A chronostratigraphic framework for the north-central Kaapvaal craton, the Bushveld Complex and the Vredefort structure

Sufficient geochronological data are available pertaining to the north-central part of the Kaapvaal craton to construct a chronostratigraphic framework for the rock units involved in the Vredefort event, from the basement rocks at ∼ 3170 Ma to the Waterberg-Soutpansberg Groups deposited between ∼ 2025 and 1425 Ma. The resulting framework forms a convenient chronological guide to the geological evolution of this area, and the timing of events such as that at Vredefort. The basement on which the supracrustal rocks were deposited stabilised from ∼ 3170 to 3060 Ma but metamorphism and metasomatism of these rocks occurred up to 2800 m.y. ago in different parts of the craton. The first unit of the Wits triad, the volcaniclastic Dominion Group, was deposited at ∼ 3060 Ma on the stabilising basement. This was followed by the dominantly clastic Witwatersrand Supergroup and the overlying Ventersdorp Supergroup. The dominantly volcanic Ventersdorp rocks are ∼ 2700 m.y. old and form the last unit of the Wits triad. A considerable time gap of ∼ 200-300 Ma separates the triad from the lowermost units of the overlying Transvaal Sequence. The age of these volcanic and clastic protobasinal rocks is poorly constrained, between ∼ 2500 and 2300 Ma. They may in fact represent eroded remnants of a more extensive depositional cycle. The mainly clastic sedimentary part of the Transvaal Sequence (Pretoria Group) has been dated at ∼ 2224 Ma. The upper part of the Transvaal Sequence (Rooiberg Group) is only constrained by the age of the intrusive Bushveld Complex. The Bushveld basic layered rocks are well dated at ∼ 2060 Ma, as are the subsequent granites at ∼ 2050 Ma. The Vredefort event, which post-dates the Bushveld Complex, is reasonably well constrained by the age of the bronzite granophyre at ∼ 2000 Ma. Deposition of the Waterberg and Soutpansberg Groups, which post-date the Bushveld Complex is poorly constrained between the age of the Bushveld granites and younger alkaline intrusives at ∼ 1425 Ma, but these sediments and volcanics were probably formed between ∼ 2025 and 1752 Ma. The Waterberg Group may therefore contain debris from the Vredefort structure. © 1990.