Diamictite in the Archaean Pongola sequence of southern Africa

Abstract

Archaean diamictite occurs in the Pongola Sequence, exposed in the southeastern part of the African subcontinent. Four diamictite units are developed in a mudrock-dominated interval which is interbedded with arenites of the 5000 m thick Mozaan Group. The most prominent of these diamictites is 80 m thick. The rock is black and comprises a homogeneous matrix supporting sparse clasts that are characteristically varied in composition. Some clasts are striated and faceted. The diamictite essentially represents a mudflow deposit which was emplaced in a marine shelf environment. Sediment was delivered to a subsiding basin by downslope mass movement from a fault-bounded, elevated margin where highland glaciers are likely to have contributed clastic detritus. The diamictite would thus represent a reworked admixture of glacially-derived debris and argillaceous gravity flow sediment related to tectonic activity along the basin margin. Whereas the final depositional mechanism involved subaqueous mass-flow, the presence of striated stones, the heterogeneous clast composition, and major element chemical data support a glacial interpretation for the diamictite. The sparse record of glaciation during the early history of the Earth warrants documentation of such Archaean diamictite with possible glacial affinities.