Aragonitic-walled benthic foraminifera (Epistomina) in the Cretaceous 'mudbelt' off southern Africa, and postmortem cross-shelf transport of tests

 

 

Ian K. McMillan

Natural History Collections Department, Iziko South African Museum, P.O. Box 61, Cape Town, 8000 South Africa. E-mail: imcmillan@iziko.org.za

 

 

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ABSTRACT

Throughout the Early Albian to Maastrichtian period there was a widespread dominance of aragonitic-walled tests in inner neritic benthic foraminifera assemblages, of which species of the genus Epistomina predominate. Fifty years ago Smitter first recognized Epistomina-dominated assemblages from outcrops at Mzamba and in the KwaZulu Basin, and realized their stratigraphic potential. These aragonitic-walled species are believed to have inhabited the palaeo-mudbelt. The changing ornamentation characteristics of both highly ornamented and smooth-walled Epistomina evolving through time led Smitter to recognize their value as marker species. They are found throughout the inner neritic region in basins experiencing slow siliciclastic sedimentation in normal marine, clay-dominated environments where fluvial input is limited (Pletmos, Gamtoos, Algoa, Thekwini and KwaZulu). Epistomina species, and aragonitic genera in general are absent in inner neritic regions of basins experiencing hyposalinity and rapid sedimentation, where fluvial input is abundant (Bredasdorp and Orange). However, although Epistomina-dominated assemblages are missing in the inner neritic (partly equivalent to the mudbelt) domains of the Orange and Bredasdorp Basins, transported tests of Epistomina are preserved in abundance in upper bathyal claystones in the two basins. The enigma of a complete absence of in situ aragonitic-shelled tests in the inner neritic domain, yet an abundance of obviously allochthonous tests, transported via debris flows to upper bathyal environments, is discussed with regard to the development of unconformities within the stratigraphic successions of southern African Cretaceous basins.

Key words: foraminifera, benthic, test, Cretaceous, assemblage, aragonite, mudbelt.

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ACKNOWLEDGEMENTS

Deep thanks for further samples from Mzamba Cliff outcrop, for discussions, and for facilities at the Ikizo South African Museum, Cape Town, are due to Herbert Klinger. The re-examination of the Makrides samples was undertaken late in 1993 while the author was employed by Soekor, and their permission to use facilities and permission to publish are acknowledged. Additional samples were kindly provided by David Broad, Petroleum Agency of South Africa. This article was carefully reviewed by John Compton (University of Cape Town) and Richard Dingle (Cambridge, England), and their comments are gratefully acknowledged. However, it should be noted that the author did not agree with all their comments. Bruce Rubidge of the Bernard Price Institute for Palaeontological Research, University of the Witwatersrand, Johannesburg kindly gave permission to use Makrides' Plate 10 from her 1979 M.Sc. thesis.

 

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Received 29 July 2008
Accepted 9 October 2008