rheology of till beneath storglaciaren sweden in order to study in situ the rheology of a deforming subglacial till various instruments were emplaced in till beneath storglaciaren sweden boreholes were used to gain access to the till beneath about 100 m of ice tiltmeters provided an estimate of the shear strain rate in the till two other instruments yielded measures of till strength in addition water pressures were recorded in boreholes and in the till a computer controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag based on a force balance calculation tilt rates varied directly with effective pressure so decreases in water pressure apparently increased the coupling between the glacier and the bed surface speed was either out of phase with tilt or varied independently of tilt thus increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed they were not a result of till deformation till strength varied directly with effective pressure which is consistent with it being a mohr coulomb or frictional material the devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed this implies that the residual strength of the till is insensitive to strain rate thus the appropriate constitutive equation for till rheology may be of the form e e k where k is a constant this is consistent with experimental data reported in the geotechnical literature

origin of a washboard moraine of the des moines lobe inferred from sediment properties abstract geometric characteristics of the washboard moraines of the des moines lobe dml of the laurentide ice sheet agree with their proposed origin as crevasse squeeze ridges but study of their sediments is required to help further test this hypothesis a 70 m long 3 5 m high section through a moraine ridge in central iowa revealed till with irregular isolated lenses of silt sand and gravel that dip to varying extents upglacier the texture and density of the till are like those of the basal till of the dml studied elsewhere in iowa and preconsolidation pressures determined from tests on till and silt of the ridge indicate that it developed subglacially rather than at the glacier margin preconsolidation pressures additionally demonstrate that pore water pressures in the bed supported most of the glacier s weight which would have contributed to till mobility fabrics based on the anisotropy of magnetic susceptibility of 3125 intact till specimens collected at 125 locations in the section indicate two end member states of strain that varied with location in the ridge and caused sediment mounding simple shear that was directed downglacier along shear planes inclined upglacier together with pure shear where an overlying crevasse allowed the sediment bed to extend upward and laterally meltwater that likely flowed along the crevasse deposited sorted sediments that were incorporated in till deformed and rotated this positive test of the crevasse squeeze hypothesis indicates that the dml was in longitudinal extension near its margin reinforcing previous arguments that the lobe surged the predominance of fabrics caused by simple shear demonstrates that crevasse filling was underway before the surge had fully halted this study should prompt caution in using similar transverse ridges such as those geophysically imaged in some submarine glacier forefields as indicators of retreat rates

microstructures and microshears as proxy for strain in subglacial diamicts implications for basal till formation ring shear experiments were used to study subglacial sediment deformation and the development of s matrix microstructures and microshears to constrain glacially induced deformation in diamicts the experiments approximated subglacial conditions and the diamict was sheared and sampled incrementally to strains of 0 7 18 33 57 and 107 at strains between 7 and 18 a steady association of s matrix microstructures developed which did not evolve further despite continued shearing the i l index a measure of microshear orientation strength parallel to the shearing direction correlates log linearly with strain which indicates its potential use as strain proxy the ring shear data were compared with the characteristics of quaternary basal tills from denmark poland and svalbard based on i l index values we conclude that these tills only experienced strain of 10 1 which is orders of magnitude lower than expected for deformation tills this suggests that pervasive subglacial deformation and sediment advection in the mobile layer at the ice bed interface may be less significant than previously assumed

distributed shear of subglacial till due to coulomb slip in most models of the flow of glaciers on till beds it has been assumed that till behaves as a viscoplastic fluid despite contradictory evidence from laboratory studies in accord with this assumption displacement profiles measured in subglacial till have been fitted with viscoplastic models by estimating the stress distribution here we present a model that illustrates how observed displacement profiles can result from till deformation resisted solely by coulomb friction motion in the till bed is assumed to be driven by brief departures from static equilibrium caused by fluctuations in effective normal stress these fluctuations result from chains of particles that support intergranular forces that are higher than average and that form and fail at various depths in the bed during shearing newton s second law is used to calculate displacements along slip planes and the depth to which deformation extends in the bed consequent displacement profiles are convex upward similar to those measured by boulton and colleagues at breidamerkur jokull iceland the model results when considered together with the long term and widespread empirical support for coulomb models in soils engineering indicate that efforts to fit viscoplastic flow models to till displacement profiles may be misguided

