The Songliao Basin, spanning approximately 260,000 km2 in Northeast China, is one of the largest oil-bearing lacustrine basins. Seawater incursion events have previously been discovered in the Turonian (lower and the uppermost Qingshankou Formation) to the Santonian and early Campanian (the first and second members of the Nenjiang Formation). We newly discovered benthic and planktonic foraminifera in the middle Campanian to Maastrichtian (Fifth Member of the Nenjiang Formation, Sifangtai, and Mingshui formations) in the ZKD-1 borehole. The Planktonic foraminifera include Archaeoglobigerina blowi, A. cretacea, and Globotruncanella sp. The benthic foraminifera include Anomalinoides sp., Gavelinella sp., and Haplophragmoides sp. Among them, A. blowi, A. cretacea, Anomalinoides sp., Gavelinella sp., Haplophragmoides sp. are widely distributed in the marine strata of the Upper Cretaceous. We tested the marine biomarker compounds including C30 24-n-propyl cholestane and C30 24-iso-propyl cholestane, indicating that the Songliao Basin experienced seawater incursion events to varying degrees during the middle to late Campanian. We also found abundant charophyte fossils in the ZKD-1 borehole, including Atopochara trivolvis ulanensis, Latochara yuananensis, Feistiella anluensis, Mesochara biacuta, M. gobica, and M. leiocarpa. Among them, the brackish water charophyte fossils of Feistiella anluensis were first found in Songliao Basin. (c) 2025 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights are reserved, including those for text and data mining, AI training, and similar technologies.