Formative Mechanics And Evolvement Of Jiuquan Basin

The study of rift basins was of interest to many geologists for last 39 years.and many mative mechanics have been put up by geologists. The formative mechanics all indicate that rift basins are controlled by hot crown of mantal, a half-garben cluster that its length usually is more than 1000 km is formed in ground surface. And the length of Jiuquan basin is less than 100 km, but the mark of fault basin is explicit.The study of rift basin require the study of the peripheral basins of Jiuquan basin.The previous fluidies indicate that the Hexi corridor and its adjacent region belong to intralcontinental evolvement stage in the early Cretaceous. According to the analysis and the comparison of Cretaceous which occur in Hexi corridor, Beishan-Alashan, Qilian moutain and northern Chaidamu basin, the author thinks that the lower Cretaceous which occur in Changma basin, eastern Beishan, Minle basin and Yingen-Ejinaqi basin can be divided into three parts, and each part can be comparable for the Chijinbao, Xiagou and Zhonggou formation individually. Based on the results, the paper analyses the Changma basin, Minle basin, Zhongkouzi basin and Zhagegaonao basin summarily.Furthermore, Yingen-Ejinaqi basin is studied by its sequences and the architecture of lower Cretaceous,sedimentary formation, extensionl tectonics type and deep structure. This study indicated that the above basins together with Yingen-Ejinaqi basin possess the same evolvement in the rifting process.According to the author' s analysis of large set of seismic data and the results of previous fluidies in Jiuquan basin, two types of extensionl faults can be recoguized.they are NE or NNE striking and NW or approximate EW striking. Meanwhile, according to the analysis of the extensionl structures in adjacent region of Jiuquan basin, the author suggest that the former were made by the regional extensionl striking of NW to SE, and the last is derivative structure of the extensionl stress field, and it not only possesses the features of striking sliding, but also that of dip. Based on above all, the author distinguishes the boundary faults and secondary faults of Jiuquan basin, and thinks that the extensionl structures move in the depositional stage of Chijinbao Fm and Xiagou Fm, and stops in the depositional period of Zhonggou Fm, both possesses the feature of synchronous evolvement. Otherwise, by means of the analysis of the relationship and shape of Qingnan subbasin and Hongnan subbasin, and the seismic data of the tectornic units, the author thinks that these units results from the earth surface breakdown caused by the topwall deforming made by the extensionl action during the sedimentary period of Chijinbao Fm and Xiagou Fm.The volcanics lying in Hongliuxia and Hanxia region formed in the early Cretaceous and are related to the extensionl tectonics occurring duiring the sedimentary period of Xiagou Fm according to the author' s analysis of their geological features and predecessors' isotopic ages. Meanwhile, this paper analyses the geochemical feature of volcanic lithology, rare-earth metal and trace elements and these features are similar to those of volcanics of the typical rift basins all over the world. Otherwise, based on the analysis of feature of geology, fabric and geochemistry of exhalative rock, the author consider that the rock related to the intense extensionl action occurring in the sedimentary period of Xiagou Fm and Chijinbao Fm, that the stronger the extensionl action is, the stronger the exhalative action is.Based on field investigation of outcrops and the data of drilled wells and well logging, the author thinks that there are five of sedimentary system type which include alluvial fan, fan delta,river and lake fan, and the author divides each type of them. Furtherore, according to the analysis about the relationship of extensionl structure to provenance. I think that the sedimentary system of alluvial fan, fan delta and lake fan usually develop at bold coast in the depositional period of Chijinbao Fm and Xiagou Fm, and river delta depo...