长江三角洲北翼全新统底界和“硬粘土层”的讨论

HOLOCENE BOTTOM BOUNDARY AND "HARDCLAY BAND" IN THE NORTHERN FLANKOF THE CHANGJIANG RIVER DELTA

本文以钻孔资料为依据,通过岩石地层学、生物地层学和^(14)C测年分析,认为应以第一海相层底板作为Q_4/Q_(1-3)界线,而不能机械地划在“硬粘土层”顶板;上更新统顶部属河漫、湖沼相沉积,后经暴露、脱水局部层段硬化而成为所谓的杂色“硬粘土层”;本区晚更新世末古地形呈由东、东南向西、西北逐渐抬升的掀斜状,并控制着全新世的海侵方向和范围。

The bottom boundary of Holocene in the northern flank of the Changjiang River Delta has been discussed based upon the analysis of biostratigraphy, rock stratigraphy and 14C dating. The absolute date of 12000a is suitable for H/P boundary. In the study area, lower limit of the first marine bed should be considered as Holocene bottom, not the upper limit of 'the first hard clay band', though the two may coincide sometimes. According to the analysis of sediment characteristics and engineering mechanical nature of the cores, the top of upper Pleistocene belongs to river marsh sediments, which became hard after being exposed and dewatered. At the end of late Pleistocene, topographic form in the area was tiltedly uplifted from southeast to northwest, which influenced Holocene transgression obviously. The direction of the transgression was also from southeast to northwest.