Marine incursion events in the Late Cretaceous Songliao Basin: Constraints from sulfur geochemistry records

Songliao Basin in NE China developed the most productive oilfield in the world sourced from terrestrial rocks. The main source rock of the basin includes member 1 of the Qingshankou (K2qn1) and members 1 and 2 of the Nenjiang Formation (K2nj1–2). However the exact reasons for the formation of the source rock, especially the K2qn1 are still controversial. Former paleontological and organic geochemical research suggested that organic matter was deposited during marine incursion events of K2qn1 but further geochemical evidence is needed. This paper explores the distinct sulfate levels that distinguish marine from fresh waters of the Songliao paleo-lake. We undertook a systematic investigation the sulfur geochemistry of the sediments from top of member 4 of the Quantou Formation (K1q4) to top of member 1 of the Qingshankou (K2qn1). The ratio of organic carbon to the pyrite sulfur (TOC/PYS) proved the previously suggested saline conditions during the deposition of K2qn1; and the pyrite sulfur isotope indicated that marine incursion may not only have occurred for K2qn1, but also for top of K1q4. The exact time for the beginning of marine incursion is to be determined. The marine incursion within K2qn1 is not strong and partially controlled by the fluctuation of paleo-lake level. The present study will be helpful for understanding the contribution of marine incursion events to the burial of organic carbon in the Songliao paleo-lake, although further studies are still needed.