The pre-Karoo geology of the southern portion of the Kaapvaal Craton, South Africa

Abstract

The nature of the geology beneath the Karoo Supergroup in South Africa has been of considerable interest to geologists since the late 1800s when it was discovered that Karoo strata unconformably overlie gold-bearing rocks of the Witwatersrand Supergroup. The first comprehensive map of the pre-Karoo geology, focusing particularly on the distribution of Witwatersrand Supergroup rocks, was compiled by R. Borchers in the early 1960s. based largely on borehole records. T. Stratten and A. B. Cadle used the SOEKOR borehole data base, supplemented by private exploration boreholes, to extend the coverage of the pre-Karoo geological maps in the 1970s. Commencing in the late 1930s. geophysical methods, notably gravity and magnetics, played an increasingly important role in deciphering not only the geology underlying the Karoo strata but also the fundamental geological structure of southern Africa. The introduction of computer processing technology to evaluate large geophysical data sets in the 1980s greatly improved the ability to resolve geological features. Apart from an update of Borchers' Witwatersrand Basin map compiled by D.A. Pretorius in 1986. most of the interpretations of the pre-Karoo geology of southern Africa carried out since then have been based largely on geophysical data by a variety of workers. In this paper we use previously available data, as well as information that has become available in recent years, to reaffirm the boundaries of the Kaapvaal Craton and to re-examine the geology beneath Karoo cover. Datasets used include regional and higher resolution gravity and aeromagnetic data, exploration borehole records. geochronological information and reflection seismic data. Our reconstruction indicates that the southern portion of the craton is geologically remarkably similar to the well-known northern part and includes outlying erosional remnants of both Witwatersrand and Transvaal Supergroups. interestingly, iron formations of the Transvaal Supergroup are extensively developed in this southern region of the craton, unlike the northern portion where their occurrence is more restricted. Although Ventersdorp Supergroup rocks were encountered in drill holes, the full distribution of these rocks is uncertain due to their weak geophysical signature. Also present in the study area is a possible satellite intrusion of the Bushveld igneous Province (the Trompsburg Complex) which was emplaced into the Transvaal Supergroup and appears to be partly overlain by rocks of Waterberg Group age. The revised pre-Karoo geological map of the craton presented here has important implications for palaeo-Proterozoic continental reconstructions and in addition opens up new exploration targets for Witwatersrand-type gold deposits in South Africa.