Palaeoenvironmental, palaeoecological and palaeobiogeographical implications of mixed fresh water and brackish marine assemblages from the Cretaceous-Palaeogene Deccan intertrappean beds at Jhilmili, Chhindwara District, central India

Abstract

The Deccan infra- and intertrappeans volcano-sedimentary sequences have been considered as terrestrial to fluvio-lacustrine deposits. Presence of planktonic foraminifera and brackish water ostracods in the Jhilmili basal Danian intertrappean beds, recorded earlier, indicates complex palaeoenvironment, palaeoecology and palaeobiogeography. The intertrappean sediments span 14 m between two Deccan basaltic flows deposited under terrestrial, palustrine and flood plain environments. In the middle of this section is a 60 cm- thick layer (unit 3) that contains fresh water ostracods, charophytes and Early Danian planktonic foraminifera. Palaeoecologically, the ostracod assemblage includes active swimmers (Cypridopsis, Cypria, Mongolianella, Paracypretta and Zonocypris) and poor swimmers (Limnocythere, Gomphocythere, Frambocythere and Darwinula). The bulk of the ostracod fauna points to a freshwater, lacustrine depositional environment. Presence of planktonic foraminifera and two brackish water ostracod species (Buntonia sp. and Neocyprideis raoi) invigorates discussion of a marine seaway into central India, with this fauna carried through the Narmada and Tapti rift zones by seasonal offshore currents (short lived transgressive phase), which created temporary estuarine conditions and deposited marine micro-biota. Above this interval, sediments in unit 4 and 5 consist of reddish and greenish grey clayey siltstone with rare fine sand layers deposited in palustrine and terrestrial conditions. The ostracod fauna shows extensive endemism which come across among the Indian Maastrichtian non-marine ostracods sustained into the Early Danian, which further proposed geographically isolated Indian plate and intercontinental dispersal of Maastrichtian to early Danian freshwater ostracods that support the Out-of-India hypothesis. Age of the Jhilmili intertrappean beds is Early Danian-Maastrichtian based on planktonic foraminifera and few brackish water ostracods.