15. Kimberlites, Kimberlite Exploration, and the Geomorphic Evolution of Botswana

An understanding of the regional geomorphic framework of Botswana played a critical role in the discovery by De Beers of economic kimberlite pipes in the Orapa area in central Botswana. Early prospecting in the eastern Botswana hardveld in the late 1950s to early 1960s resulted in the recovery of a train of diamonds in the east-flowing Motloutse River, extending to the headwaters of this drainage line. De Beers concluded that the Motloutse had previously extended further west, based on an appreciation that the eastern Botswana watershed represented an axis of epeirogenic flexure, which had beheaded the former headwaters. The implication was that the source of the diamonds recovered in this river could be located in the Kalahari sandveld to the west of the drainage divide. Subsequent extension of sampling to the west into the Kalahari was rewarded by the discovery of the Orapa kimberlites in 1967, a year after Botswana achieved independence. We discuss how these sampling programmes in turn provided a feedback loop, making it possible to refine understanding of the regional geomorphic evolution. Because kimberlite pipes typically taper in size with depth, they also provide a gauge for estimating the depth of post-emplacement erosion, providing an important constraint on landscape evolution. Soil sampling in the Kalahari environment demonstrated that bioturbation has translocated kimberlite indicator minerals (KIMs) vertically from sub-Kalahari kimberlite pipes, resulting in their concentration in the Kalahari surface sand directly above the buried kimberlite. A consequence of this vertical mixing of the Kalahari sedimentary section is that luminescence dating of Kalahari quartz grains may produce spurious “mixed” ages. In the south of Botswana, mid-Cretaceous sub-Kalahari kimberlites are overlain by fluvial conglomerates, while in the northeast of the country, pipes of comparable age are overlain by a calcretized silcrete, which has been linked to the development of the African erosion surface. The basal conglomerates and African Surface duricrusts are unconformably overlain by semi-consolidated Kalahari sand. However, both conglomerate clasts and fragments of the duricrust have been identified within craters sediments forming the upper section of the respective kimberlite pipes. This implies that deposition of the conglomerates, and development of the African surface overlapped the episode of kimberlite eruption. This, in turn, provides an important mid-Cretaceous temporal constraint on initiation of Kalahari deposition, and of the African erosion cycle.