Magnetic minerals and magnetic properties of the Siwalik Group sediments of the Karnali river section in Nepal

Sandstones and siltstones collected from the Siwalik molasse sequence (simi lar to 16 to 5 Ma) of the Karnali river section have been studied for their magnetic properties. Behavior of the specimens during demagnetization (of the NRM and IRM) and magnetic susceptibility vs. temperature data suggest t hat goethite, maghemite/magnetite, and hematite are the main magnetic miner als in the section. Goethite, carrying a recent component, is the dominant magnetic mineral in the fine-grained lithologies from the lower part. Maghe mite and magnetite, which also carry a secondary remanence, occur in the sa ndstones from the upper part. Hematite, mainly of detrital origin, is prese nt in the whole sequence. The magnetic fabric is defined by mainly oblate A MS ellipsoids and a low degree of anisotropy (P' < 1.20). The magnetic line ations (declinations: 75 degrees-130 degrees or 245 degrees-310 degrees; pe ak orientation: 290 degrees/2.8 degrees) are subparallel to the fold axes/b edding strikes/thrust fronts (WNW-ESE). The initial sedimentary-compactiona l fabric has been overprinted by a secondary tectonic fabric, which was pro bably induced by mild deformation active in the compressive tectonic settin g.