Palynofacies and sedimentology-based high-resolution sequence stratigraphy of the lignite-bearing muddy coastal deposits (early Eocene) in the Vastan Lignite Mine, Gulf of Cambay, India

摘要

Geological records of early Paleogene warming are rare in low latitudinal regions. The Indian subcontinent preserves records of this global event on western and eastern margins. We attempt to decipher paleoenvironmental setup and facies architecture of the paleo-equatorial early Eocene succession at the Vastan Lignite Mine, Gulf of Cambay, western India. The Vastan lignite succession was deposited in a low-energy coastal marsh-bay complex receiving only fine-grained muddy sediments from the weathered Deccan Traps. The lower part of the Vastan lignite deposit, designated as “Vastan Succession A”, comprises four depositional facies representing distinct environments (open bay, restricted bay, creek and channel, and coastal marsh) and one diagenetic facies. Palynofacies analysis, backed by precise sedimentological framework, records changes in terrestrial supply and fluctuating marine characters of bay and marshes. Eleven Palyno-Units are identified in distinct lithofacies sequences stacked in shallowing-upward cycles representing five parasequences that constitute a Transgressive Systems Tract (TST) deposit. Each parasequence starts with a transgressive sheet deposit, followed by shallowing-upward bay fill-marsh deposits. In the vertical succession, each parasequence acquires increasing marine character, culminating in a maximum flooding surface (shell carbonate) that represents large-scale coastal onlap during early Ypresian time. The TST is followed by a Highstand Systems Tract deposit, which shows an erosional surface at the top of the upper lignite indicating Lowstand Systems Tract and a sequence boundary at ~52 Ma. The Vastan Succession A represents TST (3rd-order cycle) deposits with parasequences and hemicycles representing 4th- and 5th-order cycles. The study demonstrates sea level rise along the Indian western coastal margin in response to early Eocene warming between ~55 and ~52 Ma with maximum transgression at 53.7 Ma.