The Mesozoic Tectonic Event And Sedimentary Environment In The Southwest Margin Of Ordos Basin

Base on extensively gathering and systematically analyzing abundant structure geology, geophysics and thermo-chronology materials in the southwest margin of Ordos, according to the combination of field geology and indoor analysis, as well as the combination of structure geology and tectonic geochronology, tectonic petrologic geochemistry, this paper comprehensively analyses the Mesozoic tectonic evolution phases, structure deformation characters, event geochronology sequences and the time limit of the key tectonic events, also the response relationship between the sedimentary source setting together with its transition in different time and the main tectonic events in the area, in the end, elementarily discuss the coupling relationship between multi-minerals and key tectonic events.The results of Mesozoic uniconformity-tectonic event and the geochronology of tectonic event by fission track analysis suggested: (1) The event of the Indosinian mainly occurred at the age among 230~194Ma±. It may divide into two events: the early and the late event of the Indosinian. The early event of Indosinian mainly occurred at the peak age of 230Ma+, while the late event of Indosinian mainly occurred at the peak age of 205Ma±. The event of Indosinian mainly occured in the form of uplifting movement, and make the relationship between low-middle Triassic and upper Triassic as well as upper Triassic and low-middle Jurassic become the parallel unconformity. The Indosinian event did not happen as the form of tectonic deformation activities. (2) The event of the Mid-Yanshanian mainly occurred at the age among 168~141Ma± with the peak age of 150Ma±. It happen in the form of deformation in all areas, and make the former Cretaceous stratum strongly folded, and make the folded stratum become the contact relationship of angular unconformity with Lower Cretaceous stratum. (3)The event of the late Yanshanian could be divided into two events. The event of the early period of late Yanshanian mainly occurred atthe peak ages of 90Ma±, and the event of the late period of late Yanshanian mainly occurred at 65Ma±. The event of the late Yanshanian mainly happened as the form of uplifting activities in all areas, and make Lower Cretaceous stratum become the contact relationship of parallel unconformity with Tertiary.The geochemistry characteristics of major and trace elements suggust: There is a Mesozoic paleouplift which is the northward bulgy and arclike distributed from Weining Beishan, Xiangshan to Luoshan, Qinglongshan, Tanshan in the middle region of interest. This paleouplift may experience the mainly development period in the late Triassic, the shrinking period in early-middle Jurassic, the composite enlarging period in late Jurassic and the revival period in early Cretaceous. Therefore resulte in that the different provenance and tectonic environment differed in different period of Mesozoic in the area of Helanshen and the area of Liupanshan of the Ordos baisn. The evolvement of provenance and environment could be composed of the compressional environment in the late Triassic, the extensional environment in early-middle Jurassic, the compressional environment in late Jurassic and mainly the compressional environment with partly the extensional environment in early Cretaceous. The basic reason of the change is the multiple phases evolvement of Mesozoic tectonic event.The multiple resources of oil, gas, coal, and sandstone-type uranium deposit accumulate in different tectonic units and in various stratum of Paleozoic to Mesozoic, but they relate to or depend on each other in the form of coupling coexistence in the basin. Especially, the first critical period of their coupling coexistence is in the middle to late Yanshanian(168~90Ma±), corresponding to an intensive tectonic-thermal activitiy and a tectonic transition in the basin. The second period is during the early Himalayan (65Ma±), corresponding to a tectonic uplifting process. The coupling coexistence of the multiple energy sources is obviously controlled by the basin tectonic evolution and transition in the critical period as well as later reformation during the Mesozoic to the Cenozoic.