Deglacial history of glacial lake windermere, UK: implications for the central British and Irish Ice Sheet

Abstract

In the UK, a combination of outcrop mapping, satellite digital elevation models, high‐resolution marine geophysical data and a range of dating techniques have constrained the maximum limit and overall retreat behaviour of the British and Irish Ice Sheet (BIIS). The changing styles of deglaciation have been most extensively studied in the west and north‐western sectors of the BIIS, primarily using offshore geophysical surveys. The surviving record in the southern, terrestrial sector is fragmentary, permitting only large‐scale (tens of kilometres) and longer timescale (c. 1 ka) reconstructions of ice‐margin movement, with limited information on deglacial processes. Here we present a high‐resolution study of the retreat behaviour for a section of the southern ice‐margin from Windermere in the Lake District, using high‐resolution two‐dimensional multi‐channel seismic data, processed using prestack depth migration. By combining the seismic stratigraphy with landform morphologies, extant cores and seismic velocity measurements, we are able to distinguish between: over‐consolidated till; recessional moraines; De Geer moraines; flowed till/ice‐front fan; supra‐/en‐glacial melt‐out till; and subsequent glaciolacustrine/lacustrine sedimentation. The results reveal a complex and active valley glacier withdrawal from Windermere that changed character between basins and produced two small, localized areas of ice‐stagnation and downwasting. This study indicates that similar active ice‐margin retreats probably took place in other valleys of the Lake District during the Late Devensian deglaciation rather than the previously held view of rapid ice‐stagnation and downwasting. When combined with the regional terrestrial record, this supports a model of early ice loss in terrestrial England compared with other parts of the UK. Copyright © 2012 John Wiley & Sons, Ltd.

Lake bed geomorphology and sedimentary processes in glacial lake Windermere, UK

“…The second ridge follows a similar orientation and is approximately 300 m in length. The major ridge features probably represent the surface expressions of ridge complexes identified in seismic data by Pinson et al (2013). These authors suggest they date to glacial retreat of the Main Devensian ice sheet.…”

“…Recent studies using high-resolution single and multi-channel Boomer profiles and 2D seismic reflection data (Vardy et al, 2010;Lowag et al, 2012;Pinson et al, 2013) have been correlated with existing core stratigraphies (Pennington, 1943(Pennington, , 1975Smith, 1959;Mackereth, 1971) to reconstruct the broad depositional history of the lake basin. Amelioration of the climate and removal of ice from the catchment during the late Pleistocene led to a marked change in sedimentation, characterised by a drape of Holocene mud forming a thick sediment overburden (Pennington, 1981).…”

“…In the South Basin, ten subbasins characterised by a smooth topography (lake bed ,28) are separated by small topographic steps (,138) and slope breaks. The major steps are interpreted as the surface expression of recessional moraines (Pinson et al, 2013).…”

“…In the South Basin, a topographic high to the south of Cunsey Beck extends approximately 100 m in length in 30 m water depth, and has a maximum width of 120 m and maximum slope of 458 (Figures 4 and 5(D)). This topographic high is interpreted as part of a recessional moraine which has been infilled by thick (up to 60 m) finer-grained deposition, which causes the dampened surface expression (Pinson et al, 2013).…”

“…Lacustrine sediments further provide a high-resolution record of sediment provenance, biological history, catchment level anthropogenic activity and climatic and environmental change. Windermere ( Figure 1) represents an important lacustrine environment, occupying a pre-glacial river valley excavated by Pleistocene glaciations (Holmes, 1968;Pearsall & Pennington, 1973;Pinson et al, 2013). Multibeam bathymetry systems provide high-resolution datasets with a significantly high level of detail, accuracy and data density, capable of identifying small-scale features.…”