北京平原顺义ZK12-2钻孔剖面第四纪磁性地层学研究

摘要

对北京平原顺义凹陷内的ZK12-2钻孔岩芯进行了详细的磁性地层学研究,结果表明,719m的岩芯记录了布容正向极性时(Brunhes)、贾拉米洛(Jaramillo)极性亚时、奥尔都维(Olduvai)极性亚时和高斯正极性时(Gauss).在系统古地磁样品采集、处理和测试的基础上,应用古地磁法、光释光法,结合岩芯特征,对ZK12-2钻孔剖面进行了详细地层划分,分别确定了下更新统、中更新统、上更新统和全新统的分布深度及地质年代,将Q/N定位于706m处,Q1/Q2、Q2/Q3、Q3/Q4分别定位于170m、66.5m、10m处.本项研究结果为该区第四纪地层划分对比、古地理环境、活动断裂相关研究提供了可靠的地层年代框架.%Magnetostratigraphy was studied in detail using ZK12-2 borehole core from the Shunyi depressuib in Beijing Plain. Here we present the magnetostratigraphic results of ZK12-2 borehole (719m) located at Shunyi Depression. A total of 648 undisturbed samples were collected from ZK12-2 borehole at an interval of 0. 2~0. 8m except for several missing intervals during drilling. 640 samples were performed on thermal demagnetization in emperature steps until 580℃. The Brunhes/Matuyama boundary was found at the depth of 170m, the Matuyama / Gauss boundary at 706m. The Gauss normal polarity chron is shown from 706 to 719m deep in the borehole. The interval from 170m to 706m is fairly correlative with the Matuyama reversal in the geomagnetic polarity timescale with two normal polarity zones corresponding to Jaramillo (200～210m) and Olduvai(464～520m) subchrons, respectively. The OSL age of 10200 800a and 117800 590a was obtained from the sample at the depth of 9. 7m and 40. 5m. The lithologic section of the borehole ZK12-2 includes the entire Pleistocene and Holocene series and part of Pliocene series. According to the correlation of the polarity column, and the OSL ages from the samples, and the lithologic section, Our definition boundary for the N/Q boundary is approximately conformable with M/G boundary. We believe that the sediments below 706m depth belong to Upper Pliocene series. The lower Pleistocene Series ranges from the depth of 170～706m,which was formed during 0. 78～2. 58Ma. The Middle Pleistocene Series and Upper Pleistocene Series correspond to the Brunhes normal polarity chron. Considering the sedimentation rates, the boundary between Middle and Upper Pleistocene Series is the depth of 66. 5m, and the boundary of the Holocene and Upper Pleistocene Series is 10m.