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Beyond the Channel-to-Lobe Transition Zone: Off-Axis Facies Distribution and Implications for Stratigraphic Pinch-Outs


Despite the channel to lobe transition zone (CLTZ) representing a small volume of deep water deposits, it has received considerable attention in the recent past. Interest lies in the effects the transition from confined to unconfined flow has on depositional patterns. However, constraints of outcrop size / sub-surface resolution do not readily allow for a comprehensive investigation of the CLTZ and its lateral equivalents. Additionally, off-axis regions of deep water systems commonly show a stratigraphic juxtaposition of levee over lobe suggesting that the CLTZ can be strongly progradational, driving channel-levee systems basinward over earlier lobe deposits for 10s of km. A stratigraphic framework established for the 120 m thick sand prone Unit D of the Fort Brown Formation, Laingsburg Karoo, South Africa, provides an ideal outcrop dataset for investigating transitions in depositional architecture developed at the channelized system axis and into more off-axis regions. Detailed mapping of a 10 km dip section through the CLTZ at Slagtersfontein and associated off-axis areas shows a complex interplay of channel-levee process, unconfined lobe deposition and system progradation. At Unit D system scale, a >50 km dip parallel transect shows a gradual decrease in levee thickness with increasing distance down-dip, driven by a diminishing availability of silt through progressive flow stripping. The decrease in levee height is accompanied by an increased incorporation of sand-rich crevasse splay lobes into the most distal reaches of levee strata at Slagtersfontein. This effect is thought to be enhanced by the reduced confinement to in-channel flow promoting levee breaching and overspill of the increasingly sand rich flows. Immediately downdip from Slagtersfontein the basal levee and clean crevasse splay sands are seen to interdigitate with hybrid event beds typical of off-axis lobes indicating an increasing dominance of unconfined flow. Higher in the Unit D succession meter-scale packages of thin-bedded siltstone levee continue to persist for several hundred meters down-dip reinforcing the dynamic nature of deposition in and around the CLTZ. Mapping of the spatial distribution of stacked channel-levee to lobe architecture provides small scale detail on connectivity and distribution of sand in the enigmatic CLTZ area, with implications for the prediction of up-dip stratigraphic pinch-outs within lobe successions.