Rock magnetic characterization of pedogenesis under high energy depositional conditions: A case study from the Mio–Pliocene Siwalik fluvial sequence near Dehra Dun, NW Himalaya, India

Abstract

Systematic rock magnetic studies accompanied by field investigations are presented for 51 pedogenic horizons from magnetostratigraphically constrained Middle–Upper Siwalik fluvial sequence (∼ 9–4 Ma) near Dehra Dun in the Himalayan Foreland Basin (HFB). Detailed sedimentologic studies infer predominantly proximal to distal alluvial fan setting with considerably high rate of sedimentation (> 55 cm/ka). Paleosol profiles in the Mohand Rao (MR) section are commonly truncated by channel activity. These paleosols display few rootlets, diffused color contrast amongst variegated pedogenic horizons, rare concretions, intense gleying at places, and frequent incursions by sand/silt layers (< 30 cm) characterizing their vicinity to channel. The mean mass specific magnetic susceptibility ( χ lf) for pedogenic horizons is ∼ 0.95 × 10 − 8 m 3/kg (Max = 1.4 × 10 − 8 m 3/kg; Min = 0.5 × 10 − 8 m 3/kg) and the associated non-pedogenic layers is ∼ 0.82 × 10 − 8 m 3/kg (Max = 1.2×10 − 8 m 3/kg; Min = 0.4 × 10 − 8 m 3/kg). The low frequency dependence of the susceptibility ( χ fd mean = 2%) and high coercivities ( B OCR mean = 425 mT) of the pedogenic layers infer the absence of finer pedogenic superparamagnetic (SP) fraction that might have depleted during post depositional burial processes. Acquisition of isothermal remanent magnetization (IRM) shows under-saturation and gentle upward trend even at high applied fields (4000 mT) advocating the predominance of hematite and goethite. Sympathetic variation in the total organic and inorganic carbon content with iron oxides stable under warm humid, oxidative arid and cold restricted conditions confirm the sensitivity of the rock magnetic parameters to ambient climatic condition. Relative variations in goethite, hematite and ferrimagnetic contents have been derived from ratios mainly based upon selective IRM saturations and their coercivities. These studies are validated by sandstone petrography and clay mineralogy for the same section. However, the variation in rock magnetic parameters shows a better amplification and allows relative quantification of the ambient climatic conditions. The assemblage of iron oxides using the rock magnetic ratios of the pedogenic layers in these high energy conditions broadly infer the prevalence of semi-humid to semi-arid climate entirely during the Late Miocene to Early Pliocene period in the HFB with an increased seasonality during early Pliocene. The present approach envisages a great scope for quantitative database under the given setup of high energy conditions that are common in the Mio–Pliocene HFB.