5:15 PM - 7:15 PM
[HCG21-P03] Sedimentary environment and climate change recorded in paleosols since Miocene to Pleistocene in the Thakkhola region, southern Tibetan Plateau
Keywords:paleosol, paleoclimate, Miocene to Pleistocene, south asia monsoon
The uplift of the Himalayas due to the collision between the Indian subcontinent and the Eurasian Plate has significantly influenced the onset and evolution of the monsoonal climate in Asia. However, there has been limited discussion on climate change, particularly in Nepal and the southern Tibetan Plateau. The Thakkhola area, where is located in the boundary area between the Himalayas and the Tibetan Plateau, is characterized by the distribution of Neogene to Quaternary fluvial deposits[1]. The paleosols developed within these deposits are expected to record the climatic conditions of the terrestrial environment at the time. In this study, we examine the sedimentary facies of the Thakkhola Formation in the Thakkhola area, central Nepal, to discuss climatic changes in the southern Tibetan Plateau and reconstruction of the sedimentary environment and its evolution from the Neogene to the Quaternary in this region.
The Thakkhola area is interpreted as a half-graben formed by the extensional stress field following the Indo-Eurasian continental collision, with terrestrial sediments deposited since the Miocene. Based on paleomagnetic dating, the depositional age of the Tetang Formation has been reported as 11-9.6 Ma [2], while that of the overlying Thakkhola Formation ranges from 8-2 Ma [3]. Pollen analysis suggests a wet environment during the deposition of the Tetang Formation, followed by a transition to a drier climate in the Thakkhola Formation [1].
The Thakkhola Formation is exceeding 600 meters in thickness. The lower part of the Thakkhola Formation is exposed in Chhusang, located 80 km north-northwest of Pokhara, a major city in Nepal. From the bottom, it comprises a red matrix-supported angular conglomerate rich in limestone and slate boulders, a cross-bedded conglomerate rich in rounded granite cobbles, along with sandstone and siltstone units, red sandstone and siltstone with trace fossils, black siltstone with root fossils, and a coarse-grained sandstone and angular conglomerate unit.
The middle to upper part of the Thakkhola Formation is exposed at Ghiling, about 10 km north of Chhusang. The Thakkhola Formation can be roughly divided into three types of sediments: braided river deposits dominated by cross-bedded conglomerate rich in rounded granite cobbles with lenticular sandstone, low-sinuosity river deposits consisting of cross-bedded conglomerates, sandstones, and siltstones, and floodplain deposits dominated by siltstone with thin layers of sandstone and conglomerate. In the upper part of the Thakkhola Formation, layers rich in calcite and carbonaceous material are observed in the floodplain deposits. On the other hand, the uppermost part is poor in carbonaceous material, and calcite precipitated around root traces.
A red matrix-supported angular conglomerate observed at Chhusang is considered to have been transported short distances, suggesting rapidly deposited fan-like sediments, while a cross-bedded conglomerate rich in rounded granite cobbles was transported long distances, suggesting main channel deposits. This conglomerate is similar to the conglomerate observed in Ghiling, suggesting that the sediments around Ghiling are regarded as a widely distributed facies in the Thakkhola area. The calcite accumulation is observed in the upper part of the Thakkhola Formation at Ghiling, which may suggest climatic conditions with fluctuating groundwater levels and seasonal dry climate [4]. However, in the uppermost part of the Thakkhola Formation, considered to be floodplain deposits, calcite precipitated in root traces and is poor in carbonaceous matter, suggesting drier climatic conditions with less precipitation. Currently, the Himalayas are located to the south of this area, and the southwest monsoon winds are intercepted, resulting in a dry climate, which is classified as a steppe-tundra climate [5]. Therefore, changes in the elevation of the Himalayas since the Middle Miocene may have caused the aridification of this region.
References
[1]Adhikari, B.R., 2009. Ph.D. Thesis, Vienna Univ., Austria, 158p. [2] Yoshida, M. et al., 1984. Jour. Nepal Geol. Soc. 4, 101–120. [3] Garzione, C.N., et al., 2000. Geology 28, 339–342. [4]Demkoa, T.M., et al., 2004. Sedimentary Geology 167, 115 – 135. [5] Ramchandra, K. et al., 2016. Theoretical and Applied Climatology 125, 799-808.
The Thakkhola area is interpreted as a half-graben formed by the extensional stress field following the Indo-Eurasian continental collision, with terrestrial sediments deposited since the Miocene. Based on paleomagnetic dating, the depositional age of the Tetang Formation has been reported as 11-9.6 Ma [2], while that of the overlying Thakkhola Formation ranges from 8-2 Ma [3]. Pollen analysis suggests a wet environment during the deposition of the Tetang Formation, followed by a transition to a drier climate in the Thakkhola Formation [1].
The Thakkhola Formation is exceeding 600 meters in thickness. The lower part of the Thakkhola Formation is exposed in Chhusang, located 80 km north-northwest of Pokhara, a major city in Nepal. From the bottom, it comprises a red matrix-supported angular conglomerate rich in limestone and slate boulders, a cross-bedded conglomerate rich in rounded granite cobbles, along with sandstone and siltstone units, red sandstone and siltstone with trace fossils, black siltstone with root fossils, and a coarse-grained sandstone and angular conglomerate unit.
The middle to upper part of the Thakkhola Formation is exposed at Ghiling, about 10 km north of Chhusang. The Thakkhola Formation can be roughly divided into three types of sediments: braided river deposits dominated by cross-bedded conglomerate rich in rounded granite cobbles with lenticular sandstone, low-sinuosity river deposits consisting of cross-bedded conglomerates, sandstones, and siltstones, and floodplain deposits dominated by siltstone with thin layers of sandstone and conglomerate. In the upper part of the Thakkhola Formation, layers rich in calcite and carbonaceous material are observed in the floodplain deposits. On the other hand, the uppermost part is poor in carbonaceous material, and calcite precipitated around root traces.
A red matrix-supported angular conglomerate observed at Chhusang is considered to have been transported short distances, suggesting rapidly deposited fan-like sediments, while a cross-bedded conglomerate rich in rounded granite cobbles was transported long distances, suggesting main channel deposits. This conglomerate is similar to the conglomerate observed in Ghiling, suggesting that the sediments around Ghiling are regarded as a widely distributed facies in the Thakkhola area. The calcite accumulation is observed in the upper part of the Thakkhola Formation at Ghiling, which may suggest climatic conditions with fluctuating groundwater levels and seasonal dry climate [4]. However, in the uppermost part of the Thakkhola Formation, considered to be floodplain deposits, calcite precipitated in root traces and is poor in carbonaceous matter, suggesting drier climatic conditions with less precipitation. Currently, the Himalayas are located to the south of this area, and the southwest monsoon winds are intercepted, resulting in a dry climate, which is classified as a steppe-tundra climate [5]. Therefore, changes in the elevation of the Himalayas since the Middle Miocene may have caused the aridification of this region.
References
[1]Adhikari, B.R., 2009. Ph.D. Thesis, Vienna Univ., Austria, 158p. [2] Yoshida, M. et al., 1984. Jour. Nepal Geol. Soc. 4, 101–120. [3] Garzione, C.N., et al., 2000. Geology 28, 339–342. [4]Demkoa, T.M., et al., 2004. Sedimentary Geology 167, 115 – 135. [5] Ramchandra, K. et al., 2016. Theoretical and Applied Climatology 125, 799-808.