Analyses Of The Early Cretaceous Environment And Climate In Northwestern Yunnan, China

Studies of paleoclimate are helpful for the understanting of present greenhouseclimate change and the prediction of future climate, and the research of theCretaceous climate is an alternative. The Cretaceous is a particular “Greenhouse”climate period in geological history, which has been demonstrated by huge marinerecords, but there have not been enough evidences from land sendiments forsupporting this viewpoint.The terristrial Cretaceous is widespread in mainland China,and is quite typical in Southwest Yunnan province. Large of the Cretaceous redterrestrial sediments, filled in both Chuxiong basian and Lanping basin in northwestYunnan, provide present study lots of foundations.Based on the detailed observation and description of lithology and strata from fieldcross sections plus observation of lithological microfacies under microscope,sedimentary environments were analysed for the Lower Cretaceous MatoushanFormation, Jingxiing Formation and Nanxin Formation in northwestern Yunnanprovince in this master thesis. According to Lithologic and sedimentary characteristics,it is proposed that the Matoushan Formation and Nanxin Formation have ever beendeposited in environments of mostly meandering river and lake, and the JingxiingFormation was formed in meandering river, delta and lake. Paleosol facies is oftenassociated within the subaerial river and lake facies in all three formations.Sedimentary subfacies were subdividedly recognized and classified.The Early Cretaceous paleoclimate in northwest Yunnan was analysed using detritalcomponent, clay mineral and carbon and oxygen isotope of pedogenic carbonate.Indexes of F/Q and L/Q estimated from detrital component are used to indicateaffection of the dryness and tectonic activity onto sedimentation. Relative ratios ofclay minerals, such as smectite, illite, chlorite and kaolinite as well as crystallinity, areindicatives of wet and dry conditions. Carbon-isotope values of calcrete are aimed todistinguish if it was made in a calcareous paleosol (special climate sedimentaryenvironment), and to correlate stratigraphy.The results show that from indexes of F/Q and L/Q sedimentation had been disturbedby tectonic activities in the Berriasian and Hauterivian of the Early Cretaceous in northwest Yunnan, and controlled by climate in theother Early Cretaceous time.Relative ratios of clay minerals, crystallinity of illite and chemical indexes supportedthat it had been in the dry-hot climate in the Valanginian-Berriasian, warm-cold andwarm-humidity climate in the Barremian–Hauterivian, and dry-hot climate in theAptian and Albian. From the-4~-11.0‰values of stable carbon-oxygen isotope ofcalcretes, it is demonstratedthat calcareous paleosol is a frequent sedimentary faciesin the Early Cretaceous in northwest Yunnan,, and the climate is mostly dry and hotduring the period.