Abstract: Petrogenesis of lacustrine dolostone is closely related with paleo-lake water conditions. Here we report the high spatial-resolution petrographic and geochemical results of a lacustrine dolomite nodule from the Qingshankou Formation, the Songliao Basin. Sedimentary and elemental signatures confirm the protogenetic origin of this nodule and its effectiveness in recording geochemical characteristics of paleo-lake water during dolomitization. The low Y/Ho ratios, middle rare earth element (MREE) enrichment and subtle positive Eu anomalies within the nodule indicate a fresh water source. However, the Sr isotope values in the core of the nodule (0.7076–0.7080) are close to contemporaneous seawater (0.7074), yet different from the modern river (0.7120) and the host black shale (0.7100). On the premise of excluding the influence of hydrothermal fluids, the significantly low strontium isotope values of the lacustrine dolomite might be caused by seawater interference during dolomitization. Our findings demonstrate that lacustrine dolomite within black shales is not only a faithful tracer of diagenetic water environment, but also a novel and easily identified mineralogical evidence for episodic seawater intrusion event (91 Ma) in the Songliao Basin, which supplements other paleontological and geochemical evidence.