| The analysis of basin sediment filling characteristics and provenance is not only of great significance to the study of basin reservoir genesis,but also plays an important role in determining the tectonic background of the source area,restoring the sediment transport process and basin filling history,and has become an important method to explore the coupling relationship between basin and mountain.In this paper,aiming at the basic geological problems that restrict the production in this area,the Middle Jurassic-Lower Cretaceous strata,which are the main storage layers of oil and gas,are selected as the research object.By means of sedimentology,geochemistry and isotope geochronology,the sedimentary facies and provenance characteristics of the study area are clarified,and the evolution of basin-range system in the southern margin of Junggar Basin is further discussed.The main understandings are as follows :1.Through the summary of stratigraphic characteristics,lithology combination characteristics and sedimentary structure characteristics,it is concluded that the delta plain subfacies and delta front subfacies are developed in Xishanyao Formation.The Toutunhe Formation mainly develops braided river delta front subfacies and shore-shallow lake subfacies;the Kalazha Formation develops alluvial fan facies;the Qingshuihe Formation mainly develops shallow lake subfacies and alluvial fan facies.2.The content of feldspar(16.67 ～ 54.08 %,average 35.61 %)and debris(5.1 % ～61.11 %,average 30.52 %)in the sandstone debris of each group in the study area is high.The debris is mainly volcanic rock debris(2.24 % ～ 54.44 %,average 24.59 %),and the quartz is mainly single crystal quartz(13.33 % ～ 49.47 %,average 31.11 %).The triangular diagram of sandstone debris components shows that the source area of Xishanyao Formation is the source area of continental plate,and the source area of Toutunhe Formation-Shengjinkou Formation is the source area of magmatic island arc orogenic belt,indicating that there were magmatic activities and orogenic activities in this period.3.The CIA,CIW and PIA indexes indicate that the weathering degree of Xishanyao Formation and Toutunhe Formation is high,which belongs to moderate weathering-strong weathering,and the weathering degree of Hutubihe Formation and Shengjinkou Formation is weak weathering.Zr / Sc-Th / Sc,La / Sc-Co / Th,La / Th-Hf and La / Yb-REE diagrams show that the source rocks of Xishanyao-Shengjinkou Formation are mainly felsic rocks.La-Th-Sc,Th-Zr / 10-Sc and Th-Co-Zr / 10 triangular diagrams show that the source area is mainly the tectonic background of continental island arc.4.Detrital zircon geochronology shows that the synsedimentary zircons of the Qigu Formation-Qingshuihe Formation in the study area are important evidence of Yanshanian regional volcanic activity in the Junggar Basin.In addition to the contemporaneous sedimentary zircons,the Paleozoic zircons also have a peak of 305 Ma.The source of zircons in this age group should be from the North Tianshan and Bogda Mountains,and the closer North Tianshan provides a relatively large source.There are few zircons in the synsedimentary period of the Kalazha Formation,mainly Late Paleozoic zircons,with a peak of 319 Ma and a secondary peak of 423 Ma,indicating that the period received material sources from the central and southern Tianshan Mountains,with the northern Tianshan Mountains as the main source area.The peak age of zircon in Qingshuihe Formation is 300 Ma and 427 Ma.The provenance of this group comes from Tianshan area,and the relative increase of zircon in ancient crystalline basement indicates that Tianshan area is strongly denuded and deeply denuded.However,considering that the northern Tianshan has a certain barrier effect on the provenance of central and southern Tianshan,it is considered that the main provenance area of Qingshuihe Formation is still the northern Tianshan.5.According to the sedimentary filling characteristics of the study area,three first-order sedimentary cycles are divided,that is,the positive cycle from the Middle Jurassic Xishanyao Formation to the Upper Jurassic Qigu Formation,the reverse cycle from the Upper Jurassic Kalazha Formation,and the positive cycle from the Lower Cretaceous Qingshuihe Formation to the Shengjinkou Formation,which can be divided into seven second-order sedimentary cycles.6.In the Early-Middle Jurassic,the climate was warm and humid.The basin-mountain pattern in the Tianshan area was characterized by a large sedimentary range of the basin and a small positive terrain of the Tianshan Mountains.The Tianshan Mountains suffered continuous erosion and decapitation,and the climate turned to drought during the sedimentary period of the Toutunhe Formation.From the late Late Jurassic to the early Early Cretaceous,the tectonic activity in the Tianshan area was enhanced,the quasi-plainization ended,the basin boundary moved northward,and the aridification of the climate reached its peak.During the sedimentary period of the Lower Cretaceous,the structure of the southern margin of the Junggar Basin was relatively stable,the basin subsided rapidly,the basin expanded,and the climate became warm and humid.In general,the Middle Jurassic-Early Cretaceous Tianshan experienced a process of denudation-roofing-rapid uplift-reerosion,and the basin experienced expansion-shrinkage-expansion,and the paleoclimate was humid-arid-humid. |
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