microstructural interpretations of modern and pleistocene subglacially deformed sediments the relative role of parent material and subglacial processes analysis of till micromorphology represents a relatively new technique that has been used most frequently to infer the importance of subglacial shear in till genesis this study aims to calibrate the technique by comparing pleistocene tills from united kingdom with a modern till the upb till from beneath ice stream b west antarctica despite the fact that all of the tills examined have been interpreted as deforming bed deposits the modern till has significantly less abundant and diverse microstructures than those found in the pleistocene tills seventeen examined thin sections of the upb till contain recognisable microstructures over only 0 30 of individual thin section area the most common microstructures are i birefringent clay patterns that are interpreted as shear zones and ii adherent matrix structures which we interpret as uncomminuted remnants of the parent glacial glaciomarine diamictons fourteen thin sections of the pleistocene tills were covered by microstructures in 10 95 of their area the pleistocene microstructures include birefringent clays and adherent matrix structures as in the upb till but also laminations and deformed pods made of chalk and sorted sediments we conclude that the same till forming process i e subglacial deformation may result in distinctly different till micromorphology this is a consequence of the fact that microstructural characteristics are strongly influenced by factors other than shear deformation we identify three controls that may be important for forming contrasting microstructural assemblages i strain magnitude ii the degree of heterogeneity of parent material and iii relative importance of sediment sorting by flowing water in the subglacial environment thus till micromorphology is sensitive to multiple factors which with proper calibration may enhance the existing capability to interpret past subglacial conditions from microstructures copyright 2001 john wiley sons ltd

submarginal debris transport and till formation in active temperate glacier systems the southeast iceland type locality abstract exhaustive sedimentological analysis of freshly exposed subglacial surfaces and moraines in southern iceland provides diagnostic sedimentological signatures of a debris transport pathways through active temperate glacier snouts and b till production in subglacial traction zones dominated by deforming layers three till end members are recognised based on stratigraphic architecture 1 thin and patchy tills over eroded bedrock 2 single push moraines and complexes and 3 overridden moraines or outwash fans typical till thicknesses are 0 10 1 40 m with each till relating to a deformation event driven by the seasonally tuned processes of glacier sub marginal shearing freeze on squeezing and bulldozing clast form trends demonstrate progressive modification towards mature forms in subglacial traction zones with till being clearly differentiated from scree and glacifluvial deposits clast macrofabric strengths are variable rarely matching those of laboratory shearing experiments except where obviously lodged clasts are abundant they also consistently record former glacier flow directions but localized variability is introduced by bedrock protuberances cavity infill clast interference and freshly imported plucked clasts within tills macrofabrics strengthen from lower b horizon to upper a horizon tills but at the outer edges of sub marginally thickening till wedges or push moraines seasonally driven cycles of squeezing flowage freeze on melt out and bulldozing give rise to a range of clast macrofabric strengths as well as superimposed deformation signatures this reflects two extremes of till emplacement including the more mobile flowing and often liquefied matrixes in push squeeze moraines and in contrast the lodgement deformation and ploughing at the thin end of sub marginal till wedges

the relationship between drumlins and other forms of subglacial glaciotectonic deformation this paper brings together the field data from 33 drumlins to show the relationship between drumlin formation and other subglacial deforming bed processes it is shown that there is a drumlin structure continuum a depositional which are similar to flutes formed by sediment flowing into the low pressure area behind a large obstacle and these conditions are mostly found within rock cored drumlins b deformational which contain different styles of deformation associated with a relatively weak core including stoss side deformation including both small and large scale brittle and ductile deformation stoss and lee side deformation compressive deformation and extensional deformation c erosional which consist of either truncated stratified sequences or homogeneous tills with distinct ice flow patterns furthermore it is suggested that if more sediment enters the deforming layer than can be removed then there will be net subglacial deposition which will result in the build up of a deforming bed till but if more sediment leaves the deforming layer then net erosion will occur under net erosional conditions any obstacle to flow may be left behind as a drumlin thus although individual drumlins have different internal structures they are formed associated with subglacial net erosion 0 1997 elsevier science ltd all rights reserved

microfabric and microshear evolution in deformed till abstract the bed deformation model of glacier movement requires that ice masses shear their beds to high strains for homogeneous basal tills that can result from such deformation however there are no sedimentological criteria for estimating strain magnitude thus a large ring shear device was used to shear two basal tills and their particle fabrics and microshears were studied at various strains up to 108 sand particle fabrics in a longitudinal flow plane became steady at shear strains of 7 39 steady fabrics were moderately strong with s1 eigenvalues of 0 71 0 74 these values are smaller than those from experiments with pebble sized particles perhaps reflecting the greater probability of sand particles colliding with larger grains fabrics parallel to flow strengthened progressively indicating that particles did not reorient with their long axes transverse to flow r1 and r2 microshears developed and persisted to the highest strains attained r1 shears became more pervasive with strain and oriented closer to the shear plane movement along both sets of shears provides an explanation for the plunge of the steady state sand fabric approximately 10 upglacier these experiments provide a basis for estimating till deformation magnitude but results will require cautious application to the geologic record where deformation processes may be ambiguous

