentary facies and interpretation of sedimentary processes operating during deposition. This high-resolution database allows detailed analysis of the heterolithic component that makes up >70% of the fill. Interpretation of sedimentary processes, lithofacies distributions, and unit thickness supports an internal levee interpretation. Similar to external levees, lithofacies distribution within internal levees is dependent on proximity to the channel; in channel-proximal locations, beds are 5-20 cm thick and exhibit normal grading from very fine-grained sandstone to siltstone. Climbing ripple cross-lamination is common and examples of multi-directional current ripple lamination are present. In channel-distal localities, sandstone content decreases markedly (from ∼50%–<5%), siltstone bed thicknesses are 2–20 cm, very fine-grained sandstone beds are less than 1 cm and ripple lamination is rare. Individual bed thickness decreases upwards as the levee becomes siltier, with fewer occurrences of ripple lamination suggesting increasing confinement or waning flow density. This facies change suggests that there are several stages to levee development through time, related to a combination of allogenic (e.g. sediment supply) and autogenic (e.g. channel migration) processes.