Basal Crevasses Research Articles

Basal-crevasse-fill origin of laminated debris bands at Matanuska Glacier, Alaska, U.S.A.

AbstractThe numerous debris bands in the terminus region of Matanuska Glacier, Alaska, U.S.A., were formed by injection of turbid meltwaters into basal crevasses. The debris bands are millimeter(s)-thick layers of silt-rich ice cross-cutting older, debris-poor englacial ice. The sediment grain-size distribution of the debris bands closely resembles the suspended load of basal waters, and of basal and proglacial ice grown from basal waters, but does not resemble supraglacial debris, till or the bedload of subglacial streams. Most debris bands contain anthropogenic tritium (3H) in concentrations similar to those of basal meltwater and ice formed from that meltwater, but cross-cut englacial ice lacking tritium. Stable-isotopic ratios (δ18O and δD) of debris-band ice are consistent with freezing from basal waters, but are distinct from those in englacial ice. Ice petrofabric data along one debris band lack evidence of active shearing. High basal water pressures and locally extensional ice flow associated with overdeepened subglacial basins favor basal crevasse formation.

Significant marine-ice accumulation in the ablation zone beneath an Antarctic ice shelf

AbstractHigh-resolution crystallographic, salinity and isotopic analyses of a 45 m ice core reveal the presence of a thick layer of marine ice near the grounding line of the Nansen Ice Shelf, Antarctica. The anomalous formation of marine ice in a zone assumed to be the site of active basal melting leads us to propose the hypothesis of large basal crevasses as a favorable environment for important marine-ice accretion. This hitherto unexplored possibility is supported by the overall field configuration and by the discrepancy in some ice properties between this core and the marine-ice sections of previous drilling projects. These findings could have important implications for the general stability of ice shelves and their disintegration processes. The specific properties of this core reveal that marine ice is post-genetically deformed.

The formation of a geometrical ridge network by the surge-type glacier Kongsvegen, Svalbard

Traditionally, geometrical ridge networks are interpreted as the product of the flow of subglacial sediment into open basal crevasses at the cessation of a glacier surge (‘crevasse-fill’ ridges). They are widely regarded as a characteristic landform of glacier surges. Understanding the range of processes by which these ridge networks form is therefore of importance in the recognition of palaeosurges within the landform record. The geometrical ridge network at the surge-type glacier Kongsvegen in Svalbard, does not form by crevasse filling. The networks consist of transverse and longitudinal ridges that can be seen forming at the current ice margin. The transverse ridges form as a result of the incorporation of basal debris along thrust planes within the ice. The thrusts were apparently formed during a glacier surge in 1948. Longitudinal ridges form through the meltout of elongated pods of debris, which on the glacier surface are subparallel to the ice foliation and pre-date the surge. This work adds to the range of landforms associated with glacier surges.

Ocean interactions with the base of Amery Ice Shelf, Antarctica

Using a two‐dimensional ocean thermohaline circulation model, we varied the cavity shape beneath Amery Ice Shelf in an attempt to reproduce the 150‐m‐thick marine ice layer observed at the “G1” ice core site. Most simulations caused melting rates which decrease the ice thickness by as much as 400 m between grounding line and G1, but produce only minor accumulation at the ice core site and closer to the ice front. Changes in the seafloor and ice topographies revealed a high sensitivity of the basal mass balance to water column thickness near the grounding line, to submarine sills, and to discontinuities in ice thickness. Model results showed temperature/salinity gradients similar to observations from beneath other ice shelves where ice is melting into seawater. Modeled outflow characteristics at the ice front are in general agreement with oceanographic data from Prydz Bay. A freshwater flux across the grounding line, derived from melting beneath the grounded ice sheet, would have to be anomalously large to produce the basal marine ice layer and account for the Ice Shelf Water outflow. We concur with Morgan's inference that the G1 core may have been taken in a basal crevasse filled with marine ice. This ice is formed from water cooled by ocean/ice shelf interactions along the interior ice shelf base.

Formation of De Geer moraines and implications for deglaciation dynamics

AbstractDe Geer moraines are very common in the Møre area, western Norway. These moraines occur below the marine limit and outside the Younger Dryas ice limit and occupy tributaries that connect the main fjords through the mountain passes. During deglaciation, ice in these tributaries flowed to the major ice streams.Sections across three De Geer moraines show that the ridges are composed of diamictons and fine‐grained sediment, partly in stacked sequences. The diamicton units are interpreted as being composed of water‐lain tills, lodgements tills and subaqueous flow deposits. The fine‐grained sediment is though to have formed in a proglacial marine environment. Clast fabric of diamictons and deformation structures in underlying sands show that depositional directions for diamicton units and the direction of deformation for the sands is perpendicular to the ridge crests. Mainly based on this evidence, the ridges are thought to have formed by push at the glacier grounding line. The formation of transverse ridges (relative to ice flow) do occur in basal crevasses on modern glaciers, as do swarms of ridges along the front of retreating glaciers. The first mechanism of deposition does not seem to explain the ridges studied in the present paper and hence the importance of this process in the formation of De Geer moraines is questioned.The De Geer moraines were deposited by ice lobes advancing from one main fjord into another; therefore by studying the drainage pattern of the tributary lobes and their sequence of deglaciation, many features of the style of deglaciation of the ice sheet across the area can be determined.The northwestern part of the area was deglaciated earliest. After that, deglaciation proceeded to the southwest parallel to the coast. Subsequently the outer and the central part of Romsdalsfjorden were deglaciated causing ice to drain towards this fjord from both the north and south. The last fjord to be deglaciated was Storfjorden in the south.

