Palaeosols in early Himalayan foreland basin sequences demonstrate latitudinal shift-related long-term climatic change

Abstract

The Palaeogene sedimentary record in the Himalayan foreland basin contains palaeosols that are interpreted as reflecting changes in climate through time with the passage of the Indian Plate from the equator to 30^° N latitude. To understand spatial and temporal variation in the occurrence of diagnostic palaeosol types, 12 exposed stratigraphic sections were investigated and studied in detail using petrographic and geochemical techniques, including scanning electron microscopy and X-ray mineralogy. Oxisol (bauxite) is a karst bauxite that occurs in crudely bedded and laminated forms. It contains gibbsite, goethite and kaolinite and shows chemical index of alteration values close to 100, demonstrating intense weathering. No gradual change in the concentrations of trace and rare earth elements is recorded. However, their behaviour suggests that the Oxisol was produced by weathering of basalt under a warm and humid climate. Histosol (coal), found stratigraphically higher than Oxisols, contains woody structures dominated by vitrinite maceral. The Histosol is interpreted as having originated from an undisturbed peat developed in a swamp of high tree density under wet tropical climatic conditions. Up-section, calcrete profiles of stages 3 and 4, containing rhizoliths, pellets, filamentous calcite and Microcodium, have developed by pedogenic processes. Large negative stable carbon (-8.5%. to -11.2%.) and oxygen isotope values (-8.5%. to -12.6%.) suggest that the studied calcretes formed under the influence of meteoric water and soil organic matter in dry sub-tropical climatic conditions. The formation of Oxisol in a warm and humid climate was possible in the equatorial region when part of the Indian subcontinent was close to the equator. The coal developed under a humid climate when the northern tip of the Indian subcontinent reached the intertropical convergence zone, while the pedogenic calcrete formed once the sub-tropical climatic zone was reached. Hence, it is suggested that these palaeosols formed in different climatic zones during northward drift of the Indian plate and that the drift brought them to their present positions between 30^° N and 35^° N within the sub-tropical climatic zone.