Sedimentary facies characteristics and dating of the late Quaternary sedimentary sequence in the nearshore coastal area of Nantong, Jiangsu Province, China

The formation and evolution of three-stage incised valleys in the Yangtze River delta during the late Quaternary have attracted much research attention. However, age constraints on the infilling sequence for the early and middle stages of incised valley formation are lacking. In this study, a 79-m-long sediment core (JC-1205) obtained from the nearshore coastal area in Nantong (Jiangsu Province) was analyzed, including measurements of down-core changes in grain size and benthic foraminiferal assemblages, optically stimulated luminescence and AMS ¹⁴C dating, and sedimentary-facies characterization, to determine the ages of valley incision and infilling and the responses of these processes to global sea-level variation. The sedimentary succession in core JC-1205 can be divided into four depositional units (DU 4 to DU 1) from bottom to top. Units DU 4 to DU 2 in the lower to middle parts of the core are interpreted as fluvial deposits with distinct contacts between vertically adjacent units. The uppermost unit DU 1 is interpreted as having been deposited in a nearshore shallow-marine environment and consists of tidal-channel, shallow-subtidal, and tidal-flat deposits from bottom to top. Late Quaternary sea-level variations are considered to have been a key control on the late Quaternary sequence at and around the core site. The early- and middle-stage valley incisions and corresponding infilling sequences (DU 4 and DU 3) were formed during the lower sea-level periods of MIS 6 and MIS 4, respectively, and the flood-plain deposit (DU 2) in the interfluve area, which is genetically linked to the late-stage valley incision, was formed during MIS 2. Fluvial incision during MIS 4 is interpreted to have eroded the underlying marine sediments that were deposited during MIS 5, and the development of relatively high terrain during MIS 3 resulted in a hiatus between DU 2 and DU 3. There is an unconformity corresponding to a hiatus of >10 kyr between DU 2 and DU 1, which was deposited during late MIS 1. This hiatus is attributed to the occurrence of tidal-channel scour, as evidenced at the bottom of DU 1, which is interpreted to have eroded the underlying deposits that presumably formed from late MIS 2 to middle MIS 1. This study provides important temporal constraints on the formation and evolution of late Quaternary incised valleys in the Yangtze River delta.