Abstract
Subglacial lakes are common features of contemporary ice masses. However, they are rarely identified in the geological record. This is due to the difficulty in discriminating between subglacial and proglacial lake sediments; a proglacial origin is typically preferred as the ‘simplest’ explanation. We hypothesise that numerous deposits currently interpreted to record proglacial lake sedimentation may actually have a subglacial origin. Here we try and find ways of distinguishing proglacial from subglacial lake sediments by investigating three sites along the Dänischer Wohld Peninsula, northern Germany, which have been interpreted to record both proglacial and subglacial sedimentation. We identify two major phases of ice activity and associated lake formation during the Late Weichselian glaciation. (1) Proglacial lake formation at ∼23 ka in front of the advancing Baltic Ice Stream. This lake was subsequently overridden and the sediments glaciotectonised as ice continued to advance to its maximum extent. (2) Retreat of ice back into the Baltic Basin at ∼19 ka and formation of a proglacial lake that persisted for ∼4 ka. We suggest that subglacial lake activity may have occurred at two of the sites between 23 and 19 ka. This is based on the presence of aggrading sediment deposits characterised by stratified/laminated diamictons and interbedded tabular to channelized sorted sediments, the juxtaposition of relatively undeformed waterlain sediment and subglacial till, absence of glaciotectonic thrusting and folding or of fining/coarsening successions and the geomorphic association with tunnel valleys to the south of the study area. The style of sedimentation and deformation provided the greatest insight into the discrimination of proglacially vs subglacially deposited glaciolacustrine sediments. The luminescence signal palaeodose distributions also offers a potentially powerful means of fingerprinting sediment transport pathways of young glacial systems.
Highlights
Glaciolacustrine sediments are commonly observed in the geological record and have classically been interpreted to record proglacial lake formation (e.g. Rust and Romanelli, 1975; Eyles, 1987; Eyles et al, 1989; Larsen et al, 2006; Livingstone et al, 2010; Carrivick and Tweed, 2013)
We explore this conundrum using the glacigenic succession of the Da€nischer Wohld Peninsula, northern Germany, in an attempt to advance our skill in the discrimination of proglacially vs subglacially deposited glaciolacustrine sediments
We hypothesise that numerous deposits currently interpreted to record proglacial lake sedimentation may have been deposited by their subglacial cousins
Summary
Glaciolacustrine sediments are commonly observed in the geological record and have classically been interpreted to record proglacial (ice-marginal or ice-fed) lake formation (e.g. Rust and Romanelli, 1975; Eyles, 1987; Eyles et al, 1989; Larsen et al, 2006; Livingstone et al, 2010; Carrivick and Tweed, 2013). Glaciolacustrine sediments are commonly observed in the geological record and have classically been interpreted to record proglacial (ice-marginal or ice-fed) lake formation If the geographic context for these sediments in relation to a former ice margin is suitable for lake impoundment, or where other lines of evidence exist e such as lake shorelines and/or deltas e such an interpretation is robust. Despite recent attempts to formulate diagnostic criteria based on our current knowledge of subglacial lake processes Despite putative examples of palaeo-subglacial lakes (e.g. Gjessing, 1960; McCabe and O Cofaigh, 1994; Rebesco et al, 1998; Munro-Stasiuk, 1999; van Rensbergen et al, 1999; Christoffersen et al, 2008), a proglacial lake origin is typically preferred as the ‘simplest’ interpretation for glaciolacustrine sediments
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