LATE PRECAMBRIAN FORMATIONS IN THE WESTERN RICHTERSVELD, NORTHERN CAPE PROVINCE

Summary

A detailed reinvestigation of rock relationships in the western Richtersveld led to a new stratigraphic classification of the Late Precambrian deposits, which were found to belong to one continuous cycle of geotectonic activity in the Orange River area and which are now grouped together in the Gariep Group.

The development of a geosynclinal basin started with basement shearing and subsequent deposition of the mixed sedimentary-volcanic Stinkfontein Formation. Further subsidence led to the accumulation of mixed clastic-carbonate shoreline and shelf deposits of the Hilda Formation, followed by the clastic Numees and Helskloof formations. At the same time basic to andesitic volcanics of the Grootderm Formation were extruded in the eugeosynclinal basin, accompanied and followed by turbidite deposits of the Holgat and Oranjemund formations.

Gravity deformation took place during and after deposition of the Gariep strata and was succeeded by orogenic activity which culminated in compressional eastward-directed folding and faulting under low-grade metamorphic conditions. The intrusion of the Kuboos and Swartbank plutons concluded the orogenic development of the Gariep Belt in the Richtersveld.

The Gariepian geotectonic cycle is part of the Late Precambrian/Early Paleozoic Pan-African Orogeny and may be a result of initial continental separation and subsequent plate collision. A model for the Gariep evolution in the Richtersveld in terms of plate tectonics is presented.