undrained triaxial strength and stress strain characteristics of a glacial till soil undrained triaxial compression tests were carried out on reconstituted and nominally undisturbed tubed samples of a lodgement till from the vale of st albans in hertfordshire england the soil is a matrix dominant chalky boulder clay of anglian age with little discernable engineering fabric electron microscope observations showed the presence of crystalline calcite in tube samples the test results were examined within the general framework of critical state soil mechanics using normalizing procedures to take account of the different states and stress histories of the samples these analyses demonstrate the practical importance of accounting for the current state and stress history in the interpretation of soil test data the present results form a self consistent pattern of behaviour differences between reconstituted and tubed samples were found only at small strain and may be attributed to cementing in tubed samples which is broken down during reconstitution and during relatively large straining in recompression and shearing

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 ofdiscrete 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

discrete element modeling of subglacial sediment deformation 1 the discrete element method dem is used in this study to explore the highly nonlinear dynamics of a granular bed when exposed to stress conditions comparable to those at the bed of warm based glaciers complementary to analog experiments the numerical approach allows a detailed analysis of the material dynamics and the shear zone development during progressive shear strain the geometry of the heterogeneous stress network is visible in the form of force carrying grain bridges and adjacent volumetrically dominant inactive zones we demonstrate how the shear zone thickness and dilation depend on the level of normal overburden stress and we show how high normal stress can mobilize material to great depths the particle rotational axes tend to align with progressive shear strain with rotations both along and reverse to the shear direction the results from successive laboratory ring shear experiments on simple granular materials are compared to results from similar numerical experiments the simulated dem material and all tested laboratory materials deform by an elastoplastic rheology under the applied effective normal stress these results demonstrate that the dem is a viable alternative to continuum models for small scale analysis of sediment deformation it can be used to simulate the macromechanical behavior of simple granular sediments and it provides an opportunity to study how microstructures in subglacial sediments are formed during progressive shear strain

a multiproxy study of a basal till a time transgressive accretion and deformation hypothesis a two part basal till at knud strand denmark reveals a uniform fabric pattern and strength petrographical composition and clay mineralogy the nature of the contact with the underlying sediments ductile deformation structures partly intact soft sediment clasts small meltwater channels and thin horizontal outwash stringers dispersed in the till indicate both bed deformation and basal decoupling by pressurised subglacial water a time transgressive model is suggested to explain the lack of vertical gradation in till properties in which debris released from the active ice sole is sheared in a thin zone moving upward as till accretion proceeds it is suggested that although strain indicators occur throughout the entire till thickness the deformation at any point of time encompassed the uppermost part of the till only allowing preservation of fragile clasts below the substantial thickness of the till up to 6 m coupled with a much smaller by more than one order of magnitude inferred thickness of the deforming bed suggests that the bulk of till material was transported englacially prior to deposition the lack of petrographical gradation in the till is attributed to effective mixing and homogenisation of material along the ice flow path copyright 2004 john wiley sons ltd

glacial marine or subglacial origin of diamicton units from the southwest and north iceland shelf implications for the glacial history of iceland abstract the extent and timing of ice advances onto the north and southwest iceland shelf is reconstructed by identifying the depositional environment of four diamicton units present in two marine cores from shelf troughs we analyze a combination of properties including sedimentological parameters microfabric analyses radiocarbon dates and foraminiferal assemblage data this multi proxy approach for understanding depositional environments of diamicton units is critical in areas where ice sheets extended into the marine environment the two cores studied contain a lower diamicton unit that is interpreted as subglacial till and an overlying diamicton unit that is interpreted as glacial marine sediment our analysis showed that in general till is classified by lower water content lower total organic carbon and coarser grain size than glacial marine sediment anisotropy of magnetic susceptibility shows that the till has a sheared fabric low inclination of the short k3 axis whereas the glacial marine sediment has a high inclination of the short k3 axis and a more random unsheared fabric grain counts on the 106 1000 m fraction in till contain a higher percentage of basalt whereas the glacial marine sediments contain a higher percentage of quartz volcanic glass and foraminifera foraminiferal assemblage data support the interpretations from the sedimentology and microfabric data radiocarbon dates on shells and or foraminifera in sediments directly above the diamicton units show that the ice retreated from these shelf areas by approximately 13 ka

