| In the mixed sedimentary strata of Xiejiawan Formation of Devonian in Longmenshan area,storms occur frequently,and the storm deposits are well preserved,which can be used as the basis for proving the paleogeographic location and paleotemperature change of the study area.Based on the measured data of outcrops in the upper and lower member of Xiejiawan Formation strata and the observation of the middle member,combined with other published paleowater temperature data and mathematical analysis,the controlling factors of storm deposition are discussed from Milankovitch cycle,carbon dioxide concentration and paleowater depth changing.Meanwhile,the mixed deposition of the formation is divided into mixed shore facies,carbonate mixed shelf facies and clastic mixed shelf facies,and the mixed shore mixed shelf sedimentary model of Xiejiawan Formation is established.Storm deposits are widely developed in the strata,among which the diagnostic criteriasof erosion surface,gutter casts,shell lag deposits,graded sequence bedding,parallel bedding,hummocky cross-stratification and other storm deposits characters are obvious.According to the storm diagnostic criterias,storm sedimentary sequences are divided into six different types,which are composed of Sa-erosion surface and gutter casts,Sb-grain sequence bedding section,Sc-parallel bedding section,Sd-hummocky cross-stratification section,Se-mudstone section.Sb can be divided into: Sb1-biological shell grain sequence bedding,Sb2-crinoid stem grain sequence bedding,Sb3-sand debris grain sequence bedding,based on the different composition.Sedimentary scale and characteristics of storm sedimentary sequence can be further divided into far-source storm deposition and near-source storm deposition.The lower member of Xiejiawan formation is dominated by far-source storm deposits,while the upper part of the lower member is dominated by near-source storm deposits,The upper member of Xiejiawan formation is dominated by near source storm deposition,and the corresponding storm deposition model is established.The 34.26 m layer in the upper member of Xiejiawan formation and 53.46 m layer in the lower member of Xiejiawan formation are measured,and more than 200 storm periods are divided respectively.The corresponding storm frequency curve and Fischer diagram are established.The Fischer diagram shows that the sea level of the upper Xiejiawan formation rises first,then falls slowly;the sea level of the lower Xiejiawan formation declines first,then rises slowly,and finally quickly The process of rapid descent.Through mathematical analysis,the eccentricity periods of 400 kaand 100ka in the strata are identified,and the controlling factors of storm deposition are discussed from three aspects of water depth,carbon dioxide concentration and astronomical orbit cycle.It is concluded that the upper and lower strata of Xiejiawan formation are controlled by the astronomical orbit cycle,and the increase of greenhouse gas will lead to the increase of storm frequency and the scale of tempestite decreases with the increase of paleodepth.At the same time,the study of the tempestite in Xiejiawan Formation restored that the Lower Devonian strata in Ganxi section of Longmenshan are located in the low latitude area of the tropic-subtropics,and the shallow sea shelf sedimentary environment.The frequent occurrence of the storm makes the stratum mixed and developed,but inhibits the growth of the biological community... |
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