Magneto‐chemical signature of the Lower‐to‐Middle Siwaliks transition in the Karnali River section (Western Nepal): Implications for Himalayan tectonics and climate

Abstract

Fluviatile sediments comprising a 600-m-thick sequence of the Lower and Middle Siwaliks in the Karnali area in Nepal exhibit a distinct zonation revealed by magnetic and geochemical properties. Four magneto-chemical zones (MCZ1-MCZ4), each about 150 m thick and 400 kyr in duration, provide new insights on Himalayan tectonoclimatic events during the Tortonian (Miocene) stage. They exhibit contrasting magnetic susceptibility and isothermal remanence due to differences in magnetic mineral types (magnetite, hematite, and goethite) and concentrations. Odd-numbered zones with higher goethite/(goethite + hematite) ratio, a moisture proxy, indicate wetter conditions in the source area, while the even-numbered zones, virtually without goethite, suggest drier conditions. Chemical indices of alteration/weathering and proxies for hydraulic sorting and mobility derived from the major element compositions also reveal contrasts among these zones. The middle of the MCZ2-MCZ3 zone, with a transitional magneto-chemical signature, is the best candidate for the Lower to Middle Siwaliks contact, rather than the field-based boundary placed 18 m up-section at the base of the thick salt and pepper sandstone bed. The transition records an increase in river energy and associated accelerated erosion of the Himalayan gneiss zone as the source of coarse-grained material. We suggest a scenario, whereby climate change from drier to wetter (with higher precipitation) conditions affects erosional processes (i.e., weathering, disaggregation and particle transport on the hillslope) prevailing in a large catchment and influencing the depositional modes.