Origin and terminal architecture of a submarine slide: a case study from the Permian Vischkuil Formation, Karoo Basin, South Africa

Abstract

Submarine mass movement deposits exposed in the Vischkuil Formation, Laingsburg Karoo Basin, South Africa, provide a rare opportunity to analyse and interpret their emplacement history and deformation processes at a scale comparable to seismic examples. An up to 80 m thick slide deposit, continuously exposed in two 2 km long sub‐parallel sections, passes from extensionally deformed material (clastic dykes and down‐dip facing low‐angle shear surfaces) down‐dip into a compressional toe zone with large (tens of metres amplitude) folds dissected by steep, up‐dip facing thrust planes. The compressional shear planes sole out onto a highly sheared décollement and cross‐cutting relationships indicate an up‐depositional dip younging in the timing of fold dissection. Lithofacies characteristics and detailed correlation of volcanic ash and other marker beds over more than 500 km2 in the bounding undeformed stratigraphy indicate a low‐gradient (<0·1°) basin floor setting. The slide is abruptly overlain by an up to 50 m thick debrite with sandy clasts supported by an argillaceous matrix. Shear loading of the debris flow is interpreted to have driven large‐scale deformation of the substrate through the generation of high shear stresses at a rheological interface due to: (i) the abrupt contact between the slide and the debrite; (ii) the coincident thickness distributions of the debrite and slide; (iii) the distribution of the most intense folding and thrusting under the thickest parts of the debrite; (iv) the preservation of fold crests with only minor erosion along fold limbs; (v) the presence of the debrite under overturned folds; (vi) the presence of laterally extensive marker beds directly above deformation units indicating minimal depositional topography; and (vii) the demonstrably local derivation of the slide as individual folded beds are mapped into undeformed strata outside the areas of deformation. The debrite is directly overlain by fine‐grained turbidite sandstone beds that show widespread vertical foundering into the debrite. This case study demonstrates that intensely deformed strata can be generated by negligible amounts of down‐dip movement in a low‐gradient, fine‐grained basin floor setting with the driver for movement and deformation being the mass imbalance resulting from emplacement of episodic debris flows. Simple interpretation of an unstable slope setting based on the presence of such deformed strata should be treated with caution.

Internal architecture of mass‐transport deposits in basinal carbonates: A case study from southern Italy

“…A similar case was documented by Van Der Merwe et al . () in the Karoo Basin where debris flows deformed underlying beds and created a gently folded horizon.…”

“…A similar example has been described by Van Der Merwe et al . () in the Karoo Basin, where a debris flow deformed the underlying unconsolidated beds. In both cases (this study and that of Van Der Merwe et al ., ), the deformation was enhanced by the overburden.…”

“…Over the last decades, several outcrop studies focusing on the internal architecture of MTDs have been undertaken in order to investigate processes occurring during the transport and emplacement of the failed material. These studies, which have mostly been carried out in siliciclastic systems and only rarely in carbonate systems (Pickering, ; Lucente & Pini, ; Pickering & Corregidor, ; Van Der Merwe et al ., , ; Butler & McCaffrey, ; Ogata et al ., ), indicate that MTDs move along a basal detachment surface that may either follow the bedding plane (Strachan, ; Van Der Merwe et al ., ) or plough through the underlying beds incorporating the underlying material into the moving mass (Strachan, ; Ogata et al ., ; Sobiesiak et al ., ). Internally, gravity‐driven soft‐sediment deformation processes result in the formation of an array of structures including dip‐slip and oblique‐slip normal faults (Alsop & Marco, ), extensional sedimentary fissures in the upper extensional head (Winterer et al ., ; Strachan, ; Spalluto et al, ), and folds, thrusts, ramps and erosional surfaces in the lower compressional part.…”

Widespread and progressive seafloor-sediment failure following volcanic debris avalanche emplacement: Landslide dynamics and timing offshore Montserrat, Lesser Antilles

“…Deformation of in-situ sediment by an over-running landslide has been postulated elsewhere (e.g., Schnellman et al, 2005;Van Der Merwe et al, 2011). However, in some instances deformation may arise through shear coupling, via forces exerted on the underlying sediment by motion of the over-running material, rather than loading-induced excess pore pressure.…”

The full range of turbidite bed thickness patterns in submarine lobes: controls and implications

“…1a), and records a late Permian shallowing-upward succession from deep-marine (Vishkuil and Laingsburg Formations; Van der Merwe et al 2011), through submarine slope (Fort Brown Formation;Hodgson et al 2011) to shelf-edge and shelf deltas (Waterford Formation; Oliveira et al 2011). The Laingsburg Formation is up to 500 m thick and comprises Unit A, a c. 300 m thick sandy basin floor fan system (Sixsmith et al 2004) overlain by a c. 40 m mudstone package, and Unit B, a c. 150 m thick base of slope channelized lobe system Fig.…”