Abstract:

The architecture and facies variability of shelf-edge deltaic deposits from Permian–Triassic strata of the Karoo Basin, South Africa, are described. The architecture of the deposits are used to propose a model of repetitive collapse and re-establishment of mouth-bar packages at the shelf edge, and associated delivery of sand to the slope and basin floor. Accretion of basin-margin sediment wedges is facilitated through deposition of sediment on the basin-margin clinoform. This sediment is delivered by deltas at the shelf edge in the form of turbidity currents, delta collapse, and other gravity-driven processes.

A well exposed 3-km-long transect that records the transition between the outer shelf and upper slope for one progradational unit is described. The strata are defined by basinward-dipping, offlapping clinoform packages of soft-sediment-deformed and undeformed sandstone and mudstone. Undeformed sandstone units are characterized by meter-thick beds of mouth-bar sandstones with internal scour incisions, 1–2 m deep. Sets of undeformed sandstone beds thicken in a basinward direction from less than 1 m to over 10 m, and in multiple locations the bed sets are truncated by slump-scar surfaces that are overlain by mass-transport deposits. Soft-sediment-deformed strata are defined by packages of contorted, broken, and overturned beds. The deformed deposits are interpreted as being the product of dewatering features, as well as slumps, slides, and debrites, all of which emanate from slump-scar features at the shelf edge. On the uppermost slope, thin-bedded prodelta turbidites are interbedded with mass transport deposits, most of which pinch out within 2 km of the shelf edge.

The clinoformal packages presented record the basinward transition between: a) prograding mouth-bar deposits on the delta topset, b) truncation of the mouth-bar deposits by slump-scar incisions, 2–6 m deep, c) deposition of mass-transport deposits on the delta front and prodelta, and finally, d) re-establishment of mouth-bar deposition above and basinward of the slump-scar and mass-transport deposits. The observed stratigraphic stacking patterns, facies transitions, and interpreted depositional systems form the basis of a model for cutting and healing of deltaic packages that describes the repetitive deltaic progradation, delta collapse, and subsequent re-establishment of deltaic deposition. This process of repetitive collapse and reestablishment records the depositional-element-scale progradation of the shelf edge and accretion of the basin margin within the same depositional sequence.

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