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

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 δ 13 C (−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, δ 13 C vs. δ 18 O plot and Mn/Sr ratio suggest that the measured δ 13 C and δ 18 O values have been significantly altered during diagenesis. The limestones from both the Kallankurichchi and Niniyur formations show large variations in 87 Sr/ 86 Sr values (0.709310 to 0.711962; 0.708280 to 0.708398, respectively) which are higher than 87 Sr/ 86 Sr ratios of the contemporary Lower Maastrichtian ( 87 Sr/ 86 Sr: 0.707760) and Danian (0.707819 to 0.707833) seawaters. The elevated 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr ratios in the Niniyur Formation resulted from variations in riverine input. One sample from the Niniyur Formation exhibits an unaltered 87 Sr/ 86 Sr ratio (0.707828) which is interpreted to indicate an age of 65.02 Ma.