A Shallow Water Case of Ordovician Marine Red Beds (South China): Evidence from Sedimentary Structures and Response to the Kwangsian Orogeny

Ordovician marine red beds (OMRBs) are widely developed along the margins of Gondwana and represent distinctive limestone facies. These red beds are known for their diverse sedimentary structures and have been described by scholars as the “fashionable facies” in geological history. However, their characteristics and classification remain controversial. Multiple hypotheses about their origin have also hindered a clear understanding of these strata. Therefore, this study focuses on the Xiangxi area (South China) and presents a detailed analysis of the sedimentary structures of marine red beds, building on previous research on OMRBs in South China. Based on genetic features, we divide the most debated “nodule-like” and “cracked” structures—previously identified by earlier researchers—into ten subtypes. Three key genetic end-members are identified among these subtypes: breccia, patch, and argillaceous band. Detailed studies using microslab analysis, scanning electron microscopy, geochemistry, and paleontology were carried out on these three end-members. The results confirm that the Ordovician marine red beds were mainly deposited in a shallow marine environment, with the red coloration primarily derived from continental sources. As the sea level rose, the color of the red beds lightened, and the dominant sedimentary structures shifted from breccia end-members to argillaceous band end-members. Additionally, this study identified a vertically penetrating argillaceous band controlled by syndepositional compressive stress, which may be linked to NW-directed compression from the Kwangsian Orogeny. Evidence from tectonic styles, biofacies migration, and chronostratigraphy supports this hypothesis.