multiple subglacial till deposition a modern exemplar for quaternary palaeoglaciology the sedimentology of a vertical succession of alternating beds of massive and fissile diamictons on a orisjokull plateau icefield outlet foreland is employed to assess the evolution of subglacial traction tills at the margins of active temperate glaciers with deformable substrates where tills appear to form sub marginal incrementally thickened wedges that display multiple till emplacement events potentially of annual scale lodged boulders display strong a axes and surface striae alignments which parallel surface flutings indicating that fluting construction and till emplacement were related to moulding by consistent glacier flow from the ssw during the little ice age in contrast clast macrofabrics at the sub boulder size not unlike those reported from other icelandic tills are not as strong as would be expected in a subglacially sheared medium indicating shear strains too low for a steady state strain signature this separation of fabric data has isolated the strain signatures of the lodgement and deformation components of subglacial traction till whereby the orientations of the largest lodged clasts record high cumulative shear strains and those of the sub boulder sized clasts record greater susceptibility to deformation of their enclosing matrix this is likely due to the effect of clast collisions in clast rich till and the perturbations set up by the numerous large boulders consistent with observations on till fabrics in flutings and around lodged clasts a b plane macrofabric data display unusually high degrees of isotropy reflective of the more variable orientations of a b planes which are thought to reflect a b plane susceptibility to dip parallel or anastomosing shear planes a wide range of clast angularity values reflects the localized input of freshly plucked and hence relatively highly angular blocks to the deforming layer a characteristic of stepped bedrock profiles beneath the snouts of mountain glaciers finally we hypothesize that the massive and fissile units may represent a and b horizons of subglacial deforming layer couplets and that each couplet could record seasonal emplacement and partial inter couplet modification near the ice margin if the latter is the case then less than 1 m of subglacial till is advected to the glacier margin per annual deformation event

numerical modeling of advective transport of saturated deforming sediment beneath the lake michigan lobe laurentide ice sheet abstract we investigated the dynamics of advective transport of saturated fine grained deforming sediment beneath the late wisconsinan lake michigan lobe lml of the laurentide ice sheet using a numerical flowline model sediment rheology is modeled using a general constitutive law for mass movement of fine grained material geotechnical tests on selected tills deposited by the lml in northern illinois suggest an effective viscosity of about 5 109 pa s for fully saturated sediment and mild nonlinearity in the stress strain relation numerical calculations of sediment flux based on conservative parameters and assumptions yield estimates on the order of 100 m3 y meter width of the lobe suggesting that advective transport could account directly for at least 25 of the total sediment flux estimated on the basis of field evidence less restrictive parameters and assumptions would increase sediment flux attributable to both advective transport as well as other ice velocity dependent sediment transport processes modeling results also suggest that maximum advective transport rates might be associated with viscosities up to an order of magnitude higher than the viscosity exhibited by fully saturated sediment these results thus suggest that subglacial sediment deformation could account for a substantial portion of total sediment flux in areas with fine grained glacial till and that transport rates are strongly sensitive to the subglacial hydrologic processes that control basal water pressure and sediment viscosity

magnetic fabrics of the douglas till of the superior lobe exploring bed deformation kinematics abstract the superior lobe of the laurentide ice sheet during its last advance into northwestern wisconsin may have moved and transported sediment primarily through deep widespread shear of its bed to high strains 100 to study the kinematics of this deformation we measured the anisotropy of magnetic susceptibility of 3750 intact samples of its basal till the douglas member of the miller creek formation collected from eight profiles at 0 2 m depth increments a benchmark for interpretations was provided by results of ring shear experiments conducted on the douglas till which provided fabric characteristics for the case of horizontal simple shear orientations of maximum susceptibility k1 were controlled by preferred orientations of silt sized magnetite grains and were similar to sand grain orientations measured in thin sections strengths of fabrics formed by orientations of k1 indicate that most of the till sampled was deformed but to variable strains deformation averaged over the 20 km spanned by the study approximated horizontal simple shear however large differences in k1 fabric azimuths 45 over lateral distances of meters to tens of meters indicate highly heterogeneous bed deformation perhaps focused in anastomosing zones with associated divergent and convergent till shear this interpretation is supported by orientations of principal susceptibilities that imply in many cases either that shear planes were steeply dipping particularly transverse to the shearing direction or that there were major components of pure shear variations in k1 fabric azimuth with depth indicate that most of the till thickness did not shear simultaneously rather till accumulated at the bed as shear direction changed in response to temporally shifting zones of shallow deformation

