Magnetic Polarity Stratigraphy of the Siwalik Sediments from Ramganga Basin, NW Himalaya

A detailed magnetic polarity chronology is reconstructed using a 3.2 Km thick sequence of molasse sediments exposed along the Kalaunia River (KR) in the Ramganga basin, NW Himalaya in western UP. Close interval (10-12 m) oriented samples collected from 281 sites subjected to thermal and alternating field demagnetization revealed 40 records of magnetic reversals from Lower and Middle Siwalik Sub-Groups exposed in the KR section. Progressive thermal demagnetization carried out at the intervals of 30, 50, and 100°C unblocked a strong secondary component at 400°C. The characteristic remanent magnetization (ChRM) component resides in haematite and is isolated successfully at 650°C. The tilt corrected mean directions for the normal and the reverse sites are D/I =324°24'/23°30' (α₉₅ = 4.42) and 140°42'/-22°24' (α₉₅ = 4.3) respectively. Finally the local magnetic polarity zones are correlated with the global GPT scale (Cande and Kent, 1995) yields the chronology of 13 to 5 Ma for the KR section with an average sediment accumulation rate of 37.9 cm/kyr.