Late Cretaceous-early Paleocene Ostracod Biostratigraphy And The Relationships Between The Stable Isotopic Compositions Of Ostracod Shells And The Paleoenvironments In The Songliao Basin, Nearthest China

The Cretaceous biostratigraphy of the Songliao basin in Northeast China has beenwell suited for the petroleum investigation. Widespread deposits in the upper part ofthe basin are mainly composed of clastic sediments which contain abundant fossilsincluding gastropods, bivalves, ostracods, charophytes, spone and pollen amongothers. These well preserved fossils provide us valuable informations about pastclimate changes and biotic responses in a greenhouse environment. However, despitemuch research having been conducted in the basin, the Late Cretaceousbiostratigraphy and palaeoenvironment are still less understood. Here, we are trying toestablish a high-resolution biostratigraphy of the Late Cretaceous based on ostracodsfrom borehole SK1(n) in the Songliao basin, Northeast China. As part of the presentstudy,45species assigned to20genera have been recovered, with one new species(Ilyocypris bisulcata sp. nov.) and5ostracod assemblages: the Cyprideagunsulinensis-Mongolocypris magna assemblage; the Ilyocyprimorpha-Limnocypridea sunliaonensis-Periacanthella assemblage; the Strumosia inanditaassemblage; the Talicypridea amoena-Metacypris kaitunensis-Ziziphocyprissimakovi assemblage; and the Ilyocypris assemblage. Moreover, the zonal fossilIlyocypris bisulcata sp. nov. of Zone5is here described for the first time from theupper Mingshui Formation, and Paleocene charophyte genera including Neochara andGrovesicahra have been found to coexist with the Zone5fauna. The first appearanceof Ilyocypris bisulcata sp. nov. possibly marks the K/Pg boundary. The IlyocyprisAssemblage coexisting with Late Cretaceous elements, and therefore it is assigned tothe transition biota from latest Maastrichtian to the earliest Paleocene.Base on study of the oxygen and carbon isotopes, we suggest that the songliaoBasin was a relatively open lake during the deposition of Sifangtai and Mingshuiformations, and the values of oxygen and carbon gradually increased about2‰fromthe Sifangtai to the Mingshui Formation. Suddenly, the values delined around thedepth of335m where the Ilyocypris genus appeared, which might be correlated with the K/Pg boundary and caused by the environmental changes during the time.