Subaqueous deltaic formation of the Old Yellow River (AD 1128–1855) on the western South Yellow Sea

Abstract

Abstract The Yellow River presently flows into the Bohai Sea, but during the period AD 1128–1855, the river (called the Old Yellow River, OYR) flowed into the western South Yellow Sea (SYS), where the dispersal of its sediment load was controlled by the interactions among alongshore currents, waves and tides, and shelf morphology. In this study, high-resolution shallow seismic profiles, vibrocores, boreholes, and water depth data, which were obtained during geological surveys in the western SYS in recent years, are used to examine the depositional pattern and stratigraphic record related to the OYR entering the sea. Our data reveal compound clinoform morphologies (double clinoforms) – a subaerial/subaqueous delta couplet – sourced by the OYR in the western SYS. The subaerial delta comprises the currently onshore delta plain and deltafront extending to ~ 17 m in water depth, while the subaqueous delta is composed of a wide, gently inclined topset and a relatively narrow, steeply sloping foreset, stretching seaward about 160-km away from the shoreline and showing a morphological asymmetry with a generally southward deflection relative to the OYR mouth. The topset is 25–100-km wide in a shore-normal direction at water depths of 10–25 m, but is generally less than 4-m thick. The foreset is much narrower and thicker than the topset, is generally 5–30-km wide at 10–45 m water depth and is mostly 4–16-m thick with a maximum thickness of 20 m; it shows seaward-dipping internal reflectors (generally 0.3–0.5°). The rollover points (topset–foreset transition) of the subaqueous clinoform range mostly between 10 and 25 m water depth. Results of 210Pb and 137Cs geochronology indicate that maximum sediment accumulation rates (1.06 to 2.16 cm/yr) correspond to the foreset region, with much lower rates in the topset and surrounding shallow-sea regions ( 97%) in AD 1128–1855 has accumulated on the subaerial delta, and that about 10–16% of the total load on the subaqueous delta. Our data also reveal that the subaqueous delta related to the OYR is underlain by a large-scale clinoform (broadly confined to ~ 2.5–9.8 cal kyr BP in age) as thick as 4–13 m with seaward-dipping lower angled (