MAGNETIC-POLARITY STRATIGRAPHY OF SIWALIK GROUP SEDIMENTS OF TINAU-KHOLA SECTION IN WEST CENTRAL NEPAL, REVISITED

Remanent magnetization of sandstones sampled at 127 levels/sites throu ghout a 1710m thick Siwaliks molasse sequence exposed along the Tinau Khola river in Nepal has been studied using thermal demagnetization. T he magnetic remanence consists of: a secondary low-temperature compone nt of normal polarity, unblocked mostly below 400-500 degrees C close to the present-day field, and a high-temperature characteristic remane nce (ChRM) unblocked mostly between 600 and 685 degrees 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 haematit e carries ChRM. Anisotropy of magnetic susceptibility data reveals pri mary depositional magnetic fabric as judged by oblate ellipsoids and s ubvertical tilt-corrected minimum susceptibility axes. A magnetic pola rity sequence established using tilt-corrected ChRM directions from 12 4 levels reveals more details not found in the polarity stratigraphy w orked out by Munthe et al. (1983). Comparison of the sequence with a s tandard polarity time-scale (Harland et al. 1989) suggests a depositio nal time range between c. 5.9 Ma (older than chron 3r) and 11 Ma (youn ger than chron 5r-2) for the section considered. According to new data , the horizon of Sivapithecus punjabicus falls close to the reversal b oundary at c. 8.54 Ma (the lower age limit of the normal polarity chro n 4Ar-1). Hence, the hominoid should be regarded as of c. 8.5 Ma rathe r than 9.0-9.5 Ma as suggested by Munthe et al. (1983).