北京平原区新5孔岩心特征及地层初步划分

Core characteristics and preliminary stratigraphic classification of the New-5 Hole in Beijing Plain

北京平原由一套巨厚的冲洪积物构成,岩性岩相变化迅速,难以依据钻孔的岩性组合建立代表整个平原区地层层序,随着测年技术飞速发展,为建立长序定年提供了可能。本文通过观察该孔松散层岩心,按照沉积物颜色、沉积环境、沉积旋回和沉积构造等特点,将岩心划分为8个特征岩性段。同时,古地磁测试结果显示,0～96.6 m为布容正极性世,96.59～246.8 m为松山反极性世,246.8～413.6 m为高斯正极性世;通过孢粉分析并结合华北地区新生代气候演化历史对比,孔深246.5 m处可作为Q/N的分界线,534.5 m作为N/E的分界线,803.4 m可以作为E/K分界的参考。以磁性地层学和气候地层学为主,以岩性地层学为辅的原则,对新5孔岩心进行了详细研究,初步对新5孔岩心进行了新生代地层层序划分,将第四系划分为晚更新统、中更新统和早更新统。Q/N定位于248.4 m处,N/E定位于511.9 m处,E/K定位于804.1 m处。

Beijing Plain is composed of a suite of considerably thick alluvia and diluvia,and the establishment of the standard stratigraphic sequences of the whole plain is difficult because of the remarkable changes of lithology and lithofacies.With the rapid development of dating technology,the establishment of long dated sequences of sediments has become possible.Through observing the core of unconsolidated strata,this paper divided the core into 8 characteristic lithologic sections on the basis of the characteristics of sediment color,sedimentary environment,cycle and structure.Also,the result of paleomagnetism dating shows that 0-96.59 m of core is the Brunhes Normal Polarity Zone,96.59-246.79 m is the Matuyama Reversed Polarity Zone,and 246.79-413.64 m is the Gauss Normal Polarity Zone.By mean of pollen analysis and a comparison with the evolution history of Cenozoic climate in North China,the authors hold that the depth of 246.5 m can be regarded as Q/N boundary,the depth of 534.5 m is N/E boundary,and the depth of 803.4 m is E/K boundary.Based on the principle of magnetic and climatic stratigraphy with the aid of rock stratigraphy,the authors studied the new No.5 borehole and divided Cenozoic stratigraphic sequence into the late,middle and early Pleistocene,with Q/N boundary located at the depth of 248.4 m,N/E at the depth of 511.9 m,and E/K at the depth of 804.1 m.