Diagenesis of siliciclastic and carbonate deposits of the Neoproterozoic Uk Formation in the stratotype section (Southern Urals, Yuryuzan river, Ust-Katav town)

Research subject. The products of epigenetic transformations of carbonate and siliciclastic rocks in the stratotype section of the Upper Riphean Uk Formation (Yuryuzan River near Ust-Katav) in Bashkirian Mega-anticlinorium were identified and studied. Material and methods. The mineral composition of deposits was determined by X-ray diffraction analysis, the rocks were studied in thin sections under a polarizing microscope, and individual samples were examined using an electron microscope. Post-depositional transformations analysis was used to assess the nature of secondary changes. Results. Data on the sequence and intensity of both terrigenous and carbonate sediments alterations were obtained. All epigenetic transformations in the rocks of the Uk Formation were divided into gradational (progressive/subsidence epigenesis) and tectonic-fluid (superimposed epigenesis). Gradational transformations include progressive transformations associated with changes in thermobaric conditions over geological time, as well as authigenic native mineralogenesis. Superimposed transformations include authigenic extraneous mineralogenesis (metasomatism and filling of voids), formation of voids, and transformations associated with local changes in temperature and pressure. The superimposed processes (mainly metasomatism) were typical for deposits that initially had the highest permeability: fine-grained sandstones, oolitic limestones, intraclastic limestones and some stromatolitic limestones – they are characterized by a plethora of epigenetic products, which relationships in each case allow us to construct the sequence of processes that took place. Conclusions . The complex of structural and mineralogical features of rocks indicates progressive transformations at the level of the beginning of late catagenesis, which is consistent with the data literature of organic matter geochemistry, evidenced that the sediments reached the mesocatagenesis substage (“oil window”). The termination of progressive epigenesis is most likely due to the formation of the Ural orogen in the late Paleozoic. The set of secondary changes indicates a significant influence of superimposed epigenetic processes on the sedimentary sequence, but mainly only within the units or layers that could serve as fluid reservoirs (aquifers).