| According to the analysis of the grain size, foraminifera, ostracods,14C dating and more than 50 other drilling works of the boreholes C214, D-dl samples which deposited at the bottom of Bohai Bay, a preliminary study on the paleo environments existed before Bohai Bay transgression was conducted.Sedimentary stratigraphy, Biostratigraphy and Chronology tests were carried according to the hole C214 core samples that deposited on/at the Bohai Bay. The hole was divided into four stratum from the top to the bottom. They were respectively marine-littoral, continental facies, marine facies and continental facies. Consequently, it is elucidated that Late Quaternary in this area has undergone four environmental evolution stages, which were are land, sea, land, marine (environmental evolution). Before 39kaB.P.Xian County transgression, the Bohai Bay was a plain land. When Xian County transgression happened, the water's temperature of the Bohai Bay was higher and the sea area was larger than that of now. Sea water was moved out of the Bohai Sea after 23kaB.P.In the Last Glacial Maximum and deglaciation before Holocene transgression happened, the Bohai Bay was bare into land for a long time, and (was) dominated by fluvial deposition. The Holocene transgression happened approximately from 8 kaB.P to 9kaB.P. and sea water began to flood the Bohai Bay as of today.By the analyses of the core include sediment,microfossils and radiocarbon dating, the information of paleoenvironment in the first interface was obtained, environmental evolution evidence in Late Quaternary was provided, the buried depth of transgression interface in Holocene was summarized. According to boreholes C214 mostly, and contrasting with another boreholes. It was shown that the Bohai Bay was plain during the periods when Bohai Sea was terrene, and it was high in the west, it was low in the east, it was gradient in the south, it was depression in the northeast, respectively. The difference between the east and the west was 21.2m and the slope was 0.7%o,like the North China Plain in the modern.Grain size analysis was based on the upper terrigenous layer of hole C214. We use it to compare with the frequency characteristics of the typical granularity component of modern lake surface samples in different sediments and the size discrimination parameters including average particle size, standard deviation, skewnes, abundance of the quaternary loess-paleosol and fluvial sediments. Basic on those, a preliminary study was conducted on the causes and the depositional environment. Identification was tried according to the size characteristics of underwater loess lacustrine deposition before the Holocene transgression that happened on the Bohai Bay, which layed a sound foundation for the comprehensive understanding of the process of lake accumulation, the deeper understanding of Late Quaternary depositional systems of the Bohai Bay and the reconstruction of Paleoenvironment. The results of the analysis of grain size characteristics show that, before Holocene transgression happened,â‘ Hole C214 nearby has been an depositional environment of the lakesideâ†'the center of the lakeâ†'underwater loess, which was consistent with the micropaleontological findings (Plain and the arid grasslands sedimentary environment).â‘¡Underwater loess deposits existed in the Lake sedimentary environment around Hole C214, which was similar with the loess of Qingzhou in Shandong province and loess of Loess Plateau in size frequency distribution and the grain size parameters, but had large difference from the fluvial samples. These findings explained that the sources was aeolian material, then had been deposited underwater, and was not transformed by strong hydrodynamic action.â‘¢Loess underwater can be identified from typical lake sediment by way of particle size analysis.We applied the materials of shallow acoustic surveying and boring stratiagraphy analysis to affirmed the distribution of paleochannel. The parameters like river width, river depth, velocity, flow and sediment discharge of paleochannel were calculated. And it is further speculated that:a river in bohai bay which flowed from southwest to northeast was a high discharge river with multi-branched meandering reach。... |
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