The largest lake transgression event (LTE) associated with lake anoxic events (LAE) and periodic seawater incursion events (SWIE) in the Songliao Basin, northeastern China, occurred during deposition of the Cretaceous Nenjiang Formation. The Yaojia-Nenjiang Formation boundary (YNB) marks the beginning of the LTE, as well as LAE and SWIE. However, there is an absence of direct radioisotopic dating, and therefore the age of the YNB, as well as the beginning of LTE, together with their relationship with other geological events, is strongly debated. Here we present a new SIMS U-Pb zircon age from the lowermost Nenjiang Formation. The bentonite bed located 9.88 m above the YNB of the X1-4 borehole was analyzed. Twenty-five analyses of 25 zircons were conducted, which produced a weighted mean age of 85.5±0.6 Ma (MSWD = 0.87). Based on the average sediment accumulation rate, the age of the YNB is suggested to be 85.7 Ma, indicating that the LTE began in the Early Santonian. The new ages provide a precise chronostratigraphic framework for climatic and geological events. Our new results imply that the beginning of the LTE, LAE and SWIE occurred almost simultaneously with short-term sea level rise, and probably had a close relationship with OAE3.

The Lower Cretaceous Dalazi Formation in the Yanji Basin, eastern Jilin Province is of particular interest because it contains key fresh water fossil taxa, oil and gas resources, a potential terrestrial Albian–Cenomanian boundary, and regional unconformities. However, the lack of a precise chronology for the non-marine strata has precluded a better understanding of the regional stratigraphic correlation and terrestrial processes. Here, we report magnetostratigraphic and U–Pb geochronologic results of a sedimentary sequence from the Xing’antun section in the Yanji Basin. Thirty-two zircons from the tuff sample were analyzed by secondary ion mass spectrometry (SIMS); the U–Pb zircon dating method yielded a weighted mean of ²⁰⁶Pb/²³⁸U age of 105.7 ± 0.8 Ma (2σ, internal error). Paleomagnetic results show that the Dalazi Formation is of normal polarity, which was correlated to the early chron C34n constrained by the SIMS U–Pb zircon geochronologic data, further demonstrating that the terrestrial sedimentary sequence of the upper Dalazi Formation is of late Albian age. The established geochronologic framework allows the regional correlation of the Dalazi Formation in the Yanji Basin to the strata from other terrestrial sequences in northeastern China. The similar geodynamic and geologic background between the Yanji Basin and other terrestrial rift basins in northeastern China suggests that the unconformity between the Dalazi and Longjing formations may represent syn-rift and post-rift stages in the Yanji Basin, and thus the switch from extension to contraction during the mid-Cretaceous, precisely constrained to ~106–101 Ma based on our new chronology and previously published high-precision U–Pb dating of the lower Longjing Formation. It is most likely attributable to the docking of the west Pacific plate along the East Asian continental margin.

Despite the abundant literature on limnic Cretaceous ostracode faunas, the database on mid-Late Cretaceous taxa is still scarce. The Songliao Basin in northeastern China preserves a diverse assemblage of ostracode fossils from the Santonian–Campanian Nenjiang and Sifangtai formations. This rather unique material is of major importance to comprehend Early to mid-Late Cretaceous (Cenomanian–Campanian) limnic ostracode faunas and therefore the evolution of Late Cretaceous basins in China. A study of this fauna was conducted to detail the taxonomy of ostracode species from members 1 and 2 of the Nenjiang and Sifangtai formations in the Songliao Basin. Well-preserved specimens of 14 ostracode species were recovered from samples of the ZKY2-1 well in southwestern Songliao Basin:Scabriculocypris liaukhenensisLiu in Netchaeva et al., 1959,Ilyocyprimorpha netchaevaeSu in Netchaeva et al., 1959,Cypridea accliniaNetchaeva in Netchaeva et al., 1959,Cypridea cavernosaGaleeva, 1955,Cypridea gracileNetchaeva in Netchaeva et al., 1959,Cypridea lepidaYe in DOFEAD, 1976,Cypridea squalidaSou in Netchaeva et al., 1959,Fabaeformiscandona?Disjuncta(Hao in Hao et al., 1974),Lycopterocypris profundaLübimova, 1956,Mongolocypris magna(Hou, 1958),Mongolocypris tera(Su in Netchaeva et al., 1959),Talicypridea obliquecostae(Szczechura and Błaszyk, 1970),Talicypridea reticulata(Szczechura, 1978) andRenicypris renalata(Su in Hao et al., 1974). Four of the species identified received emended descriptions and diagnoses. Other important results include the first discussion on sexual dimorphism inCypridea accliniaandFabaeformiscandona?disjuncta, the first study on the ontogeny ofIlyocyprimorpha netchaevae, and the reassignment ofCypridea liaukhenensisto the genusScabriculocypris. Several species recorded herein are also found in other continental far-eastern Asian basins, while genera range from worldwide to far-eastern Asian distribution; these results support strong affinities among faunas of Mongolia, China, and Japan.