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A terminal Proterozoic history of sea-level change and biogeochemical anomalies, Lesser Himalaya, India


, : A terminal Proterozoic history of sea-level change and biogeochemical anomalies, Lesser Himalaya, India. Abstracts with Programs - Geological Society of America 32(7): 94

An integrated sequence and chemostratigraphic study of terrigenous and carbonate rocks of the Infra Krol Formation and Krol Group in the Lesser Himalaya of northern India provides a unique window into sea level history and biogeochemical perturbations at the end of the Proterozoic Eon. In this study, eight regional stratigraphic discontinuities were traced across five synclines over a distance of nearly 300 km. Three of the surfaces are interpreted as sequence boundaries, while the remaining five are regional flooding surfaces identified on the basis of transitions in facies stacking patterns. Carbon isotope analyses of >1300 samples reveal an overall pattern of carbon-13 enrichment punctuated by five discrete intervals of negative carbon isotopes. Geochemical screens reveal few clearly altered samples, consistent with the interpretation of the negative excursions as primary. However, a compilation of trends based on the framework of regional stratigraphic discontinuities shows greatest variation in the interval of negative excursions. Possible explanations include 1) unusual diagenetic resetting of carbon isotopes; and 2) stratigraphic complexity, with local omission of section at surfaces. Carbon isotope fractionation between inorganic and organic phases is notably reduced in the interval across the Krol B/C transition. Of five carbon-13 depleted intervals, only one is a clear "cap carbonate" atop glacial diamictite. In some areas, however, the basal beds of the Krol C are lithologically and isotopically similar to cap carbonate facies. While a glacial-eustatic origin is permitted for the three sequence boundaries, no evidence has been found for marked lowering of sea level at other horizons, including the intervals characterized by negative carbon isotopic excursions.

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