Sedimentary Environment Evolution and Yangtze River Channel Change in Guanzhou Site, Songzi City, Hubei Province

Guanzhou Site, located beneath the surface of the central bar of Songzi reaches of the middle Yangtze River, which is the earliest and largest Neolithic cultural site found in the Jianghan Plain up to now. Since Guanzhou Site provides vital evidences for the prehistoric civilization in the middle Yangtze River, it is important to understand the relationship between geomorphological evolution and human development. Here, we use ¹⁴C dating and grain size characteristics analysis to investigate the sedimentary evolution history and constrain the drainage evolution of this region. We further discuss the causes of this site buried deep under the surface of Guanzhou Island. The chronology results show that the age of the Guanzhou Site is between 12 783 cal. a B.P. and Late 20th Century. The analytical results of grain size characteristics show that sediment deposited in the lower section (11.65-6.70 m) is fine, and the grain size changing tendency reflects the weak hydrodynamic and low-energy river sedimentary environment, which belongs to the floodplain bank accumulation. As for the middle (6.70-2.10 m) section, the coarse grain size layer and fine grain size layer occurred alternately, and the grain size curve shows the sedimentary environment changed between edge-floodplain deposition and strong hydrodynamic force, indicating that the site is in the condition of riverbed environment. The grain size characteristics of upper (2.10-0 m) section indicate it is a fine sediment layer, and the grain size curve reflects the river sedimentary environment as a weak and low energy hydrodynamic force, suggesting a floodplain accumulation. Based on the above analyses, the sedimentary evolution history and the middle Yangtze River channel evolution of the Guanzhou Site can be divided into three periods: The first period (11.65-6.70 m) is roughly from 12 783 cal. a B.P. to the Ming Dynasty or Qing Dynasty, when the former Guanzhou Island was still linked to the land, not an Island. The second period (6.70-2.10 m) is accumulation during the Ming Dynasty or Qing Dynasty, which exhibits a turbulent channel sedimentary environment. At that time, the Guanzhou Island was located on the concave bank of the river with strongly erosion. During the third period (2.10-0 m), the Guanzhou Island was finally formed as an island in the middle Yangtze River channel, separated from the land completely. From then on, the location of Guanzhou Island is similar to that of present-day, and the main channel of the Yangtze River in Songzi reaches changed to the south of the Guanzhou Island gradually.