STRUCTURAL POSITION AND DISTRIBUTION OF THE LOWER CRETACEOUS BLACK SHALE DEPOSITS WITHIN THE SILESIAN NAPPE OF THE UKRAINIAN CARPATHIANS

Abstract

The article presents the results of studying the structural position and distribution of potentially oil-generating, organic-enriched Lower Cretaceous black shale deposits of the Shypot Formation, developed within the Silesian Nappe, using the methods of geological mapping and structural analysis. We concluded, that these deposits, which are exposed in the Vicha River basin near the town of Volovets among a continuous field of Oligocene flysch, are the part of the stratigraphic succession of the Silesian Nappe, and they are not a large olistolith or an allochthonous tectonic remnant of another, more internal nappe. Shypot Formation fills a large (1 × 3 km) subvertical tectonic lens. The rocks of the Shypot Formation are represented here by medium-rhythmic flysch: sandstone beds alternating with dark gray and black foliated mudstone (black shale) layers. They are lithified products of both medium-density turbidite flows and hemipelagic deposition of clayey material with a significant amount of organic matter. In the contact zone of the tectonic lens with the surrounding rocks, the deposits are intensively deformed, almost isoclinal folds with subvetrical hinges are observed, which suggest the strike-slip movements. A tectonic mélange is recorded here, which is represented by small blocks (clastolites) of brittle sandstones, sometimes similar to Shypot sandstones, placed in a silt-pelitic ductile matrix. Foliation of the mélange matrix is also subvertical and close to the meridional and/or subcarpathian direction. The structural studies carried out show that in the zone of this contact there were compressive stresses perpendicular to the Carpathian orogen, and stress fields that caused strike-slip movements, most likely right-lateral along the Carpathian thrusts. The tectonic lens is located within the broad Latorytsa-Stryi shear zone, and probably extruded out from deep horizons by transpressive movements (compression and strike-slip) and forms a “positive flower structure”. The vertical foliation in the mélange suggests that small blocks of Shypot sandstones were extruded up together with the ductile mélange matrix during diapirism. Similar processes probably led to the rise of a large lens of Shypot sandstones as the positive flower structure. The studies suggests that the black shale deposits of the Shypot Formation, enriched with organic matter, are widespread at some depth within the Silesian Nappe, and in some places are pushed up to the level of the Oligocene flysch. As a result, the Shypot Formation and Oligocene flysch are now in direct contact along faults/mélange zones. The presence of Lower Cretaceous potentially oil- and gas-generating black shale deposits within the Silesian Nappe significantly increases the prospects for oil and gas exploration in this area.