LAUSR

Abstract Two breccia layers occur directly stacked upon each other in a silty/sandy Saalian glaciolacustrine succession near Ujście (western Poland). This complex is exceptional because of two aspects: the two breccia layers consist of distinctly more fine-grained material (mainly silt) than the rest of the succession, and they contain numerous soft-sediment clasts of sandy silt with irregular shapes. The silty clasts are interpreted to represent fragments of eroded sediment from the more marginal (proximal) bottom of the lake. The fine-grained character of the breccia matrix excludes that the process responsible for their deposition had a sufficiently high erosional capability to erode the embedded clasts. The only feasible explanation is that this enigmatic breccia consists of a mixture of sediments: the silty matrix must first have been deposited on the bottom of a lake and afterward have been eroded and redeposited by gravity flows that initially had sufficient power to erode the lake margin over which they ran down. The source of the gravity-flow silt remains enigmatic because no sediments with similar lithology are currently present nearby at a higher topographic level. We must therefore deduce that such sediments existed previously but were entirely eroded away, probably by the Weichselian ice mass that had reached this area during the next glaciation. The probable trigger of the gravity flows is evidenced by the presence of soft-sediment deformation structures in two levels around 4.5 m below the breccias. These two levels show intense folding, collapse, and load structures indicative of liquefaction and fluidization. The deformation is ascribed to shocks, most probably resulting from earthquakes that were induced by glacioisostatic rebound during the Saalian deglaciation. Similar seismic shocks are likely to have triggered the mass flows that resulted in the breccia layers.