Sedimentary Differences between ~ 1.64 Ga Shales along the Northern and Southern Margins of the North China Craton: Implications for the Mid-Proterozoic Basin Evolution

The interval at ca. 1.64 Ga represents a critical stage in mid-Proterozoic Earth history, marked by important changes in ocean redox conditions and the early evolution of eukaryotes. Coeval shales developed along the southern and northern margins of the North China Craton (NCC) provide an important archive for reconstructing marine environmental evolution during this period. However, debate persists as to whether a unified regional sedimentary pattern existed across the NCC at that time, and whether ocean connectivity had been established between its southern and northern marginal seas. In this study, we investigate the dark shale of the Cuizhuang Formation in the Xiong’er basin on the southern margin and the Chuanlinggou Formation in the Yanliao basin on the northern margin, respectively, integrating regional geological constraints, whole-rock geochemistry with total organic carbon (TOC) and total sulfur (TS), to evaluate ocean connectivity and spatial environmental differentiation at ca. 1.64 Ga. Both formations show strong overall similarity in most geochemical indicators. They are dominated by felsic detrital input, record broadly comparable weathering intensity and nutrient supply, and contain low to moderate organic matter contents. Redox-sensitive elements are weakly enriched, indicating deposition under suboxic to weakly reducing conditions rather than in a persistently euxinic basin. Although the two units are highly similar overall, subtle but systematic differences can still be identified. The Chuanlinggou Formation exhibits slightly higher chemical index of alteration (CIA), P/Ti, and Zr/Sc values, whereas the Cuizhuang Formation is characterized by relatively higher Corg/P and VEF. Overall, these characteristics suggest that the southern and northern margins of the NCC were likely influenced by broadly similar regional marine conditions at ca. 1.64 Ga. In the context of the regional geological background, these observations suggest that the Xiong’er and Yanliao basins were at least partly connected during the early mid-Proterozoic, probably through the Taihang region, and together recorded a regionally coherent yet internally differentiated marine system.