太原盆地晚新生代沉积物元素地球化学特征及古环境意义——以清徐ZK01钻孔为例

沉积岩的主、微量元素蕴含了大量的地质信息, 在古环境、古气候变化的研究中, 占据着极其重要的地位。本研究以太原盆地ZK01钻孔为对象, 通过对长度为853.5m岩芯的主、微量元素和元素比值(如化学风化指数CIA、Al₂O₃/Na₂O、MgO/CaO、Rb/Sr、Cu/Zn、V/(V+Ni)、V/Cr)变化特征进行分析, 旨在探讨太原盆地古环境、古气候变化。研究发现, 主、微量元素变化特征指示了太原盆地具有Al₂O₃、Na₂O、Ni、Cr、Cl、S亏损, CaO、Pb、N、Rb明显富集的特征; 通过对CIA指数研究表明, 太原盆地地层和大陆上地壳(UCC)及陆源页岩的风化强度序列为: 下土河组一段>红崖组>下土河组二段>陆源页岩>小白组>木瓜组>汾河组>大沟组>UCC; 从Cu/Zn、V/Cr、V/(V+Ni)元素比值变化可以判断研究区环境自下而上表现为弱还原性-弱氧化性的旋回, 且以贫氧为主; 又根据CIA、Al₂O₃/Na₂O、MgO/CaO、Rb/Sr变化特征可将盆地气候划分为3个阶段: 中新世晚期(8.1~5.3Ma), 气候变化由寒冷干燥转向温暖湿润; 上新世-早更新世(5.30~0.78Ma), 气候以温暖湿润为主, 5.30~2.58Ma气候温暖湿润, 2.58~1.64Ma气候相对稳定, 1.64~0.78Ma寒冷干燥与温暖湿润气候交替出现; 中更新世-全新世(0.78~0Ma), 基本为温暖湿润气候。研究表明, 研究区气候变化不仅反映了东亚季风由上新世前不显著至上新世中逐步建立, 直至第四纪早期稳定建立的动态过程, 还反映出青藏高原隆升等构造活动对其造成的影响。

Major and trace elements geochemical characteristics and paleoenvironmental implications of borehole ZK01 in Taiyuan Basin of the North China

Taiyuan Basin is one of the large rifted basins in Shanxi faulted basins, and serves as an important heavy industry base in the North China region. Reconstructing its Late Cenozoic palaeoenvironment and paleoclimate can not only better study the climate evolution in Shanxi Province, but also help to understand the Late Cenozoic weather characteristics in the North China, Central Asia and even the global scale. According to the stratigraphic divisions of geomagnetic data, the Late Cenozoic sediments revealed by hole ZK01 in Taiyuan Basin can be classified into 6 lithologic groups(members)from profile bottom to top: The first member(N1x1)and the second member(N1x2)of the Miocene Xiatuhe Formation, Pliocene Xiaobai Formation(N2xb), Hongya Formation(N2h); Lower Pleistocene Dagou Formation(Qp1d), Mugua Formation(Qp1m); Pleistocene-Holocene Fenhe Formation(Qf1), and through magnetic stratigraphic methods determine the age of the bottom boundary of the Late Cenozoic strata in this area to be 8.1Ma. The Sr content as well as the Cu/Zn, V/Cr, and V/(V+Ni)ratios suggest that the chemical distributions showing a certain layering in the Late Cenozoic sediments, and the sedimentary environment in study area shows a cycle of weak reduction and oxidation from bottom to top which is mainly oxygen-poor. The CIA values of all strata in the Taiyuan Basin are higher than those of the continental upper crust(47.92), whilst the CIA values of the terrigenous shale, Fenhe, Mugua, and Xiaobai formations are significantly lower than those of terrigenous shale. In addition, the CIA values of the second member of the Xiatuhe Formation, the Hongya Formation, and the first member of the Xiatuhe Formation are larger than those of the terrigenous shale. Hence it can be known that the sequence of weathering intensity is: the first member of the Xiatuhe Formation>Hongya Formation>the second member of the Xiatuhe Formation>Terrigenous Shale>Xiaobai Formation>Mugau Formation>Fenhe Formation>Dagou Formation>UCC(Upper Continental Crust). According to the elemental ratios, the mean values of MgO/CaO and Rb/Sr in each layer are 0.44, 0.38(Fenhe Formation); 0.37, 0.32(Mugua Formation); 0.64, 0.26(Dagou Formation); 0.73, 0.30(Hongya Formation); 0.46, 0.33(Xiaobai Formation); 1.19, 0.36(the second member of the Xiatuhe Formation); 1.24, 0.47(the first member of the Xiatuhe Formation). The climate evolution of Taiyuan Basin can be roughly divided into three stages: During the Pliocene(5.3~8.1Ma), the Pliocene-Early Pleistocene(0.78~5.30Ma), Middle Pleistocene-Holocene(0.78~0Ma), and the Miocene(5.3~8.1Ma), the climate changed from cold and dry to warm and humid. The Pliocene-Early Pleistocene(0.78~5.30Ma)is mainly warm and humid, with frequent fluctuations in the elemental curve at 5.30~2.58Ma. Stable climate characteristics in 2.58~1.64Ma, and cold-dry and warm-humid climates in 1.64~0.78Ma, appeared alternately, with two short periods of cold and dry climate in the Hongya Formation(about 390.00m)and Dagou Formation(at 203.65m). Since the Middle Pleistocene(0.78Ma), a cold and dry climate has appeared, whereas the whole Middle Pleistocene is basically warm and humid climate. These variations reflect the dynamic process of the East Asian monsoon from insignificant before the Pliocene to the gradual establishment in the Pliocene to the stable establishment in the Early Quaternary, which may be related to the uplift of the Qinghai-Tibet Plateau.