Late Cretaceous tectonic inversion within the S Baltic Sea – insight from regional seismic profile stretching between the West and the East European Platforms

In 2016, 850 km of multichannel seismic reflection data of the BALTEC survey have been acquired offshore Poland within the transition zone between the East European Craton and the Paleozoic Platform. Acquired data, processed up to Kirchhoff pre-stack time migration, was integrated with other seismics available in this area and calibrated by deep and shallow wells. All this data provided new information regarding Late Cretaceous inversion i.e. the last major tectonic event that shaped geology of this area. It led to uplift of basement blocks, their localized erosion and formation of syn-inversion growth strata. This phase of geological evolution could be until now hardly resolved by industry seismic data due to limited shallow seismic imaging and very strong overprint of multiples. Within the SW Baltic Sea main structure formed during Late Cretaceous inversion is NW segment of the Mid-Polish Anticlinorium (MPA) that extends for over 1000 km from the vicinity of Bornholm towards the SE Poland and W Ukraine. Identified subtle thickness variations, progressive unconformities and contourites within the Upper Cretaceous succession document complex interplay of Late Cretaceous basin inversion, erosion and re-deposition. Seismic data from within the Bornholm-Darłowo fault zone located NE from the MPA imaged system of deeply rooted steep reverse faults and associated zones of prograding wedges that testify to complex depositional pattern controlled by transpression. Farther to the E, within the Ustka and Słupsk Blocks, Precambrian basement is overlain by Cambro-Silurian sedimentary cover. It is dissected by a system of steep, mostly reverse faults rooted in the deep basement. This fault system has been so far regarded as having been formed mostly in Paleozoic times, due to the Caledonian orogeny. As a consequence, Upper Cretaceous succession, locally present in this area, has been vaguely defined as a post-tectonic cover, locally onlapping uplifted Paleozoic blocks. Our results confirmed that, contrary to previous models, at least some of these deeply-rooted faults were active as a reverse faults during Late Cretaceous, in particular those faults that continue into the inverted Christianso Block farther to the North. It can be therefore unequivocally proved that large offshore blocks of Silurian and older rocks, located presently directly beneath the Cenozoic veneer, must have been at least partly covered by the Upper Cretaceous succession; then, they were uplifted during the widespread Late Cretaceous inversion. All these results prove that Late Cretaceous inversion in this part of Europe strongly affected also large areas located much farther towards the East than previously assumed

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