Strontium isotope stratigraphy of marine Oligocene–Miocene sedimentary successions of Kutch Basin, western India

Abstract

The Kutch Basin of western India exposes one of the most complete, fossiliferous marine Cenozoic sedimentary sequences of India. Precise depositional ages of these sequences are of great importance in the reconstruction of palaeoclimatic and palaeobiogeographic histories of the basin. Due to the poorly constrained foraminiferal biostratigraphic ages, we conducted high-resolution ⁸⁷Sr/⁸⁶Sr chronology of the various fossiliferous, stratigraphically continuous units of Oligocene–Miocene strata in Kutch Basin. Besides updating the previously determined ⁸⁷Sr/⁸⁶Sr chronology, we report new numerical depositional ages for Oligocene–Miocene formations and members, and correlated these ages to the existing biostratigraphic schemes. The new ⁸⁷Sr/⁸⁶Sr data indicates a depositional age range between 28.64 + 0.29/-1.11 and 23.51 + 1.53/-1.85 Ma (Rupelian-Chattian) for the Maniyara Fort Formation, between 23.07 + 0.94/1.39 and 18.09 + 0.24/-0.57 Ma (Chattian-Burdigalian) for the Khari Nadi Formation, and between 15.11 + 0.56/-2.87 and 12.29 + 1.22/-1.9 Ma (Langhian-Serravallian) for the Chhasra Formation. The sedimentation rate, determined through numerical ages derived from ⁸⁷Sr/⁸⁶Sr, indicates an increase in sedimentation for the Khari Nadi (0.23 ± 0.04 cm/kyr) and Chhasra (0.12 ± 0.04 cm/kyr) formations, likely related to uplift of the Himalaya-Tibetan plateau and intensification of the Indian Summer Monsoon. The bulk sediment element ratios (V/Ni, Ni/Co and V/Cr) indicate oxic to suboxic palaeo-redox conditions during deposition of the Oligocene–Miocene successions of the basin. Based on new and updated ⁸⁷Sr/⁸⁶Sr chronology, the depositional environments and sedimentation rates of the Kutch sequence are correlated with changes in sea-level, sedimentation rates in the Arabian Sea and Bay of Bengal, and the global climate changes across the Oligocene–Miocene boundary.