Geological environments forming the Eocene black-shale formation of the Silesian Nappe (Ukrainian Carpathians)
Received: February 19, 2019
Institute of Geology and Geochemistry of NAS of Ukraine
Institute of Geology and Geochemistry of NAS of Ukraine

Objective. To elaborate the geological structure and stratigraphy of the Eocene black shale formation of the Silesian Nappe, to recreate the depths and processes of its accumulation, and to reconstruct paleogeography and geodynamics of the Eocene sedimentatary Silesian Sub-basin. Methodology includes geological mapping, studing the lithostratigraphy and sedimentological features of the deposits (elements of Bouma sequence et al.) in a natural sedimentary succession, as well as paleobathimetric analyses of the foraminiferal assemblages. Results. The geological position of the studied sediments has been refined: the Eocene black-shale formation (Soimy Formation) developed within the Silesian Unit, and the variegated Paleocene-Eocene clayey deposits are distributed along the borders of the Silesian Unit in the Rozluch Structure (Subsilesian Nappe) and in the olistostrome strata. It is established, that the Soimy Formation is composed mainly of both products of the different density turbidity flows and the dark to black hemipelagic mud sediments. This formation was deposited in the Silesian Sub-basin at the bathyal-abyssal depths below the calcite compensation depth (CCD) which is indicated by the deepwater agglutinated foraminifera (assemblages “Glomospira-Haplophragmoides”, “Recurvoides”, “Glomospira”, “Rhabdammina-Reticulophragmium” and fauna “A”-type). The Silesian Sub-basin was limited by the uplifts such as Bukovets (branch of Silesian Kordiliera) and Subsilesian ones, on which in the Late Paleocene, the pelagic calcareous sediments enriched with planktonic foraminifera were deposited at the bathyal depths above the CCD and above the foraminiferal lysokline. In the Eocene on these uplifts, the clayey (hemi)pelagic deposits with the agglutinated foraminifera (assemblages “Rzehakina”, “Glomospira”, “Glomospira-Karrerulina” and fauna “B-type and “Rhabdammina-Reticulophragmium”) were settled below the CCD. In the Eocene, on the south-eastern wedge-shaped ending of the Silesian Sub-basin, there was a semi-isolated deep-water area, bounded on the northeast by the Sub-Silesian uplift, on the south-west by the Silesian (Bukovetsky) elevation, and on the south by the Fore-Marmarosh accretionary prism. Into this area, the circulation of the bottom-oxygen currents was limited and organic-enriched deposits (Soimy Formation) were deposited. At the end of Eocene, as a result of the subduction of the sedimentary basis of the Outer Carpathian Basin, the flysch strata was torn off from this basis and was thrust towards the platform and tectonically covered the submarine uplifts. Thrust movements caused the shallowing of the sedimentatary Outer Carpathian (including Silesian) Basin during the end of Eocene. Scientific novelty. A new paleogeographic model of the Eocene black shale (Soimy Formation) accumulation was proposed. According to the model, these deposits were accumulated by the turbidity flows and (hemi) pelagic sedimentation in the south-eastern semi-isolated deep-water section of the Silesian Sub-basin partly limited by uplifts. Practical Value. The geological characteristic of the Eocene black-shale formation, which may be potentially oil-generating, is given. Taking into account its availability, it can considerably expand the directions of the search of such work in the Carpathians.

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