川北—鄂西地区下志留统龙马溪组上段厚层斑脱岩的新发现及地质意义

基于重庆市城口地区明中剖面上奥陶统五峰组—下志留统龙马溪组露头，对川北—鄂西地区埃隆阶—特列奇阶斑脱岩的发育特征开展系统研究，新发现多套厚层斑脱岩层。对其地质意义和科学价值进行分析，得到3点认识：①城口地区明中剖面共发现7个斑脱岩密集段（下志留统埃隆阶—特列奇阶新发现3个斑脱岩密集段），主要赋存于上奥陶统凯迪阶—下志留统特列奇阶的7个笔石带中；②埃隆阶—特列奇阶的斑脱岩密集段主要为单层、厚度超过5 cm的厚层斑脱岩，且大多出现于黏土质含量较高的层段，在自然伽马曲线上呈现峰值响应，其发育特征及自然伽马响应在川北—鄂西地区具有普遍性，可以作为该地区龙马溪组上段重要的地层对比界面；③根据龙马溪组上段斑脱岩密集段的分布特征，川北—鄂西地区在前陆挠曲发展期至少经历了埃隆阶早期、埃隆阶晚期和特列奇阶早期3次强烈的火山喷发和前陆挠曲活动，导致沉降-沉积中心向西、向北相应发生了3次大规模迁移，这使得扬子台盆区北缘海域的封闭性变弱，上升洋流大规模涌入川北—鄂西海域。川北—鄂西地区埃隆阶—特列奇阶的富有机质页岩沉积是斑脱岩密集段Ⅴ出现前后前陆挠曲和上升洋流共同控制的产物，页岩的层位随着斑脱岩密集段Ⅵ—Ⅷ的发育而向西逐渐变新；斑脱岩密集段Ⅴ与Ⅵ之间的火山活动间歇期是埃隆阶上升洋流相富有机质页岩沉积的主要时期。

A new discovery and geological significance of thick-layered bentonites in the Upper Member of Lower Silurian Longmaxi Formation in the Northern Sichuan-Western Hubei area

Based on the outcrops of the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in the Mingzhong profile of Chengkou area, Chongqing municipality, this paper carries out a systematic study on the development characteristics of bentonites in the Aeronian-Telychian strata in the Northern Sichuan-Western Hubei area, and discovers multiple sets of thick-layered bentonites. By analyzing its geological significance and scientific value, three understandings are obtained. (1) A total of seven intensive bentonite intervals have been discovered in the Mingzhong profile of Chengkou area (including three newly discovered intensive bentonite intervals in the Lower Silurian Aeronian-Telychian strata), which mainly occur in the seven graptolite zones from Upper Ordovician Katian to Lower Silurian Telychian. (2) The intensive bentonite intervals in the Aeronian-Telychian strata are mainly single-layered thick bentonite with a thickness over 5 cm, and mostly appear in the sections with high clay contents, showing a peak on the natural gamma-ray curve. These development characteristics and gamma ray responses are universal in the Northern Sichuan-Western Hubei area, so those intervals can be used as important stratigraphic correlation interfaces in the Upper Member of Longmaxi Formation. (3) According to the distribution characteristics of intensive bentonite intervals in the Upper Member of Longmaxi Formation, the Northern Sichuan-Western Hubei area at least experienced three intense volcanic eruptions and foreland flexures in the Early Aeronian, Late Aeronian, and Early Telychian stages, leading to the corresponding westward and northward large-scale migrations of the subsidence center and depocenter for three times. As a result, the sea area in the northern margin of Yangtze craton becomes less closed, and upwelling currents flowed into the Northern Sichuan-Western Hubei area on a large scale. The organic-rich shale deposits in the Aeronian-Telychian strata in the Northern Sichuan-Western Hubei area are the products controlled by foreland flextures and upwelling currents before and after the occurrence of the intensive bentonite interval Ⅴ. The shale horizon gradually becomes newer westward with the development of the intensive bentonite interval from Ⅵ to Ⅷ. The intermittent period of volcanic activity between the intensive bentonite interval Ⅴ and Ⅵ is the main deposition period for the organic-rich shale of the Aeronian upwelling current facies.