Mid-Cretaceous Biostratigraphy And Palaeoclimate Change From The Qingshankou And Nenjiang Formations In Nong'an Area, Jijin Province

The Songliao Basin is a large Mesozoic and Cenozoic sedimentary basin yielding abundant fossils, such as: sporopollen, ostracods, Esthenaus. Sporopollen, ostracods samples were collected and identified from the Lijiatuozi and Houjingou sections in the Qingshankou area. By lithology and biotic characters, the section has been correlated, the paleoeclogy and paleoclimate have been analyzed. The following results have been achieved:The Balmeisporites- Cyathidites-Classopollis sporopollen assemblage, Cypridea dekhoinensis- Limnocypridea copiosa ostracod assemblage, and Nemestheria qingshankouensis -Cratostracusmerus merus Esthenaus assemblage have been recognized from the Lijiatuozi section. Lithologically, the section has been correlated as lower Member 2 of the Qingshankou Formation, which differrs from the previous research refferring it to the Member 1 of Qingshankou Formation. Based on the ostracod paleoeclogical features such as morphology, abundance, diversity, prevervation, and on the features of sporopollen such as climate distribution, humidity type, combined with characteris of sedmentation, key minerals and fossil Esthenaus, the paleoeclogy, paleoenvironment and paleoclimate have been constructed. According to the analysis and statistics of sporopollen, the assemblage implies mild, humectation, tropical to south subtropical climate. In the early sedmentary period of the Member 2 of Qingshankou Formation, the paleoenvironment is suggested as shallow-sub deep lake, which was stable, rich in oxgen and Ca²⁺, indicating a pattern of humid and hot climate.Sporopollen, ostracods samples were collected from the Houjingou section in the Qingshankou area, Songliao basin. The author has recognized two sporopollen assemblages: Proteacidites-Cyathidites-Dictyotriteles and Lythraites-Aquilapollenites-Schizaeoisporites, two ostracod assemblages: Cypridea gunsulinensis-Cypridea ardua and Ilyocyprimorpha netchaevae- Periacanthella portentosa-Cypridea ordinate, one Esthenaus assemblages: Estheritilis mituishi; Bylithology and biotic characters, the section has been correlated to the sequence from upper part of Member 1 to lower part of Member 2 of the Nenjiang Formation. According to the analysis and statistics of sporopollen, the climate change from tropical, subtropical, humectation to subhumid climates has been proposed. In this period, a more humid and hot climate was represented by the black and oil shale deposits at the lowermost Member 2 of the Nenjiang Formation. Then, the climate tended to be dry and cool. Along with the lake trasgression in the sedmentary period of top Member 1 to Bottom Member 2 of the Nenjiang Formation, the ostracods underwent a progress of species decline, disappearance and recovery.