Paleoenvironment and Organic Matter Accumulation Mechanism of Shallow to Semi-Deep Lacustrine Shale: A Case Study of Member I of the Qingshankou Formation in the Changling Sag, Southern Songliao Basin

The Member I of the Qingshankou Formation (91.89~90.97 Ma) is the most significant lacustrine shale in the Songliao Basin, and preserves a continuous and complete record of terrestrial paleoenvironmental conditions and organic matter (OM) accumulation during the early Late Cretaceous. Although its abundant OM has traditionally been attributed to deep-lake anoxia, the accumulation mechanism in shallow to semi-deep lake settings remains unclear. To address this gap, samples from the JYY1 well in the Changling Sag were analyzed for TOC, iron species, and major/trace elements. The Al2O3/TiO2 ratios and PAAS-normalized REEs patterns indicate the provenance is predominantly felsic to intermediate rocks, broadly compatible with tonalite. Geochemical proxies (τNa, V/Cr, FeHR/FeT, Fepy/FeHR, B/Ga, Ti/Al, etc.) indicate a relatively humid climate, ferruginous to ferruginous-euxinic bottom waters, and brackish to saline conditions during the Member I deposition. In shallow to semi-deep lacustrine settings, redox conditions and detrital dilution were the principal controls on OM accumulation. The upper interval exhibits higher TOC because of improved OM preservation under ferruginous-euxinic bottom-water and weaker detrital dilution. In contrast, stronger detrital dilution and ferruginous bottom waters contributed to the lower TOC of the lower interval. These findings suggest that widespread pyrite occurrence and elevated Fepy/FeHR ratios may help identify organic-rich shale intervals and guide shale oil and gas exploration.