Clay Mineralogy of the Oldest Paleosols from the Himalayan Foreland: Implications of Diagenetic Overprinting and Paleoenvironments

In this study we investigated clay mineralogy of the oldest of the fossil soils (∼ 31 Ma age) that formed during the earliest phase of continental sedimentation in the Himalayan foreland. The paleosols within fluvial sequences of the Dagshai Formation are exposed along the Koshaliya River, NW Himalaya. Despite burial alteration, evidence of paleopedogenic processes is still well-preserved in these fossil soils in the form of structural elements (structures, pedogenic calcite, clay coatings, root traces). Clay mineralogy of the four paleosols (D5, D9, D12, D17) representing well-developed to weakly-developed paleopedofeatures of the entire Dagshai Formation is marked by sharp and strong XRD patterns for 14 Å, 10Å, 7Å, 3.5Å, 3.3Å minerals. A semi- quantitative estimate of the clay minerals shows the presence of mica, kaolinite, vermiculite, feldspar and quartz in order of decreasing abundance. Burial diagenesis (∼7.5 km depth, ∼140°C) caused significant changes in clay mineral assemblage of these paleosols in terms of transformation, illitization, and increase crystallinity.