Research On The Stratigraphic Sequence And Palaeoenvironments Of Jurassic In Daxigou Area In The Northwestern Margin Of Junggar Basin

The study area is located in the Daxigou area adjacent to the northwest of Karamay city and belongs to the Zaire mountain margin adjacent to the Junggar basin.The Triassic-Jurassic sequence is continuous,the rock assemblages of different stratigraphic systems are rich in sedimentary structure,the sedimentary facies are diverse,and the paleoclimate environment is obviously different.However,due to the strong structural transformation and strong denudation of the study area in various geological and historical periods,the accuracy of previous research work is restricted,and the stratigraphic framework in the study area is not unified,and the recognized sedimentary types,Depositional phenomena and explanations are also inconsistent.In this paper,based on the field geological survey,the standard section of Jurassic is established,and the fine progress is made.The stratigraphic sequence,rock assemblage,sedimentary facies and sedimentary environment of Triassic to Jurassic strata in the study area are analyzed.The paleoenvironment changes of Middle Jurassic in Daxigou area of Junggar Basin were studied by analyzing the main elements,trace elements and rare earth elements of the samples collected from the Xishanyao formation of Jurassic in the study area.The study found for the first time that the well-preserved paleo-weathered leaching zone between the Middle Triassic Karamay Formation and the Carboniferous represents a subsurface formation of the Lower Carboniferous from the marine volcanic-sedimentary rocks and different types of intrusive rocks.Nearly 100 million years of weathering leaching,sedimentation and erosion.The debris flow event deposits were identified at the bottom of the Sangonghe formation for the first time,and a stable and contrastive phyllite layer was found near the top of the Sangonghe formation,including 5 species,belonging to the typical terrestrial freshwater lacustrine bioassemblage.The delimitation between the Triassic-Jurassic strata is redefined,especially the Middle Triassic Karamay formation,the Lower Jurassic Badaowan formation,the Lower Jurassic Sangong he formation and the Middle Jurassic Xishanyao formation,in particular the Middle Triassic Karamay formation,the Lower Jurassic Badaowan formation,and the Middle Jurassic Xishanyao formation.The boundary between the Xishan Yao formation of the Middle Jurassic and the Tunhe formation of the Middle Jurassic.A set of grayish-white,light-gray conglomerate and sand-conglomerate strata were developed on top of the delta-shoal shallow lake fine sediments of he Middle Triassic Karamay formation.From the aspects of rock color,gravel characteristics,rock assemblage,sedimentary environment,oil and gas bearing property,regional correlation and so on,they are included in the lower member of the first set of sedimentary Badaowan formation of Lower Jurassic.The sedimentary facies and sedimentary system developed from bottom to top in the Daxigou profile are defined as follows:the shoreline-delta facies of the Karamay formation of the Middle Triassic(mainly delta front subfacies);Braided river sediments(channel subfacies and plain subfacies)of Lower member of Lower Jurassic Badaowan formation.Upper member of Lower Jurassic Badaowan formation fluvial sediments(channel subfacies and flood beach subfacies),Sangonghe formation shoreline-semi-deep lacustrine sediments(mainly shoreline and shallow lakes);The middle Jurassic Xishan Yao-Toutunhe formation is dominated by meandering river deposits,and fluvial and lacustrine sedimentary cycles are repeated.The middle Jurassic Xishanyao formation in Daxigou area is a transitional period of paleoclimate,ancient water temperature,paleosalinity and paleo-oxidation-reduction environment.From the upper part of Karamay to the early stage of Xishan kiln formation,the climate is warm and humid,the vegetation is more developed,the coal is thin,there are some fossil plants,and the higher plants grow.By the late stage of Xishan kiln formation,the ancient water temperature had obviously decreased,the climate had gradually changed to dry and cold,the water body of river and lake had evaporated seriously,the salinity of water body had increased,and continued to Toutunhe formation,the lithology was dominated by red brown mudstone.There are many layers of gypsum in the strata to form a sedimentary environment with strong oxidizability.