Ring-shear studies of till deformation: Coulomb-plastic behavior and distributed strain in glacier beds

A ring-shear device was used to study the factors that control the ultimate(steady) strength of till at high shear strains.Tests at a steady strain rate and at different stresses normal to the shearing direction yielded ultimate friction angles of 26.3° and 18.6° for tills containing 4% and 30% clay-sized particles, respectively Other tests at steady normal stresses and variable shear-strain rates indicated a tendency for both tills to weaken slightly with increasing strain rate. This weakening may be due to small increases in till porosity.These results provide no evidence of viscous behavior and suggest that a Coulomb-plastic idealization is reasonable for till deformation. However, viscous behavior has often been suggested on the basis of distributed shear strain observed in subglacial till. We hypothesize that deformation may become distributed in till that is deformed cyclically in response to fluctuations in basal water pressure. During a deformation event, transient dilation of discrete shear zones should cause a reduction in internal pore-water pressure that should strengthen these zones relative to the surrounding till, a process called dilatant hardening. Consequent changes in shear-zone position, when integrated over time, may yield the observed distributed strain

Effect of a Cold Margin on Ice Flow at the Terminus of Storglaciaren, Sweden: Implications for Sediment Transfer

The cold-based termini of polythermal glaciers are usually assumed to adhere strongly to an immobile substrate and thereby supply significant resistance to the flow of warm-based ice upglacier. This compressive environment is commonly thought to uplift basal sediment to the surface of the glacier by folding and thrust faulting. We present model and field evidence from the terminus of Storglaciaren, Sweden, showing that the cold margin provides limited resistance to flow from up-glacier. Ice temperatures indicate that basal freezing occurs in this zone at 10−1 –10−2 ma−1, but model results indicate that basal motion at rates greater than 1ma−1 must, nevertheless, persist there for surface and basal velocities to be consistent with measurements. Estimated longitudinal compressive stresses of 20– 25 kPa within the terminus further indicate that basal resistance offered by the cold-based terminus is small. These results indicate that where polythermal glaciers are underlain by unlithified sediments, ice-flow trajectories and sediment transport pathways may be affected by subglacial topography and hydrology more than by the basal thermal regime

Magnificent marine mud : a late Foxe paleoclimate record from Outer Frobisher Bay and Brevoort Basin, Baffin Island, Canada

Includes bibliographical references (pages [128]-135).High resolution seismic reflection data and three piston cores from Frobisher Bay and Brevoort Basin were examined to further refine the positions of late glacial ice margins and the deglacial chronology of this region. The seismic data reveals three acoustic units similar to those recognized on the southeast Baffin Island continental shelf (Evans, 1990 and Praeg et al, 1986) and the Labrador shelf (Josenhans et al, 1986). In Frobisher Bay, turbidtes originating form a glacial marine source make up the majority of the sediment which is distributed among five small basins adjacent to Griffin Bay/Ney Harbor, Charles Francis Hall Bay, York Sound, Jackman Sound, and the Savage Islands. Sediment deposition within the basins is controlled by bedrock topography of the seabed as well as an available sediment source from the adjacent Meta Incognita Peninsula. Detailed sedimentological data of the lithostratigraphic units within piston cores 007 and 009 (York Sound Basin), indicate a sediment source associated with the retreating local ice caps and foreign ice of the Noble Inlet and Gold Cove readvances. This is confirmed by cores collected from the basins adjacent to Griffin Bay/Ney Harbor (Duvall, 1993) and the Savage Islands (Stravers et al, 1992). . In Brevoort Basin (to the east of Hall Peninsula), seismic reflection profiles show a sequence of post-glacial and glacial marine sediments overlying a acoustically chaotic unit that is interpreted as either a subglacial till or debris flow deposit. Two small moraines on the southwest margin of the basin ABSTRACT appear to be correlative to glacial marine sediments deposited within the basin (Kaufman et al., 1993). The sedimentary record from piston core (022), collected from the basin reveals an ice marginal retreat sequence (9.9-7.7 yrs BP.) grading slowly into post glacial sediments (7.7-2.4 yrs BP.). This sequence is attributed to the initial retreat of the Gold Cove ice margin (by 9700 yrs BP) from the Hall Peninsula continental shelf, and lingering glacial input from local ice on Hall Peninsula.M.S. (Master of Science