CHEMOSTRATIGRAPHIC INSIGHTS INTO FLUVIO-LACUSTRINE DEPOSITION, YANCHANG FORMATION, UPPER TRIASSIC, ORDOS BASIN, CHINA (Invited Presentation)

The Yanchang Formation of the Ordos Basin, North-Central China, represents the depositional remnants of an aerially- and temporally-extensive fluvio-lacustrine system of late Triassic age. A chemostratigraphic study of a 300-meter-long core recovered from near the central axis of the Ordos depocenter reveals thick (<10 to 65 m) intervals of fine-grained, organic-rich lacustrine strata, interpreted to represent deep-water deposition under meromictic conditions during lake highstand phases, interspersed with thick (10-30 m) intervals of arkosic sandstones, reflective of deltaic deposition during lake lowstand phases. Along with elevated concentrations of %Al, a proxy for clay content, maximum TOC values in the deep-water lacustrine facies reach 8%, with average values of ~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 during lake highstand phases. Enrichment factors for redox-sensitive trace elements (RSTEs) are not significantly elevated throughout the TOC-rich intervals, but 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. While bulk rock C/N values are overall higher in the deep-water lacustrine facies, potentially indicating enhanced nitrogen remineralization to the water column, the stable isotopic composition of TOC (d¹³C_TOC) does not define distinct signatures between fluvio-deltaic and deep-water lacustrine facies, signifying a complex origin for bulk organic matter. 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.