SUBGLACIAL EMPLACEMENT AND DEFORMATION OF EXTENSIVE GLACIOFLUVIAL SEQUENCES UNDER THE FORMER SCANDINAVIAN ICE SHEET, ISLAND OF FUNEN, DENMARK

On the Island of Funen, Denmark, sediments in the uppermost 8-10 m consist of a 6-8 m thick subglacial sequence of stacked and folded glaciofluvial sediments overlain by a 2-3 m thick basal till. Glaciofluvial sediments consist of tabular beds of planar stratified, massive and cross-stratified sand and gravel with localized channel fills of open-work 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 deposition of sand and gravel occurring during the decoupling phase and deformation occurring during recoupling. Folded beds are typically separated by undeformed silty clay beds and laminations recording slackwater deposition within subglacial cavities.

Folds frequently display attenuated upper limbs reflecting shearing associated with ice translation from above. 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, individual sands and gravel beds were deposited and deformed subglacially during discrete episodes. 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 (save for localized cm-scale blocks) from the glaciofluvial sequence suggests either non deposition or reworking into the glaciofluvial sediments. Deposition of tabular sand and gravel beds and open work gravel filling scours within single beds suggest broad sheet-like subglacial flows (10-100s 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.