CHEMOSTRATIGRAPHIC INSIGHTS INTO FLUVIO-LACUSTRINE DEPOSITION, YANCHANG FORMATION, UPPER TRIASSIC, ORDOS BASIN, CHINA

AbstractA 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 elemen...