Sedimentological evidence of Pleistocene earthquakes in NW Poland induced by glacio-isostatic rebound

Soft-sediment deformation structures are abundantly present in two levels within Warthanian/Eemian lacustrine sediments at Siekierki, near the Polish/German border. The two ‘event horizons’ show intense folding, collapse, sag and load structures, indicative of liquidization and fluidization. The structures must have been caused by sudden shocks, most probably resulting from earthquakes that were induced by glacio-isostatic rebound, probably after the Warthanian deglaciation. Such seismites have not been previously recognized in Polish Quaternary sediments. They provide supporting evidence for glacio-isostatic movements that were interpreted up till now based on drilling, lithostratigraphic, geophysical and geodetic data. The recognition of seismites in NW Poland may help recognize other deformed ‘event horizons’ in Pleistocene sediments as glacio-isostatic or neotectonic seismites.