Tidal deposition systems of China's continental shelf, with special reference to the eastern Bohai Sea

Abstract

The continental shelf of China has undergone a large-scale marine transgression. Tidal currents have become the main active agent and have had a profound influence on the deposition and geomorphology of the shelf. The continental shelf of China has five modern tidal deposition systems: (1) the eastern part of the Yellow Sea, (2) the eastern part of the Bohai Sea, (3) off the mouth of Yangtze River; (4) to the west of Taiwan, and (5) the Strait of Qiongzhou. When the tidal current speed is above 150 cm/s, erosion dominates and reciprocating currents form deep scour furrows. When the tidal current speed is 50 to 150 cm/s, deposition is dominant, and tidal currents form tidal shoals (i.e., tidal sand ridges and tidal sand sheets) whose margins are near an absolute ellipticity value for the M2 tidal component of 0.4. When the absolute M2 value is less than 0.4, a reciprocating current is strong enough to form tidal sand ridges. When the absolute value is above 0.4, a rotating current increases so as to form a tidal sand sheet. Tidal deposition material came mainly from tidal erosion and locally redeposition of Late Pleistocene sediments, and from the neighbouring sea bottom. The tidal deposition system in the eastern Bohai Sea is a typical example. It consists of the Laotieshan Channel scour furrow, the Liaodong Shoal sand ridges and the Bozhong Shoal sand sheet. The maximum tidal current speed in the Laotieshan Channel where strong erosion occurs, is up to 250 cm/s. In the Liaodong Shoal, the maximum tidal current speed is 64–115 cm/s and the absolute ellipticity value of the M2 tidal component is less than 0.4. In the Bozhong Shoal, the maximum tidal speed is 58–79 cm/s and the absolute ellipticity value of the M2 tidal component is above 0.4. The small-scale bedforms surveyed in detail in the area show that the sand ridges and sand sheet are still under the action of tidal current processes at the present time. Based on the interpretation of tidal currents, bedforms and sediment thickness, it is inferred that the general direction of sediment transport within the study area is NNW from the Laotieshan Channel to the Liaodong and Bozhong Shoals. The small-scale bedforms in the detailed survey area indicate a local net sediment transport from south to north.