Degree Type

Thesis

Date of Award

2016

Degree Name

Master of Science

Department

Geological and Atmospheric Sciences

Major

Geological and Atmospheric Sciences

First Advisor

Neal Iverson

Abstract

The tendency for orthogonal networks of crevasses to open during glacier surges may result in rectilinear ridge networks that form by squeezing of deformable till into the bases of the crevasses. Testing this hypothesis by determining the origin of rectilinear ridge networks in the forefields of modern surge-type glaciers can, therefore, potentially help establish a landform-based criterion for determining if Pleistocene glaciers have surged.

To test whether a rectilinear ridge network in the forefield of Múlajökull, an Icelandic surge-type glacier, formed at the bases of surge-induced crevasses, ∼1000 intact till samples were collected so that their anisotropy of magnetic susceptibility (AMS) could be measured and used to infer till-deformation kinematics at the bed. Other samples were collected for determining grain-size distribution, preconsolidation pressure (i.e., the largest effective stress on the till since its deposition), and dry till bulk density.

Preconsolidation pressures in excess of the modern overburden pressure on the till supports the subglacial origin required by the crevasse-squeeze model, precluding the possibility that the ridges formed supraglacially or as marginal push moraines. High clast roundness also points to a subglacial origin. Preconsolidation pressures and till densities are higher away from the ridges than within them; effective stresses, therefore, decreased toward the ridges, in agreement with the effective stress gradient necessary to help squeeze till into the crevasses. Vigorous groundwater flow caused by subglacial pore-water pressure gradients adjacent to the crevasse produced a dike-like sand and gravel body in a transverse ridge by winnowing the fine sediment fraction from the till and helping it to locally flow upward.

AMS fabrics support a crevasse-squeeze origin for both the transverse and flow-

parallel ridges. In the transverse ridges, AMS fabrics–with principal susceptibility orientations parallel to the ridge crest (k1) low in the ridges and fabrics with ridge-parallel girdles (k1 and k2) characteristic of flow-parallel shortening higher in the ridges–indicate that a combination of upward extrusion and downglacier bulldozing formed the ridges. Principal susceptibility orientations parallel to flow lower in the ridges (k3 and a sub- cluster or k1) indicate flow-parallel shear but to strains insufficient to fully overprint the effects of flow-parallel shortening. Fabrics from one flow-parallel ridge indicate that downglacier shearing was sufficiently large to overprint fabric signatures. In another flow-parallel ridge, fabrics show a reverse-herringbone pattern that was produced by the cumulative strain of downglacier shearing and localized transverse motion associated with crevasse opening. These AMS fabric results indicate that the glacier was sliding during or after ridge formation.

These results contradict the common assertion that formation of crevasse-squeeze ridges requires stagnant ice in order to preserve the ridge. Rather, forward motion of the ice helped build the transverse ridges of this study. Rectilinear crevasse-squeeze ridge networks are more indicative of glacier surging than transverse ridges alone.

Copyright Owner

Geoffrey Thomas Gadd

Language

en

File Format

application/pdf

File Size

88 pages

Included in

Geology Commons

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