Thin-section analysis of lithified paleosols from Dagshai Formation of the Himalayan Foreland: identification of paleopedogenic features and diagenetic overprinting and implications for paleoenvironmental reconstruction. (Special Issue: Landscapes and soils through time.)

摘要

In the present study we report micromorphology of diagenetically altered paleosols (~31 Ma old) from Dagshai Formation, Himalayan Foreland. The fluvial sequence exposed along the Koshaliya River, NW Himalaya, contains four types of paleosols with decreasing abundance of well-developed paleosols from basal to upper part of the Dagshai Formation. Burial diagenesis (at ~7.5 km depth) caused compaction (54-78% current thickness compared to pre-burial thickness) and cementation of the paleosols, accompanied by internal reorganization of groundmass within peds, staining of ped walls with iron oxide, plugging of voids, disruption of textural pedofeatures, fracturing of large mineral grains and nodules, coarsening of pedogenic calcite crystals, redoximorphic features, and mineralization of root channels. Despite diagenetic alteration, evidence of paleopedogenic processes is still well-preserved in these fossil soils in the form of microstructures, b-fabrics, pedogenic calcite, bioturbation, and textural pedofeatures. Thin-section analysis helped to distinguish pedogenic and diagenetic features of lithified paleosols and to infer the paleoenvironment of the Dagshai paleosols. The paleopedological characteristics of the fossil soils suggest humid to sub-humid conditions during their formation in early Oligocene. The paleoclimate inferred here is consistent with prevalence of tropical paleovegetation (cf. Ficus L.) reported from Dagshai sediments.