ABSTRACT
The Ngezi Group comprises the Manjeri, Reliance, Zeederbergs and Cheshire Formations. Although local erosional unconformity exists in the sequences, particularly at the contact between the Zeederbergs and Cheshire Formation, the sequence is essentially conformable and represents a remarkably well preserved and near continuous record of the development of the greenstone belt.
The Manjeri Formation includes roughly 100 m of sedimentary rocks, with local variation. It records the transition from the subsidence of the underlying basement to deeper water conditions. It is laid down with an unambiguous unconformity on graniticgneiss basement in the east and on the Mtshingwe Group in the west.
The Reliance Formation includes 1 km of mafic and ultramafic pillow lavas, flows, minor tuffs and high-level intrusions. A lower komatiitic basalt member, mostly pillow lavas, flows and tuffs is overlain by acentral komatiite member of ultramafic pillows and flows and an upper extrusive komatiitic basalt member.
The Zeederbergs Formation is up to 5.5 km thick and is predominantly composed of basaltic pillows and flows, associated with thin tuffs. Some komatiitic basalts occur, sedimentary rocks are almost absent apart from minor volcaniclastic deposits.
The Cheshire Formation includes roughly 2 km of sedimentary rocks and minor igneous rocks. The top is not seen. Conglomerates, ironstones, thick siltstones and limestones occur, in a very varied assemblage, most of which was laid down in very shallow water. Among the limestones are very well-preserved stromatolitic units which display a wide variety of spectacular structures.
