LOCAL VERSUS GLOBAL CONTROLS OF ISOTOPIC VALUES IN THE RASTHOF CAP-CARBONATE: TESTING THE VIABILITY OF CHEMOSTRATIGRAPHIC CORRELATION IN THE NEOPROTEROZOIC

The Rasthof Formation, deposited along a broad, semi-restricted continental shelf, is the cap-carbonate to the Sturtian-aged (~750 Ma) glacigenic Chuos Formation and contains a variety of intriguing isotopic features. These include 1) facies-dependent variation of d13C, 2) up to 5‰ sub-centimeter scale inter-laminar variation of d13C and 3) geochemical evidence for an aragonitic primary mineralogy. The Rasthof Formation consists of two distinct facies, a lower facies consisting of <10 m of gray/black mm-scale parallel laminated organic carbon-rich, intermittently fetid, limestones and dolomicrites transitioning sharply to an upper facies of gray biolaminated dolostone. Sub-storm wave base, and likely sub-photic zone, deposition is indicated by an absence of shoaling, “upwards” orientation of stromatolites, or current formed sedimentary structures. 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. Despite some classic diagenetic indicators, pervasive diagenetic alteration of isotopic signatures is unlikely based on inter-laminar variability of >5‰ for d13C within the rhythmically laminated lower unit and relationships between concentrations of Mn and Sr. In one key section, an additional relationship exists between a change in mineralogy in the lower facies from mixed limestone and dolomite to dolomite and a short-term isotopic variation of 4.5‰ for d13C. Because carbon isotopic trends in the lower facies of the Rasthof Formation are used for correlation with other Sturtian-age cap carbonates globally, as well as inferences about paleoceanic events, it is important to determine the nature of these isotopic values. The local and facies controlled variations in isotopic values discussed here obscure primary marine isotopic values in whole-rock samples, which instead represent a broad range of microfacies- to facies-scale processes.