Identifying marine incursions into the Paleogene Bohai Bay Basin lake system in northeastern China

The extent of marine influence on the Paleogene Bohai Bay Basin in northeastern China, which accumulated mainly continental (e.g., fluvial and lacustrine) facies, remains under debate. Evidence from geochemical, paleontological, and lithological data have documented several marine incursions, but no systematic study has been conducted to investigate their timing and frequency and their significance for understanding the Paleogene tectonic evolution of northeastern China. In order to address the marine influence on sediment accumulation and to identify variations in paleosalinity during deposition of the mid-Eocene lower Es3 (Es3L) Member of the Shahejie Formation, this study analyzed multiple paleosalinity proxies (B/Ga, Sr/Ba, and S/TOC) in fine-grained siliciclastic (argillaceous) sediments of the Luo-69 drillcore from the Dongying Depression. All three proxies yielded similar paleosalinity interpretations, with peak salinity and marine influence (i.e., maximum B/Ga, Sr/Ba, and S/TOC values) occurring at the base of the Es3L Member, and progressively declining salinities, representing a shift toward brackish conditions, upsection over an interval of 185.5 m representing similar to 2.2 Myr of sedimentation. This paleosalinity record suggests gradual long-term variations that were probably due to changes in tectonically controlled gateways linking the Bohai Bay Basin to the Yellow Sea. In addition to analysis of the mid-Eocene Es3L Member, published data on the stratigraphic distribution of marine fossils, glauconite, and elemental paleosalinity proxies are applied to evaluate the history of marine incursions into the Bohai Bay lake system throughout the mid to late Paleogene (similar to 50-24 Ma). This analysis revealed four intervals of probable major marine incursions dating to the early Eocene, the middle Eocene, the early Oligocene, and the late Oligocene.