Middle Jurassic - Early Cretaceous drastic paleoenvironmental changes in the Ordos Basin: Constraints on sandstone-type uranium mineralization

Abstract

Middle Jurassic - Early Cretaceous paleoenvironmental evolution is fundamental to U (Uranium) migration and U-enrichment in the Ordos Basin. However, the detailed long-term terrestrial climate evolution across the Jurassic/Cretaceous (J/K) transition is still lacking. This study presents a comprehensive geochemical element analysis of 88 fine-grained sediments from Well Pengyang-SDI to verify the major paleoenvironmental changes including the changes in paleoclimate, paleoweathering, paleosalinity, and paleo-redox conditions. Various paleoweathering proxies (e.g., CIA) suggest that the sediments in the source area experienced a stronger chemical weathering process during the Jurassic than during the Early Cretaceous. The paleoclimate proxies (e.g., C–value, Rb/Sr, and Sr/Cu ratios) and the sharp increase in the salinity of the Luohandong Formation (LHDF) in the Ordos Basin may provide strong evidence for the continuous aridification since the Early Cretaceous. In the Lower Cretaceous non-uranium mineralization section in the basin, large U primary enrichments (high U/Th ratio) may be related to the rise in atmospheric oxygen levels during the Cretaceous. This remarkable paleoenvironmental transition controls the spatial–temporal collocation of the coal and oil in deep strata and the uranium in upper host rocks. The large-scale hydrocarbon migration from the deep strata transformed the Cretaceous aeolian sandstones into favorable uranium reservoirs. Core alteration observation and iron content testing shows that the large-scale oxygen-containing uranium surface fluid came from the western margin of the Tianhuan Depression. Finally, based on the analysis of the genetic relationships among uranium, coal, hydrocarbons, and alteration this study proposes that regions adjacent to the western margin of the Changqing oilfield with large faded alteration in the Lower Cretaceous strata are new ideal target areas for further uranium exploration.