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

a ring shear device for the study of till deformation tests on tills with contrasting clay contents abstract deformation of subglacial sediment may have had a profound influence on the geomorphic effects and dynamics of pleistocene ice sheets but data that bear on the rheology of such sediment are few and contradictory a means of studying systematically the mechanical properties of glacial tills is provided by a ring shear device that shears a large annular specimen to high strains at variable rates special features of the device allow continuous observation of the distribution of shear strain measurement of local stresses normal to the direction of shear and isolation of wall effects thus far a linear viscous putty and two tills with different clay contents 4 and 32 by weight have been tested the experiments with till indicate as expected that the presence of dispersed clay minerals reduces markedly both the residual strength and hydraulic diffusivity of till clay also reduces spatial variations in normal stress associated with grain bridges the walls bounding the specimen support only 9 17 of the total resistance to shearing strain was localized in both tills but not in the linear viscous putty strain localization in the tills indicates that their theology departs significantly from linear and bingham viscous models

laboratory study of fabric development in shearing till the importance of effective pressure and shearing rate abstract herein we present data on the shearing rate glacier velocity and effective pressure difference between the ice overburden pressure and pore water pressure in the development of magnetic fabric anisotropy of magnetic susceptibility using a rotary ring shear device a wisconsin age basal till was used in the experiments and deformed to its critical state at shear strains as high as 93 we also present data from hysteresis and high temperature susceptibility experiments to identify the magnetic carrier in the basal till results showed little change in fabric strength when varying the shearing rate in the speed range of 110 860 m year 1 moreover the effective pressure tests also showed an inconsistency in fabric between 30 and 150 kpa however a slight strengthening effect was documented thus the k 1 magnetic fabric strength is independent of the shearing rate and effective pressure this suggests that the fabric strength upon these variables cannot be used as a benchmark for estimating shear deformation to the geological record the k 1 fabric strength in this study however remained consistent with respect to other till particle fabric methods e g sand and pebble in which the same conclusion was drawn all particles align parallel to the direction of shear and plunge mildly up glacier

compaction induced inclination shallowing in synthetic and natural clay rich sediments experiments using single clay slurries consisting of acicular or equidimensional magnetite mixed with either kaolinite illite montmorillonite or chlorite were conducted to determine the effect of compaction on inclination a natural marine sediment and a reconstructed marine sediment made up of the clays used in our single clay experiments were also studied a laboratory postdepositional remanence imparted to the slurries by stirring had its inclination shallowed by compaction for both the single clay samples as seen previously anson and kodama 1987 and the natural and reconstructed marine sediments suggesting that our single clay sediments are valid analogues of natural sediments magnetic intensity also decreased by 30 50 during compaction the greatest decrease occurring for samples with the steepest initial inclination altering the ph of a slurry so that the magnetite would have a negative surface charge and be repelled by clay particles did not affect a slurry s inclination shallowing behavior suggesting that electrostatic attraction between magnetite and clay is probably not a major contributor to inclination shallowing however most of the inclination shallowing observed in our experiments occurs at low pressures when void ratios are high comparison with a clay microfabric compaction study mcconnachie 1974 suggests that volume loss at these pressures is accommodated by reorientation of clay particles or clay particle aggregates clay domains since pore spaces are large at these low pressures inclination shallowing is probably caused by magnetite particles either being attached to clay particles perhaps by van der waals forces or having been incorporated into clay domains alternating field demagnetization of compacted samples which contain magnetite with a range in grain sizes supports this model indicating that the smallest magnetic grains are slightly more shallowed by compaction than larger grains our results suggest that a sediment s clay content may be an important contributor to inclination shallowing

subglacial deformation evidence from microfabric studies of particles and voids in till from the upper urumqi river valley tien shan china microfabrics formed by flat particles and voids in till samples from the upper urumqi river valley tien shan northwestern china were analyzed using three mutually perpendicular thin sections chi squared tests were used to determine significance levels of the orientations of both particles and voids the basal tills in an end moraine on the stoss sides of two roche moutonnees on the stoss sides of a drumlin with a rock core and on the lee side of the same drumlin far from the rock core have strong particle and void microfabrics field evidence suggests that these tills were deformed in a subglacial setting thus consistent with recent laboratory studies of till fabric development deformation is believed to be responsible for the strong particle and void microfabrics in the same end moraine particle microfabrics in an overlying till were strong but void microfabrics were weak this till is believed to have been formed by dumping and rolling of debris from hill slopes and of ablation moraine from the glacier surface the weak void microfabric is interpreted to indicate that the till was not consolidated and hence not sheared under the weight of the glacier despite the strong particle microfabric basal tills on the lee sides of the roche moutonnees and on the lee side of the drumlin but near its core have weak microfabrics

