The Geologic Features And Tectonic Setting Analysis Of Cretaceous In Alashan Area, Inner Mongolian Autonomous Region

The study area are in the middle to east part of Yingen-Ejina basin in Alashanarea of Inner Mongolian Autonomous Region and partly in the north part of Gansuprovince. Cretaceous is another important tectonic evolution period after theformation of the Central Asian Orogenic Belt, when many continental rift basinsdeveloped and with several volcanic eruptions, which show the powerfulness oftectonic activities. Therefore, the study of sedimentary characteristic and volcanicactivities of Cretaceous is crucial to research the tectonic evolution.Based on the achievements of former research and our field work, the study usesthe theory of stratigraphy, sedimentology, lithofacies paleography and petrology toanalyze the characteristics of outcrops and samples which are eventually serve tostudy the stratum, sedimentary facies and tectonic implications of Cretaceous in themiddle to east of Yingen-Ejina basin and in the west of Yabulai basin. Cretaceous inthe study area can be divided into three facies: Alluvial fan facies, braided riverfacies and lacustrine facies and it can be ulteriorly divide into5subfacies and7microfacies. According to the results of pollen analysis, the Cretaceous climate ofstudy area was moderate drought.The rock geochemistry of Suhongtu formationshowed it belongs to Alkaline volcanic rocks and can be named Trachybasalt,Trachybasalt-andesite and Trachandesite. The result of trace elements test manifests itformed in continental stable massif instead of orogenic belt, and it representsextensional environment. Based on the study of basin types, filling sequence, tectonicsetting of lava, sedimentary source and facies, the tectonic evolution can be known asfollows: in the early stage of K1, the area is in fault phase；in the close of K1，it is involcanicity stage; and in K2，it transferred to depression stage.