DEPOSITIONAL CONTINUITY DEMONSTRATED BY MULTIPLE CONODONT CHRONOMORPHOCLINES THROUGH THE BASHKIRIAN-MOSCOVIAN BOUNDARY INTERVAL, NAQING SECTION, GUIZHOU, SOUTH CHINA

A succession of conodont chronomorphoclines occurs throughout the Bashkirian-Moscovian boundary interval at the Naqing section in South China. The transitional conodonts demonstrate that deposition was remarkably continuous through the boundary interval, a major criterion for selecting a Global Stratotype Section and Point (GSSP). We present the current state of knowledge for several of these chronomorphoclines, and provide an updated range chart of conodonts recovered from the Naqing section and their correlation with other regions.

Although the Moscow Basin provides the working stage terminology for international chronostratigraphy, the base of the type Moscovian is unconformable—with marine strata sitting on basal alluvial deposits. The taxon that best matches the current concept for the base of the Moscovian Stage in its type region is Diplognathodus ellesmerensis. An ancestral form with most of the characteristics of D. ellesmerensis occurs just below D. ellesmerensis sensu stricto at Naqing. More specimens are needed to completely document the chronomorphocline, but because D. ellesmerensis is found worldwide—including at the base of the Type Moscovian—its evolutionary first occurrence would provide an almost ideal GSSP definition. Other ancestor-descendent successions at Naqing include transitions within“Streptognathodus” expansus and “Streptognathodus” suberectus sensu lato, and between multiple species in Idiognathodus, Gondolella, and Mesogondolella.

The Naqing section is located along a major road cut that exposes Devonian through Permian strata in the mountains between Loudian and Wangmo in Guizhou Province, South China. The general setting represents a platform margin to slope depositional environment. The beds in the Bashkirian-Moscovian boundary interval consist of gray thin- to medium-bedded wackestones and packstones intercalated with chert. These strata dip approximately 60-70 degrees, are 100 per cent exposed, and are easily accessible along the extended road cut. Conodonts are very abundant in the boundary interval, with more than 45 species representing 11 genera. All of the numerous samples that have been collected have produced conodonts. These characteristics make the Naqing section an ideal candidate for the basal Moscovian GSSP.