TIDAL DEPOSITION SYSTEMS OF CHINAS CONTINENTAL-SHELF, WITH SPECIAL REFERENCE TO THE EASTERN BOHAI SEA

The continental shelf of China has undergone a large-scale marine tran sgression. Tidal currents have become the main active agent and have h ad a profound influence on the deposition and geomorphology of the she lf. The continental shelf of China has five modern tidal deposition sy stems: (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, de position is dominant, and tidal currents form tidal shoals (i.e., tida l sand ridges and tidal sand sheets) whose margins are near an absolut e ellipticity value for the M2 tidal component of 0.4. When the absolu te 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 ro tating current increases so as to form a tidal sand sheet. Tidal depos ition 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 exam ple. 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, i s up to 250 cm/s. In the Liaodong Shoal, the maximum tidal current spe ed is 64-115 cm/s and the absolute ellipticity value of the M2 tidal c omponent is less than 0.4. In the Bozhong Shoal, the maximum tidal spe ed is 58-79 cm/s and the absolute ellipticity value of the M2 tidal co mponent is above 0.4. The small-scale bedforms surveyed in detail in t he area show that the sand ridges and sand sheet are still under the a ction of tidal current processes at the present time. Based on the int erpretation of tidal currents, bedforms and sediment thickness, it is inferred that the general direction of sediment transport within the s tudy area is NNW from the Laotieshan Channel to the Liaodong and Bozho ng Shoals. The small-scale bedforms in the detailed survey area indica te a local net sediment transport from south to north. (C) 1998 Elsevi er Science B.V.