Stratigraphic Sequences And Basin Characteristics Of Devonian In Yakeshi,Inner Mongolia

Through comprehensive research on the Devonian strata of the study area,in this paper,the strata in the basin are divided into conglomerate facies,sandstone facies,mudstone facies and carbonate facies.The main body of clasolite is a set of sediment gravity flow deposition,which develops arkose,greywacke and polymictic conglomerate,and so on.The depositional environment is mainly alluvial fan facies,fluvial facies,fan delta,abyssal-bathyal facies,and so on.The major elements are comparatively concentrated in the passive continental margin and continental island arc zone in the geochemical analysis of Devonian clasolite samples.Al2O3 has continental high content(7.2-14.71%).there are more clay mineral content.Clastic rocks are of low maturity,reflecting that the sediment is accumulated nearby the source.The geochemistry characteristics analysis of rare earths and trace element indicates that the detrital material is quartzes.Eu is of obviously loss,with ?Eu between 0.52 and 0.71.Ce is of slighter abnormity,with ?Ce from 0.87 to 1.02.Rare earths generally shows the rightward heeling form:light rare earths concentrates and heavy rare earths is of flat distribution,which is similar to the typical upper crust.The geochemistry characteristics of basalt shows that LREE are relatively high concentrated,while HREE has a relative loss,with the ratio of LREE and HREE between 3.40 and 3.90.Eu has obvious positive anomaly,with ?Eu between 1.01 and 1.59.The ratio of LaN and YbN is between 2.41 and 2.85,whose trace element characteristics is similar to beneficiation mid-oceanic ridge(P-MORB).Given the interbedding output of basalt and radiolarian silicolite in the study area,it is confirmed to be oceanic ridge basalt.According to sedimentary facies sequence character analysis,the Devonian system is divided into 6 sequences.The sequences SQ1-SQ3 are included in Niqiuhe group,the sequences SQ4-SQ5 in Daminshan group and the sequence SQ6 in Angeeryinwula group.Lowstand system tract is generally agenesis,while transgression and highstand system tract are developed.Based on the related sequences and the developmental characteristics of system tract,the process of formation and evolution of the sequence mainly depends on the relative changes of sea level and the activity of boundary fault.On the basis of sequence development and characteristics of lithofacies and sedimentary facies,the Niqiuhe group is divided into section one,section two and the Wunuer reef limestone,and the Daminshan group is divided into section one and section two in this research.The acid volcano rock in Daminshan group belongs to the Ordovician according to systematic analysis.The original Daminshan group is subdivided into three parts:the metamorphic basic volcanic rocks and radiolarian silicolite in Toudaoqiao-Wunuer structural belt;the acid volcanic rocks which is belonging to Carboniferous developing in volcanic arcs;and the sandy clastic rocks.It can be deduced that the study area is rift basin in Devonian according to comprehensive research and analysis.With the increasing extension,lithosphere was thinned to such a extent that the mantle material uplift,thus uplifting the mantle material and generating new oceanic crust.The study area mainly experienced the stages of continental rift,intercontinental rift and nascent ocean basin.