Using sediment facies & ground penetrating radar profiles to investigate the internal architecture and genesis of De Geer moraines

De Geer moraines (DGMs) have the potential to generate very high-resolution spatial and temporal ice margin reconstructions (~annual in contrast to 100-500 years, the current state-of-the-art). Existing studies suggest that DGMs likely form annually in a sub-aqueous, ice-marginal environment whereby basal sediments are advected and deposited at the grounding-line during seasonal advances. However, there have also been suggestions of a crevasse-fill origin that challenges this temporal regularity. Whilst the spatiotemporal properties of DGMs are disputed, the balance of evidence suggests an ice-marginal depositional environment with annual/seasonal regularities. Understanding the processes related to DGM formation is therefore critical, as it underpins the ability to use DGM to delineate ice-marginal retreat at unprecedented (potentially annual) resolutions.

A recent large-scale 3D morphometry study of DGMs and Crevasse-Squeeze Ridges (CSRs) was undertaken to constrain landform metrics and explore their formation processes. The results revealed statistically significant differences across all morphometrics between the sampled DGMs and CSRs. DGMs were found to be lower-relief, narrower, slightly more asymmetrical, and more sinuous than the studied CSRs. The morphometrics of DGMs support an ice marginal depositional environment. Furthermore, tendencies for cross-sectional asymmetry suggest a unidirectional push movement involved during formation. These inferences, however, must be supported with geophysical and/or sedimentological investigations.

Here we present the results of a field study using sedimentological and geophysical (Ground Penetrating Radar) techniques to investigate the internal architecture of DGMs in southwest Finland. Sedimentological data was acquired from two excavated exposures and 55 GPR profiles were obtained from four different locations across SW Finland. Radar facies were identified and corroborated with the lithofacies units as observed in the ca. 30 m long trench excavations. Typically, these facies comprise of stacked thrusted planes of laminated clay and diamicton on proximal slopes, sheared diamicton on surfaces indicative of proglacial pushing/overriding, and gravity-driven flow deposits on distal slopes. At places, glaciotectonic structures such as dipping, faults and folds were also identified.

The results may be used to complement the existing morphometry study, constraining the main processes involved in DGM formation and validating the use of DGMs as ice marginal indicators. This can ultimately be used as a foundation to explore the climatic significance of DGM ridges, thus meriting further work to constrain the spatial and temporal properties of DGMs during deglaciation.