Characteristics of Cambrian Oolitic Rocks in Northern Sichuan, South China: Implications for the Aragonite–Calcite Sea Transition

The Cambrian Period represents a critical yet debated interval in the global transition from “Aragonite Seas” to “Calcite Seas”. This study reconstructs the physicochemical evolution of paleoseawater through microstructural analysis and trace element geochemistry of Cambrian oolitic rocks in the northern Sichuan Basin, South China. Our results demonstrate that micrite envelopes on ooid margins and early submarine cements (Stage 1) effectively least-altered signals, resisting diagenetic alteration. Consequently, the maximum values of trace element in these fabrics serve as reliable proxies for paleoseawater reconstruction. Ooids from the upper Canglangpu Formation to the Longwangmiao Formation (Lower Cambrian, Series 2) are characterized by concentric laminations with tangential ultrastructures, high Sr contents (up to 1536 ppm), and high seawater molar Mg/Ca ratios (hereafter mMg/Ca, up to 5.02). These features contrast sharply with the radial fabrics, low Sr contents (