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New Model of the Caledonian Foreland Basin in North Poland Revealed by Recent Regional Seismic Data


The Silurian succession in north Poland has recently become the target of exploration efforts for unconventional hydrocarbons. This mixed fine-grained terrigenous and carbonate sedimentary cover was deposited above the SW edge of the East European Craton within the foreland flexural basin that developed due to the continental collision and formation of the Caledonian thrust belt further to the west-southwest. Seismic data from the recently acquired high-effort regional deep reflection survey, PolandSPAN, has revealed many new features of this sedimentary basin. The underlying Cambrian – Ordovician succession was deposited on the Baltica passive margin formed during the Precambrian – M. Ordovician. Some extensional control on the regional depositional pattern might be inferred as minor thickness changes could be observed across some of the basement faults. Top of the passive margin sequence is marked by an extensive angular unconformity. Its formation was caused by onlap of the foreland basin infill that was prograding from the orogenic belt towards the cratonic interior. Deposition within the Caledonian foreland basin was dominated by fine-grained organic rich shales, generally derived from the eroded orogenic wedge and deposited in the distal foredeep basin. Basal Llandovery “hot shales”, deposited during the initial transgression over the erosional top Ashgill surface and characterized by up to 17% TOC, are regarded as the most promising target for shale gas / oil exploration. Seismically defined depositional architecture and regional seismic markers allowed for quantitative reconstruction of particular stages of development of the Silurian foreland basin. Regional seismic data has also imaged previously-unknown dense fault patterns that may have been formed during flexural extension of the lower plate during continental collision. Some of these faults have been reactivated as reverse faults during a Late Paleozoic tectonic phase connected to the earliest Carboniferous uplift of the Mazury High, and than again as normal faults during the Late Triassic extension within the peripheries of the Mid-Polish Trough developed above the Tornquist – Teisseyre Zone. New evidence from high-resolution regional seismic data improved our understanding of the tectonic history and evolution of the Caledonian foreland basin in Poland and it is hoped that continued research and analysis of such data will also contribute to successful exploration results.