CARBONATE PETROGRAPHY, KEROGEN DISTRIBUTION, AND CARBON AND OXYGEN ISOTOPE VARIATIONS IN AN EARLY PROTEROZOIC TRANSITION FROM LIMESTONE TO IRON-FORMATION DEPOSITION, TRANSVAAL SUPERGROUP, SOUTH-AFRICA

The transition zone comprises Campbellrand microbialaminated (replacing "cryptalgalaminate') limestone and shale, with minor dolomite, conformably overlain by the Kuruman Iron Formation of which the basal part is characterized by siderite-rich microbanded iron-formation with minor magnetite and some hematite-containing units. The sequence is virtually unaltered with diagenetic mineral assemblages reflecting a temperature interval of about 110° to 170°C and pressures of 2 kbars. Integration of sedimentary, petrographic, geochemical, and isotopic results makes it possible to distinguish between depositional, early diagenetic, deep burial, and metamorphic effects on the isotopic compositions of the carbonate minerals and the kerogen in the sequence. Major conclusions are that deep burial thermal decarboxylation led to 13C depletion in euhedral ferroan sparites and 13C enrichment in kerogen (organic carbon). -from Authors