Date of Award

December 2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Geosciences

First Advisor

Thomas S. Hooyer

Second Advisor

John L. Isbell

Committee Members

Richard B. Alley, John L. Isbell, Dyanna M. Czceck, William F. Kean, Rani Elhajjar

Keywords

Anisotropy of Magnetic Susceptibility, Drumlins, Fláajökull Glacier, Iceland, Ice Fabrics, Ring-shear Device, Till Fabrics

Abstract

The increased surface melting of the outlet glaciers of the Vatnajökull Ice Cap has a profound affect on the dynamics of the ice-bed couple and landform genesis. Soft-bedded glaciers are largely inaccessible, which creates a problem. One challenge is to understand the complex interactions of the glacier bed and its resultant depositional and deformational landform systems. This study investigates an outlet glacier from the Vatnajökull Ice Cap, described herein as the Fláajökull glacier system. To circumvent some of these problems, three separate projects were conducted in this dissertation: (1) magnetic fabric study of effective pressure (difference between the ice-overburden pressure and pore-water pressure) and shear rate (glacier velocity) using a laboratory ring-shear device; (2) glaciological analysis of magnetic fabrics and c-axis orientations of dirty ice veins; and (3) investigation of drumlin formation using magnetic till fabrics and field relationships. Several hypotheses were addressed for each of these studies, which include: (1) to determine if fabric strength is independent of shear rate and effective pressure. This hypothesis was tested and the results confirmed that the fabric strength (S1 eigenvalue) was independent of shear rate and effective pressure. Based on these results, effective pressure and shear rate cannot be interpreted from fabric strength evidence from glacial deposits; (2) in the glaciological study, I hypothesized that the dirty ice veins were sub-vertically sheared from the bed near the ice front, but then moderately deformed. Results from the magnetic fabrics indicate that the maximum K1 susceptibility axis (77º plunge) is approximately parallel to the vein margins verify that the injection was sub-vertical. The long axes of the recrystallized ice grains (parallel to foliation plane defined by K1 - K2) appeared to show a good correlation with the plunge of the maximum K1 susceptibility. Also, the eigenvector plunge of the c-axes was approximately normal to the shear plane, which supports previous theory that c-axes rotate away from the shear plane toward the vertical. Multi-maximum girdle fabrics from the c-axes and associated textures from thin sections (e.g. nucleated grains, bulging of grain boundaries and slips band) suggest that some deformation likely occurred after emplacement; and (3) the Boulton hypothesis was tested using magnetic till fabrics and field relationships. According to this idea, drumlins form due to hydrologic heterogeneity (permeability differences in granular materials) causing a solid nucleus to form in the bed where sediment is accreted and sheared in the direction of ice flow. At Fláajökull, the magnetic fabrics from sites B and C mimicked the glacier flow direction with the longitudinal flow plane (K1 - K3) approximately parallel to the NNW-SSE drumlin long axis. The drumlin cores consisted of outwash sand and gravels which likely acted as rigid obstacles in the bed. Ice overriding resulted in heterogeneous deformation of the drumlin cores following the deposition of the upper basal till carapace. These results support the Boulton hypothesis.

These studies demonstrate significant progress toward understanding fabric strength development of soft-bedded glaciers. In linking studies (1) and (3) the ring-shear device was used to provide insights into fabric strength development upon shear rate and effective pressure. In the third study previous ring-shear experiments, magnetic till fabrics and field relationships were used to understand modern drumlin genesis.

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