New data on red Visean calcareous algae of the Dnipro-Donetsk Depression: systematic composition and facial confinement

As a result of the study of the core material of 540 thin sections from the boreholes of the northern marginal part and the paraxial part of the Dnieper-Donetsk Depression (DDD), a complex of fossil red calcareous algae including 14 species of 7 genera, as well as 2 species belonging to 2 genera identified for the first time for the Visean deposits of the DDD has been discovered. The studied carbonate deposits represent the Hlybokynskyi, Sukhynskyi, Stylskyi, Donetskyi and lower part of Mezhivskyi horizons of the Don-Dnipro Rift (DDR). It has been found that the greatest diversity of the taxonomic composition of fossil red calcareous algae in the DDD is observed in the deposits of the Sukhynskyi, Stylskyi and the most part of Donetskyi horizons, (XIII, XIIa, XII microfaunistic horizons of the DDD). It has been established that the limestones of the DDD paraxial zone dated to the uppermost Donetskyi and lower Mezhivskyi horizons (XI MFH of the DDD) are represented mainly by foraminiferous-polydetrital and algal wackestones, less often by wacke-packstones. The association of calcareous algae with dark ferruginous fragments indicatesthat the deposits formed in relatively deep conditions of the outer shelf. Carbonate sediments of the most part Stylskyi and Donetskyi horizons (XIIa, XII MFH of the DDD) of the northern marginal zone of the DDD are represented mainly by polydetrital wackestones, sometimes they are spicular, sometimes iron-bearing, which makes it possible to assume that these deposits, like the previous ones, belong to the relatively deep-water facies of the open platform, with moderate and reduced water circulation. Carbonate deposits of the Sukhynskyi and lowermost Stylskyi horizons (XIII MFH of the DDD) of the paraxial and northern rift side zones are represented mainly by polydetrital and fine-detrital-algal wackestones with remains of ostracods, mosses, spicules, brachiopod shells, fragments of Stacheaceae, green tubular algae of the family Palaeoberesellaceae Mamet et Roux. The predominance of green siphonocladal algae in the algal association indicates a calm hydrodynamic regime below the tide level as well as the depth of the basin, which mostly does not exceed 3-5 m, a small fluctuation in the depth of the sea basin, good illumination, and thermal conductivity of the sea basin. Carbonate deposits of the Hlybokynskyi horizon (XIV MFH of the DDD) exposed by the well 10 Skorobahatky in the northern marginal part of the DDD, are represented by organogenic, detritus-muddy packstones and wackestones with remains of bivalves, foraminifera, crinoids, ostracods, green, tubular algae of the family Palaeoberesellaceae Mamet et Roux with a transition in the upper part to sandy grainstone. This indicates that sediments were formed in the shallow water conditions under constant wave action, near the base of this action that led to the washout of clay mass.