Ostracod Biostratigraphy, Paleoecology, and Their Response to Lake-Level Changes in the Lower Nenjiang Formation (Upper Cretaceous) from Borehole J79, Songliao Basin, NE China

Cretaceous paleoenvironmental reconstructions have relied heavily on marine records, whereas lacustrine deposits, though offering valuable high-resolution archives for non-marine environmental changes, remain underexplored. The Songliao Basin, the largest Mesozoic continental basin in East Asia, preserves a continuous succession of ostracod-rich sediments in its lower Nenjiang Formation, deposited during the largest Late Cretaceous lacustrine transgression (Santonian–early Campanian), yet the ostracod fauna and their ecological responses to lake-level fluctuations are poorly constrained. Borehole J79, on the Western Slope of the basin, yields abundant and highly diverse ostracod fossils from the lower Nenjiang Formation (members K2n1 and lower K2n2), making it ideal for investigating ostracod evolution in response to lake-level fluctuations. Through taxonomic, biostratigraphic, and morpho-functional analyses of shell ornamentation and carapace shapes, we identified 21 species belonging to 7 genera and established 3 biozones: the Cypridea anonyma–C. squalida, C. gunsulinensis–C. ardua, and C. stellata–C. liaukhenensis zones (in ascending order). The fauna is overwhelmingly dominated by Cypridea. Three ornamentation types (deep-pitted, shallow-pitted, and nodose) and four shell morphotypes (oval, reniform, triangular, and trapezoidal) were recognized, revealing a stratigraphic shift from deep- to shallow-water adaptations. The observed ostracod assemblages define three paleoenvironmental stages: rapid transgressive deepening (lower K2n1), lake-level fluctuations (middle–upper K2n1), and another major transgression (lower K2n2). Overall, ostracod evolution in the Songliao Basin is tightly coupled with lake-level dynamics, recording a deepening–shallowing–deepening–shallowing cycle that reflects the basin’s transgressive–regressive history.