Mineralogical and Textural Characteristics of Red Boles of Western Deccan Volcanic Province, India: Genetic and Paleoenvironmental Implications

This study presents physical, petrographical and mineralogical investigation of interbasaltic red bolewithin the late Cretaceous Deccan lava flows in Pune and Mahabaleshwar areas (India) to understand depositional and weathering conditions. The micromorphological study of red boles reveals two genetic types characterized by distinct features. The first variety, with general sharp lower and upper contacts, consisting predominantly of altered basaltic fragments and volcanic glasses along with hematite, plagioclase, pyroxene (mainly augite), accessory olivine replaced by iddingsite, zircon, quartz, Fe–Ti opaque phases (ilmenite and magnetite), represents a volcaniclastic bole bed. The second variety of red bole, with gradational lower contact and sharp upper contact, containing basaltic clasts, alteromorphs/iddingsite, abundant zeolite and glaebules in groundmass of homogeneous, red, ferruginous clay, is classified as a volcanic paleosol. They are devoid of olivine and pyroxene, with rare alteromorphs of glass. The clay mineral content of red boles was investigated using XRD and VNIR. The presence of 2.21, 2.24 and 2.29 μm absorption features in reflectance spectra of VNIR spectroscopy indicates a mixture of Alsmectite (montmorillonite) and Fe-smectite as the clay mineral constituents, which is confirmed by the XRD data. The presence of smectite and hematite in both types of red boles suggest their formation under near-neutral pH, poorly-drained and moderate weathering condition. Hematite in the red bole suggests a combination of oxidative weathering and the effect of post-burial diagenesis. The volcaniclastic red boles with partially to unaltered mafic minerals and glass shards possibly indicates limited subaerial/subaqueous alteration under oxic condition. The paleosol type red boles with pedogenic features indicate prolonged sub-aerial weathering of flow top basalt, indicating significant hiatus in Deccan volcanism. Zeolite and other alteration products possibly indicate hydrothermal alteration on red bole.