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Magnetic polarity stratigraphy of the Neogene Siwalik Group at Khutia Khola, far western Nepal

¹ Department of Geosciences, University of Arizona, Tucson, Arizona 85721
² Department of Geology, Tri-chandra Campus, Ghantaghar, Kathmandu, Nepal

The middle Miocene–Pliocene Siwalik Group was deposited in the Himalayan foreland basin in response to uplift and erosion in the Himalayan fold-thrust belt. Results of thermal demagnetization experiments on samples from the Siwalik Group in central and western Nepal demonstrate that laminated siltstones yield paleomagnetic data useful for tectonic and magnetostratigraphic studies. Sandstones and paleosols of the Siwalik Group, however, generally display highly erratic paleomagnetic behavior during thermal demagnetization. On the basis of these observations, siltstones from a well-exposed, 2423-m-thick section of the Siwalik Group in Khutia Khola, far western Nepal, were sampled for magnetic polarity stratigraphy. The Siwalik Group is composed of informal lower, middle, and upper members. Correlation of the resulting polarity stratigraphy with the geomagnetic polarity timescale indicates that the exposed section spans 13.30 to 7.65 Ma. The lower-middle Siwalik boundary occurs at 11.05 Ma, near the beginning of chron C5n. The rate of sediment accumulation increases upsection, similar to rate changes previously observed in the Pakistan Siwalik Group, and probably in response to increasing proximity of the Himalayan thrust belt. In the Khutia Khola section, a discordant declination indicates that this region has rotated about a vertical axis 16.6° counterclockwise with respect to the Indian subcontinent. Measurements of ¹³C in paleosol carbonate indicate the predominance of C₃ plants until 7.65 Ma, and the clear presence of C₄ plants higher in the undated portion of the section.

Key Words: Himalayan orogeny • Siwalik Group • Nepal • Neogene • magnetostratigraphy • geochronology • carbon-13

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