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

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Title: Magneto‐chemical signature of the Lower‐to‐Middle Siwaliks transition in the Karnali River section (Western Nepal): Implications for Himalayan tectonics and climate
Authors: Gautam, Pitambar Browse this author →KAKEN DB
Huyghe, Pascale Browse this author
Mugnier, Jean‐Louis Browse this author
Regmi, Kamal R. Browse this author
Keywords: chemical index of alteration
climate
geochemistry
goethite
magnetism
Siwalik
Sub-Himalaya
Issue Date: 21-Jan-2020
Publisher: John Wiley & Sons
Journal Title: Geological Journal
Publisher DOI: 10.1002/gj.3727
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 into Himalayan tectono‐climatic events during the Tortonian (Miocene) stage. They exhibit contrasting magnetic susceptibility and isothermal remanence due to differences in magnetic mineral types (magnetite, haematite, and goethite) and concentrations. Odd‐numbered zones with higher goethite/(goethite + haematite) 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.
Rights: This is the pre-peer reviewed version of the following article: Gautam, P, Huyghe, P, Mugnier, J‐L, Regmi, KR. Magneto‐chemical signature of the Lower‐to‐Middle Siwaliks transition in the Karnali River section (Western Nepal): Implications for Himalayan tectonics and climate. Geological Journal. [Early View], which has been published in final form at https://doi.org/10.1002/gj.3727 . This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.
Type: article (author version)
URI: http://hdl.handle.net/2115/76574
Appears in Collections:創成研究機構 (Creative Research Institution) > 雑誌発表論文等 (Peer-reviewed Journal Articles, etc)

Submitter: Gautam Pitambar

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