Sedimentary evolution of the Holocene subaqueous clinoform off the Shandong Peninsula in the Yellow Sea

Abstract

A subaqueous clinoform wraps around the end of the Shandong Peninsula in the Yellow Sea. The clinoform, previously called the Shandong mud wedge, is up to 40 m thick nearshore and thins seaward. Its origin has been controversial since it was first noted in the 1980s. For our recent investigation offshore of the northeastern Shandong Peninsula, high-resolution shallow seismic profiles covering about 1700 km were obtained, and two drilling cores penetrating the clinoform were recovered. We described the lithofacies characteristics, identified benthic foraminifera and ostracoda, measured grain size, analyzed the mineral components of the cores, and obtained AMS 14C dates on 28 samples, to decipher the sedimentary evolution of the clinoform in response to postglacial sea-level changes. The shallow seismic profiles show that the clinoform comprises three units: a lowermost seismic unit (SU 3, less than 2–3 m thick) showing a retrogradational/aggradational stacking pattern with subhorizontal internal reflectors, a middle seismic unit (SU 2, up to 35 m thick) showing a prograding reflection pattern with mostly seaward-stepping reflectors separated by several erosive surfaces, and an uppermost seismic unit (SU 1, maximum thickness 18 m) exhibiting an aggradational reflection pattern with subparallel reflectors. The three seismic units are bounded on the bottom by distinct erosion surfaces. Analyses of the two cores indicate that the clinoform comprises three depositional units, DU 1, DU 2, and DU 3 in descending order, corresponding respectively to SU 1, SU 2, and SU 3. The depositional units are distinguished by lithofacies characteristics and the downcore distributions of benthic foraminiferal and ostracod assemblages. Mineral components of the cores suggest that the clinoform's provenance is predominantly Yellow River-derived sediments. AMS 14C dates suggest that DU 3 was formed from about 11.6 to 9.6 cal kyr BP, DU 2 from 9.6 to 6.5 cal kyr BP, and DU 1 after ca. 6.5 cal kyr BP. Some major meltwater pulse (MWP) events during the deglacial transgression were registered in the clinoform stratigraphy, with the basal surfaces/sediments of DU 3 and DU 2 corresponding respectively to MWP-1B (ca. 11.6–11.3 cal kyr BP) and MWP-1C (ca. 9.6–9.2 cal kyr BP) events, and with DU 2 being largely formed after the 8.4–8.2 cal kyr meltwater event. Rapid centennial- to multidecadal-scale flooding events were presumably responsible for the erosive surfaces separating the downlapping internal reflectors in SU 2. The change of depositional pattern from progradational in SU 2 to aggradational in SU 1 is attributed to a change from strong, dominantly to-and-fro tidal currents to the modern circulation in the Bohai Sea as well as to the weakening of the tidal-current field during the highstand after about 6500 cal yr BP. As a product of complex interactions among sediment supply, sea-level changes, and hydrodynamic regimes, the clinoform can be regarded as the distal part of the tidal depositional system in the eastern Bohai Sea, which was largely derived from the Yellow River.