GEOCHEMICAL AND MINERALOGICAL INDICATORS OF STRATIGRAPHY AND PROVENANCE OF THE KAROO SUPERGROUP IN SOUTHERN BOTSWANA

The Karoo Supergroup comprises successions of sedimentary and volcanic rocks spread across southern Africa. In Botswana they unconformably overlie Archean and Proterozoic rocks but are only poorly exposed being in turn overlain by the Kalahari Beds. Rock complexes have been correlated between widely spaced boreholes based on macroscopically similar appearance and similar position in the succession. In neighbouring South Africa and Namibia these rocks are well exposed and the lithostratigraphy is well constrained by the fossil record. The Karoo units of Botswana have been correlated with these more precisely defined successions, but these correlations are unsupported by other criteria and are as such limited. Here we present preliminary results of a study of the heavy mineral chemistry along with whole rock geochemistry in an attempt to provide additional, criteria for the middle part of the Karoo suite, the Ecca Group. Analysis of samples of heavy minerals by SEM-EDAX indicates a progressively more important pyrope rich component in the garnet population (with increasing depositional age). Analysis of 35 samples for major and trace (including REE) element composition shows a close relation between the geochemical characteristics and stratigraphy. Major elements show that the deltaic material of the Kweneng Formation and Boritse Formation was sourced from recycled continental crust. The basinal mudstone and siltstone below and above fall into intermediate-mafic igneous field. Most samples have distinct negative europium anomalies (Eu/Eu*=0.49-1.27; av. = 0.74) and most values are characteristic of sediments of cratonic derivation. A clear shift in (Gd/Yb)N in the basinal pro-delta shales (the Bori Formation) is generally 2.0 or greater, which is typical of an Archean signature, whereas post-Archean rocks usually have (Gd/Yb)N 1.0 – 2.0 as seen for the strata above the delta mouth bars and channels (average 1.6). In a diagram in which (La/Yb)N plotted against CeN shows a relationship between REE fractionation and provenance, the samples from the lacustrine Kwetla Formation and Bori Formation fall into the Intermediate Igneous Provenance Field, whereas the deltaic Kweneng and Boritse Formation belong to the quartzose provenance field. This agrees with the indications from the major element discriminators.