origin of the active drumlin field at mulajokull iceland new insights from till shear and consolidation patterns abstract stratigraphic and morphologic data previously collected from the forefield of mulajokull iceland suggest that its recent surge cycles are responsible for the formation of drumlins there and that their relief reflects both deposition on drumlins and erosion between them we have tested these ideas and aspects of leading models of drumlin formation by studying past patterns of bed deformation and effective stress in basal tills of the glacier s forefield patterns of till strain indicated by the anisotropy of magnetic susceptibility ams of 2300 intact till samples indicate that till was deposited during shear deformation with shearing azimuths and planes that conform to the drumlin morphology thus till deposition occurred as drumlins grew in agreement with lidar data indicating that the degree of aggradation of the glacier forefield is largest in areas subjected to the most surges previously described unconformities on the drumlin flanks however indicate that drumlin relief at mulajokull has resulted in part from erosion given that the last surge deposited a till layer both on and between drumlins a reasonable hypothesis is that erosion between drumlins occurred during normal quiescent flow of the glacier between surges densities of till samples analyzed in conjunction with laboratory consolidation tests indicate that effective stresses on the bed during such periods were on the order of 100 kpa larger between drumlins than within them an observation consistent with subglacial channels at low water pressure occupying interdrumlin areas transport of sediment by turbulent flow in these channels or high effective stress adjacent to them causing enhanced till entrainment in ice or increased depths of bed deformation would promote the sediment flux divergence necessary to erode areas between drumlins the observation that effective stresses were higher between drumlins than within them is the opposite of that presumed in leading models of drumlin formation moreover the lack of ams fabric evidence of longitudinal compression in drumlin tills does not support some models of drumlin formation that invoke negative till flux gradients in a deforming bed

till as a glacial tectomict its internal architecture and the development of a typing method for till differentiation abstract till has been studied for over a century yet still a clear understanding of the mechanics of deposition and or emplacement of till eludes us tills in the past have been generally subdivided into sediments formed in various sub environments of the glacial system as such it has been generally agreed that tills can be separated in terms of these types based on various attributes such as clast fabric increasingly such separation into various sub types of till has become however highly debatable and tenuous based upon the use of micromorphology and a greater in depth appreciation of the internal architecture of till it has become apparent that previously established classifications of till are erroneous micromorphology reveals that deposition and or emplacement of tills is a complex tectonic process such that lodgement melt out and flow tills as previously defined do not appear to exist rather a new term better describes tills as glacial tectomicts by using micromorphology in establishing the presence of sets of microstructures a statistical method is introduced that permits differentiation of tills from other diamictons the implications for this new understanding of tills allied to statistical typing is considerable in terms of till formation glacial stratigraphy and understanding glacial environments in terms of stress histories porewater transmission subglacial deformation mechanics till rheological phase changes and the timeline of till formation

physical processes in subglacial lake whillans west antarctica inferences from sediment cores abstract the hydrologic system beneath the antarctic ice sheet is thought to influence both the dynamics and distribution of fast flowing ice streams which discharge most of the ice lost by the ice sheet despite considerable interest in understanding this subglacial network and its affect on ice flow in situ observations from the ice sheet bed are exceedingly rare here we describe the first sediment cores recovered from an active subglacial lake the lake known as subglacial lake whillans is part of a broader dynamic hydrologic network beneath the whillans ice stream in west antarctica even though floods pass through the lake the lake floor shows no evidence of erosion or deposition by flowing water by inference these floods must have insufficient energy to erode or transport significant volumes of sediment coarser than silt consequently water flow beneath the region is probably incapable of incising continuous channels into the bed and instead follows preexisting subglacial topography and surface slope sediment on the lake floor consists of till deposited during intermittent grounding of the ice stream following flood events the fabrics within the till are weaker than those thought to develop in thick deforming beds suggesting subglacial sediment fluxes across the ice plain are currently low and unlikely to have a large stabilizing effect on the ice stream s grounding zone

