Conodont diversification during the Ordovician: A perspective from North China and Tarim (Northwestern China)

Review of the literature on Ordovician conodont diversification in palaeoplates of North and Western China reveals that four diversity peaks are present in North China, occurring in the middle Tremadocian, early Floian, late Floian, and late Darriwilian, with three of these peaks (excepting that in the late Floian) also being recorded in Tarim. Three diversification intervals are present in North China, during the Tremadocian, late Floian, early and middle Darriwilian; comparable intervals are observed in the early and late Tremadocian, early Floian, and the Middle Ordovician in Tarim. The main conodont diversification episode in both palaeoplates took place in the Darriwilian, at the time of the Great Ordovician Biodiversification Event. A comparison of conodont diversity patterns in different palaeoplates (North China, Tarim, and South China) demonstrates that conodont radiation events mainly occurred within the Tremadocian, Floian, and Darriwilian. Conodont diversifications in these paleoplates also display some differences. In contrasting with Tarim and South China, North China witnessed a rapid conodont diversification during late Floian time. Conodont diversity in North China and Tarim increased continually and reached a peak in the late Darriwilian, concurrent with a prominent decreasing trend in South China. Differences of conodont diversification in these three palaeoplates may be related to their palaeogeography and tectonic history. When conodont diversifications in North China and Tarim are analysed on the background of palaeoenvironments, the main episodes are seen to be partly coincident with second order sea-level changes, particularly in North China. In general, conodont radiation correlates with large scale transgressions.     

New SHRIMP U-Pb age from the Wuqiangxi Formation of Banxi Group: Implications for rifting and stratigraphic erosion associated with the early Cryogenian (Sturtian) glaciation in South China

In South China, the Wuqiangxi Formation of the Banxi Group and its equivalents underlie the early Cryogenian (Sturtian) glacial deposits but their thickness varies from <200 m to >2000 m. In the Guzhang section of western Hunan, the Wuqiangxi Formation is only 152 m thick, and an ash bed 58 m below the glacial diamictite yielded a SHRIMP U-Pb age of 809.3±8.4 Ma. In contrast, 90 km south of the Guzhang section towards the basin in Zhijiang area where the Wuqiangxi Formation is ~2200 m thick, an age of 725±10 Ma has been reported from the top of this unit, 300 m below the glacial diamictite. These ages provide new evidence for the regional stratigraphic correlation across the Nanhua basin, and suggest unusually large (>2 km) stratigraphic erosion potentially associated with the Sturtian glaciation in South China. The magnitude of erosion may imply significant uplifting and tectonotopography at the onset of the Sturtian glaciation.