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Cyclostratigraphy is widely applied in establishing high-precision sequence stratigraphic frameworks and astronomical time scales, making it an important field in the quantitative study of sedimentology. A detailed cyclostratigraphic analysis of natural gamma-ray logging data from the upper cretaceous Sifangtai formation in Qian’an area of Songliao basin was conducted in this paper. Spectral analysis results show that Sifangtai formation exhibits sedimentary cycles of 40.2, 10.6-13.1, 3.6-3.8, and 1.79-2.4 m. The ratio of these cycles is approximately the same as the ratio of Late Cretaceous astronomical orbital periods, specifically 20∶5∶2∶1, corresponding to sedimentary cycle periods of 405 ka long eccentricity, about 106 ka short eccentricity, about 38 ka obliquity, and about 21 ka precession cycles. The COCO statistical test results indicate that the sedimentation rate of the Sifangtai formation is about 9.78 cm/ka, which aligns with these astronomical periods. Filtering the sedimentary cycle corresponding to the 405 kyr long eccentricity period, eight sedimentary cycles were identified. Using the 405 kyr long eccentricity period, an astronomical tuning was performed on the GR data sequence, establishing a floating astronomical time scale. This provides a sedimentary duration of 3.23 million years for the Sifangtai formation. By referencing the chronological stratigraphic framework of the Mingshui formation and the Sifangtai formation in the Songke-1 well in the northern basin, the sedimentary duration of the uranium-bearing sand body in the study area was determined. The results show that the optimal sedimentation rate variations of the Sifangtai formation in the study area are consistent with the lithological changes, and can be divided into two high-value zones: the top and the bottom. The bottom high-value zone corresponds to the medium to coarse sandstone section of the lowstand systems tract at the early stage of the Sifangtai formation deposition, which is the main uranium mineralized sandstone layer in the Qian’an area. The top high-value zone corresponds to the regressive systems tract at the end of the Sifangtai formation, which is in conformable contact with the Mingshui formation above it. The thickness of the sandstone revealed by drilling in this section is relatively thin and its distribution is relatively limited. In addition, the sedimentation rate of the lower red mudstone and grayish-white medium to coarse sandstone section of the Sifangtai formation is significantly lower than that of the upper gray mudstone section. This indicates that during this period, under the background of a dry and hot climate, the lake basin shrank, the braided river delta prograded, and the tectonic conditions were stable. The basin accommodation space grew slowly, characterized by overcompensated infilling, and the sedimentary system was stable, favoring the formation of large-scale sand-mud combination structures. This provides favorable strata for the formation of sandstone-type uranium deposits. Uranium mineralization in sandstone in the Qian’an area develops in the lower part of the Sifangtai formation, with the sand body deposition duration estimated to be ((75.36-76.09)±0.40) Ma. The study results indicate that cyclostratigraphic analysis can be effectively applied to the exploration of sandstone-type uranium deposits. It can accurately determine the deposition period of uranium-bearing sandstone layers, making it an effective method for regional target layer identification, comparison, and exploration target area delineation. |
