--> Channels and Scours—A Newly Described Exposure Revealing the Axis of the Proximal Leveed Channel Complex of Fan 3, Tanqua Karoo turbidites, South Africa

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Channels and Scours—A Newly Described Exposure Revealing the Axis of the Proximal Leveed Channel Complex of Fan 3, Tanqua Karoo turbidites, South Africa

Abstract

The world-class exposures of the Permian Tanqua Fan Complex (Skoorsteenberg Formation, Ecca Group) in the southwestern Karoo Basin of South Africa offer valuable insights into the architecture and process sedimentology of fine-grained turbidite fan systems. These fan systems are tectonically undeformed, exceptionally well exposed and easily accessible with present day erosion allowing 3D observation of laterally continuous (tens of kilometres) outcrops. These outcrops host an inexhaustible amount of information on deep-water architecture from lower slope to basin floor. Fan 3 of the fan complex is well exposed across 35 km, exhibiting a fully channelized facies in the south to stacked distributive lobe facies in the north. The most proximal, east-west strike section outcrops of Fan 3, which represent a broad channelized zone along the Ongeluks River, have long been interpreted as a base-of-slope leveed channel complex, feeding the distributive lobe elements to the north. A recent detailed field reconnaissance of these exposures using dGPS techniques has revealed exceptionally large and deeply incised channel elements of Fan 3. Multiple stacked channel thalweg fills, channel margin elements and large scours were identified and mapped, highlighting the highly erosive nature of sediment-laden flows at this locality, compared to adjacent channelized areas along the outcrop belt. Based on these observations, combined with existing information on facies characteristics and isopach information, we propose that the nature of channelization in the study area reflects the most proximal true axis of the Fan 3 distributary system, an area of much larger incision than previously recognized. This larger axial channel complex and its relationship with laterally adjacent sheet and channel complexes indicates that the leveed distributary system of Fan 3 is wider than hypothesized, that it had deeper axial thalwegs than previously thought, and by inference that there was a larger sediment budget for Fan 3 than we would assume based on observations elsewhere along the outcrop belt. These new observations have important implications for our understanding of the morphology and evolution of Fan 3, a sought-after and often-used field analog for current turbidite exploration targets in the Gulf of Mexico and elsewhere.