长江口区晚新生代沉积物粒度特征和沉积地貌环境演变

对长江河口区的三个晚新生代钻孔作了地层对比和粒度分析，据此探讨晚新生代长江河口的沉积地貌环境演变过程。结果表明，SG6和J18A孔上新世厚层含砾砂质沉积物粒度特征，反映了洪积扇和冲积扇相沉积环境，沉积物搬运距离短；而SG13孔缺失上新世沉积，反映古地势较高，以剥蚀为主。因此上新世长江三角洲地区古地势高差较大，侵蚀区和沉积区共存。早更新世，本区继承了上新世的冲、洪积扇沉积环境，但SG6孔泥质沉积明显变厚，J18A孔此时则以厚层含砾砂沉积为主，SG13孔也开始接受泥质沉积，反映在构造沉降作用下，冲、洪积扇体向西、南部迁移，沉积盆地范围扩大。中更新世，沉积物普遍变细且以悬浮沉积为主，显示了曲流河或曲流河冲积平原的沉积环境。晚更新世初，本区又发育含砾砂层，特别是SG13孔出现厚层含砾砂，但是粒径显著小于上新世和早更新世沉积物，反映本区再次发生显著构造沉积，河流地貌广泛发育。晚更新世中晚期和全新世沉积物以粘土和粉砂质粘土为主，悬浮沉积占优，说明本区已演变为滨、浅海沉积环境。因此在构造沉降、剥蚀和沉积的共同作用下，本区的地貌演变经历了自上新世至中更新世和自晚更新世至全新世的两次准平原化过程。

Grain- size Characteristics and Geomorphology Evolution in Late Cenozoic Era Sediments, Changjiang Estuary

On the basis of grain size analysis of the samples from three boreholes in Changjiang Estuary, the present study discusses the sedimentary and morphologic environmental evolution throughout the late Cenozoic era. Results demonstrate that, thick layers of sand and gravelly sand dominate in the borehole sediments of Core SG6 and J18A in Pliocene, indicating quick deposition of alluvial fan system. The lack of Pliocene sedimentation in the borehole of Core SG13 indicates higher topography. Therefore obvious relief occurred in the study area with both depositional and erosion areas during Pliocene. In early Pleistocene the sedimentary environment remained as alluvial fans. But the sedimentary basin extended westward and southward due to tectonic subsidence, as evidenced by the thick layers of gravelly sand in Core J18A and initiation of deposition in Core SG13. In middle Pleistocene fine sediments accumulated generally, revealing the sedimentary environment of alluvial plain dominated by meandering river. In early late Pleistocene thick layers of sand and gravelly sand dominate in the region again, but the grain size become finer as compared with Pliocene and early Pleistocene. It reveals that rivers developed again in the region because of local tectonic subsidence. In late Pleistocene and Holocene silt and clay accumulated from still water dominate in the boreholes sediments, indicating the sedimentary environment of coastal plain and shallow sea. Hence two processes of peneplaintion occurred in the region due to the tectonic subsidence, deposition and erosion since Pliocene to middle Pleistocene and late Pleistocene to Holocene.