曹娥江下游XYC孔粒度特征及沉积记录

Grain-size and sedimentary records of the XYC Core in the downstream of the Cao'e River

基于曹娥江下游星一村钻孔(XYC)1717个沉积物样品的粒度分析,探讨钱塘江南岸地区全新世沉积环境记录。初步结论如下:1基于粒度组成,XYC孔沉积物分为粉砂质砂、砂质粉砂、粉砂、粘土质粉砂等4种岩性,自下而上分为9个单元,组成多个粒度粗细变化旋回。其中,第1、4两层为粉砂质砂,砂含量高于粉砂,粘粒含量10%~15%,分选差,动力条件复杂;第3、5两层属砂质粉砂,粉砂为主但含量不超过70%,砂含量20%以上,粘粒含量低;第2、7、9三层为粉砂,粉砂含量超过70%,粘粒、砂含量均低于20%;第6、8两层为粘土质粉砂,粉砂含量高于70%,粘粒含量20%以上。2基于C-M图最大搬运动能和平均搬运动能强弱及其变化幅度分析,初步识别出4种沉积相,自下而上依次为河流相、潮坪相、浅海相、湖沼相。第1、2两层为高能且不稳定的水动力搬运环境,属河流相沉积;第3~7层,最大搬运动能减弱,搬运动力与上虞近海滩涂样品SYTT相似,为潮坪相沉积;第8~9层,搬运动能低且变幅小,属低能静水沉积环境;第8层下段为浅海相,该层上段至第9层为湖沼相。3粒度纵向变化表明,粘粒含量自下而上波动增加;砂含量自下而上波动减少,平均粒径向上变细、分选性向上变好。XYC孔记录了该地区9690~5131 cal.a BP时段海面上升,达到高海面继而又海退的过程。研究结果有利于进一步探索钱塘江流域的古环境演变特征。

Based on the analysis of grain-size constitution and distribution curves of the XYC core in the downstream of the Cao＇e River--the central section of the Ningshao plain, we arrived at the following conclusions： （1） According to the grain-size constitution, the sediments of the XYC Core can be divided into four lithologies： silty sand, sandy silt, silt, and clayey silt. They distribute along the core alternately. The core is divided into 9 layers and they compose multiple grain-size combinations. The 9 layers are marked as layer 1 to layer 9 from the bottom to the top. Among them, layer 1 and layer 4 are silty sand, and the percentage of clay is 10%-15%, which is poorly sorted and the dynamics is complicated. Layer 3 and layer 5 are sandy silt, the percentage of silt is no more than 70%, and clay is more than 20%. The percentage of clay is low. Layer 2, layer 7, and layer 9 are silt, the percentage of silt is more than 70%, and clay and sand are less than 20%. Layer 6 and layer 8 are clayey silt, the percentage of silt is more than 70%, and clay is more than 20%. The percentage of sand is very low. （2） According to the C-M graph of the max and the mean values and the analysis of its variation, there were four types of deposits from the bottom to the top： fluvial, tidal flat, shallow marine, and lakes and marshes. For layer 1 and layer 2, the hydrology was high energy and unstable, which indicates a fluvial environment; for layer 3 to layer 7, the max hydrology weakened, but the mean hydrology was strong and the range was still wide, and the hydrology was similar to the SYTT tidal flat samples. It referred to a tidal flat environment; for layer 8 and layer 9, the hydrology was weak and stable, which indicates still water environment. Based on the grain-size and biology information, it can be concluded that the under part of layer 8 belonged to shallow marine environment, but the upper part and layer 9 belonged to lakes and marshes environment. （3） Vertical variation of grain-size indicates that clay increased from the bottom to the top with fluctuation, while sand declined. The average grain diameter is decreasing and the sorting coefficient is higher. The sediments change from the bottom to the top reveals that the XYC Core and its adjacent area experienced sea level rise, and then water level decreased during 9690-5131 cal.a BP. The results of the study are helpful for identifying the paleoenvironmental evolution characteristics of the Qiantang River Basin.