The stratigraphy and sedimentation of the Bokkeveld group

Theron, J. N. (1972-03)

Thesis (D.Sc.)--Stellenbosch University, 1972.


ENGLISH ABSTRACT: A detailed stratigraphic investigation of the Bokkeveld Group along seven regularly spaced profiles, supported by heavy mineral studies, permits definition of the upper boundary of the sequence. There is a distinctive increase in the brookite percentage from the base to the top of the Group. From west to east the sequence displays cyclic deposition which gradually fades out southwardso Five sheetlike arenitic wedges in the west intertongue southwards with lutite ; only the lower three can be traced eastwards. Lithological nomenclature has been introduced. Marked consistency of the lower units as contrast ed with variability of the upper sequence forms the basis of major subdivision into a lower Ceres Subgroup and an upper Traka Subgroup. Each fundament allithological unit (formation) is described in relation to its outcrop area. Grain-size distribution, palaeocurrent orientations and other directional structures have been investigated mainly in the arenaceous units and analysed by modern computer methods . The coincidence of palaeo current orientations, isopach patterns and southward diminution of grain-size , proves that a general southward and westward directed palaeoslope prevailed during sedimentation. The grain-size distribution suggests shallow marine to beach deposition. The stratigraphi coccurrence, orientation and fossil- and -sediment association support this conclusion. In the west the elongate Clanwilliam basin, flanked by marginal shelves, merged southeastward with an east-west trending 'shelf' or basin with marked eastward axial pitch ( Agulhas 'basin'). Cratonic borderlands existed to the north (Bushman Mountain land) and west (Atlantic Mountainland) of the Clanwilliam basin. Similar borderlands flanked the Agulhas 'basin' to the north (Nuweveld and Winterberg Mountainlands) and possibly south (Antarctic Mountainland). Lithofacies and isopach contour patterns are closely related, revealing a direct relationship between structural grain and sedimentation, a marked northward increase in sand, as well as an overall predominance of the finer elastic fraction in the Bokkeveld Group. The sedimentary history of the sequence deals with a shoalwater deltaic-marine complex, subjected to marine transgressions and regressions in a tectonically unstable setting. The Bokkeveld Group, predominantly a regressive sequence, forms a separate entity of the Cape Supergroup marked by a lack of supermature sediment. The two tectonically dissimilar basins which received the sediments of the Bokkeveld Group were probably inherited weaknesses of the continent.

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