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

The middle Miocene-Pliocene Siwalik Group was deposited in the Himalayan fo reland basin in response to uplift and erosion in the Himalayan fold-thrust belt. Results of thermal demagnetization experiments on samples from the S iwalik Group in central and western Nepal demonstrate that laminated siltst ones 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 demagnetizatio n. 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, w ere sampled for magnetic polarity stratigraphy, The Siwalik Group is compos ed of informal lower, middle, and upper members. Correlation of the resulti ng polarity stratigraphy with the geomagnetic polarity timescale indicates that the exposed section spans 13.30 to 7.65 Ma. The lower-middle Siwalik b oundary occurs at 11.05 Ma, near the beginning of chron C5n. The rate of se diment accumulation increases upsection, similar to rate changes previously observed in the Pakistan Siwalik Group, and probably in response to increa sing proximity of the Himalayan thrust belt. In the Khutia Khola section, a discordant declination indicates that this region has rotated about a vert ical axis 16.6 degrees counterclockwise with respect to the Indian subconti nent. Measurements of delta(13)C in paleosol carbonate indicate the predomi nance of C-3 plants until 7.65 Ma, and the clear presence of C-4 plants hig her in the undated portion of the section.