CHRONOLOGY STUDY OF THE QUATERNARY STRATA IN YBK1 CORE IN THE NORTH WING OF THE YANGTZE RIVER DELTA

The establishment of Quaternary drilling core time rulers is an important basis for analyzing the evolution of the Quaternary environment in the plain areas. After analyzing the lithology, color, material composition, sedimentary structure and interface of the Quaternary sediments with a total thickness of 95.5m in YBK1 core in the east side of Yunhe Bridge in Shiqiao Town, Yangzhou City, dividing the stratum in detail, and using dating methods like AMS¹⁴C, OSL, cosmogenic nuclide and paleomagnetism, we finally establish the Quaternary stratigraphic-time ruler of YBK1 core. Combining with the stratigraphic structure, key bed and so on, we determine that the Quaternary sediments is overlying the Pukou group red siltstone, and it is composed of four sets of strata, which from old to new respectively are:the upper part of the early Late-Pleistocene Qidong Group, with age of 0.1~0.3Ma, thickness of 30.20m, sand bed gravel layer as the main stratum with complicated composition; the lower part of the early Late-Pleistocene Kunshan Group, with age of 0.1~0.045Ma, thickness of 1.80m, residual sand bed gravel layer as the main stratum; the middle part of the later Late-Pleistocene Gehu Group, with age of 0.045~0.01Ma, thickness of only 3.40m, littoral facies grey clay sand as the main stratum, no upper and lower parts; the Holocene Rudong Group, with large thickness of 55.10m, which can be further divided into upper, middle and lower sections with demarcation ages of~2.5Ka and~7.5Ka. The Holocene in this area is dominated by coastal sediments, estuaries and floodplains. It is the Holocene river diversion that made the stratum thickness increase suddenly. According to the lithological characteristics of YBK1 core, Early-Pleistocene and Early-Middle-Pleistocene sediments are non-existent, sediments of Late-Middle-Pleistocene to Late-Pleistocene are affected by river erosion, resulting of depositing a small amount of residual coarse grain river bed sediments. Up to Holocene, the weakening of surface hydrodynamic forces caused by river diversion produced the thicker Holocene sediments.