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Ancient basins |
1 Dipartimento Scienze della Terra, Università di Siena, Via Laterina, 8-I-53100 Siena, Italy (e-mail: gandin{at}unisi.it)
2 Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK (e-mail: davywright2{at}ntlworld.com)
A detailed sedimentological and petrographic analysis of the Neoarchaean Campbellrand and Malmani carbonates of South Africa provides evidence that collectively indicates the former existence of evaporites and the early replacement of primary sulphate deposits by calcite. Diagenetic disruption of sedimentary structures, solution collapse breccias, cross-cutting relationships, corrosion surfaces, pseudomorphs of displacive nucleation cones, flowage and nodular structures are all characteristic of evaporites and their ductile behaviour. In thin section, rosettes of length-slow chalcedony around the heads of the silicified stromatolites also testify to the former presence of sulphates. Calcite, in the unusual form of herringbone/flamboyant spar in ‘cuspate’ and nodular facies, may preserve the fold shapes of enterolithic gypsum, picked out by deformed trails of degraded organic matter and dolomitized clusters of filaments. Subvertical columns of compressed organic material in a matrix of herringbone/flamboyant calcite appear to be the result of squeezing and compression between adjacent sulphate nodules. Carbonate replacement of evaporites was largely driven by bacterial sulphate reduction in sapropels and microbialites during early diagenesis. The petrographic evidence indicates that the replacive herringbone/flamboyant calcite precipitated after enterolithic and nodular structures formed, the sulphate having been used up before the enclosed organic material was totally consumed.
