The palaeosalinity analysis of Paleogene mudstone in Lanzhou Basin，Gansu Province
Li Zhichao1，Li Yongxiang1，Li Wenhou1，Zhang Yunxiang1，Xie Kun1，Ma Yao2
1. Steta Key Laboratory of Continental Dynamics，Department of Geology，Northwest University， Xi′an 710069；2. School of Earth Sciences and Engineering，Xi′an Shiyou University，Xi′an 710065
Abstract：
The Lanzhou Basin is located on the northeast margin of the Tibet Plateau，bounded by the Qilian Mountain to the east and the Loess Plateau to the west，which outcrops mainly Paleogene Xiliugou Formation，Yehucheng Formation and lower part of Xianshuihe Formation. Much attentions have been paid for the Cenozoic environment in the Lanzhou area，as the conjunction of the Tibet Plateau cold region，eastern monsoon region and arid region of Northwest China. So it's necessary to do researches on the sedimentary environment. Samples in the Palaeogene strata was collected in the different segment of Huangyangtou area in Lanzhou Basin. Major， trace elements and X-ray diffraction was conducted on these samples. The Palaeogene sedimentary environment was revealed by quantitative semi-quantitative analysis involving calculation using boron methods，B/Ga ratio，Sr/Ba ratio and the variation characteristics of chemical index of alteration. Results indicate Adams paleosalinity ranges from 9.5‰ to 14.0‰，with the mean value of 11.8‰ and Couch paleosalinity ranges from 8.2‰ to 12.8‰，with the mean value of 10.8‰ in Xiliugou Formation，Yehucheng Formation and bottom of Xianshuihe Formation，which show that the Lanzhou Basin was a bracket water to fresh water lake. Adams paleosalinity value exhibits positive correlation with B/Ga ratio and no obvious correlation with Sr/Ba ratio. The Sr/Ba ratio is not suitable for reconstruction of paleosalinity， as Sr was absorbed by the biology in the stratum， which gives rise to the larger Sr/Ba ratio. The Environment research displays that the basin was chiefly characterized by alluvial fan，shallow water，high salinity values and humid climate during the Xiliugou age. With the increasing depth of water it gradually transformed into lake deposits and had low salinity water and warm-humid environment in the middle Yehucheng age. Meanwhile，there was an obvious dry-cold climate interval in that period，during which the lake basin evaporation capacity was greater than the recharge and salinity value increased. The lake basin gradually shrinked and was deposited in a lakeshore facies from the middle Yehucheng age，and into fluvial facies in Hanjiajing age into lacustrine facies in Ganjiatan age. The paleosalinity rose first and then fell，but the whole weathering degrees significantly reduced，which show that climate gradually became cold and dry since late Yehucheng age.
Li Zhichao,Li Yongxiang,Li Wenhou et al. The palaeosalinity analysis of Paleogene mudstone in Lanzhou Basin，Gansu Province[J]. Chinese Journal of Geology, 2016, 51(4): 1344-1356.
2016, Vol. 51 
