Department of Geology

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Browsing Department of Geology by Author "Halder, Kalyan"

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    Effects of Eocene Global Events on Marine Bivalves from Kutch and Cambay Basins of Gujarat, India
    Halder, Kalyan
    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
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    Evolution of Cambay Basin, western India during the Eocene from the study of gastropods and shell beds
    Halder, Kalyan
    During the early Palaeogene western India was flooded with extensive transgressions. The first marine sediments were deposited in the western marginal basins during the Ypresian, i.e., early Eocene. Molluscs, including gastropods, bivalves and scaphopods, appeared and flourished in these basins, within the transitional habitats between sea and land. The early Cenozoic time was important for the evolutionary history of gastropods because several of the gastropod groups, abundant in modern-day seas, either first appeared or went through rapid diversifications and distributions during this time. While comprehensive monographs about the Paleocene-Eocene gastropods are available from the European and North American basins, and even the adjacent Pakistan basins, the Indian basins remained relatively less explored. Among the western Indian basins, most of the published reports came from the Kutch Basin, a few from the Subathu and Rajasthan basins and none from the Cambay Basin. The gastropods present in the Ypresian Cambay Shale of the Cambay Basin are studied here. 32 caenogastropods and 9 heterobranchs are reported and described. 32 of them (24 caenogastropods and 8 heterobranchs) are new. 13 of them are microgastropod (< 4 mm). 3 new genera and 2 new subgenera are introduced. The fossils were collected from four lignite mines at Mangrol, Vastan, Tadkeshwar and Valia, Gujarat. The Camaby Shale is an argillaceous formation with intercalations of lignite seams and marine fossil bearing limestones. Glauconitic green shale layers and shell beds dominated by only one or a few opportunist molluscs yielded gastropods of eurytopic or marginal marine affinity. A change from the freshwater-dominated coastal swamp to a relatively open-marine condition in an overall marginal marine set-up could be deciphered from the sequence of lithologies and fossils. The composition, diversity-abundance pattern, environmental preference and trophic structure of the molluscs, especially gastropods, elucidate this transgressive sequence. The Cambay gastropod assemblage predominantly comprises endemic species and widespread genera. A similar pattern was unveiled by the Paleocene-Eocene gastropod faunas reported from all Indian and Pakistan basins, situated within the Western Indian Province (WIP). The wide distribution of the genera took place due to the faunal exchanges via the circum-tropical equatorial currents along the relict Tethys, Atlantic and Pacific oceans. The westerly flowing somewhat weak Tethys current was important for migration of western Indian faunas. However, the specific endemism is enigmatic because a large part of the fauna had planktotrophic development. The restricted nature of the basins might be a significant factor. The early developmental mode was not found to be a decisive factor in the geographic distribution of the WIP gastropods at both generic and specific levels. The maximum geographic distribution and also, temporal duration of these genera were found to be not significantly different among the WIP genera characterized by planktotrophy, non-planktotrophy (lecithotrophic/direct), and a combination of these. With greater time, the maximum geographic distribution became broader. Also, majority of these genera were found to attain their maximum geographic distribution at the early stages of their temporal ranges. The changes in the maximum latitudes of these genera support their migration towards higher latitudes during the warming and towards lower latitudes during cooling periods, in response to the climatic turnovers of the Palaeogene.