Regional variations in the carbon isotopic composition of phosphorite from the Early Cambrian Lower Tal Formation, Mussoorie Hills, India

Early Cambrian Lower Tal phosphorites of the Mussoorie Syncline have been analyzed for their carbon isotopic composition of apatite-bound carbonate and organic carbon. Two mine excavations at Maldeota and Durmala, located on two limbs of the syncline and ∼14-km apart were subjected to detailed sampling along selected stratigraphic profiles. Field and microscopic evidence suggest a shallow marine environment at the site of phosphorite formation characterized by small-scale fluctuations in the sedimentation milieu. Isotope data have been utilized to characterize differing redox conditions prevalent during phosphorite formation at Durmala and Maldeota. In addition, a marked negative carbon isotope excursion has been recorded at the dolomite–phosphorite contact defined by the Krol–Tal stratigraphic boundary. It is thought to reflect the influx of 12C- and P-rich waters from a deeper part of the sedimentary basin. Based on the isotopic composition of carbon in the phosphorite, sulfur in the associated pyrite and petrological data, it has been inferred that the Durmala phosphorite reflects deposition under a suboxic diagenetic environment and the Maldeota phosphorite formed within suboxic to anoxic sulfate-reducing conditions.