武汉地区第四纪磁性地层

STUDY ON QUATERNARY MAGNETIC STRATIGRAPHY IN WUHAN AREA,CHINA

武汉地区90%为第四系所覆盖,但目前针对第四纪年代地层的研究严重滞后。文中在武汉市城市地质调查施工的数百个钻孔中选择了地层发育齐全、适于进行磁性地层研究的SK2钻孔进行了古地磁极性测试和分析,并重点对SK2钻孔进行了磁性地层的划分,初步建立了武汉市第四纪磁性地层的年代序列。结果表明,SK2钻孔的布容与松山(B/M)界线位于孔深约29. 6m处,其古地磁年龄约0. 78Ma;松山反向带中的正极性亚时贾拉米洛(Jaramillo)、奥杜威(Olduvai)、留尼汪(Reunion)Ⅱ分别位于孔深38. 2～39. 8m、66. 5～71. 9m和75. 8～78. 4m处。依据磁性地层结果对研究钻孔进行了第四纪地层划分,SK2钻孔的1. 2～14. 1m为上更新统,15～29. 6m为中更新统,29. 6～78. 8m为下更新统。计算SK2钻孔地层的沉积速率后发现,古地磁年代1. 47～1. 57Ma BP和1. 07～1. 21Ma BP为2个沉积速率相对增大的阶段。

With the lateral and vertical expansion of cities,urban geology becomes critical for urban construction. Wuhan City,as one of the largest cities in China,was chosen by China Geological Survey as pilot city to study multiple elements of urban geological survey. 90% of Wuhan area is covered by Quaternary strata,which means that most of the city is built on Quaternary sediments. The study of Quaternary stratigraphic structure of Wuhan area is a crucial groundwork for the urban geological survey of Wuhan. Due to the badly lagging of research on Quaternary stratigraphy of Wuhan area,this study selects four boreholes from hundreds of cores in this area in the project of Wuhan Urban Geological Survey for magnetic stratigraphic study and paleomagnetic analysis. This work mainly focuses on the borehole SK2 which possesses well-developed strata and is representative for magnetic strata division. Wuhan is located in the eastern Jianghan Basin where the Quaternary sediment has a fluctuant bottom edge,forming a half-graben shape boundary in large scale,as revealed by a large amount of boreholes. The borehole SK2 is located in the western Dongxi Lake depression. SK2 reveals continuous deep Quaternary sediment except for a short gap of late Pleistocen-early Holcene. Moreover,the grain size of drill core is generally smaller than other cores nearby,which is more suitable for paleomagnetic study.In this study,we collected 117 samples with an interval of 0. 25 ~ 0. 30 m from relative fine grain layers in the borehole for paleomagnetic study. Demagnetization and sample measurement were conducted in State Key Laboratory of Loess and Quaternary Geology,Institute of Earth Environment,CAS. By comparing with other well dated cores nearby,Wuhan’s Quaternary magnetic stratigraphic chronology framework is initially established. Our data show that: 1) the Brunhes-Matsuyama boundary(B/M) of borehole SK2 is clear,locating at 29. 6 m,and sediment at 1. 2~29. 6 m belongs to Bmnhes normal polarity chron;2) Two polarity drifts at 9. 3 ~ 9. 6 m and 14. 1 ~ 15. 0 m are observed in Brunhes. These samples collected from the Xiashu Loess yield two polarity drifts,which can be compared with the Xiashu Loess at Nanjing. The comparison between Xiashu Loess from borehole SK2 and that at Nanjing suggests that negative polarity drift at 14. 1 ~ 15. 0 m is the Blake reverse polarity subchron(0. 104 ~ 0. 128 Ma BP) and the polarity drifts at 9. 3 ~ 9. 6 m(0. 07 ~ 0. 08 Ma BP) is Norwegian-Greenland Sea reversal-polarity subchron;3) Three polarity drifts were observed in Matsuyama,which is comparable with the borehole at Zhoulao and the borehole R25 in the center of the Jianghan Basin. They are at 38. 2 ~ 39. 8 m(Jaramillo: 0. 99 ~ 1. 07 Ma BP),66. 5 ~ 71. 9 m(Olduvai: 1. 77~ 1. 95 Ma BP) and 75. 8 m(Reunion: 2. 01 Ma BP);4) Based on the forementioned paleomagnetic data, the depositional age structure of this borehole is: 1. 2 ~ 14. 1 m(late Pleistocene),15 ~ 29. 6 m(middle Pleistocene) and 29. 6 ~ 78. 8 m(early Pleistocene);5) Our data show that deposition rate increased during 1. 47 ~ 1. 57 Ma BP(54. 9 ~ 61. 3 m) and 1. 07 ~ 1. 21 Ma BP(39. 8~47. 3 m) because they are fluvial sediments. This study builds the Quaternary stratigraphy and timescale for Wuhan area for the first time. However,because of the complex Quaternary sediment type in Wuhan area and the coarse sediment in the Matuyama,more comprehensive study is needed in the future to test whether our Quaternary stratigraphy and timescale is the best.