Mineralogy And Geochemical Characteristics Of Mudstones Of The Jurassic-neogene In The Southern Margin Of The Junggar Basin

In the southern margin of the Junggar Basin,the structural zoning belongs to the northern Tianshan piedmont thrust fold belt.The sedimentary strata are well developed,the color of the strata changes from bottom to top,the outcrop layer is exposed,and the clay minerals are abundant in the mudstone.It is systematically developed.The study of the dynamic evolution of shale caprock sealing and the source of sediment material,sedimentary environment and tectonic setting lay a material foundation.For this purpose,the mudstone samples in the outcropping areas of the southern margin of the Junggar Basin were chosen as research objectives,and the mudstone clay mineral composition and geochemical characteristics were systematically studied.It was found that the main mineral composition of mudstone in the study area was clay minerals 30.2%~59.4%,followed by quartz,albite and other terrigenous debris materials.Among them,the Lower Jurassic mudstones are mainly grayish black and grayish green,and the clay minerals are kaolinite,Iraqi/Montmorillonite,chlorite,and illite.The water medium is acidic and formed in a warm and humid climate.The Cretaceous Lower Jurassic mudstone is dominated by grayish green and purplish red,and the clay mineral combination types are Iraqi/Mongolian mixed layer,chlorite,and illite.The water medium is alkaline and formed in arid climate conditions.Paleogene mudstones are dominated by green and red,and clay mineral types are composed of Iraqi/Mongolian mixed layers,illite,chlorite,not containing or slightly containing kaolinite,and the water medium is alkaline and formed in arid climate conditions.The Neogene mudstones are dominated by grayish green and reddish purple.The types of clay minerals are Iraqi/Mongolian mixed layer,chlorite,illite,and no or slightly kaolinite.The water medium is alkaline and formed in arid climate conditions..The Jurassic-Neogene sedimentary rocks have lower chemical alteration index(CIA)and higher compositional variation index(ICV)characteristics,with a distribution range of 48.31 to 76.91 and 1.26 to 2.57,reflecting low chemical weathering in the source area.The low maturation of sediment composition indicates that the tectonism of the source area is intense and it belongs to the tectonic environment of rapid erosion and transportation.Rare earth elements in different areas have similar fractionation characteristics,Eu negative anomaly,Ce anomaly difference is not obvious,light and heavy rare earth fractionation is obvious,light rare earth enrichment overall content is much higher than heavy rare earth,according to Th/Sc,Cr/Th,Co/ The ratios of Th,Cr/Zr and La/Sc indicate that the parent rocks are mainly felsic,and the La-Th-Sc,Th-Sc-Zr/10,K2O/Na2O-Si O2 structures The illustration shows that the tectonic setting of the source area is mainly close to the active continental margin of the continental island arc.