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The study analyses a 7.5-m-thick sedimentary succession deposited in the largest sandur (the Gwda sandur, a glacial outwash plain) in NW Poland, during the late Weichselian (MIS 2). Although the study site is located in the distal zone of the sandur, its sedimentological features and palaeohydrological parameters reflect the presence of an energetic, powerful environment typical of the proximal zone. Three sedimentary units were recognized in the studied sedimentary succession: (1) lower unit — fine-grained sands with ripple cross-lamination and horizontal lamination; (2) middle unit — gravelly coarse-grained sands and sandy gravels with planar cross-stratification; and, (3) upper unit — sands and gravelly sands with trough cross-stratification. Although the age of deposition of the sandur is accepted to be convergent with that of end-moraines of the same phase, the sediments in the distal zone of the Gwda sandur correlate with an earlier glaciation phase/subphase. Our findings hint at a complex problem: large sandurs such as the Gwda sandur were probably deposited over a long time, and their successions might record the textural and structural features of the proximal subenvironment, even in their distal parts due to deglaciation-related shifting of the proximal zone of a sandur. This paper presents a new approach to analysing the depositional processes in large sandurs, shows a new light on glaciofluvial water supply dynamics of distal parts of sandurs, and may solve several fundamental problems related to the sandur deposition.

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.

This study describes the sedimentary successions of two glaciomarginal fans (=end moraines) deposited during MIS 6 and MIS 2 in the eastern and north-western part of Poland. Sedimentological analyses of the successions indicate they were both deposited close to the ice sheet margin and are dominated by extensive gravelly and sandy sheet beds derived from the upper flow regime with single ice-wedge structures. Qualitative and quantitative analysis of the pancake-like gravelly and sandy sediments was performed using Markov chain analysis. The presence of these, admittedly common, sedimentary successions raises two key points: why sheetfloods are so common on glaciomarginal fans instead of in-channel flows, and whether sedimentary rhythms deposited by sheetfloods represent the full depositional records, or only partial ones due to accompanying erosion. While both points remain unclear, the answers to these questions may be connected to the presence of permafrost, which occupied the foreland of the ice sheet where the glaciomarginal fans were deposited: it can control the type of proglacial river flows on glaciomarginal fans, the depth of erosion and influence the record of sedimentary rhythms.

Geomorfologiczne i geologiczne uwarunkowania centralnej cześci Miedzyrzeckiego Rejonu Umocnionego (MRU), Polska Zachodnia