The geology of an area west of Welkom, O.F.S. : degree blocks 2825A and B and 2826A

[摘要] Rocks present in thé mapped area range fromEarly Precambrian (Easement granite) to the most recentdeposits of aeolian sands, alluvium, gravel and calcrete.Outcrops of Ventersdorp rocks are scarce andwidely scattered. Nevertheless, agglomerates and felsiclavas of the Makwassie Quartz Porphyry Formation, fanglomerates,quartzites, volcanic breccias, tuff, mafic massivelavas and pillow lavas of the Rietgat Formation, quartzitesof the Bothaville Formation and plateau lavas of the Allanridgeformation could be distinguished and studied. Thelatter were all grouped together in accordance with thelithostratigraphic classification of Winter (1976) becauseof the proximity to his type area.Overlying the Ventersdorp Supergroup are shalesand/ or tillite of the Karoo Sequence. Bore holes drilledto the east (outside) of the mapped area proved thatDwyka glacial deposits are preserved in pre-Karoo valleysand/ or valleys excavated through ice movement. Here, theDwyka formation consists of tillite, varved shales anda glacio-fluvial unit, viz. the sandstone-siltstone-shaleunit. In the western sector of the mapped area. Ventersdorprocks are directly overlain by black, micaceous shales ofthe Ecca Group (Prince Albert Formation). The WhitehillFormation is probably not present as one continuous layerover the whole of the mapped area, and only occurs in afew 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 rhymic layers of shales and sandstones (deltaic deposits). The transitional sone 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. course-grained and channel-axis facies which laterally grade into finer grained and thin-bedded channelmarginal facies and levee mudstone deposits on the flood plain of a braided river. Intrusive rocks consist of post-Karoo dolerites (sills and dykes) and the kimberlite 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 occur mainly as Aeolian sheet deposits buut also as dunes of variable magnitude.The Vaal River Gravels and the alluvial sands and silts of the Vet and Sand Rivers are the most significant alluvial deposits present in the mapped area. Pans of variable shapes and sizes have iriginated through the erosion of paleo river channels by subsequent wind action.

[发布机构] University of the Free State