Fan-delta sequence in the Archean Fig Tree Group, Barberton Greenstone Belt, South Africa

Abstract

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 deposits 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 processes 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.