The conformable emplacement of the Bushveld mafic rocks along a regional unconformity in the Transvaal succession of South Africa

Abstract

A previously unrecognized regional unconformity occurs in the Proterozoic Transvaal succession of the Kaapvaal province of southern Africa. The predominantly marine and siliciclastic rocks of the lower two-thirds of the Pretoria Group are unconformably overlain by a terrestrial and largely volcanic sequence, the most voluminous portion of which is the Rooiberg Felsite. If the Bushveld Complex is mentally subtracted from map patterns, the Rooiberg Felsite also locally overlies pre-Pretoria sequences and the Archean crystalline basement. The Bushveld Complex is not lopolithic as commonly supposed. Instead, regionally condordant emplacement of the Complex along the unconformity explains both the seemingly discordant footwall features and the lack of Pretoria and older strata (other than a few xenoliths) in the hanging wall. The lack of any geochemical evidence for major crustal contamination of Bushveld magmas precludes disposing of any such hanging wall strata by wholesale assimilation. The Bushveld Complex, the unconformity, and the enclosing strata are synformally folded, making the Bushveld appear to be lopolithic, Due to this folding and multiple periods of erosion, the original extents of the Bushveld Complex, the unconformity, and the enclosing strata are now much reduced. Other smaller mafic intrusions along or immediately below the same regional unconformity probably are Bushveld-related.