In this study, the grain size, chromaticity and elemental geochemistry of sediments from a borehole reaching Pliocene of Bohai Bay have been analyzed, and the sedimentary features and weathering degree of the sediments from this borehole since 5.2~Ma have been expounded. The results showed that the ratios of CIA (chemical index of alteration), Al/Si, Al/Na, Al/K, Ba/Sr and (La/Sm)N、 (La/Yb）N were good indicators of weathering degree and material composition of sediments in North China. The grain size composition, chromaticity change and element ratio characteristics revealed that the chemical weathering degree of the sediments from 5.2~Ma to 1.6~Ma changed from medium to weak, which revealed the synchronous effect caused by Late Cenozoic global cooling and the East Asian Winter Monsoon. Since 1.6~Ma, the (La/Sm）N、 (La/Yb）N from G4 hole have suddenly increased, and the trends of other indicators are not consistent with the slow decreasing characteristics of the deep ocean oxygen isotope ratio, the CIA and 87Sr/86Sr of loess, indicating the change of sediment source. Considering the regional tectonic and climatic conditions, we assume that the change of sediment source from G4 since 1.6~Ma is related to the entry of Yellow River into the North China Plain at 1.6~Ma, and the Yellow River eventually cut through the Sanmen Gorge and enter the North China Plain with the dynamic of intensive headward erosion caused by globe climate changes in the early Pleistocene. The combined section of boreholes from Bohai Bay revealed that the Huanghua depression was basically filled, and the peneplanation process of the North China Plain mainly completed from 1.6~Ma to 0.78~Ma, which indicated that the great mass of sediments from Yellow River made a huge contribution in the forming of the North China~Plain.
DownLoad: