北京平原区东南缘钻孔TB02第四纪地层划分与沉积特征

第四纪地质对开发利用第四纪资源和城市地质安全具有重要意义。基于在北京市通州区南部永乐店镇实施的第四系钻孔TB02,通过系统的岩芯观察和编录、测井资料分析、AMS ¹⁴C测年和古地磁测年等手段,结合粒度参数指标特征,对钻孔TB02进行年代地层划分,并对沉积特征进行了探讨。结果表明:北京平原区东南缘第四系底界埋深为276.21 m,中更新统底界埋深为97.75 m,上更新统底界埋深为39.32 m,全新统底界埋深为6.40 m; 钻孔TB02沉积物以细粒物质为主,具有较好的分选性,呈现远源沉积的特征; 沉积水动力环境在2 588、1 539及781 ka发生了3次比较明显的变化。第四纪以来,北京平原区东南缘环境演化规律表现为:早更新世早期,沉积水体快速加深,以湖相沉积为主,沉积环境稳定且水动力较弱; 早更新世晚期,主要发育河流沉积,陆源碎屑供给较充足,沉积水动力较强; 中更新世以来,为泛滥平原沉积,水动力减弱,以沉积细粒物质为主,沉积地貌逐渐定型,形成了广阔的平原区。

Quaternary Stratigraphic Division and Sedimentary Characteristics of Borehole TB02 in the Southeast Margin of Beijing Plain,China

Quaternary geology is important to the development of Quaternary resources and urban geological safety. The Quaternary geological borehole TB02 in Yongledian town, the southern Tongzhou district, Beijing was selected. Based on systematic core observation and cataloging, logging data analysis, the characteristics of grain size, together with AMS ¹⁴C and paleomagnetic dating, the chronostratigraphic division was carried out, and the sedimentary characteristics were discussed. The results show that the bottom of Quaternary is located at the depth of 276.21 m; the boundaries of Lower and Middle Pleistocene, and Middle and Upper Pleistocene are located at 97.75 and 39.32 m, respectively; the bottom of Holocene is located at the depth of 6.40 m. Grain size analysis shows that the sediments are mainly fine particle components, display good sorting properties, and present the characteristics of far-source deposition; the sedimentary hydrodynamic environment has changed significantly at 2 588, 1 539 and 781 ka for three times. Environmental evolution of the southeast margin of Beijing plain is as following: In the Early Pleistocene, the paleogeography sedimentary characteristics of the whole plain is dominated by stable lake sedimentary environment with weak hydrodynamic force; in the late stage of Early Pleistocene, the whole plain forms a braided river environment, and sufficient terrigenous detritus supply and strong construction capacity; since the Middle Pleistocene, the plain enters a peneplain process with weak hydrodynamic force and mainly fine-grained sediments, and the sedimentary geomorphology gradually finalizes, forming a broad plain area.