Lithostratigraphy of the Middle Ordovician (darriwilian) of the Volkhov River: Aseri, Lasnamägi and Uhaku regional stages

The key section of the Aseri, Lasnamägi and Uhaku Regional Stages (Middle Ordovician) on the Volkhov River is formed by the Duboviki, Porogi-Valim and Vel’sy formations. The Duboviki Formation (Aseri RS) corresponds to silt-argillaceous limestone unit with Echinosphaerites aurantim infra Hecker. It is subdivided into Kavra, Mel’nitsa, Suosaari and Jhikharevo members. The new Porogi-Valim Formation consists of Porogi Subformation (the top of the Aseri RS and Lasnamägi RS) and Valim Subformation (lower part of the Uhaku RS). The PorogiValim Formation shows decreasing carbonate content towards the top and is presented by intercalation of hard limestone units (0.4–1.2 m thick) and silt-argillaceous limestone units (0.16–2.4 м). The hard limestones dominate in the Porogi Subformation, the silt-argillaceous ones – in the Valim Subformation. The Vel’sy Formation (upper part of the Uhaku R. Stage) is formed by intercalation of marls and highly silt-argillaceous limestones. A common feature of the Ukhaku RS of the Volkhov River is a combination of a high silt-argillaceous background (limestones/marls) with frequent interlayers of pure bioclastic grainstones (tempestites). The lower boundaries of the formations and subformations are marked by mineralized hardgrounds and their groups, which can be traced from the Volkhov River to western Estonia.