DEPOSITIONAL AND METAMORPHIC CONTROLS ON CARBON ISOTOPIC VARIATIONS IN THE PALEOPROTEROZOIC TIMEBALL HILL FORMATION, SOUTH AFRICA

Secular organic carbon isotope variations in the early Precambrian are often used as evidence for ancient life and environmental changes on Earth. However, there is a wide range of organic carbon d¹³C_PDB values at any point in time. The origin of these variations has not been investigated and forms the focus of this study. We analysed deep drill-core intersections of well preserved carbonaceous shale of the Paleoproterozoic Timeball Hill Formation of the Transvaal Supergroup. The succession lends itself ideally for integrated geochemical, petrographic and sedimentological studies because it comprises well defined stacked genetic increments of sedimentation (parasequences) ranging from prodelta black shale to delta front sand, lateritic oolitic ironstone and glacial diamictite, crosscut by two diabase sills, minor hydrothermal calcite veinlets and a deep groundwater alteration zone.

 

Organic carbon d¹³C_PDB values vary between -34.7‰ and -24.8‰. Careful stratigraphic evaluation of the data indicates that sharply defined positive shifts in d¹³C_org. values (up to 8‰) occur in the immediate vicinity of calcite veinlets, diabase sills and along the recent deep groundwater alteration zone. Second, upward enrichment in ¹³C_organic takes place in individual shallowing upward deltaic parasequences. These could be related to more effective aerobic degradation of organic matter in shallow delta front environments relative to deep prodelta settings. Third, we recognize a gradual overall upward enrichment in ¹³C_organic from deltaic deposits, with associated lateritic oolitic ironstones, in the lower part of the succession to glacial deposits near the top. This trend is thought to reflect a long term global change in the isotopic composition of the oceanic carbon reservoir. It is related to more effective organic carbon burial that resulted in a major climatic change from warm lateritic to glacial ice-house conditions.