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

Gautam, Pitambar and Huyghe, Pascale and Mugnier, Jean-Louis and Regmi, Kamal Raj (2019) 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, TBC . pp. 1-14. ISSN 0072-1050

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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. Oddnumbered
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.
KEYWORDS
chemical index of alteration, climate, geochemistry, goethite, magnetism, Siwalik, Sub-
Himalaya

Item Type:	Journal Article
Subjects:	Q Science > QE Geology
Divisions:	Faculty of Science, Technology and Environment (FSTE) > School of Geography, Earth Science and Environment
Depositing User:	Kamal Regmi
Date Deposited:	05 Jan 2020 23:25
Last Modified:	05 Jan 2020 23:25
URI:	https://repository.usp.ac.fj/id/eprint/11881

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