TIDAL DYNAMICS OF ANCIENT BOHAI SEA AT THE TRANSGRESSION MAXIMUM AND ITS COMPARISON WITH THE PRESENT

The sea level reached its maximum off east China at 7 000 aBP,and the Yellow River delta was not formed then.The western part of the ancient Bohai Sea was very wide at that time.Based on the previous studies of seismic profiles and coring data,paleocoastlines and paleotopography were roughly recovered in this paper.A three dimensional numerical model based on the hydrodynamic sediment model of ECOMSED was established to simulate the tidal process in the Bohai Sea during the transgression maximum and the present.The results show that the tidal currents of the ancient Bohai Sea during the period of the maximum transgression (7 kaBP) were much weaker than those in the present time,especially in the west part of the ancient Bohai Sea,as a result of greater water depth,wider coast zone and weaker tidal waves from the Yellow Sea.Under such a weak tidal hydrodynamic environment,the sediment of the ancient Yellow River easily discharged into the sea.The rapid sedimentation around the river mouth was beneficial to the formation of the Yellow River delta.In addition,less sediments were dispersed from the Bohai Sea.There were two places in the central Bohai Sea and eastern Bohai Strait respectively with very weak tidal currents.They became active depositional pools for thicker sediments to accumulate.The tidal hydrodynamic was stronger in the Liaodong Bay,marked by the coarser surface sediments.