Genesis and transformation of clay minerals in the formation of ferruginous inceptisols and vertisols in the Saptadhara Watershed of Nagpur district, Maharashtra

Spatially associated Typic Ustropepts (ferruginous) and Typic Haplusterts identified in the Saptadhara Watershed of Nagpur district, Maharashtra, were found to occur in close association with a difference. The former on the sedimentary rocks is under similar topographical conditions but at slightly lower topographic position than the latter on the Deccan basalt. In ferruginous soils, smectite is dominant followed by kaolinite whereas Vertisols are predominantly smectitic. Geomorphic and climatic history of the area suggest that although both the soils did start forming during the earlier tropical humid climate, transformation of smectite to kaolinite could not be attained. Instead, broadening of (001) basal spacing and also a (001) disordering in the smectite structure did occur probably due to hydroxy interlayering in smectite interlayers in ferruginous soil developed in alluvium and/or colluvium of sedimentary rocks. Similar clay mineralogical composition of both the solum and saprolite of the sedentary but truncated ferruginous soil indicate that both kaolinite and smectite of ferruginous soils are inherited from the underlying carboniferous sedimentary rocks. Vertisols formed through a progressive landscape reduction process and the further weathering of the smectitic parent material of these soils in aiPre-Pliocene tropical humid climate was terminated because of the change of climate during the Plio-Pleistocene transition period.