Modes of Formation, Palaeogene to Early Quaternary Palaeogenesis and Geochronology of Laterites in Rajmahal Basalt Traps and Rarh Bengal of Lower Ganga Basin

Abstract

The present research work deals with the geomorphic evolution of in situ (primary) and ex situ (secondary) types of laterites (in West Bengal, eastern part of India) which are found as distinct badland terrain and litho-stratigraphic unit, developed in between the eastern fringe of Chotanagpur Plateau (Rajmahal Basalt Traps, Gondwana sedimentary and Archaean rocks) and the western fringe of Bengal Basin (Rarh Bengal). These laterites are analyzed here to address perennial unanswered questions about their genesis. What are the different modes of lateritization to classify laterites? What types of climates produce laterites? When and where were the particular palaeoclimates that formed laterites? What can laterites tell us about palaeoenvironment, palaeogeomorphology and other global events of the past? The field studies, detailed profile analysis of lithofacies, geochemical analysis, characterization of various ferricretes, fossil records and OSL dating have emphasized on the primary and secondary origin of laterites in West Bengal. The primary laterites (Palaeogene to Neogene) are genetically related to parent rocks, viz., Rajmahal basalt and Gondwana sandstones, but the secondary laterites are re-cemented ferruginous materials of fluvial fan-deltaic depositions which are related to Early to Late Quaternary tectono-climatic evolution of north-western Bengal Basin. Here the geochronology of few secondary laterite profiles is determined by OSL dating method in three sample sites, representing Late Quaternary palaeogenesis of laterites and ferruginized sediments (150–35 ka), i.e. Late Pleistocene. The analysis of in situ and ex situ laterites signifies drifting of Indian plate through ideal tropics from Palaeogene to Early–Late Quaternary and widespread occurrence of basal chemical weathering under strong, optimum and seasonal tropical wet–dry climate than prevail today in this part of West Bengal. The litho-stratigraphic ferruginous unit of Rarh Bengal (formerly the palaeovalleys of ferruginous depositions) was developed as an inverted relief due to prolong gully erosion and neo-tectonic uplift in between the Chotanagpur Foot-hill Fault and Medinipur–Farraka Fault.