钱塘江下切河谷充填物沉积序列和分布模式

以最新钻取的SE2孔沉积物为重点研究对象,对晚第四纪以来钱塘江下切河谷充填物的沉积特征和沉积相进行了精细研究,重建了研究区地层结构和层序地层格架,总结了强潮型钱塘江河口湾和下切河谷的沉积模式。钱塘江下切河谷充填物自下而上依次发育河床、河漫滩、古河口湾、近岸浅海和现代河口湾5种沉积相类型,表现为一个较完整的Ⅰ型层序,其内部层序界面、初始海泛面、最大海泛面、海侵和海退潮流侵蚀面、体系域内海侵面发育。钱塘江下切河谷充填物自海向陆可划分为海向段、近海段、近陆段和陆向段4段,各段沉积序列和海陆相互作用程度不同。在钱塘江下切河谷充填物中海陆过渡部位首次明确划分出了古河口湾相,并对其沉积特征和分布模式进行了初步探讨;其形成时间在9000 a BP左右,具有与现代河口湾不同的沉积特征,表现为中部为潮道砂体沉积,向陆渐变为受潮流影响的河流沉积,两侧被潮坪或盐沼沉积包围,沉积物在平面上自陆向海呈现粗-细-粗的分布模式。现代河口湾平面上自陆向海依次发育受潮流影响的河流沉积、粉砂质砂坎、潮道-潮流砂脊复合体和湾口泥质沉积区,沉积物呈现粗-细-粗-细的分布模式,与大多数河口湾常见的粗-细-粗的分布格局明显不同。

Sedimentary sequence and distribution pattern of filling in Qiantang River incised valley

The present day Qiantang River(QR)estuary is a world famous tide dominated estuary，and is one of the most sensitive areas for postglacial global sea level changes in China.Filling of the QR incised valley was initiated by marine inundation during the transgression after the Last Glacial Maximum and has continued during the Holocene sea level stillstand.From bottom to top it could be grouped into five sedimentary facies: Fluvial channel，floodplain，paleo estuary，offshore shallow marine，and present day estuary.The QR incised valley deposits occur as a relatively complete type I sequence with the sequence boundary，initial flooding surface，maximum flooding surface，tidal ravinement surfaces and tidal erosion surfaces well developed.Filling of the QR incised valley consists of four longitudinal segments: Seaward segment，offshore segment，the third segment and innermost segment，each of which is characterized by a distinct stratigraphic succession，and different degrees of marine and fluvial influences.This is the first time the paleo estuary facies has been recognized，formed about 9000 a BP. It has a distinct sedimentary succession from the present day estuary，and is characterized by a large number of tidal channel sand bodies flanked by mudflats and salt marshes in the landward part of estuary，passing headward into the tidal fluvial channel.The present day estuary is mainly composed of bay mouth sediments，as well as a complex of tidal sand ridges and channels at the mouth，followed headward by a silty bar，which passes landward to a single tidal fluvial channel.The sediments of paleo estuary present a coarse-fine-coarse seaward pattern.By comparison，the deposits of present day estuary show a coarse-fine-coarse-fine pattern from land to the sea，which is in general similar to the grain size distribution in tide dominated estuaries，but a significant difference exists in that the bay mouth sediments of the present day estuary are mud dominated.