Sedimentation of tidal sand ridges off the Liuguhe River mouth in the Liaodong Bay, Bohai Sea

Abstract

The Liuguhe River mouth sand ridges in the Liaodong Bay, Bohai Sea, provide a good example for further understanding the sedimentation of small-scale tidal sand ridges. However, little is known about their morphological and sedimentary characteristics. This paper presents a comprehensive study of these sand ridges using survey data from sub-bottom profiling, side-scan sonar mapping, and acoustic doppler current profiling, along with piston core analyses of grain-size and heavy minerals. The sand ridges span widths of 1–2.5 km and heights of 4–16 m, with diminishing heights toward the sea. Their cross-sections are asymmetrical, featuring gentler and steeper slope angles of less than 1.5° and up to 4.5°, respectively. Sand waves are widely superimposed on the sand ridges, exhibiting wavelengths of 0.7–62.3 m and heights of 0.11–3.12 m. The ridge sediment consists of well sorted medium sand, while the trough sediment consists of very poorly sorted silty sand, indicating that the ridges are coarser and better sorted than the troughs. The tidal deposits are characterized by high levels of magnetite and hematite. Provenance discrimination based on heavy mineral assemblages indicates that the sediment is mainly derived from the Liuguhe River. The sand ridges have been formed since the middle Holocene and are still active at present. The tidal currents are rectilinear. Near-bed currents exceeding the critical velocity of sediment motion account for 20 % of a tidal cycle, implying that the tidal currents are capable of maintaining the development of bedforms. The longitudinal axes of the sand ridges exhibit anticlockwise rotation relative to the direction of the maximum tidal current, with an angle of up to 30°. The steeper slopes of the sand waves on both flanks of the sand ridges point to ridge crestlines, implying a tendency for sand streams to converge on the ridge crestlines. The sand ridges and adjacent troughs consist entirely of modern tidal deposits in the shoreward area, while the troughs are incised in the underlying strata with a thin layer of modern tidal deposits covering on the surface in the seaward area. Seaward decrease in sediment supplies may explain the seaward changes in ridge structures and heights.