Early Cambrian Marine Environmental Evolution of the Xinji Formation (north China Craton) Constrained by Sequence Stratigraphy and Facies-Resolved Ree Geochemistry

The early Cambrian marks a critical interval of environmental and biological change, yet the stratigraphic and geochemical records of key successions such as the Xinji Formation remain poorly constrained. The Xinji Formation of the North China Craton preserves important records of this interval, including abundant small shelly fossils and widespread phosphorite deposits. Here we integrate sequence stratigraphy, phosphate microfacies, and fabric-specific rare earth element and yttrium (REE+Y) geochemistry from nine sections to reconstruct palaeoenvironment evolution. The Xinji Formation records two transgressive–regressive cycles, with phosphorite mainly developed within the transgressive systems tract of the first sequence, indicating strong sea-level control on phosphate accumulation. Phosphate components can be divided into in situ phosphates (microsphorite and phosphate cements) and phosphatic allochems (including phosphobioclasts, phosphatic ooids, and phosphoclasts), representing primary–early diagenetic and reworked products, respectively, which provide a process-based framework for interpreting REE+Y signatures in terms of depositional and diagenetic processes. Systematic differences in REE+Y abundances and patterns among phosphate fabrics indicate strong control by sedimentary reworking and early diagenesis. PAAS-normalized REE+Y patterns are characterized by a pronounced MREE bulge and variable Ce anomalies, with predominantly negative Ce anomalies but local deviations in certain sections. This variability suggests environmentally heterogeneous conditions and indicates a likely spatially heterogeneous redox structure in shallow-marine during the early Cambrian. These results collectively record dynamic changes in sea level, redox conditions, and nutrient regimes during the early Cambrian. They enable reconstruction of early Cambrian marine environmental evolution in the North China Craton and provide important environmental context for the Cambrian Explosion.