Architecture, lithology, and lithofacies of the Tosna and Koporye formations in their type sections (Lower Ordovician of North-West Russia)

The paper specifies the architecture and lithology of the Tosna and Koporye formations of the Izhora Upland, based on two sections: predominantly black shale (the Lamoshka River, including the Koporye Formation stratotype) and sandy (the Tosna River, including the Tosna Formation stratotype). The authordistinguishes four lithotype groups: kerogen-bearing quartz-hydromica mudstone, predominantly quartz siltstone/sandstone, heterolithic laminite, and spiculite. There is proposed a shoreward-coarsening facial profile of a storm-dominated shelf including seven lithofacies. The offshore and shoreface lithofacies are presentedby black shale and sandstone, respectively. Between them there are transitional lithofacies of interbedded black shale and very fine-grained sandstone; toward the shore, the sand layers become thicker and coarser-grained, while the shale layers get increasingly thinner and rarer. The dynamics of facies changes in the Tosna and Koporye formations of the Pakerort Regional Stage aids in identifying three flooding surfaces corresponding to the lower boundaries of the Tosna Formation (FS-1, everywhere), Koporye Formation on the Lamoshka River (FS-2), and the one on the Tosna River (FS-3). The Koporye Formation in essentially black shale sections is assumed to completely replace the upper subformation of the Tosna Formation.