KATIAN (ORDOVICIAN) TO AERONIAN (SILURIAN, LLANDOVERY) GRAPTOLITE BIOSTRATIGRAPHY OF THE YD-1 DRILL CORE, YUANAN COUNTY, HUBEI PROVINCE, CHINA

The graptolite succession of the YD-1 drill core provides a new view into the lower Silurian (Llandovery) biostratigraphy of the Yichang region on the Yangtze platform of China. A thick succession of black to greenish, fine to silty shales and siltstones with a variable amount of poorly to well-preserved graptolite faunas ranges from the upper Ordovician Dicellograptus ornatus Biozone (upper Katian) to the lower Silurian Lituigraptus convolutus Biozone (mid Aeronian). The Metabolograptus extraordinarius/Metabolograptus persculptus and Akidograptus ascensus/Parakidograptus acuminatus Biozone intervals are strongly condensed and bear poorly preserved graptolite faunas. A considerable gap can be detected above the Cystograptus vesiculosus Biozone, as the succeeding graptolite fauna indicates the upper Pribylograptus leptotheca Biozone. The interval of the upper Rhuddanian Coronograptus cyphus Biozone to the Lower Aeronian Coronograptus gregarius Biozone is missing. Lituigraptus convolutus was found from 1322.85 to 950.20 m, indicating a thickness of the Lituigraptus convolutus Biozone of at least 378.47 m. The age of the highest available graptolite faunas from the 729.40-750.70 m is inconclusive, but the level may still belong to the Lituigraptus convolutus Biozone. The correlation of the succession with other successions on the Yangtze platform of China indicates the need for a considerable modification of the graptolite biostratigraphy of the Yangtze platform in the Yichang region and revision of the described faunas. The Llandovery appears to be highly incomplete in the eastern flank of the Huangling anticline and the age of the biostratigraphically younger part of the succession cannot be dated precisely with graptolite faunas.

Automated summary

The graptolite succession of the YD-1 drill core in the Yangtze platform of China provides insight into the lower Silurian biostratigraphy of the Yichang region, revealing gaps and the necessity for further modification and revision of the graptolite biostratigraphy. The study highlights the incomplete nature of the Llandovery in the eastern flank of the Huangling anticline and emphasizes the challenges in dating the biostratigraphically younger part of the succession accurately with graptolite faunas.