African Natural History
On-line version ISSN 1816-8396
MCMILLAN, Ian K.. Aragonitic-walled benthic foraminifera (Epistomina) in the Cretaceous 'mudbelt' off southern Africa, and postmortem cross-shelf transport of tests. Afr. nat. history (Online) [online]. 2008, vol.4, pp. 17-24. ISSN 1816-8396.
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.
Keywords : foraminifera; benthic; test; Cretaceous; assemblage; aragonite; mudbelt.