Browsing by Author "Stewart, Allan Graham"

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    Petrology and petrography of samples from two Algoa Basin cores
    (1973) Stewart, Allan Graham; Fuller, A O
    Ninety-four samples from two cores drilled in the Uitenhage Group of the Algoa Basin were analysed. The group of sedimentary rocks comprises the Enon, Kirkwood and Sundays River Formations and consists of continental (Enon and Kirkwood Fms.) and marine (Sundays River Fm.) deposits typical of a transgression sequence into an intermontane valley. Only the Kirkwood and Sundays River Formations were intersected in the boreholes but in one - AD 1/68 'basement' of Bokkeveld Group is reached, while the other borehole, CO 1/67, did not reach 'basement'. The Kirkwood Formation sediments are typically red shales interbedded with drab, fine sandstones (wackes) while the Sundays River Formation is typically drab fine to very fine silty sandstones and gray shales of a marine delta environment. Grain size analyses utilizing wet sieving and pippeting methods showed that two populations of grains are dominant, namely fine to very fine silty sand and clay size material. From the granulometric analyses statistical parameters (e.g. mean, standard deviation, skewness, kurtosis) were calculated and plotted in various combinations. Distinctive trends are thus revealed indicating a mixing of the two sediment populations in varying proportions. Compaction has resulted in an increase with depth in specific gravity (which varies from about 2,5 to +2,6 gm cc⁻³). Studies of the heavy mineralogy reveals a dominance of garnet (two varieties, colourless and pink) with zircon, sphene, rutile and some others in considerably lesser amounts. Opaque grains are also present, sometimes in dominant amounts. X-ray diffraction analyses of the <2 μm clay fraction showed that illite is dominant in both the marine and terrestrial deposits and that chlorite is abundant to infrequent, while montmorillonite is more prevalent in the continentally deposited rocks. Selected clay samples were photographed with both the Transmission and the Scanning Electron Microscopes. Thin sections of the arenaceous samples reveal that those which are carbonate cemented are relatively free from matrix, while those which are uncemented are matrix-rich. This latter situation can be ascribed to the breakdown, after burial, of the commonly occurring rock fragments which frequently constitute about one-third of the sandstones. The problem of red beds is considered, and the red pigmentation found in the Kirkwood Formation is believed to be due to oxidation of iron after deposition of the sediment. Intense weathering in the upland source area is not a suitable explanation for the formation of the Kirkwood Formation red beds. The basin as a whole is considered with attention being focussed on the provenance areas (believed to be the Cape and Lower Karroo Supergroups), the dispersal, depositional environments and the lithification and diagenesis. Finally, the economic potential is briefly considered and some suggestions for future research are put forward.