Abstract:

The Cambrian is an important period for global phosphate deposition, and the Cambrian Xinji Formation is an important phosphorus-bearing sequence on the southern margin of North China Craton. However, the relationship between the phosphorus-bearing stratigraphy and the sedimentary environment is poorly understood, and the phosphate enrichment mechanism needs to be further studied. Here, we studied the middle Xinji Formation at Baofeng, which contains a typical phosphorus-bearing layer. Field observations and laboratory analyses were conducted to reveal its depositional environment and explore its phosphorus source and enrichment mechanism. The results show that phosphorus nodules from the Xinji Formation are black, irregular flat disk-shaped, with size of 2 to 20 mm, and are distributed in the fine sandstone beds. Collophanite is the main mineral in the nodules. There are two forms: (1) cryptocrystalline rounded colloidal phosphorite with no internal structure and locally cracked; (2) microcrystalline oolitic colloidal phosphorite with fine veins. The depositional environment of the Xinji Formation was likely littoral-shallow marine transition. Phosphorus was likely derived from deep oceanic phosphorus-rich water and small shell biological activities. Formation of the phosphorus nodules was likely dominated by chemical deposition with indirect biological involvement. It has undergone syn-deposition and early diagenesis stages, and large amounts of phosphorus nodules were deposited in turbulent shallow water. Compared with the Cambrian phosphorus-bearing facies in the Yangtze Craton, the phosphate-bearing stratigraphy is clearly diachronic, and phosphate deposition occurred after the sedimentary hiatus. Initial deposition of the Cambrian phosphate was dominated by biological and chemical processes, and then the depositional environment and paleotectonics controlled the phosphorus-bearing stratigraphy and promoted the re-enrichment. We point out the relationship between phosphorus enrichment and the Early Cambrian sedimentary environment in the southern North China Craton, and provide abundant data for studying large-scale phosphate deposition.

Key words: Southern North China Craton margin, Xinji Formation, phosphorus-bearing stratigraphy, sedimentary environment, enrichment mechanism