Geomorphology and palaeoclimatology of some Upper Cretaceous palaeosols in central India

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The Late Cretaceous (Maastrichtian) Lameta Formation exposed around Jabalpur in central India comprises a pedogenically modified fluvial succession. Accumulation of early diagenetic calcite cement during penecontemporaneous pedogenesis has transformed the fluvial sandstones into calcareous sandstones. The microscopic features of the palaeosol material include floating grain fabric, root traces, laminated micrite-microspar, micrite envelopes and corona structures, illuvial clay envelopes, poikilotopic spars and minor amounts of siliceous grain and void cutans. The palaeosol profiles of the Lameta Formation can be classified into eleven different types, nine of which are comparable to present-day mature aridisols and two to inceptisols. Nine to thirteen palaeosol profiles are present in the individual sections of the Lameta Formation within the study area. Each profile comprises two to four distinct pedohorizons arranged in vertical succession. There are six types of pedohorizons present in the area of study, namely, partly altered hosts, light-coloured and poorly cemented horizons, horizons with root traces and root-related alterations, horizons of subspherical glaebules, horizons of platy-fused glaebules and subhorizontal root traces and horizons with solution features. They show many characteristics similar to the present-day soil horizons, for example those termed C, Bk, K, A and E by pedologists. The palaeosol characteristics show correspondence to the nature of fluvial channel migration and the depth of the groundwater table (controlled by the geomorphic setting). A shallow groundwater table favoured the development of rootmat-bearing and gleyed horizons. Horizons with vertical root traces and glaebules occur where the groundwater table was low. Mature and thin palaeosols developed in those parts of the Lameta fluvial tract that were characterized by stable channels and wide floodplains. In contrast, thin and comparatively immature soils occurred in the parts where laterally migrating channels were operative. The characteristics of the Lameta palaeosols suggest that a hot and semiarid climate prevailed for most of the time represented by the Lameta Formation, but short-term wet spells occurred during the initial and the terminal phases of the Lameta accumulation. The low latitudinal palaeoposition of India and the absence of the palaeo-monsoon circulation might have produced the hot and semiarid climatic setting during this time.