永定河形成时代研究

FORMING TIME FOR THE YONGDINGHE RIVER

通过对永定河冲洪积物的研究发现,永定河形成前后的沉积环境明显不同。钻探资料表明永定河形成前本地区沉积环境主要为湖泊相,当时湖水宁静,沉积物主要为泥岩和粉砂。永定河形成时洪水裹携大量碎屑形成巨厚的泥石流堆积,沉积环境为冲洪积相,主要为砂砾石、砂、粉砂和粘土互层,形成巨大的冲洪积扇。通过新5孔钻孔岩芯273块古地磁样品的系统剩磁测试,获得了该钻孔岩芯的古地磁极性序列,对比国际地磁极性年表,认为该钻孔岩芯剖面0～96.59m以正极性带为主,对应布容正极性时,其地质时代为0～0.78Ma;96.59～246.79m则以反极性带为主,对应松山反极性时,其地质时代为0.78～2.58Ma;246.79～413.64m以正极性带为主,则对应高斯正极性时,其地质时代为2.58～3.58Ma,属于新近纪上新世。研究推测永定河最早的沉积物为上新世形成,距今3.33～3.58Ma。

As the most important river in Beijing City, the Yongdinghe river carried much alluvium and proluvium and shaped the Beijing Plain. Lots of borehole data show that the sedimentary environment was obviously different before and after the formation of the Yongdinghe River. Before its formation,the lake facies dominated this region, depositing mudstone and siltstone. After its formation,the alluvial and pluvial facies dominated this region,forming vast alluvial and pluvial fans consisting of interbedded gravel, sand, silt, and clay. This paper provided a detailed magnetostratigraphic record based on 273 samples from New Borehole 5 in the Beijing plain area. Stepwise alternating field（AF）demagnetization is used to isolate the magnetic component in these samples. The pattern of magnetic polarity sequence given by the record is well matched with the geomagnetic polarity timescale. 0 - 96.59m. The rock core of depth 0 - 96.59m is mainly of normal polarity, corresponding to the Brunhes chrons with an age of 0 to 0.78Ma. The rock core of depth 96.59 - 246.79m is mainly of reversed polarity, corresponding to the Matuyama chrons,with an age of 0.78Ma to 2.58Ma. The rock core of depth 246.79 - 413.64m is mainly of normal polarity,corresponding to the Gilbert chrons,with an age of 2.58Ma to 3.58Ma. The gompholite at the bottom of the borehole should be the oldest sediment by the Yongdinghe River, so it should be formed about 3.00MaB. P. ,that is,in the Pliocene Epoch.