Biostratigraphy and biofacies of the Middle Darriwilian (Ordovician) conodonts from the Laoshidan section in the western margin of the North China Craton

• Three Histiodella-based zones are recognized and internationally correlated.
• Conodont fauna from the Laoshidan section suggests an early to mid Dw2 age.
• Four conodont biofacies are identified using multivariate techniques.
• Estimating sea-level changes only through the conodont biofacies is irresponsible.

Middle Ordovician conodonts from the Sandaokan and Zhuozishan formations of the Laoshidan section at the western margin of the North China Craton, consist of 39 identified and six indeterminate species belonging to 21 genera, and three taxa left in open nomenclature at both genus and species levels. Of particular biostratigraphic importance is the presence of morphologically advanced species of Histiodella associated with several age-diagnostic endemic taxa, making it possible to recognize international conodont biozones in the main part of the sequence. Three Histiodella-based zones are recognized in the studied section; including the Histiodella cf. holodentata, Histiodella kristinae and Histiodella bellburnensis zones, in ascending order. They can be directly correlated with the conodont zonations established in western Newfoundland, Canada and Tarim, western China, and compared well with the conodont biozones of Baltoscandia and Yangtze, central China. However, the same stratigraphic interval on the North China Platform is barren of conodonts. Multivariate statistical studies on these conodonts allow recognition of four conodont biofacies: Scalpellodus biofacies, Panderodus biofacies, Drepanoistodus biofacies and Phragmodus biofacies. Turnovers of the conodont biofacies are related to either sea-level changes or mixing of shallow water conodonts due to down slope transportation. Excluding the effect of the allochthonous conodonts, the transgressive-regressive patterns demonstrated by the conodont biofacies compare closely to the published sea-level curves for the western margin of the North China Craton.