鄂尔多斯盆地南缘上二叠统——中下三叠统地球化学特征及其古气候、古环境指示意义

为探索陆相湖泊环境记录中二叠纪—三叠纪之交古气候演化的信息,以鄂尔多斯盆地南缘陕西铜川石川河剖面上二叠统石千峰组(P3s)和三叠系的刘家沟组(T1l)、和尚沟组(T1h)、纸坊组(T2z)为研究对象,对界面上下地层开展了系统的矿物学、岩石学和地球化学研究。通过主量和微量元素、碳氧同位素以及TOC测试对古盐度、古氧化还原环境和古气候演化规律进行分析。测试结果显示在二叠系—三叠系(P/T)界线附近主、微量元素和碳、氧同位素发生较明显的变化,气候环境代用化学指标的波动指示了从晚二叠世至早中三叠世鄂尔多斯盆地的古气候、古环境的变化。研究结果表明上二叠统石千峰组形成于河流-三角洲沉积环境,氧化程度相对低,古气候相对温暖湿润;下三叠统刘家沟组与和尚沟组的紫红色砂泥岩代表水体较浅的河流-三角洲相,强氧化环境,气候干旱炎热;而到中三叠统纸坊组沉积期,湖平面上升,元素的迁移作用加强,氧化程度变弱,气温降低,气候转为半干旱半湿润气候。碳同位素分析结果显示,鄂尔多斯盆地陆相P/T界线上δ^13CPDB存在显著负偏,与全球范围内的海相P/T界线具有一致性,说明在华北地台陆相P/T界线上同样存在气候突变和生物灭绝等重大地质事件的沉积记录,与海相地层可对比。

Geochemical Characteristics and Paleoclimatic Implications of the Upper Permian and Middle-Lower Triassic Strata in Southern Ordos Basin

To explore the paleoclimate evolution of the terrestrial Permian-Triassic system, mudstone samples were collected from the Permian Shiqianfeng Formation (P₃s) and the Triassic Liujiagou (T₁l), Heshanggou (T₁h) and Zhifang (T₂z) formations in the Shichuanhe section of Tongchuan (Shaanxi Province) in the southern Ordos Basin. The samples were analyzed for their mineralogy, petrology, and geochemistry. The paleosalinity, paleoredox and paleoclimate evolution were studied by the major-trace elements, total organic carbon (TOC), and carbon and oxygen isotopes. It is shown that the geochemical and carbon-oxygen isotope compositions near the P/T boundary had changed greatly. Fluctuation of climatic and environmental chemical indicators reflects the paleo-climatic/-environmental changes in the Ordos Basin from the Late Permian to the Middle-Late Triassic. It is shown that the Shiqianfeng Formation was deposited in a fluvial-deltaic environment that was relatively reduced, and that the paleoclimate was relatively warm and humid. The purplish-red clastic rock of the Lower Triassic Liujiagou and Heshanggou formations were deposited in shallow water, hot and arid fluvial-deltaic environment under strongly oxidizing conditions.During the deposition of the Middle-Triassic Zhifang Formation, the water level rose and chemical transfer was enhanced, and the depositional environment may have been semi-arid/-humid and reducing. Carbon isotope evidence shows that the P/T boundary in the terrestrial Ordos basin has significant negative δ¹³C_PDB features, which are comparable with the global marine P/T boundary. This indicates that major geological events such as abrupt climate changes and mass extinctions were also recorded in the North China continental P/T boundary, consistent with the marine stratigraphic record.