AN OVERVIEW ON THE FIRST HARD SOIL LAYER (FHSL) OF THE LATE PLEISTOCENE IN THE YANGTZE RIVER DELTA

The top strata of the Late Pleistocene Series in the Yangtze River Delta generally consist of hard clay, with a thickness up to 1 ~ 10 meters, which formed under dry-cold climate conditions during the Last Glacial Maximum, then transformed into hard clay through a long period of weathering leaching and diagenesis. The First Hard Soil Layer (FHSL) in the Late Pleistocene is an important marker bed for dividing the Holocene and Pleistocene Series, which also serves as the main bearing stratum of pile foundation for medium and high-rise buildings in Shanghai. However, the understanding on the formation reason, material source and secondary evolution of the FHSL are still controversial due to the lack of specific sedimentary marks and fossils. For example, the origin on the FHSL exists hydro-genesis and aeolian-genesis, provenance for the FHSL is proximal or distal, and the secondary evolution of the FHSL is also debated. The deposition and evolution of the hard soil layer experienced complex climate changes of cold warm dry wet and transgressive regression cycles, which can be divided into three stages; the deposition stage of early cold dry environment, the weathering and soil forming stage of warm wet environment before Holocene transgression in the middle stage, and the burial and transformation stage after transgressive regression in the later stage. To solve disputes on the FHSL, it is necessary to establish a continuous high-precision chronological scale, to trace maker bed regionally, and to study evolution processes and mechanism of hard soil layer.