FORMATION AND ABANDONMENT OF A PALAEO-RIVER CHANNEL IN THE LONGHU LAKE DEPRESSION IN JINJIANG COUNTY, FUJIAN PROVINCE AND ITS RELATION TO PALAEO-EARTHQUAKE

Study of 78 boreholes, in addition to field investigation, have been carried out in the Lonhu Lake and its coast in Jinjiang County of Fujian Province. In the depression of Longhu lake palaeo-river channels were developed before the lake formed and the inlets and outlets of the channels were defined with the sediment records from the 78 boreholes. We found that the formation of palaeo-river channels and the lake were all resulted from palaeo-earthquakes in the Holocene. The uplifting of the block in the lower reaches of the channels caused the formation of the Longhu Lake. The earliest lacustrine deposits in the Longhu Lake appeared 2 430±90 aBP.It coincided with the great earthquake happened in the Shenhu Bay region in Late Holocene from 2 800±100 to 1 460±90 aBP.which resulted in the subsidence of the palaeo-forests in Shenhu Bay into the sea. The bottom of the lower reach of the river channel was about 3 meters higher than that of the upper reach. It suggests that the Longhu Lake was formed by the earthquake in (2 800±100)~(1 460±90)aBP. which blocked the river channel. In this regard, the Longhu Lake is a seismic fault blocked lake only formed in Late Holocene. The palaeo-river was formed 7 000 aBP in early Holocene as the result of another great earthquake which made the palaeo-forests of Shenhu Bay basin sunk for the first time into the sea. The earthquake brought about great environmental changes in the region of the Shenhuwan Bay.