Mykolaiv Sands in Opole Minor and beyond : sedimentary features and biotic content of Middle Miocene (Badenian) sand shoals of Western Ukraine

The Mykolaiv Sands are a huge lithosome of Middle Miocene (Badenian) age, accommodated within the Fore-Carpathian Basin in the Western Ukraine. Typically developed in the area of Opole Minor, it spreads across adjacent regions of Opole to cover an area of about 1300 km2. The varied sedimentary structures and ubiquitous burrows, indicate their development as a stack of sand shoals or related bodies, up to a few tens of metres thick, some of which temporarily reached sea level. Amidst the shoals, storm scours intermittently formed channel-like infills, some with residual lags at the base. The reversed density stratification and/or an increasing gravity gradient involved mass movements, some of which may have been triggered by seismic shocks focused at the shore or the adjacent hinterland of Podolia and Volhynia. Special attention is paid to the diverse fossils, all taphonomically filtered (aragonite shells and chitinous carapaces being lost), but which locally are mass-aggregated. They typify particular sand sets/bodies, to form allochthonous assemblages, some members of which (the cirripedes Scalpellum and Creusia, the shark Hemipristis, the ray Myliobatis) are newly recognized in the Ukrainian part of the Fore-Carpathian Basin. The others enrich considerably the faunal content of the Middle Miocene (Badenian) Paratethyan basins, either in terms of taxonomic diversity, or the eco-taphonomy of selected taxa (the starfish Astropecten, diverse echinoids). The whole faunal content of the Mykolaiv Sands may owe its profuse development to the global Middle Miocene Climatic Optimum of early Badenian age.