Late Cretaceous-early Paleocene Charophytes From Songliao Basin, North China: SK1(N) Core

Songliao Basin is one of the biggest lacustrine systems in Asia during Cretaceoustime. Well-preserved Cretaceous lacustrine deposits provide us with unique materialto study the paleoenvironment and paleoclimate of this period. Widespread deposits inthe basin are mainly composed of clastic sediments which contain abundant fossilsincluding charophytes, gastropods, bivalves, ostracods and vertebrates.The drilling program (SK1) has yielded2485.89m of continuous cores (96.46%recovery) and provides significant charophytes material for Cretaceous research. Thefossil record of Charophyta revealed in this study includes46organ-species and6indeterminate organ-species in27genera. After detailed biostratigraphic study ofcharophytes,4assemblages have been recognized in Sifangtai and MingshuiFormations: Atopochara ulanensis-Hornichara anguangensis assemblage,Atopochara ulanensis-Raskyaechara gobica assemblage, Hornicharaprolixa-Gobichara desert assemblage and Grovesichara changzhouensis-Neocharasinuolata assemblage. Based on charophyte assemblages, the Sifangtai Formation islimited to middle Campanian; and the Mingshui Formation is late Campanian toMaastrichtian in age. The appearance of gyrogonites from the4th assemblage isdifferent from others, which is characterized by its larger size, curved ornamentationson cellular spirals and apical rosette. It is likely that the upper part of the MingshuiFormation belongs to Paleocene, the K/Pg boundary is, therefore, within the upperpart of the Mingshui Formation. What’s more, we have discovered a new specie ofostracoda, Ilyocypris sp., which is of great significance.The study of stable C and O isotopes in lacustrine carbonates has led to insightsinto past climates. And we have collected some ostracodes and charophytes fossils tostudy for stable isotopes. Some experts have proved that the oxygen isotope ofcharophyte gerogonites is in equilibrium with that of water in which they live in for agiven temperature. Therefore, we can use them to elucidate the geochemistry of theancient water body in which they lived.