PALAEO WATER DEPTH RECONSTRUCTION OF THE NEOGENE DEPRESSIONAL LACUSTRINE BASIN IN HUANGHEKOU AREA OF BOHAI BAY BASIN, NORTHERN CHINA

The chronostratigraphic framework of the Neogene of Huanghekou area in the Bohai Bay Basin, northern China was re-determined according to a comprehensive division result combining palaeomagnetic test of Well W5, research results of the Neogene stratigraphic framework of the Bohai Bay Basin and International Chronostratigraphy (2013 version). A modern lake, the Dongting Lake, due to its similar features as the Neogene lacustrine basins in the Huanghekou area, was chosen for correlation, sampling from bottom sediments, tests of water depth and geochemistry elements. Four elements Al, V, Ni, Ga with R² larger than 7 were chosen to establish an original quantitative relationship(F_P)among geochemistry elements and water depth of bottom sediments of a modern lake using mathematic methods. Through identification of Candoniella albicans from bottom sediments of modern lakes and analysis of their relationship with measured water depth, water depth of ~2.4m was determined as an accumulation depth of Candoniella albicans and as the water depth obtained from environmental virable parameter of formula F_P. Candoniella albicans found from the Neogene of Well W10 in the Huanghekou area are consistent with those sediment samples from the Dongting Lake. Combining the determined water depth of ~2.4m and measured data of drilling cuttings geochemistry elements at depth where Candoniella albicans were found from the drilling well, finally the relationship formula (F_F)was established through environmental varible calibration to the original quantitative relationship (F_P). The quantitative palaeo-water depths of each drilling well will be obtained for the first time in this area, when the measured results of geochemistry elements of chip samples from 4 drilling wells in the Huanghekou area are directly iterated to this final relationship(F_F). Palaeo-water depth reconstruction of the Neogene indicated an evolution of shallow-deep-shallow with average water depth of 1.7-3.0m and the largest of~4.8m. Theevolution charactersof palaeo-water depth with shallower water, frequent fluctuation and alternative occurrence of lacustrine and continent, is closely related with developing environment of shallow water delta found in this area and corresponds well to the Neogene palaeoclimate changing regularity.