Microstructures and compositional variation in the intravolcanic bole clays from the eastern Deccan volcanic province: Palaeoenvironmental implications and duration of volcanism

Abstract

Clay minerals associated with intra-volcanic bole horizons of varied colours and thicknesses contain montmorillonite, halloysite and kaolinite, show distinct microstructures and microaggregates. In kaolinite, Fe3+ ions substitute for Al3+ at octahedral sites. Most of these clays are dioctahedral type, show balance between net layer and interlayer charges. The interstratified illite — smectite (I/S) mixed layers containing variable proportions of montmorillonite. Illite contains sheet-like, well oriented microaggregates. The parallel stacks of chlorite sheets show chlorite/smectite (C/S) mixed layers. Progressive enrichment of Fe and depletion of Al ions with the advancement of kaolinization process is observed. High order of structural and compositional maturity observed in these bole clays, indicate long hiatus between the two volcanic episodes. It is observed that the bole horizons suffered rigorous weathering, approximately 5 fold higher than the respective parent lava flows under the alternate wet and dry spells of climatic changes. Translating the production time estimates of clays associated with the 21 bole horizons, found in the eastern Deccan volcano-sedimentary succession, it is assessed that the minimum time required for their formation is approximately 7 my. Clay minerals that occur across the stratigraphic sequence show cyclic changes in the climate, favouring longer duration of volcanic activity. Late Maastrichtian Lameta beds post-date Deccan volcanism to 70 Ma or earlier to this. Considering the formation of bole clays in terms of time, it is possible that the volcanic activity started much earlier in the late Maastrichtian, and continued after 65 Ma or even later. Thus, the clay stratigraphy based estimates lend support to a prolong duration.