Sunday, October 29, 2017

GSA Talks Ice Sheet Grounding-Zone Wedges

The first half of last week, Sunday through Wednesday was taken up at the Geological Society of America meeting in Seattle. Hundreds of talks and presentations to choose from over a three day period. Deciding on which talks to attend was at times difficult. 

One of the early talks I went to was titled Sedimentary Processes at Ice Sheet Grounding-Zone Wedges: Examples From Antarctica and Washington State (U.S.A) (Demet and others, 2017). This talk had some practical applications to my own work on landslides.The talk was a brief version of Demet (2016) thesis work on some of the shoreline bluffs of Whidbey Island.

When the Puget ice lobe retreated the sea invaded Puget Sound and for a time the glacial front consisted of a front of grounded ice (ice in contact with the ground) and a sheet of floating ice. Demet and others (2017) recognized several locations where the grounding line may have been present on Whidbey Island as the ice retreat paused and suggests that the grounding line area became an area where there were some minor readvances of the ice sheet.

A few weeks ago I observed some highly distorted and complex glacial sediments along a steep shoreline bluff along the Strait of Juan de Fuca. Seeing deformed glacial units is not unusual, but the degree of deformation and the cross cutting relationships were perplexing. The area was a mix of glacial outwash, glacial tills and glacial marine drift that was not following the pattern of what is the more typical sequence of units. Understanding the units and just what units I was looking at was a critical component of assessing the potential bluff failure mechanisms. In particular, at this site the presence of poorly compacted glacial marine drift was of interest because it appears to be the cause of some of the larger scale failures that are present on portions of this particular bluff. 

The pattern of units appears to be consistent with the observations made by Demet and others (2017). Perhaps a more diligent comparison would be in order.

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