Cambrian Furongian–Middle Ordovician conodonts in the northeastern margin of the South China Block (Chuzhou, Anhui province) and their paleogeographic implications

Abstract

Abstract. The South China Block is crucial for the global study of Cambrian to Ordovician conodont biostratigraphy, but research on its northeastern margin is relatively scarce. Chuzhou, located at the intersection of the South China Block, North China Block, and the Qinling–Dabie orogenic belt, boasts a significant thickness (>1100 m) of carbonate-dominated sedimentary succession during the Cambrian Furongian to Middle Ordovician period. In this study, detailed field measurements and sample collection were conducted on two well-exposed sections in the Chuzhou area. Nine conodont biozones and three assemblages were identified from the Furongian Stage 10 to the Middle Ordovician Darriwilian, including the Cordylodus proavus, Cordylodus lindstromi, Iapetoganathus fluctivagus, Cordylodus angulatus, Rossodus manitouensis, Colaptoconus quadraplicatus, Paltodus deltifer, Paroistodus proteus, and Triangulodus bifidus zones, as well as the Juanognathus anhuiensis–Protopanderodus gradates, Nasusgnathus giganteus–Paroistodus parallelus, and Dapsilodus virtualis–Protopanderodus robustus assemblages. The analysis of conodont biogeographic zonation indicates that conodonts in the study area were predominantly from the North American Midcontinent Province (warm-water biota) and North Atlantic Province (cold-water biota) from Stage 10 to the middle Tremadocian and from the late Tremadocian to the Darriwilian, respectively. This conodont biogeographic transition is comparable to that in the Tarim, North China, and Qinling blocks but precedes that in the interior of the South China Block, likely associated with the Early–Middle Ordovician global paleogeographic and surface oceanic models that led to the changeover of ocean currents in the study area from warm to cold. Furthermore, the Iapetognathus fluctivagus zone, defined as the Ordovician lowest boundary, was first discovered in the Lower Yangtze region and revealed the distribution of this species in the South China Block. The discovery of Parapanderodus lanceolatus in the early Tremadocian suggests a possible exchange of seawater between the Yangtze and Tarim blocks.