Quartz grains reveal a record of a glacial lake-outburst flood dynamics: Example from the southern periphery of the Scandinavian ice sheet at the last glaciation in NE Poland

• Cracked and non-abraded quartz grains dominate in the Suwałki Megaflood deposits. • High-energy environment revealed by significant grain shape and surface differences from typical sandur sediments. • A GLOF-related index (IG) indicates the highest energy deposition in middle profile sections. • GLOF-deposits show a minimal aeolian influence except on megadune crests and uppermost parts of profiles. • Six distinct grain groups have been identified, reflecting the dynamic depositional environments of GLOFs. Quartz is one of the most abundant minerals on the Earth’s surface. Quartz sediment grains record environmental changes in their shape and on their surfaces. These properties allow a reconstruction of depositional processes. The deposits of glacial lake-outburst floods (GLOFs) are usually rich in quartz (except for in Iceland), which can be used to decipher flood processes. A GLOF of exceptionally high discharge is known from north-eastern Poland (i.e., the Suwałki Megaflood) and was responsible for the deposition of sediments forming megadunes. These bedforms were the subject of a quartz grain microscale study to define the grain variability and differences from other water-laden or aeolian environments. Our results reveal that cracked and non-abraded quartz grains dominate, indicating a high-energy environment and revealing a significant difference from thatof normal sandur (outwash) sediments. Quartz-type-based indices were used to help distinguish differences in sedimentation type and transport dynamics. Our results show that aeolian and periglacial processes did not seem to contribute in any meaningful way to the formation of the megadunes, except for their crests and the post-flooding deposition being rich in silica precipitated grains formed under periglacial conditions.