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 ℃; resistant to AFD up to 150 mT) or in maghemite (unblocking temperature 150-400 ℃). 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 ℃) 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

Detrital thermochronology and sediment petrology of the middle Siwaliks along the Muksar Khola section in eastern Nepal

“…The location of the Muksar Khola section is indicated with a star. Fujiwara, 2000;Ojha et al, 2009). The lower Siwalik Formation is composed of thin beds (1 m thick) of fine sandstone inter-layered with reddish silt layers (50 cm thick).…”

“…The stratigraphy and depositional age of the different Siwalik units in the Mio-Pliocene Himalayan foreland basin are fairly well constrained through magneto-and biostratigraphy (Appel et al, 1991;Gautam and Rösler, 1999;Gautam and Fujiwara, 2000;Corvinus and Rimal, 2001;Ojha et al, 2009). Because collision and exhumation were not synchronous from west to east along the Himalayan chain (e.g.…”

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

“…This fact is taken as an evidence for the primary detrital remanent magnetization (DRM) (Tauxe and Kent, 1984;Gautam and Appel, 1994). A tentative polarity sequence based on this ChRM and its correlation with the GPTS (Cande and Kent, 1995) suggests an age of 16-5 Ma for the 3560 m section (Gautam and Fujiwara, 1999). 4.4 Low-field magnetic susceptibility (κ) and its variation with temperature The magnitudes of κ for the KR section vary between 3 × 10 −5 and 10 −3 SI (Fig.…”

“…Detailed analysis of the magnetic remanence, in connection with magnetostratigraphic dating of the KR section, revealed two components: a normal polarity component of recent field (RF) origin and a dual-polarity characteristic remanence (ChRM) (Gautam and Fujiwara, 1999). In general, the RF component resides in goethite in varicolored muddy specimens (LS lithologies), but in maghemite in the gray faciesespecially the "salt and pepper" sandstones (MS lithologies).…”

“…1(a)). A recent magnetostratigraphic study (Gautam and Fujiwara, 1999) suggests that the sediments comprising a 3560 m thick sequence were deposited during a time span of over 11 Myrs (∼16 to 5 Ma). Hence, the KR section has the oldest Siwalik sediments in Nepal.…”