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

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 similar 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 degrees). The rollover points (topset-foreset transition) of the subaqueous clinoform range mostly between 10 and 25 m water depth. Results of Pb-210 and Cs-132 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 (<0.64 cm/yr). Marine erosion due to abandonment of the delta after 1855 and artificial dike building around the abandoned delta coast since the 1930s have impacted the sediment accumulation in the delta front in the subaerial delta and subaqueous delta. Estimates of sediment budget suggest that about 34-52% of the total load primarily from the OYR (>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 similar to 2.5-9.8 cal kyr BP in age) as thick as 4-13 m with seaward-dipping lower angled (<0.3 degrees) internal reflectors in comparison to those in the overlying foreset deposits. Energetic marine hydrodynamics (waves, tides, and alongshore coastal-currents) and sufficient sediment supply from the OYR are interpreted to be responsible for the across-shelf subaqueous delta morphology. (C) 2013 Elsevier B.V. All rights reserved.