Post-onverwacht group stratigraphy in the SE part of the Archaean Barbeton greenstone belt

Abstract

This paper examines the younger sedimentary sequences in the SE part of the 3.55 - 3.2 b.y. Barberton greenstone belt, southern Africa. A new stratigraphy is built on the recognition of five lithofacies and the analysis of lateral and vertical variations in the sedimentary sequences, taking into account the existence of numerous structural breaks. Two distinct sequences, one overlying the other, are recognised. Sequence 1 is characterised by the dominance of chert over single crystal quartz sand-sized grains. It comprises 3 lithofacies whose characteristics and vertical arrangement are typical of submarine fan deposits. Sequence 1 is informally called the Diepgezet Group. In sequence 2, single crystal quartz sand-sized grains generally predominate over chert grains, though near its base a change from chert to single crystal quartz grain dominance can be observed. Sequence 2 consists of alluvial and possibly marginal marine deposits and is informally called the Malolotsha Group. The Diepgezet Group conformably overlies cherts and volcanic rocks belonging to the Onverwacht Group, and is in turn generally unconformably overlain by the Malolotsha Group. There is an angular discordance of up to 90° between the Malolotsha and Diepgezet Groups. Angular unconformities also occur within the Malolotsha Group itself. Locally the Diepgezet and Malolotsha Group appear to be conformable. These relationships place constraints on the timing of the deformation in this part of the belt.