Sedimentary mechanism analysis of “Lower Coal and Upper Uranium” strata structure in Junggar Basin, Northern China

Coal and sandstone-type uranium deposits in the Mesozoic coal-bearing basins in Northern China often show the stratigraphic structure of “lower coal and upper uranium”, indicating a good co-occurrence relationship between coal and uranium deposit. In order to precisely restrict the role of sedimentary factors in coal accumulation and uranium enrichment, and to explore the sedimentary mechanism of coal and uranium co-occurrence, we selected the Zhundong coalfield and its adjacent areas in the eastern Junggar Basin, where there are two sets of “lower coal and upper uranium” strata, as the research object. Based on the stratigraphic sequence analysis and isochronous stratigraphic framework construction, detailed sedimentary characterization of coal measure strata and an in-depth discussion of sedimentary evolution law are carried out. The results show that the Jurassic strata in the Zhundong coalfield and its adjacent areas is a complete second-order sequence consistent of multiple third-order depositional cycles of retrogradation-progradation, and the top and bottom are limited by regional unconformities. The retrogradation-progradation sedimentary cycle is the main reason for the superposition of coal measure strata and uranium reservoir. It controls the change of sedimentary environment and the spatial configuration of materials by accommodating spatial changes, and progradation corresponds to stronger coal accumulation and better uranium reservoir sand body than retrogradation. Moreover, The second-order sedimentary cycle has stronger compulsion than the third-order sedimentary cycle, which restricts “lower coal and upper uranium” formation structure of Badaowan Formation in the regressive environment in the second-order cycle to be much smaller than that in Xishanyao-Toutunhe Formation in the progradative environment. All these results will provide a theoretical basis for the prospect prediction of uranium mineralization in other coal-bearing basins, especially for the identification of new uranium reservoirs and new prospect areas.