CLAY MINERALOGY AND GEOCHEMISTRY OF THE INTRABASALTIC PALAEOSOLS (BOLE BEDS) OBTAINED FROM THE SCIENTIFIC DRILLING PROJECT IN KOYNA, INDIA: A MULTI-PROXY PALAEOCLIMATIC STUDY

Intrabasaltic palaeosols (bole beds) obtained from scientific drilling project in the Koyna region of India were studied for their palaeoclimatic interpretations using clay mineralogical and geochemical analyses. In most of the bole beds the dominant clay mineral is montmorillonite along with hematite which gives the red hue to the boles. However the clay minerals like illite, smectite, dickite, palygorskite, kaolinite, sepiolite etc. are also present in the bole beds from different depths. With the help of major element geochemistry the Chemical Index of Alteration (CIA), the original felsic and mafic components were calculated which were compared with the clay mineral assemblages. The abundance of montmorillonite almost all the bole beds (along with smectites in few) suggest their formation in semi-arid seasonal climate with alternating wet and dry periods. The presence of illite in few suggest transformation of smectite due to increased temperature resulting due to the emplacement of the upper lava flow while kaolinite points towards warm and humid conditions with high leaching environment under acidic conditions. The occurrences of palygorskite indicate arid or semi-arid alkaline conditions. The CIA values suggests that the bole beds are results of incipient weathering of the underlying parent basalts while the dominance of original mafic components indicate that, for most of the bole beds, the conditions were not much acidic. The proportion of hematite is quite variable (not in proportion to the total clays) which could be due to the changing redox conditions. The degree of chemical weathering and the proportions of original mafic and felsic components do not correlate well with the abundances of either clay minerals or hematite. This suggest that the formation of different clay minerals were mostly independent of the degree of chemical weathering but they represent changing climatic conditions of bole bed formation in different episodes of Deccan volcanism.