The application of stone counts in the glacigene Permo-Carboniferous Dwyka Formation, South Africa

Abstract

Stone counts, based on the lithological identification of clasts in the +10 mm size fraction, in the glacigene Permo-Carboniferous Dwyka Formation proved useful in delineating the source areas for the basal diamictite unit. However, the method was found less suitable for higher stratigraphic units where erosion of older diamictons led to the mixing of debris from several source areas. By means of this technique, denudation by ice of the northeastern source area ( “Ghaap” Plateau + Namaqua Mobile Belt ) was able to be reconstructed. The stone counts supplemented stratigraphic studies in assisting lithostratigraphic subdivision of massive diamictite sequences. Recognition of a valley and a platform lithofacies association, with the former having debris derived from subtillite material distributed throughout the sequence and the latter with it restricted to a thin basal zone, was also substantiated by stone counts. A severe limitation on the application of stone counts in pre-Pleistocene glacial sequences can be the lack of indicator rocks, so it is suggested instead that clasts forming diagnostic rock suites be used in defining the source areas. Palaeontological, isotopic and radiometric age data on the clasts may also assist in such studies.