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Miocene Evaporites of the Carpathian Foredeep Basin and Their Role in Formation of Gas-Bearing Contractional Structures — 250 Years of Geological and Geophysical Studies


The Carpathian foredeep basin developed in front of the Carpathian orogenic wedge and is filled by the Miocene gas bearing sedimentary succession, with evaporitic formation at its base. Underground exploitation of evaporites in the Kraków region (Wieliczka and Bochnia salt mines) started at the turn of the 12th to 13th centuries. Illustrations of tectonic structures of the Miocene evaporites published by Schober in 1750 show complex contractional deformations. Recent study in Wieliczka salt mine showed that the large-scale intrasalt structure is dominated by north-vergent asymmetric folds and associated thrust/shear zones. This is repeated at smaller scales, where anhydrite-mudstone layers within the rock salt form folds with long, gently-moderately south-dipping backlimbs and short, steep to overturned forelimbs. Fold axes trend E-W but have highly variable plunges. Boudins and folds cut by late extensional faults indicate complex spatial and temporal variations in the local stress field. In late 18th and early 19th century first regional cross-sections of the Wieliczka region were published by prominent geologists such as R. Townson, F.S. Beudant, G.G. Pusch, L. Zejszner and R.I. Murchison. A cross-section published by Hrdina in 1842 best captured much of what is accepted today for the overall internal geometry of the Wieliczka area, with three clearly defined tectonic slivers (contractional duplexes) developed within the evaporite-bearing succession. Recently, due to the availability of high-quality 2D and 3D seismic data geological studies of Miocene salt tectonics along the Carpathian front entered a new phase. The foredeep evaporites facilitated the development of wedge tectonics and formation of complex triangle zones with their cores filled by strongly deformed Miocene evaporites. Lateral variations in structural style of the frontal Carpathian orogenic wedge were controlled by thickness and facies variations of the evaporitic succession and the morphology of the pre-Miocene substratum. Deep erosional paleovalleys, incised following Paleogene inversion of the Permo-Mesozoic Polish Basin, controlled deposition of the Miocene foredeep evaporites. They also formed morphological buttresses that led to local concentration of compressional stresses and triggered the formation of complex fore- and backthrust zones detached within the Miocene evaporites. Structures formed due to salt-related compressional tectonics host numerous commercial gas accumulations.