Widespread syn‐sedimentary deformation on a muddy deep‐water basin‐floor: the Vischkuil Formation (Permian), Karoo Basin, South Africa

Abstract

The ∼380‐m‐thick mudstone–siltstone‐dominated Vischkuil Formation represents the initiation phase of a 1.3‐km‐thick prograding basin floor to slope to shelf succession that marks a significant increase in the rate of siliciclastic sediment supply to the early Karoo Basin in the Permian. In the upper Vischkuil Formation three well exposed, widespread (∼3000 km2) 10–70‐m‐thick intervals of deformed strata are encased within undeformed sediments. Such chaotic mass movement deposits that are mappable over areas comparable with seismic‐scale mass transport deposits are commonly associated with submarine slope settings. However, the surrounding lithofacies and the correlation of distinctive marker beds indicate that these deformation intervals developed in a distal low gradient basin floor setting. The deformed intervals comprise a lower division of tight down‐flow verging folds dissected by thrust planes that sole out onto a highly sheared décollement surface that are interpreted as slides. The lower divisions are overlain by an upper division of chaotic lithofacies with large contorted clasts of sandstone supported by a fine‐grained matrix interpreted as a debrite. The juxtaposition of these lithofacies, the distribution of thickness of the divisions, and their close kinematic relationships indicate that the emplacement of the debris‐flows triggered and drove the underlying slide, in a low‐gradient distal setting. Individual beds in the deformed intervals can be mapped laterally into undeformed strata indicating limited movement of the slide. Therefore, widespread zones of syn‐sedimentary deformation in deep‐water settings do not necessarily indicate a slope setting and should not be used as single criterion to determine depositional setting. When associated with major debrites they may be developed on a flat basin floor.

Sequence stratigraphy of an argillaceous, deepwater basin-plain succession: Vischkuil Formation (Permian), Karoo Basin, South Africa

“…Each of these packages abruptly truncates undeformed siltstone-dominated turbidites and mudstones across a sharp, regionally mapped surface. Van Der Merwe et al (2009) interpreted that the emplacement of the debris-flow initiated and drove the slide, which moved above a dé collement surface.…”

“…The lower Vischkuil Formation comprises w190 m of undeformed graded siltstone beds and mudstone lithofacies (Fig. 2), while the w220 m thick upper Vischkuil Formation is dominated by similar facies but includes three widespread (>3000 km 2 ) 10-85 m thick syn-sedimentary deformation units, termed (from base to top) DU1, DU2 and DU3 (Van Der Merwe et al, 2009).…”

“…2). The three deformation units include slides, slumps and debrites (Van Der Merwe et al, 2009) that extend over more than 3000 km 2 .…”

“…4). Each deformation unit comprises a lower folded and faulted division, and an upper chaotic division, interpreted by Van Der Merwe et al (2009) as a slide and debrite respectively. We interpret the bases of the deformation units as sequence boundaries (Figs.…”

“…The Vischkuil Formation, a well exposed w380 m thick distal basin-plain succession that includes three major regional deformation units (DU1-3) of mass transport deposits (MTDs), is overlain by a 1 km thick basin floor fan -slope -shelf succession, in the SW Karoo Basin of South Africa (Van Der Merwe et al, 2009;Fig. 1).…”