A comparative study of the geohydrology of archaean formations and associated rocks in the North-Western Transvaal and the North-Western Cape Province

Doctoral Thesis


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University of Cape Town

An area of 4 400 km² in the North-western Transvaal is compared to an area of 75 000 km² in the North-western Cape Province with regard to geology, physiography, and hydrology. In the Transvaal the Archaean Formations range from the Swaziland System to the Dominion Reef System, with younger intrusives and a cover of Tertiary to Recent ferricrete, calcrete and soil. In the Cape Province the comparable formations are the Kheis System to the Soetlief Formation, younger intrusives and a Tertiary to Recent cover of wind-blown sand, calcrete and soil. The available literature on the geological formations and structure is reviewed. Outcrops are scarce in the Transvaal in an almost featureless tree-covered plain with ill-defined drainage, except for the two perennial rivers. In the Cape Province relief is higher and drainage better developed towards the perennial Orange River. Large portions of the Bushmanland Plateau are nevertheless, covered by sand and calcrete, without well-developed drainage. The rain falls during summer in both areas, and the average annual rainfall is 526 mm in the Transvaal, and 176 mm in the Cape Province. In both areas the farms and villages are almost entirely dependant on ground-water supplies. The importance of geophysical methods in the selection of borehole sites is stressed, and a summary is given of the more important geological and geophysical procedures and methods in both areas. This included magnetic, electromagnetic, and electrical resistivity methods. A summary is given of previous hydrological investigations in both areas. It was fragmentary, and only a few reports from the Cape Province could be traced. The results of a total of nearly 1 500 boreholes drilled in Archaean Formations in the Transvaal, and nearly 3 800 boreholes drilled in these formations in the Cape Province are analysed with respect to different parameters. Boreholes are classified according to the geological formation in which they occur, and the physiographical conditions ruling in the area. In the different formations in the Transvaal, the (percentage of successful boreholes ranged from 30 per cent in the Dominion Reef System to 38 per cent in the Archaean Granite. In the Cape Province it ranged from 29 per cent in the Grey Gneiss to 45 per cent in the Kheis System, except for the adamellite with 6 per cent and the Soetlief lava with 90 per cent successful boreholes. In both areas certain geological and physiographical controls were determined by means of which borehole results can be improved. Analyses of geophysical data, especially apparent resistivity at the rest level, sometimes gave positive results. Depths at which water was struck, rest levels, depth of weathering, and other parameters are compared for different formations and areas.