Groundwater Flow Under a Paleo‐Ice Stream of the Scandinavian Ice Sheet and Its Implications for the Formation of Stargard Drumlin Field, NW Poland

Abstract

Subglacial groundwater flow was an integral part of glaciological systems of past ice sheets, but its impact on the origin of active-ice landforms remains unexplored. Using numerical experiments, we attempt to constrain groundwater flow dynamics under a major paleo-ice stream of the southern Scandinavian Ice Sheet and its impact on the formation of the Stargard drumlin field. Flow models show a total reorganization of groundwater dynamics under the advancing ice stream to a depth of up to~200 m. A mosaic of intervening groundwater recharge and discharge areas originates, whereby the same areas may experience multiple shifts in flow directions. A prominent time-and space-transgressive pressure pump recharges groundwater in a subglacial zone up to about 20 km within the ice margin and discharges it in front of the ice sheet. The simulated groundwater flow pattern suggests that drumlins occupying a portion of the ice stream area occur preferentially where groundwater upwells and discharges at the ice/bed interface. Consistent with the geological composition, a possible drumlin-forming mechanism involves an excess of pressurized water under the ice reducing the strength of the subglacial deposits and facilitating glacial erosion, erosion by turbulent meltwater flows or both, streamlining antecedent deposits into drumlin shapes. This study emphasizes the importance of causal feedback between subglacial groundwater flow and the ice sheet dynamics and suggests an impact of the former on the formation of streamlined subglacial landforms in general.

Till kinematics in the Stargard drumlin field, NW Poland constrained by microstructural proxies

“…Our data suggest that shallow but intensive bed deformation, especially its ductile component related to high porewater pressure, was an important factor facilitating fast flow of the Odra palaeo‐ice stream. Although the specific relationships between the deposit (till) and landforms (drumlins) remain elusive, the lack of correspondence between the microclast orientations and the drumlin directions suggests that the till pre‐dates the drumlin‐forming event, which is consistent with earlier studies (Hermanowski et al, 2019; Hermanowski and Piotrowski, 2019) that considered the Stargard drumlins as erosional remnants of antecedent deposits.…”

“…A recent hypothesis invokes removal of deposits from the inter‐drumlin areas by a subglacial deforming layer so that drumlins remain at the basal interface as antecedent erosion remnants (Eyles et al, 2016, 2018). The erosional hypothesis – no matter what erosional agent is invoked – is gaining momentum (McCracken et al, 2016; Iverson et al, 2017; Hermanowski et al, 2019) as it avoids the necessity of accounting for the textural and structural diversity of drumlin‐forming deposits while explaining the spread of drumlin shapes and sizes by locally variable subglacial conditions (Halberstadt et al, 2018; Hermanowski and Piotrowski, 2019; Mulligan et al, 2019). Another approach is the concept of equifinality, which proposes that different processes can produce drumlins sharing similar geomorphic characteristics (Möller and Dowling, 2016, 2018).…”

“…This paper is a follow‐up of two recent studies of the Stargard drumlin field (SDF) in north‐western Poland (Hermanowski et al, 2019; Hermanowski and Piotrowski, 2019) which applied geomorphological, sedimentological and numerical methods to constrain its origin. Both studies suggested that Stargard drumlins were created by selective glacial erosion of an older terrain and thus yielded support to the unifying theory of drumlin formation.…”

Dynamics of the last Scandinavian Ice Sheet’s southernmost sector revealed by the pattern of ice streams

“…The presence of glacial lineations associated with the Baltic Ice Stream Complex has been reported in the Bothnian Sea (Greenwood et al 2016) and to either side of the study area, outside the southern Baltic Sea sector (Kleman & Borgstr€ om 1996;Kjaer et al 2003;Houmark-Nielsen 2010;Kalm 2012). Hermanowski et al (2019) also report over 1000 streamlined subglacial bedforms in NW Poland, to the NW of the study area.…”

“…1). Streamlined bedforms present in the Lower Odra region were not included in this analysis as they have been described in detail by Hermanowski et al (2019Hermanowski et al ( , 2020 and Hermanowski & Piotrowski (2019).…”