Study On Jurassic Sedimentary Environment Of Shiguai Basin In Daqingshan Area

Shiguai Basin,located in the hinterland of Daqingshan,is a relatively complete and well exposed continental coal bearing Jurassic strata.The paleoenvironmental information contained in it is an indispensable basic data for the paleoclimate and environmental correlation study of Mesozoic continental basins in northern China.The Jurassic in Shiguai Basin develops Wudanggou Formation,Zhaogou Formation,Changhangou Formation and Daqingshan Formation from bottom to top.Systematic studies on clastic composition,rock structure,sedimentary structure,section structure and petrography show that Wudanggou Formation is unconformity on the gneiss of the Archean Wulashan group with a measured thickness of about 130.3 m.The lithology is composed of gray-white conglomerate,gravelly coarse sandstone,thin grayish-yellow fine sandstone,gray-black shale with thin coal.Cross bedding,horizontal bedding and sedimentary ring are developed.The sedimentary environment is alluvial fan braided river delta sedimentary facies;braided river delta sedimentary facies can further identify delta front sub-facies,underwater distributary channel and inter distributary channel microfacies;the Zhaogou Formation and the underlying Wudanggou Formation are in integrated contact.The measured thickness is about 153 m.The lithology in the lower part is yellowish-green gravelly coarse sandstone,lithic greywacke,dark-gray mudstone and silty mudstone.The lithology is grayish-green medium-thick coarse sandstone,fine silty sandstone,and many coal seams and coal lines with thin mud,forming a rhythmic layer in the middle part.The upper part comprises grayish-white gravelly coarse sandstone,yellow-green medium-fine sandstone and grayish-yellow mudstone containing plant fossil fragments.Horizontal bedding,cross-bedding and channel scouring filling structures are developed.The sedimentary environment is Alluvial fan-Braided River Delta-Lake-meandering river facies.Braided river delta sedimentary facies can further identify two sub-facies of delta plain and delta front,and two microfacies of underwater distributary channel and swamp;lake facies can further identify two sub-facies of shore shallow lake and semi-deep lake;meandering river sedimentary facies can further identify three sub-facies of the riverbed,dike and floodplain,and four sub-facies of riverbed detention,beach deposition,crevasse fan and river overflow swamp microfacies;The Changhangou Formation is in contact with the underlying Zhaogou Formation.The actual measured thickness is about 45 m.The lithology is mainly dark gray mudstone with argillaceous limestone.The surface weathering is grayish-green and yellow-white.The sedimentary environment is lacustrine facies.Two sub-facies,shallow shore lake and deep lake semi-deep lake,can be further identified;the Daqingshan Formation is thrust napped on the Zhaogou Formation with an actual measured thickness 204.6 m.The lithology is a purplish red composite conglomerate,gravelly medium-coarse sandstone,purplish-red mudstone and yellow-green siltstone.Horizontal bedding,parallel bedding,plate-shaped cross-bedding and channel erosion filling structures are developed.The sedimentary environment is alluvial fan braided river delta facies,and braided river facies can be further recognized braided river delta sedimentary facies can further identify braided river delta front sub-facies and underwater distributary channel and interdistributary bay microfacies.Major and trace element geochemical studies show that: 1)paleosalinity indicators(the discriminant diagrams of V/Ni and Sr/Ba values,and the discriminant diagrams of Sr with Li and Ga values,respectively)indicate that Wudanggou Formation,Zhaogou Formation and Daqingshan Formation are freshwater environment,and Changhangou Formation is freshwater brackish water environment;2)The paleo redox index(discrimination diagram of V/Cr and U/Th,Ni/Co,V/(V+ Ni)values respectively)indicates that the study area is an oxidation environment,in which the Zhaogou Formation is weak oxidation environment,and the oxidation degree of Wudanggou Formation to Daqingshan Formation is gradually enhanced;3)The paleowater depth of Wudanggou Formation,Changhangou Formation and Daqingshan Formation is 1.3-8.93 m,indicating shallow water environment;the paleowater depth of Zhaogou Formation is 1.18-32.47 m,indicating shallow water semi deep water environment,and the water body of Zhaogou Formation is the deepest;4)Paleoclimate index: climate index C from old to new is 0.51,0.12-0.60,0.50,0.13-0.34 respectively;chemical alteration index(CIA)is 92,67-86,51,69-76 respectively;Sr/Cu value is 1.94,2.77-36.32,7.11,3.04-12.43 respectively;Rb/Sr is 0.53,0.12-0.67,0.24,0.49-0.72 respectively.The climate environment changed from warm and humid of Wudanggou Formation to semi-arid and hot of Daqingshan Formation.Based on the analysis of Jurassic sedimentary facies and the study of paleoenvironment and Paleoclimate in Shiguai Basin,the change law of Jurassic paleoclimate in Shiguai Basin is compared with that of Erlian Basin,Ordos Basin,Yili Basin,Junggar Basin and Qaidam Basin.The results show that the sedimentary environment of the Jurassic continental basin in northern China is influenced by regional gas with the climate becoming hotter and drier,evaporation strengthened,water body became shallower,salinity increased and oxidation became strong.