Conodonts and radiolarians through the Cisuralian–Guadalupian boundary from the Pingxiang and Dachongling sections, Guangxi region, South China

Abstract

Serrated gondolellid conodonts (Jinogondolella) are used to define the Cisuralian–Guadalupian (C–G) or Kungurian–Roadian (K–R) boundary in the stratotype sections of West Texas and in some sections of South China, but in other sections Jinogondolella is absent. This absence has been interpreted to be the result of strong provincialism during this interval and geographic clines have been demonstrated between an Equatorial Warm Water Province (EWWP) and a North Cool Water Province (NCWP). Results from a succession of deep-water limestones in the Pingxiang section, southwestern Guangxi, China, suggest that the distribution of warm-water and cool-water lineages may not in all cases be separated over large geographic scales, but may be segregated in some areas by local changes of water-mass temperature or depth and, in some cases, fossil assemblages from these lineages may be mixed. Two contemporaneous gondolellid evolutionary lineages are represented within warm-water and cool-water conodont assemblages from limestones spanning the Cisuralian–Guadalupian (C–G) boundary in the Pingxiang section, southwestern Guangxi, China. The warm-water conodont lineage begins with the first occurrence (FO) of Mesogondolella saraciniensis, followed by Mesogondolella siciliensis, then M. idahoensis lamberti and finally Mesogondolella pingxiangensis sp. nov. The latter new species lacks serration, but has a platform outline and lateral profile similar to J. nankingensis nankingensis found elsewhere in South China and they may be closely related. Although less common in the samples, a coeval cool-water lineage begins with M. idahoensis idahoensis and develops successively with the evolution of J. nankingensis gracilis; serration is also lacking in the latter species. The lack of serration on these forms is ascribed to some form of environmental control. The contemporaneous appearance of Mesogondolella pingxiangensis and Jinogondolella nankingensis gracilis is correlated with the C–G boundary. This correlation is supported by the co-occurrence of the zonal indices of the cool-water lineage with important zonal radiolarians in a C–G boundary section at Dachongling, southeastern Guangxi. Here the radiolarian assemblages are assigned to the Albaillella foremanae Zone, Pseudoalbaillella globosa Zone and P. fusiformis Zone in ascending order. The first appearance of J. nankingensis gracilis within this succession defines the base of the Guadalupian to be at the base of the P. globosa Zone. Conodont correlation between carbonate ramps of platform margins and radiolarian-bearing pelagic environments has determined that the first appearance of J. nankingensis gracilis precisely defines the base-Guadalupian boundary in the cool-water settings.