Early Cretaceous Paleoclimate In Jiuquan Basin And Liupanshan Basin, Northwestern China

As a special period in geological history, the Cretaceous is a typical representative of extreme"greenhouse climate"pahse. Researching paleoclimate of the Cretaceous can help human beings predict that in future. The continental Cretaceous is widespread in mainland China, which provides material basis for study of the coeval climate. It is characterized by fluvial and lake lithofacies in Jiuquan Basin and Liupanshan Basin northwestern China, and its biostratigraphic and chronostratigraphic works are relatively well done. These features plus good outcrops enable us to analyze the Cretaceous climate over thereA variety of methods and techniques have been used for paleoclimate study. Besides of some special sediment for the purpose in sedimentology, clastic composition, relative content of individual clay mineral, and carbon-oxygen isotope are also alternatives, which contain much paleoclimate information. Parameters of F/Q and L/Q are used to indicate the dryness and tectonic activity; relative contents of clay minerals, such as smectite, illite, chlorite and kaolinite are indicatives of wet and dry, hot and cold conditions; carbon-isotope fractionation regimes can be analyzed for interpretation of climatic and environmental changes.Results of F/Q and L/Q ratios from the two basins indicate that tectonic activities had been influencing the deposition to a certain degree in the early Early Cretaceous, but the paleoclimate had mostly controlled the basin sediments thereafter. Clay mineral relative contents and crystallinity show that it was wet and dry climate in the Valanginian-Berriasian; arid climate in the Barremian–Hauterivian, dry-cold, warm-humidity, and dry-hot climate in the Aptian, and alternation of dry-hot and dry-cold climate in the Albian . The carbon-oxygen isotopic values of lake carbonates suggest that the lakes were of open freshwater in the Aptian-Albian, and became relatively closed by weakened fluvial effects, decreased rainfall and water salinization in the late Aptian and Albian. The positive excursion ofδ(13)~C (2.79‰) can be compared to the OAE1a (Selli) at 120 Ma in the early Aptian.