FAN-DELTA SEQUENCE IN THE ARCHEAN FIG TREE GROUP, BARBERTON GREENSTONE-BELT, SOUTH-AFRICA

The Fig Tree Group of the Barberton Greenstone Belt in South Africa records the onset of tectonism and significant terrigenous and volcaniclastic sedimentation following a period of predominantly effusive volcanism. In the southern part of the greenstone belt the lower Fig Tree conformably overlies the Onverwacht Group and can be grouped into either: (1) volcaniclastic deposites consisting of pyroclastic and reworked volcanic detritus, or; (2) non-volcanogenic, terrigenous deposits of sandstone and conglomerate. These petrologic groups are further divided into six lithofacies. The lithofacies within the lower Fig Tree Formation can be grouped into two facies associations: (1) a subaqueous, delta-front association made up of fine-grained airfall pyroclastic and slightly reworked pyroclastic debris along with resedimented, land-derived sandstone and conglomerate, and (2) a subaerial association that is interpreted as forming from deposition of sandstone and conglomerate by flashy, ephemeral-stream proceses on an alluvial-fan surface. The stratigraphic sequence coarsens upward from predominantly siltstone and fine-grained sandstone at the base to pebble and cobble conglomerate at the top. This sequence is interpreted as forming from the progradation of a fan delta into a body of relatively shallow, quiet water. Composition of clasts in the conglomerates indicates that immediately underlying greenstone-belt rocks or coeval volcanic rocks were the source for these coarse sedimentary accumulations. Erosion of uplifted greenstone-belt terrain was apparently only to shallow levels. Both tectonic and volcanic activity played a role in producing several cycles of fan-delta progradation.