北京大兴凸起南部PGZ05钻孔剖面第四纪磁性地层学

对北京大兴凸起南部PGZ05孔(孔深255 m)全孔岩心进行了磁性地层、岩石地层、AMS14C测年研究,在系统性的古地磁样品采集(440块岩样)、处理和测试基础上,分析认为255 m的岩心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)。在此磁性年代框架上,结合AMS14C测年及岩性特征,对PGZ05钻孔剖面进行第四纪地层划分,确定下更新统、中更新统、上更新统和全新统的分布深度及年代,将更新统的底界(Q/N)定位于181. 35 m处,亦为一重要岩性(棕红色半固结黏土)界限,中更新统、上更新统、全新统的底界分别定位于75. 60 m、57. 20 m、17. 35 m处。研究成果为大兴凸起南部第四纪地层划分与对比、古地理环境演变及区域地层研究提供了可靠的地层年代学框架,具有重要地质意义。

Magnetostratigraphy of Borehole PGZ05 in Southern Daxing Uplift,Beijing Plain

Quaternary deposits in the southern Daxing uplift in the Beijing Plain provide an important archive for Quaternary stratigraphic changes. Here we present the magnetostratigraphic results of borehole PGZ05 (255 m). A total of 440 undisturbed samples were collected from the borehole at a 0.2-0.5 m interval except for the sand and gravel lay. The results show that the Brunhes/Matuyama and Matuyama/Gauss boundaries occur at 75.60 m and 181.35 m deep, respectively. The Gauss normal polarity chron occurs at 181.35 to 255.00 m deep. The 75.60 to 181.35 m interval is fairly correlative with the Matuyama reversal in the geomagnetic polarity timescale, with two normal polarity zones corresponding to the Jaramillo (90.90-100.60 m) and Olduvai (149.70~165.50 m) subchrons, respectively. The AMS¹⁴C age of (2,370±30) a BP and (23,190±100) a BP were obtained from the samples at 12.20 m and 31.45 m deep. The lithologic section of the borehole PGZ05 includes the entire Pleistocene and Holocene sequences and part of the Pliocene sequence. According to the M/G boundary and lithologic section of the borehole, we suggest that the N/Q boundary is located at 181.35 m deep, and it represents an important lithological interface. We propose that the sediments below 181.35 m depth belong to the Upper Pliocene series. The Lower Pleistocene sequence occurs at 75.60-181.35 m deep and were deposited during 0.78-2.58 Ma. The boundary between the Middle and Upper Pleistocene sequence is located at 57.20 m deep, whilst the Holocene-Upper Pleistocene boundary is located at 17.35 m deep.