The Early Diagenesis of the Sediments in the Estuary of Yangtze River and its Adjacent Shelf

Abstract: The early diagenesis of terrigenous elastics in the area of the Estuary of Yangtze River and the adjacent shelf is studied. Especially, the process and mechanism are deeply analyzed in a comparative way. to provide evidences for recognizing that of the similar sediments formed in geological time. l.The study makes it clear that the process during the chemical reactions of sediment pore water is a series of metastable balance stages.The change in concentration of major ions in the pore water at a vertical section and the relative relation with reactors are analyzed with the principle of metastable chemical balance.The formation of authigenic minerals (such as glauconite. pyrite, calcite etc.) and cements and the solution of clastic minerals in the early diagenetic stage are demonsttrated.The assemblage of minerals produced in ths stage together with the typomorphic characteristics constitutes an important evidence to recognize the ancient similar diagenetic stages. 2.In the esturine depositional areas where abound with argillaceous sediment and organic matters; bacteria is predominant, sediment rate is high and bacterial decomposition of organic matter is strong. resultion in zonation of chemical reactions which is apparent in the vertical section from top to bottom: oxidizing zone, nitrate reducing zone, sulphate reducing zone and carbonte reducing zone, each with the respactive chemical reaction process and products. Thus, the structural section of biochemical zones gives another important sign for discovering the ancient similar earlier diagenesis. 3. In the zonation of earlier diagenesis of argillaceous sediment, bacteria play a main part.The study shows that when bacteria decompose organic matter and produce carbonate, the δ¹³C value is relatively low (-12‰-15‰); and only when the carbon isotope is fractionated (in carbonate reducing zone) δ^{13/C value becomes higher regularly.With the changes of δ13C values, it is also possible to obtain signs for identifying the ancient similar diagenetic stages. 4.Compared with argillaceous sediment, sand sediment are characterized by poorer organic matter, weaker bacterial activity faster sedimention and restricted ion diffusion is the early diagenesis.lt is the flowing of pore water under the compression effect which becomes an important role.Sand sediment in the estuary restrained by wedges of saline and fresh water appears often surrounded by pH gradient zones which influence directly on formation of cements; or a geochemical trap zone which is significant for oil and gas reservoir.}