Magnetic-Polarity Stratigraphy of Siwalik Group Sediments of Tinau Khola Section In West Central Nepal, Revisited

Abstract

SUMMARY Remanent magnetization of sandstones sampled at 127 levels/sites throughout a 1710 m thick Siwaliks molasse sequence exposed along the Tinau Khola river in Nepal has been studied using thermal demagnetization. the magnetic remanence consists of: a secondary low-temperature component of normal polarity, unblocked mostly below 400–500°C. close to the present-day field, and a high-temperature characteristic remanence (ChRM) unblocked mostly between 600 and 685°C., represented by both normal and reverse polarities. Demagnetization behaviour and isothermal remanence acquisition indicate that the secondary component resides on goethite and maghemite/magnetite whereas specular haematite carries ChRM. Anisotropy of magnetic susceptibility data reveals primary depositional magnetic fabric as judged by oblate ellipsoids and subvertical tilt-corrected minimum susceptibility axes. A magnetic polarity sequence established using tilt-corrected ChRM directions from 124 levels reveals more details not found in the polarity stratigraphy worked out by Munthe et al. (1983). Comparison of the sequence with a standard polarity time-scale (Harland et al. 1989) suggests a depositional time range between c. 5.9 Ma (older than chron 3r) and 11 Ma (younger than chron 5r-2) for the section considered. According to new data, the horizon of Sivapithecus punjabiecus fells close to the reversal boundary at c. 8.54 Ma (the lower age limit of the normal polarity chron 4Ar-1). Hence, the hominoid should be regarded as of c. 8.5 Ma rather than 9.0-9.5 Ma as suggested by Munthe et al. (1983).