2011 GSA Annual Meeting in Minneapolis (9–12 October 2011)
Paper No. 20-7
Presentation Time: 9:40 AM-9:55 AM


BENEDIKTSSON, Ívar Örn, Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, Reykjavík IS-101 Iceland, iob2@hi.is

Glaciotectonic end moraines display wide varieties of morphologies and structural styles. They reflect the interaction between fast-flowing glaciers and their forelands and provide an archive for glacier dynamics. Studies in Iceland show that very dynamic glaciers produce large glaciotectonic end moraines within as little as a day. The studies also demonstrate that variations in morphology, architecture and formation of the moraines are integrally related to the subglacial ice-flow mechanism and the properties of the foreland wedge.

At Brúarjökull, which flows at a rate of >120 m/day during a surge, substantial amount of fine-grained subglacial sediment is compressed (ductile) and transported downglacier due to detachment from the bedrock, caused by overpressurized porewater. This results in the formation of an upglacier-tapering marginal sediment wedge on top of which a single-crested glaciotectonic end moraine is formed instantaneously at the very end of the surge. The formation of the moraine is associated with the blow-out of subglacial porewater in front of the ice and a resultant pressure drop which initiates the final deformation phase (brittle) and stalls the glacier advance. In areas of course sediment, subglacial drainage is more efficient and porewater pressures lower. Thus, no decoupling occurs at the sediment-bedrock interface and no marginal wedge develops. Instead, single- but broad-crested end moraines are formed by submarginal thrusting of a relatively stiff sediment sequence.

Cross-section balancing at Eyjabakkajökull suggests that the glacier-foreland coupling occurs 2-6 days before surge termination to form large and multi-crested moraines through a combination of folding and thrusting of fine-grained sediments and gravel. Similarly, the architecture of single- to multi-crested, fine-grained end moraines at Múlajökull is dominated by multiple narrow, overturned, and overthrusted anticlines and shear zones in the proximal and central parts, but inclined and open anticlines in the distal part.

Structural analysis of the end moraines suggests that the deformation is polyphase. Generally, fine-grained sediments deform in ductile manner while course grained sediments are subject to brittle deformation. The timeframe for the end-moraine formation is on the order of 1-6 days.

2011 GSA Annual Meeting in Minneapolis (9–12 October 2011)
General Information for this Meeting
Session No. 20
Glacial Geology and Cryogenic Processes
Minneapolis Convention Center: Room L100A-C
8:00 AM-12:00 PM, Sunday, 9 October 2011

Geological Society of America Abstracts with Programs, Vol. 43, No. 5, p. 65

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