Upper Devonian сalcareous algae of the Northern part of the Lviv Paleozoic Trough (systematic composition, facial timing and stratigraphic significance)

As a result of the study of core material in the volume of 211 thin sections from five boreholes of the Northern part of the Lviv Paleozoic Trough, a complex of fossil green, red, and charophyte algae was revealed, which consists of 20 species of 13 genera of green, red, and charophyte algae, among which five species of five genera are identified for the first time for the Devonian deposits of the Lviv Paleozoic Trough. The carbonate deposits belong to the Frasnian and Famennian stages and are represented mainly by sandstones, argillites, siltstones, limestones, dolomites, marls, and white chalk. Carbonate deposits are represented mainly by pelitomorphic wackestones, packstones, wackestone-packstones, grainstones, grainstone-packstones, micrite, bioclastic wackestones, and microbioclastic pelitomorphic wackestones. Limestones often have a pelitomorphic texture, enriched with sand material or dolomitized. The algae are confined primarily to pelitomorphic wackestones of the Waulsortian type, which indicates the intermediate facies of the shelf margins, which were formed by the open circulation of sea waters. To a lesser extent, algae occur in packstones, grainstones, wack-packstones, and are found predominantly along the remains of foraminifera, crinoids, bryozoans, ostracods, and sponge spicules. It is revealed that the algal assemblage of the Famennian stage is depleted and represented mainly by the green tubular algae of the families Palaeoberesellaceae Mamet et Roux and Dasycladaceae Kützing. Higher up the section, a few charophyte algae of the genus Planoumbella Platonov are found. The assemblage of the Famennian stage is significantly enriched, both numerous and systematic. Numerous charophyte and red algae appear, especially in the upper part of the section, which corresponds to the stratigraphic level of the zone C1ta of Donbass. These deposits were considered for a long time to be Tournaisian until at the All-Union Congress in 1986 the Devonian-Carboniferous boundary at the base of the Gattendorfia (Acutimoceras) goniatite zone and the Siphonodella sulcata conodont zone was drawn, which represents the top of the C1ta zone of Donbass. At this level, the richest and most varied algae assemblage is revealed. Above this level, representatives of the genus Parachaetetes Deninger and charophyte algae are not found. The prevalence of the green siphoncladal algae in the Frasnian association indicates a calm hydrodynamic regime below the level of the tide, as well as the depth of the basin, which does not exceed 3-5 m, slight fluctuation in the depth of the sea basin, good illumination, and warm water. The significant distribution of charophyte algae in the Famennian time indicates a decrease in salinity in the paleobasin due to the flow of fresh water from the Kovel uplift.