FACIAL ZONING OF SILURIAN OF THE SOUTHWESTERN SLOPE OF THE EAST EUROPEAN PLATFORM AND DISTRIBUTION OF THE REEF FACES ON THE SHELF OF THE BALTICA PALEOCONTINENT

Abstract

Silurian deposits occurring within Volyn-Podillya Plate and Dobrogean Foredeep of Ukraine were investigated and correlated with the coeval strata of Moldova and Romania (Moldovian Platform). The occurrence of the Silurian strata, the patterns of thickness changes, and their petrographic composition were studied, which became the basis for establishing the features of the structure and localization of the reef facies. In order to better reflect the nature of this facies, it is proposed to give it the name “migrating reef facies”. During the time of reef formation in the Silurian (Late Wenlock-Middle Pridoli), it changed its boundaries, sometimes expanding towards the shelf edge, then narrowing towards the coast.Transgressive-regressive cycles had a decisive influence on the distribution of the facies within the southern shelf of the Baltica paleocontinent (western margin of the East European Platform). It was established that the water depth of 100 m (the upper limit of the oxygen minimum zone) during the Silurian (Late Wenlock-Middle Pridoli) was the boundary between the open-shelf facies, i.e. the sedimentation of the organic-rich muds and reef formation. The Silurian reef extends throughout the study area from the Black Sea to the border of Ukraine with Poland and Belarus, and further to the Baltic Sea. This is a continuous reef strip with a length of ≈ 2300 km and a maximum width of 150 m, the boundaries of which, both in section (Upper Wenlock-Middle Pridoli) and in area, shifted. The reef that formed in the Silurian on the southern paleoshelf of Baltica has clear analogies with the Great Barrier Reef of Australia (reef size, basin width, temperature indicators, areas of the continents).