The Chemical Characteristics And Paleoenvironmental Analysis Of Devonian Karamaili Area Of The Junggar Basin

The Karamaili area in Xinjiang has always been the focus of many scholars.In particular,the Devonian is the key period for the development of the Karamaili Ocean.In this paper,the sedimentary facies and sedimentary environment of the Middle-Lower Devonian Karamaili Formation and the Upper Devonian Keankuduke Formation in the eastern part of Fuyun County Animal Husbandry Station are studied through the field profile measurement,geochemical analysis,detailed lithologic facies-sedimentary facies interpretation and geochemical mapping of the outcrop section,and combined with previous research results.The contents of Si and Al are relatively uniform in the study area,and Si,Mg,Na and P are relatively enriched.The elements of Ca,K and Ti are in a state of loss;the changes of trace elements are extremely complicated,and the occurrence states of some elements are different among the samples.There are some good regularities of the rare earth elements in the samples of Karamaili and Keankuduke Formations.The light rare earth elements are relatively enriched,and the degree of differentiation is close to that of North American shale,the degree of differentiation is medium,and the degree of differentiation of heavy rare earth elements is weak.Combined with the characteristics of the lithofacies and sedimentary facies of the section,the section is divided into seven lithologic sections.The Karamaili Formation is divided into six lithologic sections,the sedimentary facies mainly includes continuous sedimentary environment changes from coastal shallow sea to semi-deep sea and deep sea facies,with a small amount of turbidity deposits.The Upper Devonian Keankuduke Formation is the 7th lithologic section,which can be divided into upper and lower parts.The alluvial fan facies sediments developed in the lower part,and the littoral and shallow lacustrine sediments developed in the upper part.Based on the previous research data and the sedimentary facies analysis of Karamaili Formation and Keankuduke Formation in this paper,the indicates that in the early Late Devonian,the ocean basin closed and the ocean disappeared.Using geochemical parameters such as Sr/Ba,Mg/Al,V/Cr,Sr/Cu,?Ce,?Eu,and according to the features of lithology profile,the paleosalinity of the sedimentary water body should be gradually increased,from the bottom to the top of the Karamaili Formation profile,but it will decrease in the earlier stage,and the trend is that the salinity is gradually reduced when the Keankuduke Formation was deposited.For the paleo-water depth,the water depth of the Karamaili Formation is shallower before the sedimentation,and then gradually rises.From the Karamaili Formation to the Keankuduke Formation,the depth of water body tends to shallow.The depth of water gradually increases when the Keankuduke Formation is deposited.Combined with Sr/Cu and other paleoclimatic chemical indicators,during the formation of the Karamaili Formation and Keankuduke Formation,and gradually become relatively dry and hot.As for the study of paleoredox facies,it is believed that the formations should be formed in a relatively anoxic environment.But when Karamari Formation was deposited,the reducibility of water body first increased,then decreased.When Keankuduke Formation was deposited,the reducibility of water body increased.Based on the geochemical diagrams of La/Yb-REE,Co/Th-La/Sc,La-Th-Sc,Th-Hf-Co,etc,it is concluded that the source rocks in the study area are mainly felsic volcanic rocks,and the sedimentary substance comes from the upper crust.The tectonic setting of the sediment source area is dominated by the continental island arc environment,with some tectonic changes.