Middle Ordovician carbonate facies development, conodont biostratigraphy and faunal diversity patterns at the Lynna River, northwestern Russia

The Ordovician Period has emerged as a highly dynamic time in Earth history. Comprehensive work on chrono­, chemoand biostratigraphy has resulted in an overall well­constrained systemic framework, but several local successions around the globe still await detailed analysis in many respects. Herein we perform a high­resolution analysis of abiotic and biotic signals in the Lynna River section, a key locality in northwestern Russia. As this section has been pivotal in documenting the temporal evolution of the Great Ordovician Biodiversification Event on Baltica, the macroscopic and microscopic characteristics of the local succession reveal important paleoenvironmental information that ties into the global development during the Middle Ordovician. The results add particularly to the understanding of the characteristics and large­scale sedimentary ‘behavior’ of the Baltoscandian paleobasin. Microfacies vary consistently with the macroscopic appearance of the rocks, with intervals characterized by competent limestone being associated with coarser carbonate textures and intervals dominated by marly beds associated with finer textures. Along with carbonate textures, fossil grain assemblages vary in a rhythmic (~cyclic) manner. The local rocks are commonly partly dolomitized, with the proportion of dolomitization increasing up­section. Regional comparisons suggest that the changes in overall macro­ and microfacies were strongly related to variations in sea level. New high­resolution conodont biostratigraphic data largely confirm previous regional correlations based on lithostratigraphy and trilobite faunas, and enable more robust correlations worldwide.