Volume 33 1997

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Browsing Volume 33 1997 by Author "Bamford, M. K."

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    Cretaceous fossils from the Orapa Diamond Mine
    (Bernard Price Institute for Palaeontological Research, 1997) Rayner, R. J.; Bamford, M. K.; Brothers, D. J.; Dippenaar-Schoeman, A. S.; McKay, I. J.; Oberprieler, R. G.; Waters, S. B.
    The Orapa kimberlite pipe, situated in north-central Botswana, is well-known for its rich reserves of diamonds. It is indeed one of the largest and richest diamond mines in the world. The kimberlite magma transporting the diamonds from the upper mantle erupted through a sequence ofKaroo-aged rocks before the deposition ofthe Kalahari Sands. This eruption has been radiometrically dated at early Late Cretaceous (Cenomanian-Coniacian). When volcanism ceased, a succession of epiclastic crater lake sediments was deposited above the kimberlite plug. Analysis of these sediments, which mostly comprise the results of mudflows and debris flows and fmer sediments during quiescenttimes, suggests that most of the sediments within the crater were deposited rapidly as mass flows, and were therefore mobilised soon after the volcanic eruption. Buried within the fine-grained sediments is a unique assemblage of fossils including flowering plants and many whole-bodied insects. The fossils are commonly exquisitely preserved in extremely fine-grained mudstone. Interpretation of the sedimentary facies and fossils is that the mid-Cretaceous climate of central Botswana was temperate, seasonal and wet, and the area surrounding the crater was forested. The fossils represent the recovery of the biota of the area after the violent eruptions of Orapa and other nearby kimberlite fissures and pipes. The fossils have contributed considerably to our understanding of mid-Cretaceous insects and flowering plants and suggest intimate relationships between the two at an early stage in the radiation of flowering plants. It seems that southern Gondwana (including southern Africa) was a centre of diversification for both insects and angiosperms in the mid-Cretaceous.