Palaeoclimate of the Paleogene Dongying Formation in Bozhong Sag of Bohai Bay Basin

It was showed that the sedimentary environment of lacustrine basins is more susceptible to basin size than that of marine basins, leading to frequent changes in sedimentary environment and large differences in sediment supply and types. Based on the comprehensive analysis of clay minerals, spore pollen, major elements and trace elements, the sedimentary environment of Dongying Formation and its evolution in Bozhong Sag were studied. The results showed that during the sedimentary period of Dongying Formation, the study area was in a subtropical warm and humid climate as a whole. In addition, the vertical distribution of spore pollen and algae was divided into four zones and each one represents different sedimentary environments. Combination zone I is characterized by a large number of spore pollen and algae, including juglanspollenites verus, ulmipollemites minor, abietineaepollenites and pinuspollenites, taxodiaceaepollenites hiatus, leiosphaeridia and granodiscus, and it indicates a relatively hot and dry climatic; Combination zone II mainly contains juglanspollenites verus, ulmipollemites minor, abietineaepollenites and pinuspollenites, pediastrum and leiosphaeridia. Compared with I, the number of spore pollen and algae decreases, but the number of pediastrum increases a lot, which represents warm and humid climate. The combination zone III is dominated by abietineaepollenites and pinuspollenites and chaetosphaeridium, and the species and quantity decrease significantly. It indicates a relatively hot and humid climate at that time. Combination zone IV contains a small amount of spore pollen and algae, and it means the climate was warm and humid at that period. The frequent changes of sedimentary environment in lacustrine basins are influenceded by regional tectonic environment and basin scale. Using S/A and E-T parameters, the paleoclimate in the study area was delineated and 10 sedimentary environment interfaces were identified, which further illustrated the characteristics of frequent fluctuations of sedimentary environment in lacustrine basins.