The Stebnyk Formation (Miocene) in the Boryslav-Pokuttya and Sambir nappes of the Ukrainian Carpathians: a record of environmental change in the Carpathian Foredeep

Abstract

The late Early–Middle Miocene Stebnyk Formation is a ~600–2000 m thick unit of the Boryslav-Pokuttya and Sambir nappes, which contains a molasse succession of the Carpathian Foredeep incorporated within the marginal part of the Outer Eastern Carpathian accretionary wedge. In the valley of thePrutRiver, between Deliatyn and Lanchyn, the Stebnyk Formation covers the alluvial fan deposits of the Sloboda Conglomerate and the deltaic deposits of the Dobrotiv Formation in the south, and the lagoonal salty clays of the Vorotyscha Formation in the north. The Stebnyk Formation is built of mainly rose, greenish and grey calcareous mudstones intercalated with several variable sandstone beds, including thick-bedded packages. The occurrence of tetrapod footprints and raindrop imprints, as well as the overall red-be character prove prevailing continental conditions during deposition of the formation, which is interpreted as sediments of a delta plain with distributary channels filled by the thick sandstone beds and associated by intercalations of thinner beds referred to channel levees and crevasse splays. The upper part of the Stebnyk Formation contains marine microfossils of the NN4 Zone, and locally to the NN5 Zone, corresponding to the early Badenian transgression in the region. In the regional scale, the Stebnyk Formation shows a polarity of facies, with a high contribution of conglomerates and thick-bedded sandstones in the lower part in the north west and fining to the south east, with transportation from the west and north west. The sediments accumulated in an elongated subsiding zone between the rising Carpathian orogen and the forebulge elevation of the foreland, in a warm and semi-dry climatic conditions corresponding roughly to the Mid-Miocene Climatic Optimum. The accumulation was balanced by a subsidence caused by sinking of the platform slab and by sedimentary load.