5:15 PM - 6:45 PM
[HCG23-P07] Evolution of sedimentary environments and climatic conditions in the Miocene to Pleistocene based on paleosols of Thakkhola area, central Nepal
Keywords:paleosol, paleoclimate, Miocene to Pleistocene, south asia monsoon, Tibetan Plateau, Central Nepal
The formation of the Himalayan and Tibetan Plateau, uplifted by the collision of the Indian subcontinent with the Eurasian plate, had a major influence on the establishment and variability of the Asian monsoon climate. Several grabens running north-south are distributed on the boundary between the Himalayas and the Tibetan Plateau. One of these grabens, the Thakkhola region, is currently classified as a steppe climate because the surrounding mountains block the southwest monsoon winds and create a rain shadow. The Thakkhola area contains Neogene to Quaternary fan, fluvial, and lacustrine deposits. Paleosols recognized in these deposits are expected to record the establishment of a monsoon climate and the evolution of climatic classification due to the formation and uplift of the Tibetan Plateau. In this study, we analyze sedimentary facies and paleosols of the upper Miocene to Plio-Pleistocene Thakkhola Formation to trace the evolution of climatic classification on the southern edge of the Tibetan Plateau. The paleosol horizons were classified as follows: layer A, a dark gray horizon with accumulated organic matter; layer B, a clay-accumulated horizon that develops below layer A; layer Bt, a more clay-accumulated horizon inside layer B; and layer C, which retains its primary structure.
The Thakkhola Formation in the study area can be roughly divided from below into gravel-bed braided river deposits, gravel-sand meandering river deposits, gravel-bed braided river deposits flanked by siltstones, gravel-sand braided river deposits, and sand braided river deposits. Paleosols of gravel-bed braided river deposits flanked by siltstones river and floodplain deposits are characterized by A, Bt, and B horizons, fine roots replaced by iron oxides, gleization, and are corresponding to Inceptisol. Paleosols of gravel-sand braided river deposits are characterized by Bt and B horizons, layers reddish browned by iron oxides, and opaque mineral coatings around the particles, and are corresponding to Inceptisol. Paleosols of sand braided river deposits are characterized by rhizoliths with calcium carbonate precipitated around the roots and are classified as Entisol. Clear paleosol horizons indicates warmer climatic conditions, while rhizoliths formation indicates drier climatic conditions. Therefore, the upper Miocene to Pleistocene Thakkhola Formation changed from a gravel-bed braided river to sand braided river, and from warmer climatic conditions to drier climatic conditions. The rise in elevation and the establishment of rain shades due to the uplift of the Himalayan and Tibetan plateau since the Miocene may have contributed to the change in regional climate.
The Thakkhola Formation in the study area can be roughly divided from below into gravel-bed braided river deposits, gravel-sand meandering river deposits, gravel-bed braided river deposits flanked by siltstones, gravel-sand braided river deposits, and sand braided river deposits. Paleosols of gravel-bed braided river deposits flanked by siltstones river and floodplain deposits are characterized by A, Bt, and B horizons, fine roots replaced by iron oxides, gleization, and are corresponding to Inceptisol. Paleosols of gravel-sand braided river deposits are characterized by Bt and B horizons, layers reddish browned by iron oxides, and opaque mineral coatings around the particles, and are corresponding to Inceptisol. Paleosols of sand braided river deposits are characterized by rhizoliths with calcium carbonate precipitated around the roots and are classified as Entisol. Clear paleosol horizons indicates warmer climatic conditions, while rhizoliths formation indicates drier climatic conditions. Therefore, the upper Miocene to Pleistocene Thakkhola Formation changed from a gravel-bed braided river to sand braided river, and from warmer climatic conditions to drier climatic conditions. The rise in elevation and the establishment of rain shades due to the uplift of the Himalayan and Tibetan plateau since the Miocene may have contributed to the change in regional climate.