Review on the Benxi Formation, Biostratigraphy, Age and Implications of Sea Level Changes of North China during the Pennsylvanian

As the first strata deposited in the North China Block (NCB) following a prolonged ~150-myr erosion from the end of the Ordovician to the Early Carboniferous, the Benxi Formation plays a crucial role in understanding tectonic evolution, environmental changes, transgressive-regressive cycles, and ecological patterns in North China during the Late Paleozoic Ice Age (LPIA). Composed by marine-continental transitional strata, it unconformably overlies Ordovician dolomite. The weathering crust at the unconformity surface is ferruginous and argillaceous with limy pebbles, forming one of most important iron and bauxite ore deposits in China. It has a wide distribution across the NCB, yet exhibits significant spatial variations in thickness and marked temporal discontinuities. While Late Paleozoic strata in North China have been extensively studied, the Benxi Formation has received relatively less attention, partly due to its limited limestone distribution and overall thin thickness, particularly in recent decades. Several limestone layers occur locally in the upper to middle part of the Benxi Formation, which have undergone relatively extensive biostratigraphy studies of foraminifera during the last century, along with partial conodont biostratigraphy researches. These studies, especially for conodonts, haven’t been updated for nearly two decades, resulting in insufficient temporal constraints to this formation. We summarize research and recent advances on the litho-, bio-, and chemo-stratigraphy of the Benxi Formation. Conodont samples from the Benxi Formation have been newly collected in this study at Heidaigou section, Jungar Banner, Inner Mongolia, to provide updated biostratigraphy and temporal constraints. A westward transgression is identified in limestones of the Benxi Formation and discussed in detail. This process is also documented in other regions, such as Tarim, Europe and Brazil, occurring at the late Bashkirian to the Moscovian, representing a significant global transgressive event after the peak of LPIA.