Cambrian–Ordovician sedimentary environmental evolution and dolomitization in the Lower Yangtze region of China by sedimentary microfacies and geochemistry

The Lower Yangtze region in China hosts extensive Early Paleozoic dolomitic sequences, yet their sedimentary environments and genetic mechanisms remain poorly constrained. This study integrates carbonate microfacies analysis with geochemical data (major, trace, and rare earth elements) from three studied sections (Mufushan, Lunshan, Tangshan) and two drill cores (Sure 1 and Sure 2) to elucidate the depositional evolution and dolomitization of the Cambrian–Ordovician Guanyintai, Lunshan, and Honghuayuan formations. Totally 15 types of carbonate microfacies were identified, revealing a transition from supratidal sabkha (e.g., powder-crystal dolostone with geopetal structures) to shallow shoal environments (e.g., dolomitized oolitic grainstone). Geochemical signatures (MgO–CaO covariation, Sr depletion, and REY patterns) indicate that the Guanyintai Formation dolostone formed via reflux penetration of Mg 2+ enriched brines in shallow burial settings, while the Lunshan Formation experienced superimposed burial and hydrothermal dolomitization, evidenced by saddle dolomite textures and positive δEu anomalies. Contrarily, the Honghuayuan Formation limestones exhibit significant terrigenous input (elevated Al 2 O 3 , Zr, and SiO 2 ). This study establishes a dual-phase genetic model for Cambrian–Ordovician dolostone in Lower Yangtze region, emphasizing the interplay of evaporative reflux and hydrothermal activity. These findings provide critical insights for predicting dolostone reservoir distribution and optimizing hydrocarbon exploration in analogous marine carbonate systems.