Late Hirnantian (latest Ordovician) carbonate rocks and shelly fossils in Shiqian, northeastern Guizhou, Southwest China

Abstract

The Kuanyinchiao Formation has generally been regarded as representing the only carbonate sediments deposited during the Hirnantian (Late Ordovician) interval in South China, where the Hirnantian GSSP is situated. Reinvestigation of several key Ordovician-Silurian boundary sections in Shiqian, northeastern Guizhou Province, Southwest China, reveals a lithologically distinct carbonate unit with abundant shelly fossils, including tabulate and rugose corals, conodonts, brachiopods, trilobites and stromatoporoids, many showing close Silurian affinities and suggesting a level stratigraphically much higher than the Kuanyinchiao Formation. This observation, together with further evidence that graptolites indicative of the Akidograptus ascensus biozone are present in immediately overlying rocks at a nearby section, implies that these shelly fossiliferous strata are of latest Hirnantian age (possibly straddling the Ordovician-Silurian boundary). Hence they were most likely deposited after the Hirnantian glaciation, rather than representing glacial cool water sediments (the Kuanyinchiao Formation) as was previously thought. Ordovician-Silurian boundary sequences in South China typically reflect global glacio-eustatic sea-level changes. Owing to their unique lithology and palaeontology, these newly recognized carbonate rocks, here formally named the Shiqian Formation, add substantially to our knowledge of Ordovician-Silurian stratigraphy in this region.