Paleoweathering and Paleoclimatic Reconstructions using Neoproterozoic Paleosol in the Eastern Margin of the Pranhita-Godavari Basin, India

Abstract

A thin, weakly-developed palaeosol horizon within the Neoproterozoic Sullavai sandstone in the eastern margin of the Pranhita-Godavari basin was studied. Field observations, thinsection studies and geochemical analyses of the palaeosol horizon were carried out to reconstruct the palaeo-weathering and palaeoclimatic conditions. The palaeosol developed on sandstone parent rock. Morphological features of this palaeosol are not distinct, perhaps due to very ancient nature as well as thin occurrence. A few of which that can be mentioned include weakly developed peds and calcareous nodules of size <1 cm -.2 cm. The lower part of the profile preserves incipient parent rocklamination. Major micromorphological features of this palaeosol include weakly-developed sub-angular blocky structure and redoximorphic features showing redox enrichment and redox depletion of Fe oxides and oxyhydroxides. XRD analysis of the palaeosol reveals the presence of glauconite and illite, suggesting deposition of sediments under shallow marine conditions, and prevalence of cold climate, respectively. This is also supported from Chemical Index of Alteration (CIA)and presence of illite. Further, Chemical Index of Weathering (CIW), Clayeness, and Salinization data of the palaeosol clearly suggests an environment where chemical weathering was feeble and cold (11-14°C) palaeo-temperature. XRF results show abundance of K, Ca and Mg which suggest the area was poorly drained along with cooler palaeoclimate during the late Neoproterozoic. Al2O3 concentration at the middle of the soil profile suggests weakly-developed B-horizon, however it is hardly observed in the field. Parent material, cold climate and less time of exposure only resulted in the formation of a thin and weaklydeveloped soil profile. This ancient soil may represent an unconformity at the basin boundary suggesting a local regression and exposure of the fluvio-marine deposits along the basin boundary. It further needs detailed such studies at various spatiotemporal basinal scales.