A MIDDLE PERMIAN (WORDIAN) AVULSION SYSTEM, KAROO BASIN, SOUTH AFRICA: IMPLICATIONS FOR THE ECCA-BEAUFORT TRANSITION

The Waterford Formation records deposition during the Middle Permian in the Karoo Basin, South Africa. This is a transitional stratigraphic unit between the open water Ecca Group deposits and the fully terrestrial Beaufort Group, and has been interpreted as representing the progradation of deltas into the basin following continental deglaciation. The present study focuses on the characterization of an avulsion system (S32º 37.703’ E20º 21.754’) in close proximity to a main distributary channel in the Waterford Formation south of Sutherland in the Northern Cape Province. The exposure can be traced for 350 m along strike and is located 500 m from the main channel outcrop from which it originated.

The avulsion sequence consists of ten lithofacies including: (A) basal, massive, coarse siltstone; (B) very fine sandstone with ball-and-pillow structures; (C) massive, sandy siltstone; (D) very fine, rippled sandstone; (E) soft sediment deformed, lenticular, fine sandstone; (F) massive, fine sandstone comprising the major channel fill; interbedded (G) tabular, very fine sandstone and (H) siltstone; (I) fine sandstone in lenticular barforms; and (J) fine sandstone restricted to load structures.

To determine the environment into which the avulsion event occurred, TOC and TON data were collected from the basal siltstone. Several excursions exist, particularly in the TOC:TON ratio, which is interpreted as recording deposition under high discharge flooding into prodeltaic mud. Bifurcate avulsion channels are in erosional contact with this facies and are overlain by interbedded sandstone and siltstone, which are capped by lenticular sandstone barforms. The combination of lithofacies, TOC:TON data, and stratigraphic sequence relative to the main channel allows for an interpretation of the depositional conditions and environmental setting of this Wordian avulsion system.