Geochemistry Characteristics Of Mesozoic-Cenozoic Sediments Along Northwestern Margin Of Tarim Basin

The evolution of the New Tethys has been attracting the interest of many geologists around the world for it separated Eurasia and Gondwana during the Mesozoic era. In addition to creating the Alps-Himalayan orogenic system, the diminishing and closing of the New Tethys may have played a major role in changing the Cenozoic global climate and causing the Asian inland arid. The Tarim Basin is one of the few areas in China where Mesozoic and Cenozoic marine strata are exposed, which have been investigated on lithology and paleontology by many researchers to find out whether the New Tethys existed and its specific evolution history in the Tarim Basin area. Though various attitudes have been acquired, fully understanding of the issue has been limited owing to the lack of detailed and systemic work of multi-disciplines. This paper is targeted at Kuzigongsu, Wuqia nanshan and Heiziwei profiles in Kashar-Wuqia area along northwestern margin of Tarim Basin and focuses on systemic analysis of the element concentration and organic carbon content in continuous deposits since Cretaceous, which are combined with some strata and paleontology results from other researchers to discuss the process of marine transgression and regression of the New Tethys and the stratigraphic age of its final diminishing in northwestern Tarim Basin. Combining the results of strata and paleontology from previous researches, we discuss the process of marine transgression and regression of the New Tethys and the stratigraphic age of its final diminishing in northwestern Tarim Basin. This can not only greatly augment the research contents, verify the key geological events and confirm the evolution history of the New Tethys, but also provide some significant reference for finding out the changes of sedimentary environment and conducting oil-gas exploration in northwestern margin of Tarim Basin. Three main conclusions can be obtained from this research.1. The high values of Sr/Ca ratio, Mg/Ca ratio, Sr/Ba ratio,B concentration and organic carbon content in the continuous sequences since Cretaceous lie in the Kukebai Formation and Yigeziya Formation of Late Cretaceous, Qigenmu Formation of Paleocene, Kalatar Formation-Wugen Formation of Eocene, indicating the climax horizon of four marine transgression and showing there are four cycles of marine transgression and regression along the northwestern margin of Tarim Basin from Cretaceous to Paleogene. 2. No numerous marine paleontology fossils are found in the strata lying above the oyster layer in the uppermost part of Bashibulake Formation, which implicate that the New Tethys finally retreat from the northwestern margin of Tarim Basin in Oligocene at Bashibulake Formation. The changes in element concentration and organic carbon content of successive deposits since Cretaceous are combined with litholgy and paleontology characteristics, which reveal that the research area was marine paleoenvironment from Late Cretaceous to Oligocene and residual marine(lagoon)-salified lake paleoenvironment from late Oligocene to Miocene.3. The variation tendencies of concentrations of some elements such as Cl, S, Ca, Mg and so on implicate the seawater salinizing process only reached sulfate phase in evaporitic environment from Cretaceous to Plaeogene. There are less sulfur content and more calcium content in Kezilesu Group of Early Cretaceous to Kukebai Formation of Late Cretaceous, which display the carbonate depositing dominated the period(a considerable amount of limestone formed). There are more sulfur and calcium contents in Wuyitage Formation of Late Cretaceous to Bashibulake Formation of Oligocene, which illustrate sulfate depositing dominated this period(hugely thick gypsum bedding exposed in Tuyiluoke Formation-Aertashi Formation).