DIAMICTITE IN THE ARCHEAN PONGOLA SEQUENCE OF SOUTHERN AFRICA

Archaean diamictite occurs in the Pongola Sequence, exposed in the sou theastern part of the African subcontinent. Four diamictite units are developed in a mudrock-dominated interval which is interbedded with ar enites of the 5000 m thick Mozaan Group. The most prominent of these d iamictites is 80 m thick. The rock is black and comprises a homogeneou s matrix supporting sparse clasts that are characteristically varied i n composition. Some clasts are striated and faceted. The diamictite es sentially represents a mudflow deposit which was emplaced in amarine s helf environment. Sediment was delivered to a subsiding basin by downs lope mass movement from a fault-bounded, elevated margin where highlan d glaciers are likely to have contributed clastic detritus. The diamic tite would thus represent a reworked admixture of glacially-derived de bris and argillaceous gravity flow sediment related to tectonic activi ty along the basin margin. Whereas the final depositional mechanism in volved subaqueous mass-flow, the presence of striated stones, the hete rogeneous clast composition, and major element chemical data support a glacial interpretation for the diamictite. The sparse record of glaci ation during the early history of the Earth warrants documentation of such Archaean diamictite with possible glacial affinities.