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Cenozoic tectonic evolution of the Poznań-Oleśnica Fault Zone, central-western Poland

Widera M., Ćwikliński W., Karman R.

Acta Geologica Polonica

2008

Vol. 58, no. 4

455-471

The Poznań-Oleśnica Fault Zone (P-OFZ) of central-west Poland is an over 150 km long, up to 10 km wide, and up to 200 m deep graben that developed during Early Oligocene to Pliocene times on the flank of the NE-dipping Fore-Sudetic Homocline. Fault systems of this extensional structure appear to reach pre-Zechstein basement in the area ofthe Fore-Sudetic Homocline that forms an integral part of the Bohemian Massif. The P-OFZ was affected by several stages of subsidence, separated by periods of inversion and/or tectonic quiescence between the Triassic and Cenozoic. Structurally, this dislocation zone can be linked with the Variscan faults, reactivated at that time. During the second half of the last century, the northern parts of the P-OFZ graben were intensively explored by cored boreholes because of their lignite content. Conventional back-stripping methods could not be applied to the tectonic analysis of the POFZ due to the limited thickness of the graben fill, its poor dating and the presence of relatively thick lignite seams. Therefore, phases of accelerated subsidence were determined by comparing the thickness of individual lithostratigraphic units within the graben and on its flanks. The total subsidence of the different graben segments was quantified by determining the aggradation coefficient (AC) and by taking the consolidation coefficient (CC) of lignite seams into consideration. Subsidence analyses indicate that the initial Early Oligocene extensional phase of the P-OFZ was followed by an Early.Middle Miocene extensional subsidence phase and a probably latest Miocene.Pliocene final extensional phase.

Lokalizacja wschodniego uskoku głównego w mezozoicznym podłożu Poznania

Widera M., Karman R.

Przegląd Geologiczny

2007

Vol. 55, nr 11

965-970

The paper deals with the major eastern fault in the Mesozoic substratum of the City of Poznań. In this area, only few boreholes reached the top of the Mesozoic succession. It is the reason why this fault cannot be mapped. Therefore, it is indicated the dependence between the relief of the Mesozoic substratum and the morphology of the lignite seam base in adjacent areas where numerous boreholes reached the top Mesozoic surface. This dependence was best expressed at times when the major fault in the substratum was active after peat sedimentation. Subsequently, the structural map of the base of the first Middle-Polish Lignite Seams, pierced by many boreholes, was analyzed. In this way, using an indirect method, location of the major fault in the Mesozoic substratum of the City of Poznań was quite precisely determined.