Rock Magnetism of Late Cretaceous to Middle Eocene Strata in the Lesser Himalaya, Western Nepal: Inferences Regarding the Paleoenvironment

Dhan Bahadur Khatri,Weilin Zhang,Qingquan Meng,Dawen Zhang,Tao Zhang,Xiaomin Fang,Khum N Paudayal

doi:10.3389/feart.2021.744063

Abstract

The growth of the southern piedmont of the Himalayan boundary and its depositional setting has changed since uplift of the Himalaya due to continental Indian-Eurasian collision, which has resulted in variation in magnetic minerals in marine- and terrestrial-facies sediments. In this paper, we utilize rock magnetism data from the late Cretaceous to middle Eocene strata, including the Amile and Bhainskati formations from the Lesser Himalaya (western Nepal), to understand the mechanism controlling magnetic susceptibility (χ). The active tectonics strongly influenced saturation isothermal remanent magnetization (SIRM), HIRM, and hysteresis loops, forming both low-coercivity minerals in sediments with low χ from the terrestrial facies (zones I, IIIA, and V) and high-coercivity minerals in the sediments with high χ from the marine facies (zones II, IIIB and IV). Thermomagnetic κ-T curves and frequency-dependent χ (χfd%) values show that sediments with low χ and high χ carry magnetite with coarse non-superparamagnetic (SP) grains and hematite with SP grains, respectively. Comparing the χ data with the lithologic, sedimentary environments, geomorphic features, and sea level data, we propose that low χ values were mainly produced by an increase in terrigenous detrital influx during the regression period of the Tethys Sea, while high χ values formed in marine sediments, which prompted the appearance of ferromagnetic-antiferromagnetic and paramagnetic minerals during the transgression of the Tethys Sea.

Highlights

The Himalayan orogeny and tectonic stress produced the existing Himalayan arc as a result of the Indian-Eurasian collision since the late Cretaceous time (Beck et al, 1995)
On the basis of χ, frequency-dependent χ, χARM, saturation isothermal remanent magnetization (SIRM), high field isothermal remanent magnetization (HIRM), redness, hematite content, and lithology, the Palpa section can be divided into five-rock magnetic zones (Figure 3)
We argue that the local geomorphology played a significant role in the development of magnetic mineralogy and that the nature of sediment influx was influenced by local drainage patterns and seawater conditions in the depositional basin, with enhancement by active tectonics

Summary

INTRODUCTION

The Himalayan orogeny and tectonic stress produced the existing Himalayan arc as a result of the Indian-Eurasian collision since the late Cretaceous time (Beck et al, 1995). In Nepal, the late Permian to Eocene sediments (called the Gondwana System) typically contains both marine facies, including fossiliferous argillaceous limestones and siltstone/sandstone with molluscs, foraminifers, coral and vertebrate fossils, and terrestrial facies, including ferruginous quartz arenite, conglomeratic quartz arenite, glacial diamictite, interbedded sandstone, and shale. These lithostratigraphic units are sparsely distributed in the Barahachhetra and Katari areas, Tansen, Tulsipur, and the Birendranagar areas extending from east to west, indicating the existence of scattered depositional basins with variable thicknesses.

RESULTS

DISCUSSION

DATA AVAILABILITY STATEMENT

Findings

CONCLUSION