伊犁盆地南缘中侏罗统西山窑组沉积环境讨论

伊犁盆地南缘是中国重要的煤和砂岩型铀矿富集区,中侏罗统西山窑组是主要的赋矿层位。基于对野外剖面的实测、室内岩矿鉴定、碎屑粒度统计分析和钻测井等资料的解释,明确了伊犁盆地南缘中侏罗统西山窑组的沉积环境类型、砂体成因、展布及其煤系地层分布的耦合关系。研究结果表明:伊犁盆地南缘西山窑组为曲流河三角洲平原-前缘沉积环境,分流河道微相的砂岩以高含煤屑或碳质、中粗结构、富岩屑和长石、分选性较差、中等磨圆为特征,具有较低的成分成熟度和结构成熟度。粒度曲线以三段式为主,显示了三角洲平原水道跳跃、滚动和悬浮搬运的特征。电阻率曲线纵向上表现为粒度向上变粗的逆粒序。砂体厚度多在20~40 m之间,砂地比值在0.3~0.8之间。分流河道砂岩与分流河道间沼泽相的煤层或泥炭在平面上毗邻、剖面上交互产出,古植物茎干化石和植物印痕多处存在。曲流河三角洲沉积环境和三角洲砂岩与煤系地层共生耦合关系的提出,对伊犁盆地南缘煤铀富集规律的深入研究提供了沉积学支持。

Discussion on the Sedimentary Environment of the Middle Jurassic Xishan-yao Formation in theSouthern Margin of the Yili Basin

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 environ-ment types, genesis and distribution of sand bodies, and its relation with coal-bearing stratum distributions in the Mid-dle Jurassic Xishanyao Formation at the southern margin of the Yili Basin were identified. Results show that the Xish-anyao 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.