Study on the three stages of transgression and paleochannel of western Bohai Bay since the Late Pleistocene

The comprehensive studies of sedimentary characteristics, micropaleontology and dating(AMS ¹⁴C, OSL)of boreholes indicate that western Bohai Bay has been affected by three stages of regional transgression events since the late Pleistocene, and its sediments is mostly characterized by marginal seashore lake marsh facies. At the same time, the boundaries of the greatest influence of three stages' transgression were traced, namely Tangguantun-XichangTun-Liangtou boundary representing the third and the largest transgression in the early period of the Late Pleistocene, Dahuangwa-Sanhuzhuang boundary representing the second transgression in the late period of the late Pleistocene, and the first transgression in Holocene affecting the entire research area. The maximum impact boundary of the first transgression needs to be further traced to the west, and the predecessors' data show that it can extend to Wenan County. Furthermore, four stages of paleochannels were developed since the late Pleistocene, and they affected and restricted the sea invasion layer and the influence scope. The interaction between transgression and paleochannel resulted in the formation of marine and continental intersedimentation framework and the offshore alluvial plain since the Late Pleistocene. The formation age of the four paleochannels was defined by the use of chronology, deep-sea oxygen isotope MIS(Marine isotope stages)and the marker beds of three stages'(1, 3, and 5)transgression. It is shown that the four stages, paleochannels were formed(from bottom to up)in the Early Holocene and the Late Holocene. The above results can provide important geological data for the study of paleogeography, land and sea changes and hydrogeololgy-engineering-environmental geology in the region.