Development process and environmental response of the first terrace in the Maqu reach of the Yellow River in the western margin of the Zoige Basin, eastern Tibetan Plateau, China

Through field investigation in the Zoige Basin of the eastern Tibetan Plateau, a natural and complete sedimentary profile was found on the steep cliff of the first terrace of the right bank of the Yellow River in the Maqu reach. Careful field observation, analysis of particle size characteristics, and optically stimulated luminescence dating were conducted. The results showed that the paleo-riverbed pebble layer and the floodplain sandy silt layer at the bottom of the sedimentary profile constituted a typical binary sedimentary structure layer, in which the shallow depression bog mud layer that formed during 5.0-4.2 ka was lenticularly sandwiched in the floodplain sandy silt layer, indicating that the first terrace of the Yellow River in this reach began to form at 5.0 ka. The climate was dry and cold during 4.2-4.0 ka, and strong wind on the plateau surface transformed the floodplain sediments into aeolian sand. The paleoflood slackwater deposits sandwiched in the aeolian sand layer indicated that the source of the Yellow River has experienced several catastrophic flood events during this period, inundating the first terrace ground. During 4.0-2.8 ka, mud bogs formed in shallow depressions of the terrace under the relatively humid climate. After 2.8 ka, sandstorms drove the accumulation of coarse silt over the terrace ground, forming aeolian loess. The upper part developed into modern subalpine meadow black soil with granular structure through biological weathering since 2.0 ka. The study results revealed the formation age and its surface development process of the first terrace of the Yellow River in the Zoige Basin, which has important scientific significance for in-depth understanding of the water system evolution and environmental changes in the source area of the Yellow River.