The Origin And Evolution Of The Desert In Southern Ordos In Early Cretaceous: Constraint From Magnetostratigraphy Of Zhidan Group And Magnetic Susceptibility Of Its Sediment

Global warming resulted from greenhouse effect has already become the most serious global environmental problem in the present century, owing to the natural factor and human disturbance, land desertification around the desert is becoming more and more serious. Cretaceous is the latest typical greenhouse Earth state, thus the systematic research on the desert evolution in these period is of great significance to understand the modern desert.The desert in the Ordos Basin is an important part of desert belt across China continent in the Cretaceous. In this study, based on the high resolution magnetostratigraphy of lower Cretaceous-Zhidan Group in the southern of Ordos Basin, it could be inferred that the original remnant magnetic-carrying minerals are mainly hematite and somewhat maghemite, there are 25 long normal polarity zones and 25 short reversed zones in Zhidan Group with a thickness of about 420.23m, including Yijun Formation, Luohe Formation and Huanhe-huachi Formation. Combined with the obtained biological fossil data, the observed polarity zones could be reasonably correlated with the M5n through M16n of GTS2004, thus yield the age control of lower Zhidan Group between 142.06 Ma and 129.73 Ma, then the ages of Yijun Formtion, Luohe Formation and Huanhe-huachi Formation are 142.06-141.48 Ma, 141.48-133.94 Ma and 133.94-129.73 Ma, respectively. The deposition environment of the lower Zhidan Group is alluvial fan, fluvial, desert and shore-shallow lake facies, while the desert deposition is in the Luohe Formation between 141.48 Ma and 135.37 Ma.Based on the measurement of magnetic susceptibility and the previously obtained pollen data in sediments, we found that the climate environment inferred from sediments of the lower Zhidan Group has a better correlation with global temperature change in early Cretaceous and it shows a arid and heat condition on the whole. Its climate change could be further divided into two substages:the early relative arid and warm substage (142.06 Ma-133.94 Ma), the last relative humid and warm subsage (133.94 Ma-129.73 Ma). Three stages are reconstructed in Ordos Basin at early Cretaceous based on the magnetic susceptibility feature and the location of anemoarenyte, they are development, expansion and subduction, correspondingly to 141.48 Ma-140.64Ma,139.51 Ma-137.84 Ma and 136.26 Ma-135.37 Ma.