Geometry of the Bushe-Poladpur Contact in the Western Deccan Volcanic Province, India

ABSTRACT The boundary between the Bushe Formation (Lonavala Subgroup) and the Poladpur Formation (Wai Subgroup) of the Western Deccan Volcanic Province (WDVP) marks a significant milestone in Deccan volcanism. It has a proximity to the Cretaceous-Palaeogene (K-Pg) Boundary. The distinct morphological, chemical, petrological and magnetic susceptibility differences in the lava piles below and above this boundary, documented by earlier workers, suggested that dramatic changes occurred in the eruptive styles and lava dynamics across it. We have studied the exposed Bushe - Poladpur contact (BPC) across all its known exposures in the WDVP. Absence of a marker horizon or a break in volcanism represented by a regional interflow (sedimentary/palaeosol/redbole) horizon between the two formations makes its field recognition difficult. It is shown that the changes in volcanic style and rate across the BPC were not abrupt, but were a progressive, gradual change. The reconstructed ‘surface’ of the BPC has non-rhythmic undulations of up to 200 m within lateral distances of 50 km both along and across the strike of the surface, unlike earlier projections. The BPC surface is inconsistent with several earlier assumptions and may need synvolcanic and post-eruptive tectonism to explain its geometry.