Reconstructing sedimentary processes in a Permian channel–lobe transition zone: an outcrop study in the Karoo Basin, South Africa

Geological Magazine, vol. 160, pp. 107–126

Abstract

Abstract Turbidity currents commonly bypass sediment in submarine channels on the continental slope, and deposit sediment lobes farther down-dip on the flat and unconfined abyssal plain. Seafloor and outcrop data have shown that the transition from bypass to deposition usually occurs over complex zones referred to as channel–lobe transition zones (CLTZs). Recognition of these zones in cores and outcrop remains challenging due to a lack of characteristic sedimentary facies and structures. This paper focuses on Unit E of the Permian Fort Brown Formation in the Karoo Basin, South Africa, in the Slagtersfontein outcrop complex, which has previously been interpreted as a CLTZ. This study integrates thin-section micrographs, sedimentary facies, bed-set and stratigraphic architecture, and palaeoflow directions to achieve a multiscale analysis of CLTZ features. A novel process-based facies scheme is developed to evaluate deposits in terms of the depositional or erosional tendencies of the flows that formed them. This scheme allows bypass to be distinguished from depositional zones by the spatial distribution of certain sediment facies. Areas of net sediment bypass were predominantly marked by erosive sediment facies and a larger variability in palaeoflow direction while depositional areas showed a lower variability in palaeoflow directions. Metre-scale structures in the bypass-dominated area reveal seafloor erosion and scour formation. Field relations suggest the presence of a ∼500 m long mega-scour in the CLTZ. The characteristic structures documented here are applicable for identifying CLTZs in sparse datasets such as outcrops with limited palaeogeographical context and sediment cores obtained from subsurface systems.

Authors 6

1. Florian Pohl corresponding

   University of Plymouth

   Affiliation as printed

   School of Biological and Marine Sciences, University of Plymouth, Drake Circus, Plymouth PL 4 8AA, UK

2. JT Eggenhuisen

   Utrecht University

   Affiliation as printed

   Faculty of Geosciences, Utrecht University, P.O. Box 80021, Utrecht, TA 3508, The Netherlands

3. Jan de Leeuw

   Utrecht University

   Affiliation as printed

   Faculty of Geosciences, Utrecht University, P.O. Box 80021, Utrecht, TA 3508, The Netherlands

4. Matthieu Cartigny

   Durham University

   Affiliation as printed

   Faculty of Geography, Durham University, Lower Mountjoy South Road, Durham DH1 3LE, UK

5. HL Brooks Aachen EMR – Geological Institute

   RWTH Aachen University

   Affiliation as printed

   EMR – Geological Institute, RWTH Aachen University, 52062 Aachen, Germany

6. YT Spychala

   Leibniz University Hannover

   Affiliation as printed

   Leibniz University Hannover, Institute of Geology, Callinstr. 30, 30167, Hannover, Germany