Introduction
Consists of major lithologies are (1) limestone and dolomite, (2) shale and interbedded shale carbonate, (3) siderite-rich banded iron-formation, and (4) iron oxide-rich banded iron-formation. The oldest rocks are limestones and lesser dolomite with abundant cryptalgalaminae and intraclastic textures. Interbedded with the limestones and dolomites are carbonaceous shales, some of which are unusually ferruginous or pyrite rich. These carbonates and shales are overlain by meso- and microbanded siderite-chert iron-formation which grades upward into magnetite-, chert-, and carbonate-rich iron-formation.The averages for major and trace elements and rare earth element contents of the limestones, dolomites, and shales are distinct from those of the two types of iron-formation.
Basic stratigraphy can be summarised as:
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Top is Rooiberg Group, but some believe it is part of Bushveld
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Pretoria Group – quartzites (outcrops), mudrock, sandstone and shale. Rooihoogte at bottom is conglomerates.
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Below this is Chuniespoort, which includes the Malmani Subgroup. It contains of limestone and dolomite, with chert. The Malmani is divided in Oaktree, Monte Christo, Lyttelton, Eccles, Frisco in dolomites and Penge Formation in ironstones (Thabazimbi).
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Below Chuniespoort is the Black Reef Formation, consisting of conglomerates and quartzites/mudrock.
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In N-Cape is the Griekwaland Basin with Postmasburg Group – banded ironstones and dolomite.
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Geohydrology
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Black reef: low permeability –no primary aquifers, but good yields in some fractures – Vryburg >14l/s.
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Chuniespoort: dolomite not permeable (porosity <3%). Important is the secondary structures – water moved along joints & faults, CO2 in water dissolves calcite and dolomite – KARST network. When water lowered in karst, result in sinkholes. Area divided in compartments due to dolorite dykes. Strong springs (as high as 1157l/s near Ventersdorp –springs >250 l/s common). The different formations in this group have different geohydrological properties – the Oaktree and Lyttleton is poor in chert, with little water, whilst the Monte Christo and Eccles are rich in chert with abundance of water.
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Pretoria: Weathering in quartzite and shale important. On contact with diabae sills good water. Lavas (ongelukslawa) good aquifer where weathered – as deep as 300m.
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Northern Cape: Banded ironstone, only along faults and joints. In areas with lava (tillite), water in weathered areas.
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Source: http://web.uct.ac.za/
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