KUNGURIAN CLINOFORM ARCHITECTURE, FROM THE VRYHEID FORMATION, KWAZULU-NATAL, SOUTH AFRICA

The middle Kungurian Vryheid Formation immediately post dates sediments of the Late Palaeozoic Glaciation and contains clinoforms 10 to 40m thick, interpreted as climbing shoreface ridge fields that formed in an ancient wave-dominated submarine delta. We have classified the Vryheid Formation clinoform using (Allen’s (1980) geometric bedform classification.

E1 Surface: This is sharp or erosive, often with a thin gravel or cobble lag, and represents a basal transgressive ravinement surface.

E2 Surface: These may be sharp, erosive or mudstone-draped and form a masterbedded unit. This bedding is horizontal at the top of the bedform but becomes increasingly inclined (7 to 20º) down dip. The upper E2 bedding may truncate the tops of the underlying inclined E2 surfaces and comprises coarse-grained sand to gravel. E2 surfaces are associated with tempestites and down-cutting, orthogonal, linear channels (filled with tempestites or massive sandstone). The inclined master bedded unit comprises fine-to-coarse-grained sand. E2 surfaces are often wave rippled with crests normal to the master-bedding.

E3 Surfaces: These form the bases of large-scale climbing bedforms.

E4 Surfaces: These surfaces are the smallest scale features and represent small-scale bedform bases.

There is an alternation between the bedform hierarchal development and erosion, which creates the E2 masterbedding, thus requiring a two-stroke environmental model. The first process is bedform climbing in a stable unidirectional current (although some reversal was present). This alternated with erosion and tempestite deposition and the formation of the orthogonal channels (palaeo-gutters) that represent palaeo-storm-return flow. Hence the environment was one of unidirectional flow episodically interrupted by high-swell activity. Such an environment would have been similar to that seaward of modern wave-dominated coastlines (eg. New Jersey and southeast African Shelves) which contain modern clinoform arrangements with similar geometries.