Abstract: Varanger Ice Ages in India Potentially Revealed by Integrated Sequence and Carbon Isotope Stratigraphy
KAUFMAN, ALAN J., The University of Maryland, College Park, MD 20742-4211, [email protected]; GANQING JIANG and NICHOLAS CHRISTIE-BLICK, Lamont Doherty Earth Observatory of Columbia University, Palisades, NY 10964-8000
The exceptionally thick (> 2 km) Neoproterozoic Krol carbonate platform of northern India crops out for over 250 km along the Lesser Himalaya thrust and fold belt. Integrated sequence and carbon isotope stratigraphy across the basin reveals several laterally persistant paleo-karstic surfaces, as well as first-order trends in d13C — documented by close stratigraphic sampling in multiple sections between coeval rocks of varying facies — which are comparable to those recognized in units of similar age worldwide. At the base of the Krol platform are glaciomarine diamictites attributed to the Varanger Ice Age (ca. 580-600 Ma). These are sharply overlain by a fine-grained, finely-laminated cap carbonate. Like all cap carbonates above Neoproterozoic glacial sediments, this unit, up to 15 meters thick, has negative d13C values, ranging from -2.5 at the base to near 0 at the top. Above this level are several discrete intervals that contain carbonates with varying degrees of 13C depletion; in contrast, most other carbonates in the succession have consistently positive d13C values. In two instances, the low d13C carbonates occur just below interpreted sequence boundaries, suggesting the possibility of secondary alteration associated with long periods of exposure. However, two other negative d13C excursions appear unrelated to physical breaks in the succession. One of these is present in the Krol B Formation, a widespread deposit of purple to green siltstones and shales, which is capped by a fine-grained, laminated, and silicified dolomite. Like this carbonate, thin dolomites within the unit are also significantly depleted in 13C . As these negative C-isotopic excursions have elsewhere been used to identify periods of global ice ages in units that lack glacial diamictites, we consider the possibility that the deep water deposits of the Krol B Formation were deposited during a second pulse of Varanger glaciation.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah