Sediment grain size distribution patterns of the late Quaternary on the back side of northern Yangtze River Delta and their environmental implications

On the north wing of the Yangtze River Delta, grain size data, such as grain size parameters, frequency distribution and the probability accumulation curves, are acquired from the core of CSJA3, in addition to lithology, sedimentary structures, and pollen and foraminifera assemblages. The results support the conclusion that grain size data can effectively reflect the change in sediment environment since MIS6. During the interglacial stage, clay components are getting less as the sand increasing, while the mean grain size becomes coarser. In the glacial stage, however, it is opposed. Six sedimentary environments are recognized from the bottom to top: U6 (flood plain), U5 (river-tidal flat), U4 (river floodplain), U3 (tidal flat-littoral and shallow sea), U2 (flood plain), and U1 (lagoon-tidal flat). Three transgressive layers are found in the core of CSJA3, i.e. layer U5, U3 and U1, corresponding to the Taihu transgression(MIS5), the Gehu transgression(MIS3) and the Zhenjiang transgression(MIS1) respectively. Among them, the MIS3 transgression is larger than MIS5 transgression in scale, which is inconsistent with the global pattern of sea level change, probably caused by tectonism in addition to global climate fluctuation.