Neogene magnetic polarity stratigraphy of some fluviatile Siwalik sections, Nepal

Abstract

S U M M A R YThe magnetostratigraphy of five new sections through the Neogene Siwalik Group of Nepal is presented. Rock magnetic experiments and detailed thermal demagnetization experiments prove that haematite is the carrier of a primary DRM br a PDRM. After stepwise thermal demagnetization of specimens from all sections, directions of characteristic remanent magnetization were obtained. The results show a positive reversal test and exhibit inclination errors of about 20". Due to gaps in exposures and extremely variable demagnetization behaviour of the haematite-bearing sediments, the resulting polarity sequences are poorly defined for some parts of the sections. However, doublesampled parts of some sections yield similar results. A correlation with the Surai Khola section (Appel, Rosler & Corvinus 1991) is proposed, based on rock magnetic parameters. AMS results from three of the sections prove the existence of primary sedimentary magnetic fabrics in these sections with systematic orientations of minimum and maximum susceptibility axes. Correlation with a standard polarity timescale indicates that the ages of all new sections lie within the age limits of the Surai Khola section.

Depositional chronology and fabric of Siwalik group sediments in Central Nepal from magnetostratigraphy and magnetic anisotropy

“…With welldefined normal (D/I: about 0/20) or reverse polarity (D/I: about 180/ -20) after bedding tilt-correction, it passes the reversal test and records inclination shallowing such that the ratio of the tangents of inclinations of the recorded ChRM and the expected field equals to about 0.45. Similar inclination errors are recorded by detrital remanence in sandstones sampled from the present-day riverbeds (Rösler et al, 1997).…”

“…For the sake of uniformity, even the Surai Khola section was labelled in this fashion in Rösler et al (1997). Because of diachronous nature of even the identical lithostratigraphic boundaries found in different sections, proliferation of nomenclature has little meaning in chronological sense.…”

“…The experiments conducted are: NRM measurements with stepwise alternating field demagnetization (AFD: up to 150 mT) and thermal demagnetization (ThD: up to 700C), measurements of the initial magnetic susceptibility and its changes during stepwise heating, isothermal remanent magnetization (IRM) acquisition and demagnetization, and study of opaque minerals in polished thin sections (Appel et al 1991, Gautam and Appel 1994, and Rösler et al, 1997. Figure 5 shows the NRM data plotted against magnetic susceptibility for 3 sections.…”

“…The next generous effort was made by M. Yoshida in Arung Khola-Binai Khola where over 12 sections distributed over an area of 5,000 km 2 were measured for the polarity record and the finer lithostratigraphic divisions were dated using magnetostratigraphy (Tokuoka et al, 1986). With the development of cryogenic magnetometers and sophisticated laboratory procedures capable of extracting the weak primary magnetization, relatively continuous sequences of the Nepalese Siwaliks have been important targets for the world"s paleomagnetic community (Appel et al, 1991;Harrison et al, 1993;Gautam and Appel, 1994;and Rösler et al, 1997).…”

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

“…During the last two decades, it has been demonstrated that the chronology of the Mio-Pliocene fluvial sediments of the Siwalik Group from Pakistan, India and Nepal can be deciphered through correlation of magnetic polarity reversal patterns to the geomagnetic polarity time scale (GPTS) (Barndt et al, 1978;Opdyke et al, 1979;Tauxe and Opdyke, 1982;Johnson et al, 1982Johnson et al, , 1983Tokuoka et al, 1986;Ranga Rao et al, 1988;Opdyke, 1990;Appel et al, 1991;Harrison et al, 1993;Gautam and Appel, 1994;Burbank et al, 1996;Rösler et al, 1997). These data are invaluable for regional chronological correlation throughout the Sub-Himalayan belt.…”