Chemostratigraphic insights into fluvio-lacustrine deposition, Yanchang Formation, Upper Triassic, Ordos Basin, China

A chemostratigraphic study of a 300 m long core recovered from the southeastern central Ordos depocenter reveals thick (< 10-65 m) intervals of fine-grained, organic-rich lacustrine strata, interpreted to represent deepwater deposition under meromictic conditions (incomplete watermass overturn) during lake highstand phases, interspersed with thick (10-30 m) intervals of arkosic sandstones, reflective of fluvio-deltaic deposition during lake lowstand phases. Along with elevated concentrations of % Al, traditionally a proxy for clay content, maximum total-organic-carbon (TOC) values in the deepwater lacustrine facies reach 8%, with average values of approximately 3%. The fine-grained, organic-rich facies is also characterized by elevated S (up to 6%) and As concentrations, both proxies for pyrite, an indicator of more stagnant, reducing conditions in the hypolimnion (the volume of the lake below the surface mixing zone) during lake highstand phases. Enrichment factors for redox-sensitive trace elements (RSTEs) are not significantly elevated throughout the TOC-rich intervals, but they are punctuated in thinner intervals. Punctuated enrichments in RSTEs reflect episodes of enhanced suboxia/anoxia in the hypolimnion during lake highstand phases resulting from sustained meromixis (protracted episodes of incomplete watermass turnover). Although dramatic shifts between fluvio-deltaic and deepwater lacustrine deposition are recorded in the Yanchang strata, no evidence indicates that the lacustrine system ever built up a significant salt concentration, suggesting that an outlet was maintained throughout much of the depositional history. A chemofacies framework for the Yanchang Formation is developed based on hierarchical cluster analysis and ranking of major element chemostratigraphic results. The outcome of chemofacies analysis is similar to the lithofacies analysis, subdividing the stratigraphic record of calcite-cemented sandstone/siltstone lithologies (fluvio-deltaic facies) and silty to finer grained mudstone lithologies (deep-water lacustrine facies) based on changes in elemental concentrations that equate to shifts in the relative abundance of mineral contents (e.g., clays, quartz, feldspars, pyrite, and carbonates).