The Sedimentary Characteristics And The Proto-basin Analysis In Jurassic Of The Southern Yili Basin,Xinjiang

Yili basin located in the Tianshan orogenic belt is a large inland intermountain depression basin which is formed by the hedge extrusion between the southern Tarim and the northern Kazakhstan plates. The Jurassic sand-body in southern margin of Yili basin hosts sandstone-type uranium deposit. The distribution of sand-body directly influences the development of sandstone-type uranium deposits. The research on the distribution characteristics of sedimentary structures and Jurassic sedimentary facies is helpful to understand the distribution of the sand-body. Since the Jurassic, Yili basin has experienced lots of compressional tectonic events, and thus its Jurassic proto-basin topography has been changed. Therefore, in this contribution,we restore the proto-basin topography to better understand the distribution characteristics of Jurassic sedimentary facies.In this thesis, using the sedimentology and structural analysis of sedimentary basin theory, on the basis of field geological survey, we comprehensively use the drilling data, combines rock sample size characteristics and analyze the existing geophysical data of the study area, to shed light on the sedimentary characteristics and sedimentary facies marker of Jurassic residual formation to divide the southern margin of the basin tectonic units, and to restore the Jurassic basin original appearance in different periods at the southern margin of the basin.The Lower-Middle Jurassic Badaowan formation, Sangonghe formation and Xishanyao formation and Middle Jurassic Toutunhe formation are developed at the southern margin of Yili basin, but the upper Jurassic not development. The Lower-Middle Jurassic is dominated by continental deposits with transgression of continental facies. Study area develops alluvial fan-delta front depositional system and fluvial-deltaic depositional system. Badaowan formation is a thick layer of purple conglomerate and is a set of alluvial fan deposits, which overlaps underlying Triassic. The Sangonghe formation consists of a gray thick sandstone clipping mudstone, a discontinuity coal bedding which belongs to a deposit of delta front facies. The Xishanyao formation is mainly composed of gray thick sandstone and mudstone, with development of multi-layer coal seam, suggesting it was a shallow water environment, braided river delta front facies and plains facies. The Toutunhe formation is dominated by mudstone, reflecting water become shallower and it is braided river delta plain sediment.During Early Jurassic Badaowan period, the Chabuchar mountain uplifted, and the basin experienced north-south rifting. A plurality of alluvial fan formed in the piedmont then the basin began to expand which defines a strong extension phase. During Early Jurassic Sangonghe period lake further expanded, and study area belongs to a delta front facies, in which defines a weak extension phase. During Middle Jurassic Xishanyao period, water retreat and water entering short cycle developed many times. Then lake water becomes shallow, and the basin experienced regional sustainable and swamp, formed a thick layer of sandstone clipping mudstone layer and multilayer thick coal seam. The study area is a braided river delta front facies and plains facies. During Middle Jurassic Toutunhe period internal water of basin gradually retreat in a braided river plain facies of a braided river plain deposition.The basin experienced a weak uplifting phase at this point.