Paleogene–Neogene tectonic evolution of the lignite-rich Szamotuły Graben

The Szamotuly Graben covers the southernmost part of the Permo-Mesozoic Poznan–Szamotuly Fault Zone. Along this regional discontinuity there are several salt structures, including the Szamotuly diapir, over which an extensional graben formed in the Paleogene and Neogene. The graben is located north of Poznan in central-western Poland, and is NW–SE-trending, ~20 km long, 3–5.5 km wide, and up to 160 m deep. It is filled with Lower Oligocene and Neogene sediments, including relatively thick lignite seams. Data from boreholes allow the assignment of the graben-fill sediments to appropriate lithostratigraphic units. Furthermore, analysis of changes in the thickness of these units provides evidence for periods of accelerated graben subsidence or uplift relative to its flanks. As a result, two distinct stages of tectonic subsidence and one inversion in the Paleogene–Neogene evolution of the Szamotuly Graben have been distinguished. Thus, relatively significant subsidence occurred in the Early Oligocene and the middle Early–earliest Mid-Miocene, while slight inversion took place in the middle part of the Mid-Miocene.