Sedimentary environment and organic matter enrichment mechanisms of Lower Cambrian Niutitang Formation shale in northwestern Hunan

To further reveal the sedimentary environment and organic matter enrichment mechanisms of the Lower Cambrian Niutitang Formation shale in the Upper Yangtze region, methods such as whole-rock X-ray diffraction mineralogy, organic geochemistry, and elemental geochemistry analyses were comprehensively applied. The mineral composition and paleoenvironmental evolution sequence of the Niutitang Formation shale in the Fenghuang area, northwestern Hunan, were investigated, and the main controlling factors and models of organic matter enrichment were discussed. The results showed that from the bottom to top of the Niutitang Formation in the study area, the carbonate mineral content gradually increased, the clay mineral content gradually decreased, and the felsic mineral content remained relatively stable. Meanwhile, the paleoenvironment exhibited a distinct three-stage evolution. The paleoclimate shifted from warm-humid to dry-cold and then back to warm-humid. The paleoproductivity showed a high-low-high trend. The water redox conditions transitioned from dysoxic-anoxic to oxic and back to dysoxic. The bottom water restriction gradually intensified, and the sedimentation rate followed a low-high-low pattern. During the early sedimentary stage of the Niutitang Formation in the study area, the warm-humid climate-driven high productivity, dysoxic-anoxic preservation conditions, and relatively low sedimentation rate collectively promoted organic matter enrichment. In the middle stage, the dry-cold climate, low productivity, oxic environment, and high sedimentation rate were unfavorable for organic matter enrichment. In the late stage, the return of warm-humid climate, the recovery of productivity, enhanced water restriction, the transition to dysoxic conditions, and the decrease in sedimentation rate once again created favorable conditions for organic matter enrichment. It is concluded that the dynamic coupling of water redox, paleoclimate, paleoproductivity, and sedimentation rate is the key factor controlling organic matter enrichment in the study area.