Pleistocene glaciolacustrine breccias of seismic origin in an active graben (central Poland)

Varved layers in the partly consolidated glaciolacustrine Early Saalian deposits exposed in the Bełchatów mine (central Poland), show specific types of deformations. Two types dominate: half-varve deformations constituting the uppermost, clayey part of a single varve, and multi-varve deformations affecting a few varves. The latter are found either as clast-supported breccia beds with varve fragments in a quasi-horizontal position within a clayey matrix, or as deformed beds resulting from subsequent plastic deformation and folding. The deformed beds are interpreted as seismites derived from earthquakes related to the deep graben structure. The magnitudes of the earthquakes have determined the type of deformations. The seismites are thought to have originated intraformationally.