Carbon Isotope Stratigraphy of the Phosphorite-Bearing Precambrian-Cambrian Transition Series of the Lesser Himalaya (Uttar Pradesh, India)

M. Schidlowski Max-Planck-Institut für Chemie, Postfach 3060, D-55020 Mainz, Germany

D. M. Banerjee Geology Dept., University of Delhi, Delhi 110007, India

F. Siebert Tauw Umwelt GmbH, D-68199 Mannheim, Germany

The conspicuous negative excursion of marine d13Ccarb values at the Precambrian-Cambrian (PC/C) boundary reported from many parts of the world (Schidlowski et al., 1975; Tucker, 1986; Magaritz et al., 1987; and others) has been confirmed also for the PC/C transition in the Lesser Himalaya. From positive values between +2 and +6” [PDB] in the terminal Neoproterozoic of the Krol Group, d13Ccarb plummets to -4 to -8” in the chert-bearing black shale-phosphorite series of the basal Tal Fm representing the lowermost Cambrian (Tommotian). After a first identification of this trend during a reconnaissance study (Aharon et al., 1987), recent work performed on several isotope profiles from
the Mussoori Syncline (Garhwal Himalaya) has furnished definitive proof that the carbon isotope stratigraphy of the Himalayan PC/C boundary section conforms with trends observed in respective sections from other localities. Specifically, the onset of black shale-phosphorite sedimentation at the base of the Cambrian was found to be beset with an acute negative isotope signal (comprising d13Ccarb, d18Ocarb and d13Corg) that tends to stabilize in the lower part of this series. According to a plausible geodynamic model (Donnelly et al., 1990), the coincidence of sedimentary phosphate formation with a negative carbon isotope excursion can be reasonably explained as a corollary of geological events starting with a preceding oceanic anoxic event (OAE). The re-establishment of a well-mixed oxic Late Proterozoic ocean was apparently coupled to the breakup
of the Precambrian megacontinent(s) that consequently generated an array of epicontinental seas and bays, notably in the realm of the newly-formed paleotethys. In the wake of restored marine circulation patterns, these shelf areas were inundated by previously stagnant, phosphate-rich seawater, turning the shelves into regions of high primary productivity. Mixing of the formerly trapped water bodies with aerated surface waters also resulted in a rapid oxidation of the Corg-component of the recycled waters, with upwelling currents thus spilling back isotopically light bicarbonate into the marine carbon cycle which, in turn, caused a marked negative shift in the d13C values of subsequently formed carbonates. Hence, the observed coincidence of phosphogenesis, high productivity (with black shale formation) and the
occurrence of isotopically light carbonates within the PC/C transition series was most probably caused by a sequence of geotectonic and paleooceanographic events lasting from Neoproterozoic to early Cambrian times.


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