The stratigraphy and sedimentation of the Bokkeveld group

[摘要] ENGLISH ABSTRACT:A detailed stratigraphic investigation of the BokkeveldGroup 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 thebase to the top of the Group.From west to east the sequence displays cyclic deposition whichgradually fades out southwardso Five sheetlike arenitic wedges inthe west intertongue southwards with lutite ; only the lower three can be traced eastwards.Lithological nomenclature has been introduced. Markedconsistency of the lower units as contrast ed with variability of theupper sequence forms the basis of major subdivision into a lowerCeres Subgroup and an upper Traka Subgroup. Each fundament allithological unit (formation) is described in relation to itsoutcrop area.Grain-size distribution, palaeocurrent orientations and otherdirectional structures have been investigated mainly in the arenaceousunits and analysed by modern computer methods . The coincidence ofpalaeo current orientations, isopach patterns and southward diminutionof grain-size , proves that a general southward and westward directedpalaeoslope prevailed during sedimentation. The grain-size distribution suggests shallow marine to beach deposition. The stratigraphi coccurrence, orientation and fossil- and -sediment association supportthis conclusion.In the west the elongate Clanwilliam basin, flanked bymarginal shelves, merged southeastward with an east-west trending 'shelf'or basin with marked eastward axial pitch ( Agulhas 'basin'). Cratonicborderlands existed to the north (Bushman Mountain land) and west(Atlantic Mountainland) of the Clanwilliam basin. Similar borderlandsflanked the Agulhas 'basin' to the north (Nuweveld and WinterbergMountainlands) and possibly south (Antarctic Mountainland). Lithofacies and isopach contour patterns are closely related, revealing a directrelationship between structural grain and sedimentation, a markednorthward increase in sand, as well as an overall predominance ofthe finer elastic fraction in the Bokkeveld Group.The sedimentary history of the sequence deals with ashoalwater deltaic-marine complex, subjected to marine transgressionsand regressions in a tectonically unstable setting.The Bokkeveld Group, predominantly a regressive sequence,forms a separate entity of the Cape Supergroup marked by a lackof supermature sediment.The two tectonically dissimilar basins which received thesediments of the Bokkeveld Group were probably inherited weaknessesof the continent.

[发布机构] Stellenbosch University