Olistostrome in Oligocene of the Krosno (Turka Subnappe) and the Duklya-Chornohora Nappes of the Ukrainian Carpathians

Abstract

Olistostrome horizons in the Oligocene deposits of the south-western part in the Bytlya-Svydovets subcover of the Krosno nappe or in so-called Pre-Duklya folds are described in a number of works. There is not enough information about olistostrome in the northern part of the Krosno nappe in its Turka subcover and in the outer part of the Duklya-Chornohora nappe (Stavniany subcover). In the mentioned tectonic units olistostrome is localized in the Lower Verkhovynian deposits of Oligocene above the marker bed of stripped limestones in the region of the Smozhe populated area of the Skole district of the Lviv Region, Torun and Lopushna of the Mizhgirria region of the Transcarpathian Region as well as near Lyuta Village of the same region. Matrix is mainly composed by the grey carbonate flysh of the Krosno lithotype. Olistostrome horizons are presented by strongly crumped chaotic non-sorted formations. In its composition also are present more ancient rocks than matrix, olistolites of the Upper Cretaceous-Lower Oligocene age as well as redeposited rocks formed as a result of washout of more ancient deposits. Some geologists consider, according, to M. G. Leonov hypothesis (1978), that olistolites came off the front part of the Duklya nappe during its overthrusting to the north-east. Materials collected during geological surveys and later observations deny such a mechanism. This is proved by the following: a) overthrust of the Duklya nappe couldn’t cause the formation of the Krosno olistostrome. Because olistostrome is also fixed in Oligocene of the Stavniany subcover of the given nappe of the Lyuta Village and the Mlaky ravine in the section of the Lyuta River. b) Olistostrome is traced at the strathigraphic level at a relatively narrow time interval. It means that sources of removal of olistolites were functioning not so long that contradicts the thesis on the permanent overthrusting movement. c) If olistostrome was caused by the given overthrust, so it would (and olistolites) be observed continuously, but not discretely. d) In sandstones of-the Lower Verkhovynian subsuite the presence of nummulites was fixed, and in argillites of Oligocene the microfauna of Cretaceous-Eocene age – this in the evidence of the washout event. e) Over olistostrome are developed normal bedded high sections of-the Lower Verkhovynian subsuite, thus the overthrusting of the scales in Oligocene was absent. f) In the composition of the Bytlya olistostrome are present rocks that are absent in the composition of Duklya-Chornohora nappe. In the light of data mentioned above, the alternative thesis about cordillera as a source of removal of olistolites is rather grounded and non-alternative. It is possible that as sources of removal was a number of islands that were uplifted higher that sea level at the beginning of the Upper Verkhovynian time. In places the tongues of olistostromes into the Turka sub cove olistolites in the Smozhe and Torun Village possibly due to the presence of long alluvial fans. It is probable that cordillera occurred at the boundary between the Krosno and Dusynian basins of sedimentation that differed by the conditions of sediment forming that was manifested by the presence of two lithotypes of Oligocene of Krosno and Dusynian.