A geological investigation of upper Transvaal sequence rocks in the northern portion of the Rooiberg fragment

Abstract

The stratigraphy , sedimentology and geochemistry of. the upper Transvaal Sequence rocks in the northern portion of the Rooiberg Fragment are researched. The Leeuwpoort Formation of the Pretoria Group was deposited in a fluvial palaeoenvironment. The basal arkosites in this unit reflect braided stream deposition, whereas the overlying shaly arkosi tes represent upward- fining cycles of point bar sedimentation. The uppermost main Leeuwpoort shale probably formed in a floodplain palaeoenvironment. The overlying Smelterskop Formation arenites indicate high energy flood conditions in a proximal fluvial palaeoenvironment. The extrusion of andesi tic lavas acted as an agent of change during the deposition of Smelterskop Formation sedimentary rocks. The upper and lower contacts of the Smelterskop Formation are disconformable and conformable, respectively. However, non-deposition of the main shale of the upper Leeuwpoort Formation and erosion by succeeding Smelterskop sedimentation in some areas, created local disconformities at the base of the Smelterskop Formation. The scattering of Si02 binary plots recorded in the geochemistry of the Rooiberg Group rhyolites indicates extensive alteration of these rocks. Petrogenetic conclusions drawn from the geochemistry must therefore be treated with caution. However, empirical data indicates two lava types, defined by major and trace element geochemistry and geographical position.