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.