Фациальные модели и седиментационные сценарии аккумуляции редких и редкоземельных элементов в визейских терригенных и угленосных отложениях Мелекесской впадины (Камский угольный бассейн, Волго-Уральский субрегион)

The research covers the investigation of stratigraphically consistent intervals in well 1955 (Lower Nurlat area, Melekess Depression), for which facies features (lithology, textures, bioturbation intensity, ichnofossils) as well as indicators of water energy and oxygen content in the sediment were recorded. The methods included a detailed description of the section with assessing the ratio of fractions (sorting), determining bioturbation nature and intensity, features of interbedding and contacts with over- and underlying rocks, and post-sedimentary changes. A comprehensive study of the Visean terrigenous and coal-bearing deposits at the eastern edge of the Melekess Depression made it possible to develop facies models and sedimentation scenarios for rare (RE) and rare earth elements (REE) accumulation. The established relationships between changes in facies conditions, the degree of bioturbation and elemental composition permit to consider the Melekess Depression a regional model for the facies-geochemical zonality of the Visean basin. The developed approaches can be used to predict the enrichment intervals RE and REE in the Visean thicknesses of the Volga-Ural subregion and in other coal-bearing basins with similar structures of the sedimentary cycle.