Effects of Eocene Global Events on Marine Bivalves from Kutch and Cambay Basins of Gujarat, India

Abstract

A series of hyperthermal events coupled with extensive rise in the global mean sea level (GMSL) mark the beginning of Eocene epoch. This time is also characterised by important tectonic rearrangements, especially the long journey and subsequent collision of the Indian plate with the Eurasian plate. The early Eocene (Ypresian) sedimentary deposits from the two western Indian basins, Cambay and Kutch, record five early Eocene hyperthermals. A concomitant marine incursion during the paired events of Eocene Thermal Maximum 2 (ETM2) and H2 has been recorded from both the basins. This led to the deposition of marine mollusc bearing units in an otherwise terrestrial succession of clays and lignite. The stable isotope analyses, sedimentary features and nature of bivalve assemblages indicate a brackish water dominated coastal marginal marine environment throughout the marine succession. A low faunal diversity along with high abundance of certain forms indicate a stressed environment. 36 bivalve species, including 18 new species and two new genera, have been documented from the fossiliferous strata of Cambay Shale. A global palaeobiogeographic analysis based on all Paleogene bivalve genera revealed that the tropical and subtropical latitudes of the Northern Hemisphere, especially Europe and America, hosted the most diverse bivalve assemblage throughout the Paleogene. The Paleocene-Eocene transition exhibited faunal migration to higher latitudes and relatively higher rate of extinction from high latitudes in response to the Paleocene-Eocene Thermal Maximum (PETM). The diversity declined severely in the Oligocene due to global cooling. It also caused latitudinal range contraction of bivalves from high latitudes and a significant extinction from lower latitudes. High endemism and narrow geographical ranges of the Eocene bivalve species within the Western Indian Province (WIP) indicate low dispersibility and rapid allopatric speciation. The westerly flowing Tethyan palaeo-current through the relict Tethys was the main dispersal agent of the Ypresian WIP bivalves until the Lutetian. The collision of Indian and Eurasian plates in the late Eocene (~40Ma) blocked this pathway. In response, a counter flow of the South Equatorial Current established a relation between WIP and south-east Asian localities since the Bartonian