Discussion on the Sedimentary Environment of the Middle Jurassic Xishanyao Formation in theSouthern Margin of the Yili Basin

Key words: 
• southern margin of the Yili Basin /
• middle Jurassic /
• Xishanyao Formation /
• sedimentary environment /
• meandering river delta

Abstract: The southern margin of the Yili Basin is an important area with enrichment of coalmine and sandstone-type uranium deposits in China and the Middle Jurassic Xishanyao Formation is the main ore-bearing strata. Based on field geological section, indoor microscopical identification, grain size analysis, and drilling data, the sedimentary environment types, genesis and distribution of sand bodies, and its relation with coal-bearing stratum distributions in the Middle Jurassic Xishanyao Formation at the southern margin of the Yili Basin were identified. Results show that the Xishanyao Formation on the southern margin of the Yili Basin is dominated by meandering-stream delta plain-delta front. The sandstones of the distributary channel microfacies are highly coal-bearing or carbonaceous, medium-coarse size, rich in debris and feldspar, poor selection and moderate roundness, with relatively low compositional maturity and structural maturity. The three-stage particle-size curve is the main feature, showing the characteristics of the jumping, rolling, and suspension transport in the delta plains. The resistivity curve appears in the longitudinal direction as an inverse grain size with a coarser grain size. Moreover, the thickness of the sand body is mostly between 20-40 m, with the sand ratio between 0.3-0.8. The coal seam or peat in the marsh facies between the distributary channel sandstone and the distributary river channel is adjacent to each other in the plane and alternant vertically. There are many fossils of ancient plant stems and plant imprints. The symbiotic coupling relationship between the depositional environment of the meander river delta, the delta sandstone and the coal-bearing strata has provided sedimentary support for the in-depth study of coal and uranium enrichment in the southern margin of the Yili Basin.