The sea-level highstand of the Changjiang River estuary in the Holocene revealed from tidal bore deposits

Sea level changes have an important impact on the natural environment and community economy of coastal areas. Understanding the evolution of sea level in the past could help predict the future sea level rising. In this study, a natural Holocene soil outcrop (CJGC profile) was scrutinized. The profile was located in a construction site of Changjian Square in Yangzhou City, east Jiangsu Province in ancient estuary of the Yangtze River. The tidal bore deposits contain euryhaline mollusc shells in the lower part of the profile, which provides excellent geological materials for studying the sea-level highstand and the maximum transgression in the Holocene. In addition, we studied the sedimentary facies and conducted optically stimulated luminescence (OSL) dating, from which the evolution of sedimentary environment in the study area since the middle Holocene was reconstructed. Results show that this site has undergone sedimentary environmental changes of land →tidal beach→river mouth bar→floodplain, which clearly shows a change from transgression to regression. The OSL ages show that the sea-level highstand and the maximum transgression of ancient Changjiang River estuary in the Holocene appeared at ~5.6 ka in elevation no less than 1.3 m above sea level, which is consistent with the sea-level highstand of the same period found elsewhere in many places of the world.