subglacial clast behaviour and its implication for till fabric development new results derived from wireless subglacial probe experiments this study has investigated the three dimensional movement of clasts within deformation till using embedded wireless probes these probes were part of an environmental sensor network which measured subglacial properties temperature water pressure resistivity case strain and tilt six times a day and relayed that data via radio to the glacier surface where they were forwarded and broadcast on line the systemwas installed at briksdalsbreen norway and operated from august 2004 until august 2006 approximately 2000 probe days worth of data were collected with an increase in performance 41 more readings during the second year the probes showed similar patterns of water pressure rises throughout the two years but with slightly different magnitudes and timings these changes in water pressure could be related to clast behaviour the probes decreased their dip over the year and the rate of change was related to an increase in glacier velocity after initial changes in dip the probes experienced changes in orientation followed by rotation about the a axis this continuous rotation was similar to the motion suggested by jeffery 1922 the motion of ellipsoidal particles immersed in a viscous fluid proceedings of the royal society of london series a 102 161 179 for the behaviour of clasts within a viscous material in addition some probes also showed short frequent dip oscillations in spring and autumn which were interpreted to reflect stick slip events similar to lodging and demonstrated how local conditions can interrupt the predicted rotation pattern overall it is demonstrated that when water pressures were high decoupling occurred associated with basal sliding and dip oscillations and when water pressures fell the ice and sediment were coupled and till deformation occurred these events happened during summer and autumn it is this combination of lodgement and deformation that builds up both a complex but predictable fabric and a resultant composite till sedimentology

fabric signature of subglacial till deformation breidamerkurjokull iceland the foreland of breidamerkurjokull iceland is the only locality where tills known to have undergone subglacial deformation are exposed till on the foreland has a two tiered structure consisting of a dilatant upper horizon c 0 5 m thick and a compact lower till these horizons correspond to the ductile deforming a horizon and the brittle ductile b horizon observed below the glacier by g s boulton and co workers the relationship between known strain history and a variety of macrofabric elements is examined for these two genetic facies of deformation till the upper horizon exhibits variable a axis fabrics and abundant evidence for clast re alignment reflecting ductile flow and rapid clast response to transient strains in contrast the lower horizon has consistently well organized a axis fabrics with a narrow range of dip values recording clast rotation into parallel with strain axes during brittle or brittle ductile shear the data indicate that till strain history imparts identifiable macrofabric signatures providing important analogues to guide the interpretation of pleistocene tills

differential burning recrystallization and fragmentation of archaeological bone abstract this paper presents research on the conditions under which progressive levels of burning may occur to archaeological bone and how burning damage changes bones crystal structure and susceptibility to fragmentation a k a friability experiments were conducted to simulate common patterns of high temperature bone diagenesis and fragmentation previously documented in paleolithic shelter sites bones buried up to 6 cm below the coal beds of the experimental fires were carbonized but calcination occurred only with direct exposure to live coals analysis by infra red spectroscopy reveals that marked changes in crystallinity accompany the macroscopic transformations in colour and friability of modern fire altered bone specifically a monotonic non linear decrease in mean fragment length across six colour categories was observed when samples were agitated or trampled and a concordant decline in bone identifiability first with respect to skeletal element and ultimately the recognizability of bone tissue itself these findings help qualify the behavioural and taphonomic implications of fragmented burned bones in archaeological sites especially with regard to potential stratigraphic associations between artefacts and hearth features in sites and the intensity of space use by human occupants the identification of burning damage on archaeological bone is a separate issue however it was found that the molecular signatures of recrystallization in modern burned bones partly overlap with recrystallization caused by weathering after only 1 to 2 years of exposure in an arid setting and by partial fossilization of archaeological bones over the long term while infra red and x ray diffraction techniques effectively describe heat induced changes in modern bone mineral and are an important aid for modelling diagenetic processes these techniques did not reliably identify burning damage to archaeological bones cross referencing readily visible colour phases with hcl insoluble fraction data proves much more effective and economically feasible for the latter purpose

till fabric associated with deformable beds this paper investigates the development of till fabric associated with glaciers overlying deformable beds first the till fabrics from both modern and ancient soft bed and hard bed subglacial environments are investigated and then soft bedded sites with specific strain histories are examined the results indicate that lodgement tills had a strong fabric whilst there is a wide range of much weaker fabric strengths associated with soft bed tills it is suggested from a theoretical study of soft bed till deposition that longitudinal and vertical processes in the subglacial deforming layer are important in fabric development field sites were chosen to isolate these two factors and fabrics investigated in areas of known relative shear strain and known relative thickness of the deforming layer the results indicate that 1 for a fixed deforming layer fabric strength does increase with increasing deformation except at very high strains where the development of a transverse fabric can lead to a reduction in the overall fabric strain and 2 lodgement tills and soft bed tills associated with a thin deforming layer have a high fabric strength whilst soft bed tills associated with a thick deforming layer have a low fabric strength where the deforming layer is thin clasts are oriented parallel with the glacier direction either by lodging against a hard bed or ploughing against a soft bed however where the deforming layer is thicker and shear strains are high melt out processes encourage a parallel fabric but the increased shear strain and changes in the deforming layer thickness cause the clasts to develop both a parallel and transverse fabric these processes combine to reduce the overall fabric strength till fabric studies are a further sedimentological technique to separate the depositional processes associated with a homogeneous till

