The geology of an area west of Welkom, O.F.S.: (degree blocks 2825 A and B and 2826 A)
Behounek, Norbert Josef
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Rocks present in the mapped area range from Early Precambrian (Easement granite) to the most recent deposits of aeolian sands, alluvium, gravel and calcrete. Outcrops of Ventersdorp rocks are scarce and widely scattered. Nevertheless, agglomerates and felsic lavas of the Makwas'sie Quartz Porphyry Formation, fanglomerates, .quartzites, volcanic breccias, tuff, mafic massive la.vas and pillow lavas of the Rietgat Formation, quartzites of the Bothaville Formation and plateau lavas of the Allanridge Formation could be distinguished and studied. The latter were all grouped together in accordance with the lithostratigraphic classification of Winter (1976), because of the proximity to his type area. Overlying the Ventersdorp Supergroup are shales and/or tillite of the Karoo Sequence. Bore holes drilled to the east (outside) of the mapped area proved that Dwyka glacial deposits are preserved in pre-Karoo valleys and/or valleys excavated through ice movement. Here, the Dwyka Formation consists of tillite, varved shales and a glacio-fluvial unit, viz. the sandstone-siltstone-shale unito In the western sector of the mapped area. Ventersdorp rocks are directly overlain by black, micaceous shales of the Ecca Group (Prince Albert Formation). The V/hitehill .Formation is probably not present as one continuous layer over the whole of the mapped area, and only occurs in a few isolated pockets. As a result the Prince Albert Formation is in most cases overlain by grey shales of the Tierberg Formation which grades from shales and mudstones (marine deposits) upwards into rhythmic layers of grey shales and sandstones (deltaic deposits). The transitional zone between the Ecca Group (Tierberg Formation) and the overlying Beaufort Group is exposed on the farm Basberg 416. The Beaufort Group in this area shows the typical features of a fluvial deposit, viz. coarse-grained channel-axis facies which laterally grade into finer-grained and thin-bedded hannelmarginal facies and levee mudstone deposits. The latter sediments were most probably deposited on the flood plain of a braided rivero Intrusive rocks consist of post-Karoo dolerites (sills and dykes) and the kimberiite intrusions at Rovic Diamond Mines. Nodular and laminated calcrete deposits are the most abundant calcrete deposits in the area under investigation, Aeolian sands cover large tracts of this area and occurmaiicly as aeolian sheet deposits but also as dunes of variable magnitude. The Vaal River Gravels and the alluvial sands and silts of the Yet and Sand Rivers are the most significant alluvial deposits present in the mapped area. Pans of variable shaper, and sizes have originated through the erosion of paleo river channels by subsequent wind action.