Glacial geomorphology of the southernmost sector of the last Scandinavian Ice Sheet (Wielkopolska, western Poland): The process-form imprints of ice streams, ice lobes and diversified types of ice sheet retreat

Wielkopolska region in central-western Poland is crucial for reconstructing the dynamics of the southernmost sector of the Scandinavian Ice Sheet (SIS) during the local Last Glacial Maximum (LGM). These reconstructions suggested that during the advance to the local LGM, this SIS sector was characterised by a combination of ice streams and dynamics lobes, some of which exhibited surge-like behaviour. Such vigorous and complicated behaviour is demonstrated in a complex set of landsystems arranged in several distinct zones. This palimpsest of palaeoglaciological signatures likely records SIS's spatially and temporally diverse thermal regimes during its advance and retreat from the local LGM position. The central part of the area is occupied by one of the best-preserved complexes of mega-scale glacial lineations (MSGLs), which further confirm deformable bed conditions under advancing SIS. The region also contains a diversified and complex suite of moraines and hummocky terrain recording different types of ice sheet retreats.

Wielkopolska lies at the southernmost extent of the former Scandinavian Ice Sheet and hence comprises landforms developed during the advance toward the local Last Glacial Maximum (around 25-21 ka BP) and retreat of the ice to subsequent major standstills at approximately 17 ka BP and 16 ka BP. This study aimed to develop an updated map of glacial landforms to decode the landform-sediment signatures of ice streaming overlain by frontal recession and areal ice sheet decay and provide an updated palaeoglaciological reconstruction of the southernmost sector of SIS. Detailed mapping based on LiDAR-derived digital elevation model (DEM) and high-resolution satellite imagery and aerial photographs enabled us to identify several overprinting glacial landsystems of active ice streams, surging lobes, frontal recession, ice-cored moraines development and degradation, kame and kettle topography. Widespread evidence of surge-diagnostic features identified in three distinct zones suggests that the ice sheet experienced repeated flow instabilities, consistent with a broader scenario of a highly dynamic and unstable southernmost sector of SIS. It is also emphasised by the occurrence of transitory and cross-cutting palaeo-ice streams. We also identified several inter-ice stream zones characterised by ribbed moraines, glaciotectonic deformations, cupola hills formation, and rafts displacement. Landforms recording ice sheet recession indicate complex patterns of different ice margin responses, including (1) active recession interrupted with short-term readvances/oscillations similar to modern-day active temperate Icelandic glaciers; (2) development of large scale glacifluvial ramps and fans; (3) development of ice-cored moraine fields analogous to modern-day polythermal glaciers on Svalbard; (4) development of ice stagnation areas. Widespread evidence for subglacial meltwater activity is probably linked to the transitory, surging, and cross-cutting nature of the palaeo-ice streams in the region, thereby indicating switching in ice sheet dynamics in response to the build-up, migration and marginal outburst of subglacial water.

The research was funded by the National Science Centre, Poland (Project number 2015/17/D/ST10/01975)