Genesis of De Geer moraines in Finland

The proposed model of the genesis of De Geer moraines in Finland is based mainly on the distribution of the moraine areas in relation to the palaeohydrological environment, and on the composition and two- and three-dimensional fabrics of the material. In order to shed some light on the geochronological problem involved, the distances between successive ridges in a number of De Geer moraine series are also compared with clay varve data on deglaciation rates in Finland. Granulometrically, petrographically and morphometrically, De Geer moraine till does not differ significantly from tills of surrounding areas. Two-dimensional fabrics stress orientations at right angles to the ridge crest, and three-dimensional analyses reveal the slope-conformable character of pebble inclinations. Generally, spacings of the De Geer moraine ridges do not confirm annual deglaciation rates as conveyed by clay varve data. It is suggested the moraines were formed in basal crevasses of the ice, following active flow phases of a local surging nature. During the quiescent phase of the surge cycle, the ice subsided into a water-soaked till mattress, filling up basal cavities from the proximal or the distal side. Finnish De Geer moraines are thus neither annual nor end moraines. They are subglacial bedforms with no geochronological meaning, except that they formed during one single surge cycle. As the marginal zone of the ice sheet calved and thinned, it was lifted by proglacial water and the whole group of moraines became free of ice almost simultaneously.

Formulation of ice shelf dynamic boundary conditions in terms of a Coulomb rheology

Coastal boundaries where fast flowing ice shelves shear past stagnant, grounded ice are typically riven with surface crevasses, seawater‐filled basal crevasses, and tidal strand cracks. Here we formulate a boundary condition describing stress transmission through these fractured boundaries in terms of the Coulomb law. As a result of this formulation, agreement between finite element simulations of the Ross Ice Shelf flow and field observations is improved over agreement obtained with formulations which do not account for ice failure. Our results additionally suggest that shear stress transmitted through ice shelf boundaries is lower than previously thought.

On the Disintegration of Ice Shelves: The Role of Fracture

AbstractCrevasses can be ignored in studying the dynamics of most glaciers because they are only about 20 m deep, a small fraction of ice thickness. In ice shelves, however, surface crevasses 20 m deep often reach sea-level and bottom crevasses can move upward to sea-level (Clough, 1974; Weertman, 1980). The ice shelf is fractured completely through if surface and basal crevasses meet (Barrett, 1975; Hughes, 1979). This is especially likely if surface melt water fills surface crevasses (Weertman, 1973; Pfeffer, 1982; Fastook and Schmidt, 1982). Fracture may therefore play an important role in the disintegration of ice shelves. Two fracture criteria which can be evaluated experimentally and applied to ice shelves, are presented. Fracture is then examined for the general strain field of an ice shelf and for local strain fields caused by shear rupture alongside ice streams entering the ice shelf, fatigue rupture along ice shelf grounding lines, and buckling up-stream from ice rises. The effect of these fracture patterns on the stability of Antarctic ice shelves and the West Antarctic ice sheet is then discussed.

On the Disintegration of Ice Shelves: The Role of Fracture

AbstractCrevasses can be ignored in studying the dynamics of most glaciers because they are only about 20 m deep, a small fraction of ice thickness. In ice shelves, however, surface crevasses 20 m deep often reach sea-level and bottom crevasses can move upward to sea-level (Clough, 1974; Weertman, 1980). The ice shelf is fractured completely through if surface and basal crevasses meet (Barrett, 1975; Hughes, 1979). This is especially likely if surface melt water fills surface crevasses (Weertman, 1973; Pfeffer, 1982; Fastook and Schmidt, 1982). Fracture may therefore play an important role in the disintegration of ice shelves. Two fracture criteria which can be evaluated experimentally and applied to ice shelves, are presented. Fracture is then examined for the general strain field of an ice shelf and for local strain fields caused by shear rupture alongside ice streams entering the ice shelf, fatigue rupture along ice shelf grounding lines, and buckling up-stream from ice rises. The effect of these fracture patterns on the stability of Antarctic ice shelves and the West Antarctic ice sheet is then discussed.

Till Fabrics of the Cross-Valley Moraines of North-Central Baffin Island, Northwest Territories, Canada1

Analyses of 103 till fabrics taken from proximal and distal slopes of distinctive cross-valley moraines in the Isortoq Valley of north-central Baffin Island show variations in fabric pattern with respect to slope and location within the valley. Changes in dip patterns and fabric strengths are also associated with variations in moraine morphology. Four distinct morphological moraine types are delimited and are called simple-linear, s-shaped, hooked, and asymmetric. There is no gradation in type along a single moraine. Orientation and dip strength are found to vary with moraine type as well as between distal and proximal slopes. Dip patterns of elongate pebbles from proximal and distal sites are also shown to vary with slope, location within the valley and moraine type. These results indicate that the hooked, asymmetric, and simple-linear moraines are formed by either dissimilar processes or changes in intensity of the same process. S-shaped moraines differ morphologically from simple-linear but have the same fabric characteristics. The possibility of annual deposition is examined and concluded to be possible though in no sense proven. The cross-valley moraines are thought to form at the base of an ice cliff grounded in a glacial lake. The influx of summer meltwater provides a possible trigger mechanism and explains the association between the moraines and water-deposited kame features. The asymmetric moraines were deposited by overriding and pushing. Moraines at right angles to the ice cliff were formed by the squeezing of till into basal crevasses or melt-water tunnels.