Organic petrology and geochemistry of Lower Cretaceous lacustrine sediments in the Chaoyang Basin (Liaoning Province, northeast China): Influence of volcanic ash on algal productivity and oil shale formation

Airfall volcanic ash deposited in the form of tuff layers is widely distributed in oil-shale bearing basins in western Liaoning Province, northeast China. In order to investigate the impacts of volcanic ash on algal productivity and the formation of organic-rich oil shales, the organic petrology and organic geochemistry of lacustrine sediments of the Lower Cretaceous Jiufotang Formation were investigated. Molecular geochemical data (the pristane/phytane ratio, the gammacerane index and the relative amount of β-carotane) indicate that they were formed in suboxic/anoxic depositional environments, with salinity fluctuations from freshwater to saline. Oil shales in the semi-deep to deepwater lake facies were mainly deposited under still and anoxic bottom water conditions, concomitant with water column salinity stratification. The relative abundances of diterpenoids and regular steranes indicate a limited terrigenous conifer plant contribution and a predominantly algal contribution to the organic matter. Blooms of algae contributing to high primary productivity, and anoxic depositional environments contributing to organic matter preservation, were key to organic-rich oil shale formation. Fish bones, ostracod shells, and alginite are present below the thick tuff layer. Alginite richness gradually increased from very low just above the tuff layer, to a higher concentration further above the tuff layer, indicating that during major volcanic eruptions, large quantities of volcanic ash led to water column poisoning, causing the sudden mass mortality of some aquatic organisms that were sensitive to turbid, acidified and toxic water. The increased proportion of brackish/saline-water dinoflagellates, which is corroborated by the high concentration of telalginite, abundant dinoflagellate-specific biomarkers (C31 4,22,23,24-tetramethylcholestanes), high values of the C27/C29 ααα-20R and C28/C29 ααα-20R sterane ratios, 4-methylsterane index, dinosterane index, and tetramethylsterane index, suggest that brackish/saline-water dinoflagellates show a positive response to volcanic ash input. Some specific brackish/saline-water dinoflagellates are present in organic-rich tuff laminae, indicating that during periods of minor volcanic eruptions or in intermissions between major eruptions, a minor volcanic ash input was more likely to result in rapid and efficient volcanic fertilisation effects and high-quality oil shale formation. Although the lacustrine oil shale system of the Jiufotang Formation shares some similar petrological and geochemical characteristics with the classical saline lacustrine oil shale system of the Green River Formation in the United States of America, the significant influence of airfall volcanic ash on algal productivity makes the formation mechanism of oil shales within the Jiufotang Formation strata quite different from that of oil shales within the Green River Formation.