Incremental subglacial meltwater sediment deposition and deformation associated with repeated ice-bed decoupling: a case study from the Island of Funen, Denmark

Abstract

On the Island of Funen, Denmark, sediments that are 4–7 m below the land surface consist of a 2–4 m thick sequence of stacked and folded glaciofluvial sediments overlain by a 2–3 m thick basal till. The glaciofluvial sediments consist of tabular beds of planar-stratified, massive and cross-stratified sand and gravel with localized channel fills of open-framework gravel. Sand and gravel beds exhibit curvilinear sheath folding often leading to eye folds. This folding is attributed to repeated glacier-bed decoupling and recoupling, with subglacial deposition of sand and gravel by meltwater occurring during the decoupling phase and deformation occurring during recoupling. Sand and gravel beds are typically separated by silty clay beds and laminations recording slackwater deposition within subglacial cavities. Folds frequently display attenuated upper limbs reflecting shearing associated with overlying ice movement. On lower fold limbs, only localized thrusting is observed, suggesting water saturation of sands and gravels during deformation, and relatively low strain rates. Folds are frequently truncated by overlying sand and gravel beds. No evidence of shearing is found along these contacts which are interpreted as erosional surfaces. Thus, we propose that the glaciofluvial sediment sequence records incremental, step-wise subglacial deposition and deformation rather than wholesale deformation following emplacement of the entire glaciofluvial sequence. Specifically, we envisage that individual beds were deposited and folded subglacially, with ice-bed decoupling occurring during sediment deposition followed by recoupling and shearing of the upper bed surface by the ice base to produce folds with attenuated upper limbs. Incremental deformation and folding were facilitated by elevated porewater pressure within the sediments. Silty clay beds helped to maintain elevated porewater pressure by acting as local aquitards. A general absence of till from the glaciofluvial sequence (save for localized cm-scale blocks) suggests either non-deposition or reworking into the glaciofluvial sediments. Deposition of tabular sand and gravel beds and open-framework gravel filling scours within single beds suggest broad sheet-like subglacial flows (10–100 m wide) that become increasingly channelized. We propose that the sedimentary sequence records a highly dynamic glaciohydraulic system characterized by repeated localized subglacial cavity development, and storage and release of meltwater leading to glacier decoupling that may have facilitated fast flow of the Baltic Ice Stream.