Formation of the Yangtze Shoal in response to the post-glacial transgression of the paleo-Yangtze (Changjiang) estuary, China

Abstract

The Yangtze shoal lies east of the Yangtze (Changjiang) River estuary, China, at water depths of ~25–55 m and is one of the largest subaqueous, sheet-like sand banks in the world. Previous studies have not reached a consensus regarding its origin and timing of formation, mainly due to the scarcity of coring and high-resolution seismic profiling in the shoal. We recovered a 70.20-m-long sediment core (CRE-1402) through the sand shoal and acquired 1586 km of high-resolution shallow seismic profiles across the estuary and shoal. We conducted systematic analyses of the core, including its sedimentary characteristics, down-core changes in benthic foraminiferal assemblages, and 14C and optically stimulated luminescence (OSL) dating. We also analyzed the seismic stratigraphy and integrated these results with data from other previously well-studied cores onshore and offshore of the Yangtze River mouth. Our results indicate that the stratigraphic architecture in the paleo-Yangtze estuary (i.e., the area from the present-day delta plain to the subaqueous delta and Yangtze Shoal) was controlled mainly by changes in sea level during the post-glacial transgression. The post-14 ka sedimentary successions in the paleo-estuary rest unconformably on fluvial or coastal deposits that formed mainly before MIS 1, and are separated by three distinctive bounding surfaces corresponding to MWP-1A, MWP-1B (?) or the rapid sea-level rise at ca. 11.5 ka shortly after the Younger Dryas cold period, and the 8.2 ka event, respectively, and the maximum marine flooding surface (MFS). The depositional unit of the Yangtze Shoal ranges in thickness from 6 to 32 m. The shoal exhibits large-scale, SW-dipping (generally 0.5°–2°) foresets with local cross-cutting of foresets, which are composed mainly of very fine- to medium-grained sands. The shoal is underlain by muddy intertidal to subtidal flat deposits that formed during the Younger Dryas, with an erosional surface corresponding to MWP-1B (?) or the rapid sea-level rise at ca. 11.5 ka between these and the shoal deposits. The Yangtze Shoal came into being mainly between ca. 11.5 and 7.0 ka in the form of tidal sand ridges actively moving southwestwards under intense and reciprocal tidal currents on a tidal shelf. The tidal sand ridges have evolved into sheet-like tidal sand banks with relatively low relief as a result of tidal remobilization under rotary tidal currents since ca. 7.0 ka when the MFS formed. Therefore, the Yangtze Shoal can be considered to comprise quasi-active tidal sand banks.