TY - JOUR

T1 - Reconstructing sedimentary processes in a Permian channel-lobe transition zone

T2 - an outcrop study in the Karoo Basin, South Africa

AU - Pohl, F.

AU - Eggenhuisen, J. T.

AU - De Leeuw, J.

AU - Cartigny, M. J.B.

AU - Brooks, H. L.

AU - Spychala, Y. T.

N1 - Funding Information: This contribution is part of EuroSEDS (Eurotank Studies of Experimental Deepwater Sedimentology), supported by the NWO (Netherlands Organization for Scientific Research) (grant no. NWO 864.13.006), ExxonMobil, Shell and Equinor. We thank the local farmers of the Laingsburg region for permission to undertake field studies on their land, especially Henk Steyn. Katherine Maier, Cristian Carvajal, Andrea Fildani and Editor Peter Clift provided detailed and constructive reviews that helped us to improve our paper.

PY - 2023/1/29

Y1 - 2023/1/29

N2 - 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.

AB - 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.

KW - deep marine

KW - scour fields

KW - sediment bypass

KW - sediment facies

KW - turbidity current

UR - http://www.scopus.com/inward/record.url?scp=85151532927&partnerID=8YFLogxK

U2 - 10.1017/S0016756822000693

DO - 10.1017/S0016756822000693

M3 - Article

AN - SCOPUS:85151532927

VL - 160

SP - 107

EP - 126

JO - Geological magazine

JF - Geological magazine

SN - 0016-7568

IS - 1

ER -