Shelf‐margin clinothem progradation, degradation and readjustment: Tanqua depocentre, Karoo Basin (South Africa)

Abstract

Degradation of basin‐margin clinothems around the shelf‐edge rollover zone may lead to the generation of conduits through which gravity flows transport sediment downslope. Many studies from seismic‐reflection data sets show these features, but they lack small‐scale (centimetre to metre) sedimentary and stratigraphic observations on process interactions. Exhumed basin‐margin clinothems in the Tanqua depocentre (Karoo Basin) provide seismic‐reflection‐scale geometries and internal details of architecture with depositional dip and strike control. At the Geelhoek locality, clinothem parasequences comprise siltstone‐rich offshore deposits overlain by heterolithic prodelta facies and sandstone‐dominated deformed mouth bars. Three of these parasequences are truncated by a steep (6 to 22°), 100 m deep and 1·5 km wide asymmetrical composite erosion surface that delineates a shelf‐incised canyon. The fill, from base to top comprises: (i) thick‐bedded sandstone with intrabasinal clasts and multiple erosion surfaces; (ii) scour‐based interbedded sandstone and siltstone with tractional structures; and (iii) inverse‐graded to normal‐graded siltstone beds. An overlying 55 m thick coarsening‐upward parasequence fills the upper section of the canyon and extends across its interfluves. Younger parasequences display progressively shallower gradients during progradation and healing of the local accommodation. The incision surface resulted from initial oversteepening and high sediment supply triggering deformation and collapse at the shelf edge, enhanced by a relative sea‐level fall that did not result in subaerial exposure of the shelf edge. Previous work identified an underlying highly incised, sandstone‐rich shelf‐edge rollover zone across‐margin strike, suggesting that there was migration in the zone of shelf edge to upper‐slope incision over time. This study provides an unusual example of clinothem degradation and readjustment with three‐dimensional control in an exhumed basin‐margin succession. The work demonstrates that large‐scale erosion surfaces can develop and migrate due to a combination of factors at the shelf‐edge rollover zone and proposes additional criteria to predict clinothem incision and differential sediment bypass in consistently progradational systems.

Transport and deposition of mud in deep‐water environments: Processes and stratigraphic implications

“…Ash beds potentially derived from the Choiyoi volcanic province (Northern Patagonia) are interbedded in these formations, with a higher density in the Collingham Formation (McLachlan & Jonker, ; Veevers et al ., ; Viljoen, ; McKay et al ., ). These mudstone‐rich formations are overlain by the well‐studied, sandy basin‐floor fans of the Skoorsteenberg Formation (Wickens, ; Johnson et al ., ; Hodgson et al ., ) and the upper slope to shelf deposits of the Waterford Formation (Wild et al ., ; Dixon et al ., ; Poyatos‐Moré et al ., ; Gomis‐Cartesio et al ., , ; Fig. C).…”

Clinoform architecture and along‐strike facies variability through an exhumed erosional to accretionary basin margin transition

“…The presence of incision surfaces at the base of Unit G mantled by mudstone clast conglomerates (Figure b) is consistent with several phases of erosion and sediment bypass, forming a lag deposit. The overlying fining‐ and thinning‐upward heterolithic section of sub‐unit G1 (Figure a) either represents a landward stepping of the depositional system due to transgression and increased accommodation, or the final infill of a deeper incision (see Gomis‐Cartesio, Poyatos‐Moré, Hodgson, & Flint, ). The presence of hummocky‐cross stratification in sub‐unit G2 (Figure c) indicates deposition above storm wave base and the absence of sediment bypass indicators suggests a setting that transitioned from bypass to accretion‐dominated.…”

Facies and architectural variability of sub‐seismic slope‐channel fills in prograding clinoforms, Mid‐Jurassic Neuquén Basin, Argentina

“…In this section, we will briefly describe the nature of the shelf portion of the topset of the clinoform units, with an emphasis on a slope channel identified just beyond the shelf edge on the slope. In outcrop, the shelf edge is often sharp but as a zone of interplaying fluvial, tidal, wave and gravity controls, and this zone is sometimes termed the ‘shelf‐slope transition’ (Dixon, ; Gomis‐Cartesio, Poyatos‐Moré, Hodgson, & Flint, ; Gomis‐Cartesio, Poyatos‐Moré, Hodgson, & Flint, ). Details of topset shelf deposits and the shelf‐slope transition is further described in De Almeida Jr. et al (), in review, where interaction of the shelf deltas with shelf‐edge incisions is documented along both depositional dip and strike sections.…”