MARINE TRANSGRESSIONS INTO THE BOHAI BAY LACUSTRINE SYSTEM DURING THE PALEOGENE

The extent of marine influence on the Paleogene Bohai Bay Basin in northeastern China, which accumulated mainly continental (fluvial and lacustrine) facies, remains under debate. Evidence from geochemical, paleontological, and lithological data have documented several marine transgressive events, but no systematic analysis has been undertaken to investigate their timing and frequency. In the present study, we used B, Ga and sulfur (or sulfate) data from the Luo 69 drillcore in the Dongying Depression as proxies to discriminate the salinity of the watermasses (fresh, brackish, salty) in which sediments accumulated. The concentrations of B and S are high and that of Ga low in seawater relative to freshwaters. Our results show increased B/Ga ratios and sulfur concentrations at several levels within the lower part of the Es3L mudstone interval, indicating marine incursions. These geochemical features are associated with increases in dolomite, suggesting that dolomite formation was favored during marine transgressions. Through an integrated analysis of data on the stratigraphic distribution of marine fossils, dolomite, glauconite, and B/Ga and S signals, we conclude that four major marine incursions into the Bohai Bay lake system occurred during the Paleogene. Marine transgressions are also positively correlated with TOC content and organic matter quality, demonstrating their importance for development of petroliferous source rocks in the Bohai Bay Basin.