Sedimentary-structural Evolution And Reformation Of Ningnan Basin

The Ningnan Basin Group is the sediment body without basin shape by the late reformation,whichMainly located at the south of Ningxia province.The spread area generally refers to the Tertiary which isMassively uncovered and distributed in the south of the Yin Chuan basin.Most of the strata is red,scattered and ruleless.It has important position for its geography and geotectonics.The residual basin adheres to the southwest of the Mesozoic Ordos basin to the east and the northeastern horn of the Tibet Plateau to the west,which echos the Cenozoic faulted basins in the neighboring area of the Ordos basin at a distance,and is the most immediate response to the uplift of Tibetan plateau.Hence,the study on the formation, evolution and disappear of the Ningnan Basin Group is crucial and irreplaceable to the Cenozoic reformation and the regional geodynamic environment of the Ordos Basin,the recovery of original Mesozoic basin,the oil-gas exploration in this area,as well as the uplift of Tibetan plateau.Many predecessors haveMade exploratory research on the Cenozoic in Ningnan area,but the period they have studied is limited and the work is also relatively simple.The study of the formation,evolution and late reformation of the basin is rarely,especially the regional geodynamic environment and tectonic attribute of the Cenozoic basin's formation.So there are still a series of problems should be further resolved.The characteristics of distribution,lithologic characteristic,sedimentary facies, environment evolution of current Tertiary and the characteristics of pre-Tertiary distribution and its contact with Tertiary have been identified through the three geological sections measured and other field sections investigation.It can be considered that during the Sikouzi period,the basin is composed by a number of interconnected or disconnected small fault depressions,the lake basin gradually increases at the Qinqshuiying period,and reaches to its peak at the Hongliugou period when the region has been basically connected to each other or become a larger group of lack basins,then the basin area become reduction at Ganhegou period.The 15 polarity and 14 anti-polarity records of Sikouzi have been gained by theMagnetostratigraphy method,which can be well compared with the standard polarity sequence.Multidisciplinary analysis indicate that the time-limit of Sikouzi group which is the first set of Ningnan Cenozoic sedimentary strata is from 47.9Ma to 29.4Ma B.P.namely in the Eocene and Early Oligocene.Moreover,the analysis and comparison is Made with the predecessors' study in this area.It is considered that the deposition of Tertiary is in the tensional fault subsidence the same to the other fault subsidence of Ordos block in this period which is controlled by normal faults.After researching and analysis the geological structure,such as fracture,fold, unconformity contacts and inversion structures,is involved or formed by the Tertiary.It is considered that the Ningnan basin and peripheral Cenozoic faulted basins of Ordos block formation and development in the similar dynamic environment and the occurrence with a synchronous consistency,through the uniform contrast of the basins linked with regional,and research and analysis the unity and differences of tectonic evolution and activity of each other.The uplift and expansion of the Qinghai-Tibet Plateau has been strongly affected this area in Late Miocene.The development and evolution of Ningnan basin is obvious differentiation with the other peripheral faulted basins.The basin gradually disappearing by the reversal reformation.The intensity of reversal is weaker in the north than south,and the time is earlier at the south than north.