Late Cretaceous paleoclimate change and its impact on uranium mineralization in the Kailu Depression, southwest Songliao Basin

Abstract Paleoclimate significantly influenced continental sandstone-type uranium mineralization. To better reveal the Late Cretaceous continental climate change and its influence on uranium mineralization, the Late Cretaceous sedimentary successions of four boreholes drilled in the southwest of Songliao Basin, Northeast China, were selected, and 21 samples were collected for sporopollen analysis. Based on the variations in the relative abundances of different sporopollen taxa, the following four palynological assemblages (PAs) have been recognized in ascending order: the Schizaeoisporites–Classopollis–Tricolporollenites assemblage, the Taxodiaceaepollenites–Exesipollenties–Cranwellia assemblage, the Schizaeoisporites–Classopollis–Lytharites assemblage and the Biasccate–Taxodiaceaepollenites–Aquilapollenites assemblage. The geologic era of four PAs was assigned to late Turonian to early Campanian (about 86.7–84.4Ma) according to the geological range of some important elements and the correlation with relevant assemblages of other areas. In this paper, we introduced a sporopollen–climate transforming methodology, which used the percentage of drought taxa, hygrophilous taxa, and thermophilic taxa to indicate humidity and temperature stratigraphic trends. On the one hand, the percentage of thermophilic taxa presented a continuous decreasing trend but remained a relative high abundance along the whole sedimentary sequence. This was indicative of a hot subtropical-tropical climate with declining temperature. On the other hand, the percentage of drought taxa and hygrophilous taxa exhibited cyclical fluctuations between humid and semi-arid climates. A semi-arid climate existed in PA I, then the climate changed into humid conditions in PA II. Subsequently, the climate has evolved into semi-arid, semi-humid conditions as result of the strengthened drought in PA III. Finally, the climate became humid once more in PA IV. Generally, the synsedimentary paleoclimate of Yaojia Formation (which equates to the PA II and PA III) was mainly semi-humid conditions. Moreover, the main prospect target layer was Yaojia Formation in the Qianjiadian area. Comparison with the typical sandstone-type uranium deposits in North China, we inferred that the humid paleoclimate was an essential conditions for uranium mineralization, because the humid climate was favorable for the formation of reduced sandstones, providing significant geochemical reducing barriers and pre-concentration of uranium for the epigenetic uranium mineralization.