The evolution of sedimentary environment since late Pleistocene in Laizhou Bay, Bohai Sea

There have existed three transgression-regression events and sedimentary evolution processes by sea level changes since late Pleistocene in the southern coast of Laizhou Bay, Bohai Sea. It is obvious that a multi-source fluvial delta sedimentary system may be more dominant for the sedimentary environment. Based on previous researchers' survey and collected data, the authors carried out research work such as division of sedimentary units, sedimentary facies analysis and stratigraphic division and correlation, with the purpose of establishing sedimentary stratigraphic framework of the Laizhou Bay. Besides, the authors focused on analyzing the sedimentary processes of the Laizhou Bay since Late Pleistocene. It is concluded that the main provenance of borehole 07 is Bailang River and the Yellow River, which have played an important role in the multi-source fluvial delta sedimentary system of the southern coast of Laizhou Bay since Late Pleistocene. The evolution can be divided into two glacial periods and three interglacial periods, corresponding to three transgression events and two regression events in Laizhou Bay since late Pleistocene. By geochemistry and provenance analysis, sediments have the characteristics of generation, segment and correlation. In 124.6-72.0 ka B.P., 60.0-24.4 ka B.P. and 10.2-4.0 ka B.P., there occurred three warm-wet periods, respectively corresponding to Cangzhou transgression, Xianxian transgression, and Huanghua transgression; In 72.0-60.0 ka B.P. and 24.4-10.2 ka B.P., which were two dry and cold periods, there was a continental sedimentary environment, corresponding to Wurm early glacier age and Wurm late glacial age. During the transgression period, there always developed littoral-neritic facies in marine area, which was gradually transitional to delta and fluvial facies in the land field, with sediments showing retrogradation superposition pattern. Contribution of short runoff rivers (Bailang river) was reduced gradually from land to sea. During the regression period, fluvial and delta facies were formed in most areas of Laizhou Bay. Terrigenous input of short runoff rivers increased, with sediments showing progradational sequences. From late Pleistocene to Holocene, the region went through sedimentary evolution of neritic facies-delta-continental facies-tidal flat-neritic facies-delta-continental facies.