Characteristics of the Yangtze River incised valley in the last glacial maximum in Nanjing–Haimen reach, China

Abstract

The Yangtze River is the largest river in the Asian monsoon region, research on the Yangtze River’s paleochannel in the last glacial maximum (LGM) will provide an important basis for an in-depth understanding of the hydrological characteristics and processes of large rivers in the Asian monsoon area during the last glacial period. The Nanjing–Haimen reach of the Yangtze River in Jiangsu Province was selected as the research subject. Five cross sections of incised valley depositional sequences were established on a section of the Yangtze River in the Nanjing–Haimen reach, with 173 bore data and eight documents from the references. The cross sections came from the following Yangtze River Bridges: Nanjing No. 4, Runyang, Yangzhong, Jiangyin, and Sutong. In the analysis of seven drilling core samples near the Nanjing No. 4 and the Yangzhong Bridges, carbon-14(14C) dating was used for eight age results and electron spin resonance for one age result. The results indicate that in the LGM the incised valley was cut down into bedrock at the upper reach of the Zhenjiang, and the paleochannel of the Yangtze River flowed through old riverbed sedimentary facies at the lower reach of the Zhenjiang. The Nanjing reach during the LGM had a deep incised valley at a depth of −63 to −94 m. The deep incised valley was relatively narrow and steep with a smaller width–depth ratio (B 1/2/H). At the Zhenjiang reach, the ratio of width to depth increased. At the lower reach of the Yangzhong, the incised valley became wider and a bifurcated incised valley system of the Yangtze River was formed. From the Nanjing reach to Haimen near eastern China region, the sediment of the ancient riverbed of the Yangtze River is mainly coarse sand and gravel that gradually tapers from upstream to downstream. Sediment of the incised valley displays a clear pattern, from pebble gravel and coarse sand to fine sand, from bottom to top.