Geological nature of the Pokaran Boulder Bed: Palaeoenvironmental, palaeoclimatic and stratigraphic implications

Abstract

The Pokaran Boulder Bed forms the basal lithofacies of Neoproterozoic-Cambrian Marwar Supergroup. The boulder bed denotes specific sedimentary milieus and palaeoclimatic settings during the terminal Proterozoic period and occurs in three distinct forms, viz., massive conglomerate, boulder spread and stratified conglomerate. These three forms of the boulder bed independently rest over the basement of Malani rhyolite and/or granite (>698 Ma) and mark an erosional unconformity at the base of the Neoproterozoic Marwar Supergroup (1000-540 Ma). It is suggested that the massive conglomerate is a product of weathering and disintegration of the basement rocks operating under semi-arid to arid conditions. The boulder spread is envisaged to be glaciogenic, whereas the stratified conglomerate is a product of glaciofluvial action. The stratigraphy of Marwar Supergroup is believed to be linked to the global Neoproterozoic glaciation. The base of Marwar Supergroup is interpreted to be late Neoproterozoic (< 610 Ma). The occurrence in the same basin of climatically incompatible glaciogenic Pokaran Boulder Bed and Hanseran Evaporite vis-a-vis diamictite-evaporite fill is visualized to be the effect of ice rings orbiting the earth. This has implications for stratigraphic correlation of such unique sequences.