Subaqueous giant (15 m) and supergiant (40 m) dunes from the Lower Permian Vryheid Formation of the Karoo Supergroup, northern Natal, South Africa

Abstract

The marine, Lower Permian Vryheid Formation contains a distinctive macrobedform suite. This comprises giant crossbeds up to 12 m thick (large dunes), masterbedded units up to 15 m thick (giant dunes), stacked masterbedded units 40 m thick (supergiant dunes) and a shelf stratigraphy up to 100 m thick. Tidal currents, augmented by an oceanic current, were responsible for bedform migration. Two categories of giant dunes are noted: (1) a fine-grained-type (associated with tidalites and tempestites) which forms a sheet sandstones, and (2) a coarse-grained-type (associated with tidalites) which occurs as discrete pods. It is suggested that the distal fine-grained-type formed in a setting exposed to oceanic storms whereas the proximal coarse-grained-type developed in a sheltered setting. With migration into more open areas the coarse-grained-type could develop into the fine-grained-type. It is further suggested that the giant dunes climbed to form supergiant dunes up to 40 m thick. The supergiant dunes are further stacked into stratigraphic units 40–100 m thick which may be due to supergiant dune climbing, or possibly tectonic effects, to produce a regressive shelf sequence.