晚第四纪长江三角洲高分辨率层序地层学的初步研究

A PRIMARY STUDY ON HIGH-RESOLUTION SEQUENCE STRATIGRAPHY OF LATE QUATERNARY IN THE YANGTZE RIVER DELTA AREA

根据钻孔资料分析了长江三角洲冰后期层序的构成：古河谷内由河床相、浅海相—河口湾相组成，海侵面在滞留沉积与潮坪堆积物之间，最大海侵面位于浅海相和前三角洲泥质层中。古河间地层层序包含滨海相、浅海相和潮坪相，其海侵面与层序界面重合，最大海侵面在浅海泥质层中。在古河谷和古河间的沉积相组合不同，但二者同在一个海侵海退旋回中形成，属同一层序。

Six sections(62 boreholes)demonstrate the postglacial stratigraphic structure in theYangtze River Delta area. There were two geomorphic units in the study area during thelowest-sea-level period in the last glaciation,i.e,paleovalley and paleointerfluve.A widespread paleosol horizon was formed on the paleointerfluves. But in the paleovalley,the paleosol was absent,instead,there was an erosional surface at the base. The top sur-face of the paleosol and the erosional surface constituted the base boundary of the post-glacial sequence. The sediments in the incised valley are fluvial-channel facies,ocean-affected flood-plain facies ineritic-predeltaic facies and deltaic facies in upward succession, Fluvial facies(including fluvial-channel and flood-plain facies)were developed during the postglacialsea-level rise,in a manner of retrogressive aggradation. The transgressive surface is thedemarcation of the fluvial lag deposits and the sediments of the retrogressive aggrada-tion,which is diachronous. The maximum transgression in the Yangtze River Delta area occurred about 7500a B,P,,A huge estuary,with its apex at Zhenjiang and Yangzhou,was formed at that time. Large volume of muddy sediments,rich in marine microfossils,were deposited in the estuary. Accompanying the prograding of the delta,similar muddy sediments of predelta overlay the neritic-estuarine sediments and constituted united muddy layer. These two kinds of muddy sediments are hardly distinguishable in both sedimentary characteristics and microfossils,so the ￣(14)C date of 7500a B.P. is used as the maximum flooding surface,which is located within the muddy layer.The sediments be-low the maximum flooding surface are transgressive systems tract,which contains typi-cal terrigenous facies,ocean-affected terrigenous facies and typical neritic facies; while the sediments above the surface are high-stand systems tract,and the main body is the deltaic facies.The transgressive surface is located between the top surface of the paleosol and thelittoral facies at the paleointerfluves,but locally located between the paleosol and fresh-water marsh.The maximum flooding surface is located within the neritic facies. Thestratigraphic boundaries and the systems tracts at the paleointerfluves are comparable tothat in the incised valley,except lithology and lithofacies.