Prototype Basin Analysis Of Eastern And Southern Part Of Ordos Basin In Middle-late Triassic

During the Mesozoic,the Ordos Basin was a superimposed basin which evolved on the basis of the Late Paleozoic sedimentary basin in North China.Within its evolution epoch,the basin experienced many major tectonic movements,which caused long-term unbalanced overall uplift and fall of the basin and the intense and uneven denudation of the strata.The incomplete or even denudation of the Mesozoic strata in the middle and eastern parts of the basin and thick Cenozoic strata cover in the south made restoration of the original basin face great challenges.Compared with the western margin of the basin,the discussion on the Mesozoic sedimentary boundary in the southeastern part of the basin mostly stays in theoretical analysis,lacking the support of actual material and data.Yanchang Formation in the Middle Late Triassic is the most important oil and gas production strata of Ordos Basin in Mesozoic.The lack of understanding of its original sedimentary range limits the development of oil and gas exploration.Therefore,on the basis of summing up the previous research results,combining the division,correlation,sedimentary facies analysis and provenance study of the Middle Late Triassic residual strata in the eastern,southern and adjacent areas of the modern Ordos Basin,using the first-hand measured data and analytical test data,this thesis analyzed the eastern and southern boundaries and provided new evidence on the restoration of the original sedimentary appearance of the basin.The distribution,lithology and thickness of the Middle Late Triassic strata in the eastern,southern and adjacent areas of the Ordos Basin have been ascertained through the field survey and observation of typical field profiles.The comparing results indicate that the Late Middle Triassic Tongchuan Formation and the Late Triassic Yanchang Formation in Qinshui area and Ningjing area,the Late Middle Triassic Chunshuyao Formation and the Late Triassic Tanzhuang Formation of Jiyuan area,the Late Middle Triassic Taishanmiao Formation and the Late Triassic Taizishan Formation in Nanzhao area are synchronous strata of the Yanchang Formation in the modern Ordos Basin.From north to south,the thickness of strata gradually increases,which is consistent with the tectonic setting.The conclusion of traditional provenance analysis is that there is a strong correlation between the adjacent area and the interior of the Ordos Basin.The detrital zircon U-Pb dating results show that the provenances of the eastern and adjacent areas of the Ordos Basin are mainly derived from the metamorphic basement and Indosinian granite in the Yinshan-Daqingshan area,and the provenances of the Southern Ordos Basin and Jiyuan area are mainly derived from the Yinshan-Daqingshan area and recycling sediments in pre-Triassic strata of North China Block in the early to middle epoch of Middle Late Triassic.in the late stage,there increased the provenance of the southern margin of North China Block.The provenance of Nanzhao,Lushi and Luonan areas mainly came from the North Qinling and South Qinling Mountains,while that of Nanzhao increased the provenance of the southern margin of North China in the late stage.Sedimentological studies show that deltas,braided river deltas and lacustrine deposits dominate the eastern and southern parts of Ordos,with meandering and braided rivers locally developed.Ningjing area developed braided river facies,Qinshui area developed braided river delta,delta and shallow lake facies,Jiyuan area developed delta and lake facies.Nanzhao,Lushi,Luonan area were characteristic of deep lake facies and Nanzhao also developed alluvial fan facies momently.On both time and space scales,the transitional change of sedimentary facies from adjacent area to the modern Ordos Basin conforms to Walter's law,and the change of stratum thickness also conforms to the tectonic evolution background of the North China Block in the Middle Late Triassic.Comprehensive analysis shows that the Ordos Basin and its southeastern adjacent areas should belong to the same basin during the Middle Late Triassic.Because no marginal facies has been found in the east,the sedimentary boundary is not well defined,but at least to the west of the Taihang Mountains.Combined with previous research data,the boundary can be roughly to Shijiazhuang area.Because of the discovery of conglomerate at the bottom of Nanzhao area and the occurrence of gravel-bearing quartz sandstone in Zhouzhi Liuyehe area,it is presumed that the boundary can reach to Shangdan fault at least.Based on the study of Hf isotope of tuff in Nanzhao area,it is considered that the South China Block and the North China Block were experiencing a collision process from east to west at about 234 Ma.Taking the Nanzhao-Zhashui area as the turning point,the eastern part was in the stage of post collision and in the west,Mianlue Ocean was still subducting.Under this tectonic background,the basement and sediments of the Greater North China Basin responded,and finally the restoration of the sedimentary features of this Greater Basin in the Middle Late Triassic was completed.In addition,under the above-mentioned tectonic background,during the Middle-Late Triassic,the Ordos Basin,the southeastern part of Shanxi Province and the western part of Henan Province are unified large-scale inland lake basins,and regional subsidence took place.A set of dark oil shale layers was formed in the center of the lake basin,that is,Yanchang Formation,the most favorable source rock of Mesozoic in the Ordos Basin.This horizon is also abundant in the western Henan area in the southeastern part of the Ordos Basin.Therefore,this horizon is also the key exploration target in these southeastern adjacent areas.