The geology of the farms Josefsdal, Dunbar and part of Diepgezet in the

Abstract

This thesis is an integrat~;d analysis of structural, sedimentological, petrographical and metasomatic aspects of an area located in the south-eastern part of the 3.3 - 3.5 Ga Archaean Barberton greenstone belt. It is based on detailed mapping, detailed measured stratigraphic and structural sections and microscope analysis of thin sections. AI: 10000 geological map of the area is derived from the field observations. Discovery of previously unrecognised structural and metasomatic events renders the existing stratigraphy of the belt outmoded. A new stratigraphy is derived for this part of the belt. Based on the internal analysis of vertical facies sequences within tectonic slices, previously thought to represent continuous stratigraphic sequences, and structurally restored cross sections, the original s~ratigraphy thickness of the belt in this area is estimated to have been about 3000 metres. The lowermost part of the stratigraphy is represented by the upper part of the Onverwacht Group which comprises successively a discontinuous layer of serpentinite, a layer of pillowed and massive lavas and a volcaniclastic layer. The sedimentary rocks are totally silicified while the underlying mafic to ultramafic rocks are mostly silicified.A shallow and a deep water environment can be recognised within rocks of the Onverwacht Group which is interpreted as Archaean oceanic crust. The Onverwacht Group is conformably overlain by the Lower Group which comprises ferruginous sediments, chert-arenites and conglomerates deposited as a submarine fan. It is unconformably overlain by an Upper Group of conglomerates and quartz-arenites interpreted as an alluvial sequence. Seventy five per cent of the stratigraphy is affected by metasomatism which is interpreted as related to subaerial and subaqueous hydrothermal activity.

Four phases of deformation of which the first two involved thrusting have been recognised in the study area. Dl affects only the lower part of the stratigraphy, by repeating thin stratigraphic units above well defined thrust-decollement planes and with recumbent folding. D2 has imposed the main structural trend on the area, which is the result of an imbricated thrust fan. Using two major decollement zones, a minimum of 75% shortening across the sedimentary basin has been estimated. The Barberton greenstone belt is interpreted as an Archaean sequence emplaced by obduction during D2 probably related to collision tectonics.