Cretaceous Desert In China And Its Paleoclimate

By combining sedimentology with atmospheric sciences, based on the tie between the general atmospheric circulation and climate and the knowability of desert, the depositional system, temperal and spatial distribution of the Cretaceous deserts in China and their controlling factors are studied, the general atmospheric circulation and climatic features are discussed and the intensity and framework of the general atmospheric circulation and the unequability of the Cretaceous climate are focussed, which provide direct geological evidences for the climatic variation under the greenhouse effect. Through paleolithogeographical study, it is found that the Tarim Basin, Erdos Basin, Sichuan Basin, Simao Basin, Jianghan Basin, Changtao Basin and Subei Basin are Cretaceous desert basins located at the Cretaceous mid-low latitudes and controlled by the subtropical high pressure arid climate and planetary wind system and are the component parts of the Cretaceous subtropical and tropical desert belt. The diachronism of the desert shows that the desert belt had shifted southwards and eastwards through time resulted by the drifting of the arid climatic belt. Based on the measurements of plaeowind directions and paleomagnetism, the frameworks of the paleowind belts and the general atmospheric circulation are reconstructed and it shows shat the westerlies and northeast trades were prevailing, the convert belt of westerlies and northeast trades was not only drifting south and northwards repeatedly with short cycles but also drifting southwards with long cycle, providing atmospheric circulational evidences for the arid belt drifting. The existing possibility of monsoons is discussed and it is proposed that there not only occurred planetary monsoon but also ocean-continent monsoon and the ancient East Asia Monsoon had been existing as early as the Cretaceous. Based on the dune sand grain sizes and thredhold wind velocity, the intensity of the Cretaceous general atmospheric circulation is estimated which refutes the views that the atmospheric circulation would be sluggish under the greenhous effect. The analysis on the cyclicity of the desert sediments indicates that the hydrologic cycle had undergone not only long and middle cyclic variations but also short cyclic and or seasonal variations even instantaneous changes. The variations of the hydrologic cycles and wind diretions suggests that the Cretaceous climate were not equable but very seasonal. The discovery of the evidences for unequable climate affords precious data for the unequablity of the Cretaceous climate from continent center at mid-lowlatitudes which is a great supplement to the previous indirect data mainly from continental margin and oceans at mid-high latitudes. Meanwhile, the desert system subdivision is discussed and important types of the desert sediments are studied which, to a certain degree, widens the scope of the sedimentological study in China.