Integrated Sequence and Chemostratigraphy of the Neoproterozoic Krol Platform, Lesser Himalaya, India

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9614070 Christie-Blick We propose an integrated sequence and chemostratigraphic study of the Neoproterozoic (and possibly lower Cambrian) Krol platform in the Lesser Himalaya of northern India. The Krol is part of a succession more than 6 km thick that includes glacial-marine stata of probable Varanger age at the base (Blaini Formation) and terrigenous rocks of early Cambrain age (Tommotian to Botomian) at the top (Tal Group). The Neoproterozoic and early Cambrian interval is characterized globally by marked excursions in (13C, ranging from +12 to -5 for carbonate carbon. These have been taken to reflect secular variations in the isotopic composition of sea water, and that the excursions can be used for global correlation. However, excursions in carbonate (13C as low as -5 are difficult to explain in terms of whole-ocean chemistry, and may indicate a) a surface to seafloor isotopic gradient that was larger than the ~2 per mil of the modern ocean and b) that stratigraphic trends may be due in part to changes in the paleoenvironment of deposition (water depth). The Krol platform can be used to address two questions of importance to Neoproterozoic chemostratigraphy and our understanding of the origin of the isotopic excursions: 1) What are the lateral variations in (13C as a function of facies (and paleowater depth)? 2) Are differences between published (13C measurements for the Krol and data from potentially correlative strata in Siberia, southern China, Namibia and the Mackenzie Mountains (Canada) due to inadequate attention to diagenesis in the Krol in previous studies, miscorrelation or the fact that the Krol represents an unusually complete and thick succession of carbonate (2km) from which to obtain samples? Preliminary work shows that the Krol may be particularly amenable to sequence stratigraphy because available outcrop appears to encompass the transition from shallow to deep ramp settings, as well as "interfingering" of the carbonate rocks with nearshore sandstones. The co mbination of chemostratigraphy with sequence stratigraphy will allow us to establish an independent time-stratigraphic framework with which to look at both "vertical" and "lateral" variations in (13C, and to investigate the role of diagenesis in different parts of the platform. The study should also resolve whether the Precambrian-Cambrian boundary is located at the top of the Krill (as suggested by published carbon isotopic data); and it may allow us to "fingerprint" several sequence boundaries with respect to (13C, and hence to compare the timing of these surfaces and sequence boundaries interpreted in other carbonate-rich successions of broadly the same age.