| The muddy coast of Bohai Bay which belongs to Tianjin city and Hebei province is one of the most important sections of the China muddy coasts. It has north China's largest coastal opening city, Tianjin, Tianjin Binhai New Area which belongs to the national developmental plan and World's sixth, China's fourth harbor, Tianjin New Harbor. It is the economy centre of the circum Bohai Sea, and also the most active region for the development of economic in China.As a link-up between land and sea, muddy coast is relatively fragile for geo-environmental changes. Thus, to better understanding the coastal geologic frame work is a key factor, by which a harmonious society between human being and nature and sustainable development are being perfected.The thesis is mainly focused on the shallow sea area of Northwest Bohai Bay. Methodologically, a number of approaches are used, including: (1) field investigation and core logging and digital photographing, (2) grain size analysis, (3) OSL and AMS 14C dating, (4) diatom analysis, (5) mollusk, foraminifera and ostracode analysis. By the comprehensive study of Borehole CH19, the purpose of this thesis is to reveal the environmental changes (land and sea interaction, sea level change, sedimentary environment change) since the late Pleistocene on the study area, giving the general background materials for the development of Tianjin Binhai New Area and reclaimed land project in Caofeidian.Borehole CH19, 39°03′49.1″N / 117°54′19.5″E, given by portable GPS positioning, is 30m in depth and with a high recovery of the core, 93.74%. A systemalic of sampling is carried out, including 275 samples for grain size analysis, 11 samples for OSL dating, 3 samples for AMS 14C dating, 30 samples for diatom analysis and 59 samples for mollusk, foraminifera and ostracode study.The results of the grain size analysis show that the core sediments are composed of clay-silt, sand-silt, silt-sand, and the major part is clay-silt. The sediments'grading is bad; the skew has a big range and the frequency curves show wide peaks. OSL and AMS 14C dating ages indicated this core formed from Late Pleistocene to the Holocene.By the diatom analysis, the results reveal that the study area belongs to shallow sea environment after the Holocene maximum marine transgression. Its double folding characteristics show that two relatively dramatic environmental changes existed in the year about 3?4ka and 2ka before present. During such two stages, strong depositional dynamic and changeable environment occured because of few diatom valves are visible. The mollusk, foraminifera and ostracode were found in the whole core.And when they were both found in the same layers with diatom, studied primarily, such four fauna almost indicated the same environment conditions. It indicates that it is a feasible method that using diatom on Borehole environment study in Bohai Bay.By the comprehensive study above mentioned, the Borehole CH19 reveals the paleo-environmental changes from bottom upwards as intertidal zone?estuary→lacustrine→fluvial→fluvio-lacustrine→lacustrine→intertidal zone→shallow sea (rip current channel)→shallow sea in the study area since the late Pleistocene. Exhibit the evolvement from the sea to land and back to sea of the study area.The comprehensive study also indicates the study area come into Holocene in the year 10 ka before present; and the maximum marine transgression began and then become strong. Then the study area has belonged to the shallow sea environment until now.About 3-4 ka before present, the environment in the study area changed. Rip current channel was quickly filled up by storm surges and rip current in the shallow sea, and the early middle Holocene sediments were eroded away. After that, the sedimentary environment is relative stable. About 2 ka before present, the study area came into the late Holocene. The sedimentary environment changed again (the sealevel change? tectonic? captured river?). And then, the environment become relative stable, the salinity and the water depth decreased.
|
PDF Full Text Request