关键词: 海底古森林, 全新世, 强古地震, 福建深沪湾

Abstract: The submerged paleo forest in Shenhuwan Bay, Fujian Province, is accepted to be the vestige of strong paleo earthquake. The fossil trees were formerly growing in low lying land filled with late Pleistocene gravel bearing sandy clay during Holocene. A strong earthquake occurred 7,000 years B.P. had caused the subsidence of the trees onto the seafloor and the development of lagoon environment, while another strong earthquake occurred 2,000 years B.P. had given rise to the further subsidence of the trees to a greater depth. The evidences of these two events are as follows: 1. Earthquake landform composed of loose sediments was buried by littoral facies deposits on the seafloor, and its remnants were left on the beach. This may indicate that the land was subsided rapidly below the sea level during the earthquake, while several island like highlands composed of red and yellow gravel bearing sandy clays of late Pleistocene age remained above the sea level after the earthquake. After long period erosion by sea water, red brown and yellow land spots were left on tidal zone or shoal, while isolated blocky highlands surrounded by marine and eolian deposits were remained on the coast. 2. Many paleo trees had killed simultaneously, some of which were collapsed and broken. Some broken trunks laid down on the sea floor, and some broken branches were still connected with the trunks. All these may indicate a sudden subsidence caused by strong earthquake, which had given rise to the rapid burying of the paleo trees. 3. Evidence from seismic facies sediments. The ZK 1 and ZK 2 drill holes on the shoal have revealed that the Holocene sediments contains a 20～30m thick bed of carbonized, half carbonized and un carbonized broken wood chips with a little amount of gravels. This bed has not been eroded and transported by the sea water, containing marine facies sands of fine to medium size and small yellow clay balls. The formation of this bed seems to closely related to the sudden subsidence caused by strong earthquake, which had given rise to the transgression of the sea and the transportation of fine to medium grained sands into the pre existing slope wash and alluvium. 4. Evidence from the rate of crustal movement. The Shenhuwan paleo earthquake occurred 7,000 years ago. According to estimation, the rate of sedimentation during the period of 7,000～6,000a B.P. was 11 times as high as that during the period of 6,800～2,000a B.P. The rate of crustal movement during the former period was 25 times as high as that during the latter period. 5. Four sag ponds possibly produced by earthquake are distributed along the fault in the coastal area on the southwest margin of the Shenhuwan graben. The earthquake of late Holocene occurred about 2,000 years ago. This event can be confirmed by the facts that the elevation of littoral deposits at Shenhuwan Bay at that time differed from the sea level for about 4m, and that elovian sand deposits of continental facies were remained in the marine strata on the seafloor. According to the estimation, the subsidence of Shenhuwan Bay caused by these two earthquakes reached up to 3m, and the magnitudes of the two events are postulated to be above 7.

Key words: Submarine fossil-tress, Holocene paleo-earthquake, Sudden crustal subsidence