FACIES-DEPENDENT ISOTOPIC VARIATION IN THE RASTHOF FORMATION, CONGO CRATON

The Neoproterozoic Rasthof Formation comprises a cap carbonate overlying Sturtian- aged (~750 Ma) glacigenic facies of the Chuos Formation or crystalline basement of the Congo Craton. Facies change within the Rasthof and determination of stable isotope values of carbonate demonstrate facies-dependent variation of d13C. The Rasthof Formation consists of two distinct facies. The lower unit consists of <10 m of gray/black mm-scale parallel laminated organic carbon-rich, intermittently fetid, limestones and dolomicrites. There is a sharp transition from the lower mechanically laminated unit to an upper unit of gray biolaminated dolostone. The upper unit consists of between 40 to 80 m of contorted, broadly arching, overhanging sub-meter scale stromatolites. An absence of shoaling, orientation of stromatolites, or current formed sedimentary structures indicates a sub-storm wave base and possibly sub-photic zone depositional environment. The abrupt facies transition is characterized by an equally abrupt +2-3‰ d13C shift immediately below the facies change and is duplicated in widely spaced (>100 km) sections. Inter- and intra-laminar variability of >3‰ for d13C is present within the rhythmically laminated lower unit, indicating absence of isotopic homogenation. In addition to demonstrating a relationship between facies and isotopic values, mineralogic, petrographic, and isotopic studies of six shelf and slope sections also identify a negative 4.5‰ shift of d13C values at the transition between calcite and dolomite intervals in the laminated unit and positive d13C values in bioherms of the underlying glacigenic facies of the Chuos Formation. A possible explanation for the facies-dependent d13C variation is local methanogenesis within the upper stromatolitic unit. The strong facies-d13C relation suggests that isotopic fractionation is controlled locally (in the benthic or early diagenetic environment) and thus does not provide a compelling basis for correlation of a presumed secular trend in surface ocean values shared with other sections globally. Results do provide direction for investigation into causes of facies-dependent variation, association between negative d13C shifts and mineralogy, and applicability of intra-laminar isotopic analysis for determination of primary isotopic values.