Magnetic polarity stratigraphy of Siwalik Group sediments of Karnali River section in western Nepal

Abstract

The remanent magnetization of siltstones and sandstones sampled at 476 levels/sites throughout a 3560 m thick molasse sequence belonging to the Siwalik Group (0–2015 m: Lower; 2015–3560 m: Middle) has been studied by stepwise thermal demagnetization. This section is exposed along the Karnali River in Nepal. The natural remanent magnetization (NRM) usually consists of two components: a viscous or thermoviscous component of recent field origin, and an ancient characteristic component (ChRM). The former component is of normal polarity and resides either in goethite (unblocking temperature < 150 °C; resistant to AFD up to 150 mT) or in maghemite (unblocking temperature 150–400 °C). Goethite contributes up to 90 per cent of the total intensity in the finer variegated muddy samples belonging to the lower half of the section. Maghemite content is significant in the grey mud-free lithologies from the upper half of the section. The main component, unblocked in the high-temperature range (commonly 610–680 °C) and believed to reside in haematite, presumably of mostly detrital origin, represents a characteristic remanence (ChRM). The tilt-corrected ChRM directions at individual sites show antipodal clusters (ratio of normal- to reverse-polarity sites: 0.62), and yield mean inclinations recording significant inclination shallowing—a feature well recorded in the Siwaliks. This ChRM is interpreted to represent a largely primary detrital remanence. The ChRM data from 430 sites yield the Karnali River magnetic polarity sequence, whose correlation with the geomagnetic polarity timescale (Cande & Kent 1995) suggests a depositional age of 16 Ma (younger than chron C5Cn.1n) to 5.2 Ma (around the top of chron C3r) for the 3560 m section sampled. Hence, the Karnali River exposes the oldest part of the Siwalik Group in Nepal. Estimates of the sediment accumulation rate (SAR) average to 32.9 cm kyr−1 for the 10.8 Myr time span of deposition.