Strontium stratigraphy of the Oligocene-Early Miocene shellbeds of the Kutch Basin, western India, and its implications

Abstract

The Kutch Basin is unique among the western Indian sedimentary basins because of its near?complete sequence of post?Palaeozoic rocks. Due to extensive marine influence, the Oligocene-Early Miocene formations of the basin, namely Maniyara Fort, Khari Nadi and Chhasra, contain numerous shellbeds. Although age assignments of these formations exist based on foraminiferal biostratigraphy, detailed numerical age of the lithounits are yet to be established. We have identified a total of eleven distinct shellbeds (oldest SB 01 to youngest SB11) from this interval primarily containing bivalve fossils. Using 87Sr/86Sr of selected oyster and pectinid shells with pristine shell characteristics, we report the age of four shellbeds. The ages of SB 01, SB 04, SB 06 and SB 10 are 24.37, 17.31, 16.85 and 15.38 Ma, respectively. Our dates suggest a Chattian (24.37 Ma) age for SB 01 from the Bermoti Member, validating the previous biostratigraphical estimates from the Maniyara Fort Formation. The Chhasra Formation, however, shows a younger range of ages (17.31-15.38 Ma) characterized by a transition from the Burdigalian (SB 04-SB 06) to the Langhian (SB 10) stages. These dates have important implications in the study of sequence stratigraphy, Palaeobiogeography and tectonic history of the Kutch Basin. A surface with subaerial exposure is found in SB 08 (between 16.85 and 15.38 Ma) that corresponds to a global eustatic sea?level decrease (Mi2). Our new dates will also help evaluate the response of marine fauna to the closure of the Tethyan seaway around 19 Ma due to the formation of -Gomphotherium Landbridge-. The dated shellbeds enable us to identify pre? and post?closure fauna and assess the effect of biogeographical separation on these fauna. These dates have important implications in evaluating the regional geological record of western India in the context of various global events.