Establishment of parental legacy of a Vertisol from West Bengal

Abstract

It is known that for the formation of a Vertisol, huge amount of smectite is required. Logically so, we find Vertisols in the Deccan Trap basaltic areas or in areas with parent material rich in bases. In addition to this, we find Vertisols in Krishna and Godavari deltaic regions in Andhra Pradesh and some areas in Tamil Nadu. Their parent material has its origin in basalt. Review of soil research in the Bengal basin shows extensive areas with soils having considerable shrink-swell activity (Shankarnarayana, 1982; Singh et al., 1998). During various soil survey programmes and the soil resource mapping programme of West Bengal, extensive areas were classified as “Vertic” intergrades at the subgroup level of classification (Haldar et al., 1992) presumably caused by the presence of 30-60% smectites in these soils (Ghosh and Kapoor; 1982; Ghosh and Datta, 1974). However, the occurrence of Vertisols was observed in 2004 during a GEFSOC project progamme in Chuchura, West Bengal. These Vertisols have characteristics similar to Vertisols occurring in Deccan Trap basaltic areas. It is difficult to reconcile that the IGP with dominantly micaceous parent material could give rise to so much of smectites. Detailed analysis including mineralogical analysis of the clays showed the presence of huge amount of low-charge dioctahedral smectites that are similar to the smectites of Vertisols of the Deccan Trap basalt. Detailed geomorphological studies of Singh et al. (1990) indicate that the alluvia in the Bengal basin was carried by the various rivers and their tributaries from the vicinity of the Rajmahal Traps (which have their origin in basalt and various intertrappeans and dolerite; Wadia, 1957). Thus the deposited alluvia could be a differential mixture of the alluvia from the Himalayan (micaceous) origin as well as that from Rajmahal Trap (basaltic) origin. This resulted in the formation of some major soil landforms and geomorphic units in the Bengal basin (Singh et al., 1998). In view of this geomorphic history and the similar nature of smectites between Vertisols of Deccan basalt area and Chuchura, it is most likely that the Vertisols of Chuchura, West Bengal have been developed in the smectite-rich alluvium that has been carried from the Rajmahal Traps.