Carbon, oxygen and strontium isotopic signatures in Maastrichtian-Danian limestones of the Cauvery Basin, South India

Abstract

A petrographic, carbon, oxygen and strontium study of the carbonate succession of the shallow marine Kallankurichchi and Niniyur formations of the Cauvery Basin, Tamil Nadu, India was conducted to understand the isotopic variations in seawater during Maastrichtian-Danian. The limestones from both the Kallankurichchi and Niniyur formations show large variations in Mn and Sr concentrations and high Mn/Sr ratios indicate alterations of primary isotopic signatures during shallow burial diagenesis. The limestones of both the Kallankurichchi and Niniyur formations show negative δ13C (−4.73 to −0.49‰ VPDB; −5.63 to −1.87‰ VPDB; respectively) and −18O values (−8.89 to −3.66‰ VPDB; −8.56 to −5.41‰ VPDB; respectively). The carbon and oxygen isotope composition, δ13C vs. δ18O plot and Mn/Sr ratio suggest that the measured δ13C and δ18O values have been significantly altered during diagenesis. The limestones from both the Kallankurichchi and Niniyur formations show large variations in 87Sr/86Sr values (0.709310 to 0.711962; 0.708280 to 0.708398, respectively) which are higher than 87Sr/86Sr ratios of the contemporary Lower Maastrichtian (87Sr/86Sr: 0.707760) and Danian (0.707819 to 0.707833) seawaters. The elevated 87Sr/86Sr ratios in the limestones of the Kallankurichchi Formation suggest that these limestones were significantly modified by pore fluids during meteoric diagenesis. The observed large fluctuations in 87Sr/86Sr ratios in the Niniyur Formation resulted from variations in riverine input. One sample from the Niniyur Formation exhibits an unaltered 87Sr/86Sr ratio (0.707828) which is interpreted to indicate an age of 65.02 Ma.