"Paleopedology of Siwalik Paleosols of Kangra Sub-Basin, NW Himalaya: Implication for Weathering and Climate change 11 Ma to 6 Ma"

The Himalayan Mountain range is the most prominent and active intracontinental range in the world. The outer part of this range is marked by the Himalayan foreland basin (HFB) in south that resulted due to the thrust loading and subsidence with synorogenic sedimentation from the hinterland. The lowermost part of the HFB is marked by the molassic sediments of “Siwalik” which thins out to the south. The time frame between 11Ma to 6 Ma (mid to late Miocene) is critical to understand the changes related to monsoon and vegetation as there is no detailed record or systematic study of late Cenozoic weathering and paleopedogenesis in fluvial sediments of the HFB. In the present study, we present a high-resolution paleopedological record of the paleosols along a traverse of ~ 1.8 km in the Kotla-Brail section of the Kangra sub-basin of the HFB. In the field, the paleosols are characterized by 1-2 m thick Bw, Bt, Bk, Bss, Bk horizons, rhizocretions, pedogenic carbonates (PC), and Fe/Mn mottles and concretions. The paleosols in the Lower Siwaliks show a dominance of 2.5 Y and 5 YR hue, whereas in the Middle Siwaliks they are defined by 5 YR and 7.5 YR hue. Micromorphology of these paleosols confirmed varying degrees of weathering, and paleopedogenesis showing a blocky structure, clay coatings, biogenic activity, and diffused impure micritic nodules as PC in the Bw and Bt, Btk horizons. These pedofeatures are more strongly developed in paleosols of the Lower Siwaliks than in comparison to weakly-moderately paleosols of the Middle Siwaliks.

Clay mineralogy determined based on XRD study of the total (<2 µm) fine clay (<0.2 µm) fractions of these paleosols is characterized by the dominance of smectite, vermiculite, and mixed-layer minerals in paleosols of the Lower Siwalik. The clay mineral assemblage shows a  decrease in the abundance of smectite and increase of kaolinite towards the transitions to Middle Siwalik at ~10 Ma. This also shows transformation of the smectite and vermiculite to interstratified clays at about ~8 Ma. After ~8.5 – 8.0 Ma, the paleosols are again marked an increase of the amount of smectite, vermiculite, and mixed-layer minerals in paleosols of the Middle Siwalik towards their transitions to Upper Siwalik. The varying intensity of weathering, paleopedogenesis, and clay mineral assemblage of the paleosols in the Lower and Middle Siwalik suggest fluctuating climatic conditions that evolved from initial semi-arid to sub-humid at ~11 Ma that to higher precipitation at ~8.5 to 8 Ma then again to semi-arid to arid conditions at ~6.5 Ma.

 

Keywords: Himalayan Foreland Basin (HFB), Siwalik, Paleosols, Micromorphology, Clay Minerals