Sedimentary Characteristics And Palaeoclimatic Implications Of The Cretaceous Aeolian Erg System In The Northeastern Chuxiong Basin,China

The Cretaceous greenhouse,a long-lasting typical warming interval in the history of the earth,and the superposed influence of regional tectonic activities triggered a widespread arid-climate belt in East Asia accompanying with extensive paleo-desert outcrops and evaporite deposits.Many Cretaceous desert basins in East Asia have been reported in recent years.Located at the arid-climate belt in the northern hemisphere,the Chuxiong Basin developed widespread aeolian sandstones with high-angle tabular cross-bedding,which has been ignored by previous studies.Only an arid and hot climate during the Late Cretaceous has been proposed based on the gypsum strata in the Upper Cretaceous Jiangdihe Formation.Focused on the Upper Cretaceous red clastic rocks deposited in the Chuxiong Basin of central Yunnan,the main object is to reconstruct the palaeo-wind and recognize the inner structure within the aeolian dunes through multiple approaches including outcrop analysis,grain size analysis,optical microscopy,scanning Electron Microscopy(SEM)and Energy Spectrum(EDS)analysis.It is also possible for reinterpreting the depositional environment of the Upper Cretaceous sediments and for providing an important database of the inland aridification evolution influenced from the regional circulation pattern and paleotopography effect in East Asia.The main results and observations are reported here as follows:(1)Several aeolian facies including aeolian dune,interdune,sand sheet,dry valley and paleosol have been identified according to the microstructure observations,macrostructure observations and recognitions on a hierarchy of bounding surfaces from the Upper Cretaceous red clastic rocks in northeast Chuxiong Basin.Simultaneously,the inner structures within the aeolian dunes are also interpreted and suggest that there are distinct aeolian facies,which are dominated by composite transversal dunes,in Matoushan and Zhaojiadian Formations.(2)The quartz grains from the Upper Cretaceous red clastic rocks in northeast Chuxiong Basin develop representative aeolian surface characteristics,such as dish-shaped impact,crescent impact,dissolution pit(cave),siliceous sphere and scale.Based on the combination of mechanical and chemical interactions on the quartz surface,the evolution rule of micro-morphology on the quartz grains under different sedimentary environment has been systematically summarized.And the quartz grains from the playa sediments of Jiangdihe Formation shows "double phase" characteristics relating both to strong mechanical and chemical interactions.(3)The reconstructions of the palaeo-wind directions of the Chuxiong Basin for the Middle-Late Cretaceous present that the palaeowind directions in aeolian dunes of the Chuxiong Basin,Sichuan Basin,Thailand's Khorat Basin,and Lanping-Simao Basin together form a closed circular flow ring with clockwise outward divergence.It is evident that large-scale closed annular air circulations existed near the surface.(4)Based on the paleogeographic pattern for Middle to Late Cretaceous Chuxiong Basin,the sedimentary environment within the Basin is analyzed.In addition,the effects of paleotopography in the southeast margin of the Tibetan Plateau in late Mesozoic on the formation of eolian sediments in Chuxiong Basin and the process of inland drought in South China are further discussed.It is believed that the landform barrier caused the water vapor from the sea into the inland effectively,and was an important factor for the widespread drought and desertification in East Asia.