Distinct palaeosols suggest an appraisal of climate and environmental conditions prevalent at the onset of the early Miocene, Kutch Basin (India)

The bottom-most part of the early Miocene ( i.e ., Aquitanian age) Khari Nadi Formation shows development of a distinct, about 160-cm-thick, pedocomplex wherein evidence of conspicuous ichnofabric, microfabric, and discrete pedogenic ratios indicates alternating phases of marine incursion and intermittent, a particular type of pedogenesis. The earlier literature posited a steady base-level rise throughout the early Miocene, particularly during the time of deposition of the Khari Nadi Formation. The evolution of the basal part of the pedocomplex marks a low-order lowstand systems tract (LST), which was earlier unreported for the formation. Differential preservation of primary sedimentary structures and marine-trace-fossil suites, as well as varying superimposition of pedofeatures, indicates surface instability and polyphase pedogenesis. The pedocomplex developed in an overall hydromorphic set-up due to the presence of characteristic redoximorphic features (e.g., both matrix and intrusive pedofeatures). Variations in rhizohaloes and rhizocretions, as well as changes in organic matter and jarosite-natrojarosite content throughout the pedocomplex, indicate that palaeosol units developed under different stages of waterlogging and progressively changed from an estuarine/tidal wetland soil to a coastal wetland soil. The conspicuous dissolution features in the lower part of the pedocomplex (e.g., named as S2 layer) mark the possible development of gypsum, indicating the occurrence of a short, abrupt arid climate phase, subsequently followed by a humid climate resulting in its dissolution. The global Mi1 glaciation likely triggered the arid conditions. Hence, the basal pedocomplex of the Khari Nadi Formation records an unconventional nature of pedogenesis developed during a low-order regression within the 3 rd -order transgressive phase and offers a valuable early Miocene palaeoclimatic archive for India and the world.