| This research area is located in the south of the Animaqing Mountains and is under the jurisdiction of Maqin County and Maduo County of Guoluo Tibetan Autonomous Prefecture in Qinghai Province and a low degree of geological research with poor natural conditions.Based on sequence stratigraphy,statistics of detrital components of sandstone,detrital zircon isotope chronology,and petro-geochemistry,this paper conducts a comprehensive study of the Changmahe Formation in North Bayan Har Basin,Qinghai Province,to determine and classify strata.Analyzing the sedimentary characteristics,makes the attribution of stratigraphic of the Bayan Har Basin unity;analyzing the provenances,determines the basement properties of the Bayan Har Basin,and inverts the early-middle Triassic subduction collisions.The process of evolution provides sufficient evidence for the North Bayan Har Basin.In the study area,the lithology of the strata is single,and the outcrops are poor and discontinuous.This work uses the sedimentary cycle method to accurately determine the Changmahe formation and divide it into five sections.Each member represents a quasi-secondary cycle.Each quasi-secondary cycle is dominated by variegated metamorphosed glutenite-sandstone-siltstone under weak oxidizing-weak reducing environments,with dark grey,grayish green silty slate,and slate,the upper part is the dark gray-grey metamorphic medium-grained sandstone-fine sandstone-siltstone and silty-slate slate-slate interbed in the reduction-weak reduction environment.It indicates that the distance between the source material denuded area and the sedimentary area is gradually increasing under the medium and high sedimentation rates.The five quasi-secondary sedimentary cycles are stacked in a vertical order to form a secondary sedimentary cycle of the Changmahe Formation,which is an upwardly slender,upwardly shallower depositional cycle in the overall performance.Moreover,the quasi-secondary sedimentary cycle of carbonate rocks in the upper part of the deposit is developing.In the Changmahe Formation,there are many types of lithofacies units and a rich sedimentary structure,forming a set of debris flow deposits.The sedimentary system is dominated by fan deltas and coastal facies,reflecting a relatively shallow water environment.Combined with the basic sequence types and overlap rules in the region,it can be seen that the provenancerce material supply rate is slow,indicating that the sea level is decreasing and is becoming shallow.It is further determined that the sedimentary environment is dominated by the Shore shallow sea.The study of detrital grain composition in the study area indicates that the source rock is of multi-source origin and is mainly derived from the upper crust felsic rocks.The analysis of primary,trace,and rare earth elements in sandstone shows that the source rocks have features of continental island arcs and active continental margin type provenances.The detrital zircon age(256.4-238.4Ma)is shown as the Early-Middle Trisonian Olenekian Late-Anisian metaphase,and the zircon Hf isotope indicates that the provenance is an intrinsic circulatory product,and its provenance zone is associated with the eastern Kunlun orogenic belt and the western Qinling orogenic belt is more suitable.In the process of geological evolution,the formed basin may be a forearc faulted basin. |
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