长江三角洲第一硬质黏土层粒度多元统计分析及沉积环境判别

长江三角洲第一硬质黏土层成因是长期争论的问题。对上海市青浦区第一硬质黏土层全样粒度多元统计分析,并与南京周家山下蜀黄土、长江悬浮颗粒物对比,探索长江三角洲第一硬质黏土层成因。第一硬质黏土层机械颗粒组成均一,频率曲线呈正偏态、中等或尖锐峰形的双峰分布,频率累积曲线呈"S"型分布,具有风成沉积特性。粒度特征分析为沉积环境判别提供沉积学证据时,综合运用多元统计分析可以使分析结果更准确。聚类分析将硬黏土与典型风成沉积物下蜀黄土粒度分布归为一类,长江悬浮颗粒物单独为一类,表明硬黏土与下蜀黄土成因一致。下蜀黄土与长江悬浮颗粒物粒度参数判别函数,将绝大部分硬黏土样品判定为风成沉积。粒级—标准偏差曲线结合现代风场,显示硬黏土粒度特征存在高空西风和低空西北季风两个主控因子。表明长江三角洲第一硬质黏土层是由西风和西北季风控制的风成成因堆积物。

Multivariate Statistical Analysis of Grain Size of the First Hard Clay Layer in the Yangtze River Delta and Sedimentary Environment Discrimination

The origin of the first hard clay layer in the Yangtze River Delta is a debated issue. To explore its origin, its grain size was examined by clustering analysis and discriminant analysis, and compared with the Xiashu loess in Nanjing and suspended particles in the Yangtze River. The grain size characteristic is uniform, with positive skewness, medium or sharp peak shape, bimodal distribution, S-shape cumulative frequency curve distribution, and with aeolian deposition characteristics. When the grain size analysis provides sedimentological evidence for a sedimentary environment, multivariate statistical analysis improves its accuracy. The first hard clay and Xiashu loess fall into the same classification; the suspended particles in the Yangtze River are in a separate class. The application of clustering analysis shows that the hard clay and Xiashu loess have the same aeolian genesis, as confirmed by discriminant analysis. Standard deviation curves of the hard clay, together with the present-day wind field, show that the grain size characteristics of the hard clay have two controlling factors: high-altitude westerly winds and the East Asian winter monsoon. All analyses indicate that the first hard clay layer in the Yangtze River Delta is an aeolian deposit transported by westerly winds and the northwestern monsoon.