oa South African Journal of Geology - Geometrical relations between the Transvaal Supergroup, the Rooiberg Group, and the mafic rocks of the Bushveld Complex

Volume 101 Number 4
  • ISSN : 1012-0750
  • E-ISSN: 1996-8590



The Rustenburg Layered Suite of the Bushveld Complex has been considered to result from intrusion, either discordantly across the sedimentary layering of the Pretoria and Rooiberg Groups, or along a regional unconformity at the boundary between these two groups. The latter model is based on the observation that the thickness of sedimentary rocks of the Pretoria Group in the roof of this intrusive body is much less than that eliminated from the floor by the intrusion. This conclusion is only valid if the emplacement of the intrusive body was parallel to the layering of the sedimentary rocks. A geometrical analysis shows that if there is an angular discordance between the sedimentary rocks and the intrusive body, a considerable thickness of apparently missing strata is predicted without there having been any erosion. These strata are preserved, but are truncated beneath the intrusive body. It is calculated from the known dips and discordances between the sedimentary rocks and the intrusive body in the eastern and Potgietersrus limbs of the Bushveld Complex that 3.5 and 6.5 km, respectively, of strata will not be preserved at surface. These figures match the thicknesses of apparently eliminated strata in these two areas. It is therefore considered unnecessary to appeal to a major period of pre-Rooiberg Group erosion, as discordant emplacement of the mafic rocks of the Bushveld Complex into the Transvaal Supergroup and Rooiberg Group adequately explains the observed field relationships.

Die Rustenburg Gelaagde Suite van die Bosveld Kompleks het ontstaan as gevolg van indringing of diskordant oor die sedimentêre gelaagdheid van die Pretoria en Rooiberg Groepe, of langs 'n regionale diskordansie by die grens tussen hierdie twee groepe. Laasgenoemde model is gebaseer op die waameming dat die dikte van die sedimentêre gesteentes van die Pretoria Groep in die dak van die intrusiewe liggaam veel minder is as dit wat van die vloer deur indringing verwyder is. Hierdie gevolgtrekking is slegs geldig as die inplasing van die intrusiewe liggaam parallel aan die gelaagdheid van die sedimentêre gesteentes. 'n Geometriese analise toon dat wanneer daar 'n klinodiskordansie tussen die sedimentêre gesteentes en die intrusiewe liggaam is, 'n aansienlike dikte van skynbare verlore strata voorspel word sonder dat enige erosie plaasgevind het. Hierdie strata is bewaar, maar onder die intrusiewe liggaam afgesny. Dit word uit bekende hellings en diskordansies tussen die sedimentêre gesteentes en die intrusiewe liggaam in die oostelike en Potgietersrusflanke van die Bosveld Komplex bereken dat 3.5 en 6.5 km strata, respektiewelik, nie op die oppervlak bewaar sal word nie. Hierdie getalle stem ooreen met die skynbare verwyderde strata in hierdie twee gebiede. 'n Belangrike periode van erosie voor die Rooiberg Groep word dus as onnodig beskou omdat diskordante inplasing van die mafiese gesteentes van die Bosveld Komplex in die Transvaal Supergroep en Rooiberg Groep die waargenome veldverhoudings voldoende verduidelik.

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