Magnetic polarity stratigraphy of foreland basin sediments of Nepal Himalaya

The synorogenic sediments of Siwalik group and Dumri formation in sub-Himalayan region have been deposited within the flexural foredeep of the Himalayan foreland basin system since early Miocene, eroded from developing fold-thrust belt to the north (Tokuoka et al., 1986; Hisatomi, 1990; Critelli and Ingersoll, 1994; DeCelles et al., 1998a). The aim of our project is to date the Mio-Pliocene Siwalik group and late Oligocene to early Miocene Dumri formation across Nepal, in order to put all our other petrographic and isotopic work in a geochronologic context, to construct a sediment accumulation history, and compare it to the chronology of thrusting/unroofing of the Himalayan fold-thrust belt in Nepal. The Siwalik group and Dumri formation in Nepal contain only sparse fossils, hence it is practically impossible to use biostratigraphy for age calibration. Magnetostratigraphy has proved a most useful tool for determining the age of Siwalik group sediments through the northern Indian sub-continent. Efforts to date sediments in Nepal are relatively new and have been concentrated in the western half of the country (Appel et al., 1991; Harrison et al., 1993; Gautam and Appel, 1994; Ojha et al., 2000). From previously published work, it is obvious that there was no uniformity among paleomagnetists regarding preferred lithology for sampling. Previously sampled lithologies have included sandstone (Appel et al., 1991; Gautam and Appel, 1994), bedded siltstone (Harrison et al., 1993), and undifferentiated mudrocks (Opdyke et al., 1979). Given the diversity of sampling strategies, we decided to revisit previously sampled areas and rigorously test the paleomagnetic behavior of the different lithologies. We sampled Siwalik group sediments exposed at Tinau Khola, Surai Khola, Khutia Khola, and Muksar Khola as well as older (>15 Ma) foreland basin deposits belonging to the Dumri formation along the Tansen road and laterally equivalent Suntar formation in Swat Khola. We analyzed a pilot set of samples from those sections in different kind of lithologies. Our results show that wherever possible, bedded siltstone should be sampled for paleomagnetic analysis. Paleosols yield paleomagnetic results of highly variable quality, whereas sandstones generally display very erratic behavior during thermal demagnetization.