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Chemostratigraphy of the Triassic Yanchang Fluvio-Lacustrine Succession, Ordos Basin, Shaanxi Province, China


A core-based study of a fluvio-lacustrine succession from the southeast Ordos Basin of North-Central China constrains the depositional and hydrographic history of the Triassic lake system. Elemental analysis (using handheld XRF) and mineralogical analysis (using portable XRD) constrain stratigraphic changes in mineralogy and provide insights into the conditions under which organic matter accumulated. In addition, measurements of TOC reveal strong mineralogical controls on organic matter accumulation. The 305-meter-long drill core contains four major (>20m thick) and three minor (10–20m thick) mudrock intervals, with intervening sandstone units. Some units possess interbedded mudrocks and sandstones, but are grouped by their dominant composition. A total of 458 TOC samples were analyzed from the core, at an average sample spacing of 0.6m. Values of TOC generally range between 3% and 6% in the lacustrine mudrock intervals, with some samples reaching 8%. TOC values in the sandstone units average just less than 1%, and based on hand sample observations, may largely reflect wood/charcoal contributions. Hand samples from outcropping fine-grained strata suggest lacustrine macrophytic material may represent a significant proportion of the TOC. A total of 207 XRD samples were analyzed, at an average sample spacing of 1.5m. The dominant mineralogy of the shale units is composed of illite, quartz, albite, kaolinite, and pyrite, with highly variable calcite. The arkosic sandstone units are dominated by albite and quartz, with calcite cement. A total of 2283 major and trace element analyses were undertaken on the slabbed face of the drill core, at an average sample spacing of 0.1m. Major element chemostratigraphic changes largely indicate variations in bulk mineralogy. For instance, the chemostratigraphic pattern of %S is largely reflective of measured stratigraphic shifts in pyrite content, which tends to possess higher concentrations in the mudrocks. This pattern is also observed in the arsenic (As) record, given its chalcophyllic affinity. In general, trace element compositions are indicative of the relative importance of the various minerals, and do not largely reflect variations in the evolution of lake bottom waters (e.g., redox changes). However, the increased pyrite in the mudrock-dominated intervals suggests that bottom waters potentially were at least periodically anoxic.