Cambrian small skeletal fossil and carbon isotope records of the southern Huangling Anticline, Hubei (China) and implications for chemostratigraphy of the Yangtze Platform

Abstract

Rich small skeletal fossil assemblages are described from the Yanjiahe Formation and Tianzhushan Member of southern Huangling Anticline. The Anabarites trisulcatus– Protohertzina anabarica Assemblage Zone (Small Skeletal Fossil Zone 1) is recognized in the middle to upper Yanjiahe Formation, while the thin topmost conglomeratic limestone unit yielded a mixed assemblage resulting from reworking and redeposition of source material of Small Skeletal Fossil Zones 1 to 3 (including Anabarites trisulcatus, Protohertzina anabarica, P. unguliformis, Watsonella crosbyi, Aldanella attleborensis, Anabarella plana). Our revised biostratigraphic correlation indicates the existence of longer hiatuses in the Yanjiahe Formation and between the Yanjiahe and Shuijingtuo formations. The obtained δ13Ccarb values in our profile deviate in some details from those of some previous reports. Based on the new biostratigraphic correlation, the δ13Ccarb trend records values oscillating between −3 to +4.4‰ in the upper Anabarites trisulcatus - Protohertzina anabarica Assemblage Zone, which is unusual for the Yangtze Platform. This interval of positive δ13Ccarb values in the upper Yanjiahe Formation and the Dahai Member of eastern Yunnan are here interpreted to reflect the impact of a local aquafacies. Moreover, it is here argued that the selection of a carbon isotope peak as a primary index tool in the Laolin section for defining the Cambrian Stage 2 must be treated with caution, as this isotope excursion is influenced by a regional water mass and does not reflect an open marine seawater signature, underlining other technical and theoretic concerns on the general application of chemostratigraphy in Cambrian shallow-water carbonate platforms. Overall we suggest that chemostratigraphic correlations need to be supported by biostratigraphic tools.