an investigation of subglacial processes at the microscale from briksdalsbreen norway the subglacial processes at briksdalsbreen norway are examined by a combination of sedimentology thin section and scanning electron microscope sem analysis of till samples from an exposed subglacial surface and from beneath the glacier studies of a fluted surface indicate that subglacial deformation is occurring on a field scale with flutes forming behind most clasts 0 6 m high at the thin section scale 0 014 2 0 mm it is seen that deformation is by rotation and attenuation and is dependent on till texture at the sem scale 0 1 0 4 mm it is seen that erosion is controlled by abrasion and percussion which produces distinct grain styles as part of an erosional continuum overall it is shown that rotation and attenuation is a dominant process at all scales and that the clast interactions associated with different scale perturbations within the shear zone control erosion and deposition as well as landform and fabric production

clast fabric development in a shearing granular material implications for subglacial till and fault gouge elongate clasts in subglacial till and in fault gouge align during shearing but the relation between clast fabric strength and cumulative shear strain for such materials is effectively unknown this relation was explored in experiments with a large ring shear device in which a till and a viscous putty that contained isolated clasts were sheared to high strains as expected rotation of clasts in the putty is closely approximated by the theory of g b jeffery who derived the orbits of rigid ellipsoids in a slowly shearing fluid clast rotation in the till however is strikingly different rather than orbiting through the shear plane as predicted by jeffery most clasts rotate into the shear plane and remain there resulting in strong fabrics regardless of the aspect ratios and initial orientations of clasts this divergent behavior is likely due to slip of the till matrix along the surfaces of clasts which is a natural expectation in a granular material but violates the no slip condition of jeffery9s model these results do not support the widespread belief that subglacial till deformation results in weak clast fabrics thus many tills with weak fabrics thought to have been sheared subglacially to high strains like many basal tills of the laurentide ice sheet may have been sheared only slightly with little effect on either ice sheet dynamics or sediment transport in addition these results indicate that in simple shear the rotation of clasts in till and in fault gouge is best analyzed with the model of a march who treated inclusions as passive markers

on the interpretation of subglacial till fabric abstract the modal distribution of stone long axis fabrics and their respective eigenvalues can be used to infer the genesis of subglacial till in this paper we offer a two axis diagram that compares fabric modality to fabric isotropy s3 s1 and addresses the problem of eigenvectors falling between the modes of some well developed till fabrics with low eigenvalues our simple five fold scheme of modality categories includes 1 unimodal clusters 2 spread unimodal 3 bimodal clusters 4 spread bimodal and 5 polymodal to girdle like fabrics and requires the analyst to study equal area lower hemisphere schmidt plots of the fabric data after assigning the fabric to a morality category isotropy is calculated and both results are plotted on the graph whic helps to separate two main fields of subglacial till 1 lodgement and subglacial meltout tills and 2 deformation fill on the basis of selected published fabrics from tills at modern glaciers as well as our own pleistocene till data lodgement and subglacial meltout tills tend to have unimodal or bimodal fabrics in contrast deformation tills and tills that experienced multiple processes tend to have polymodal to girdle like fabrics some overlap occurs between fields be cause of the complex nature of till formation i e because pure end member till facies are rare and most tills are hybrids we strongly recommend that schmidt plots be visually analyzed and used in conjunction with eigenvalues when studying till however fabric data alone is not enough multiple criteria including structural lithologic and stone morphologic data from the till must also be considered before drawing conclusions on till genesis furthermore if eigenvectors fall between fabric modes then they cannot be used to indicate former ice movement directions finally our new modality isotropy diagram may have wider applications

characterization of pebble fabrics in modern terrestrial glacigenic sediments pebble fabric data are available from several facies of glacigenic sediments deposited by modern glaciers where sedimentary processes can be observed or inferred with relatively little ambiguity over 100 samples from contemporary environments illustrate fabrics characterizing melt out till deformed and undeformed lodgement till sediment flow deposits and ice slope colluvium lodgement till fabric variability is related to the two layer structure of these sediments a structureless friable upper layer with low shear strength and high consolidation coefficient overlying a very compact material of horizontal platy structure fabric strength assessed by eigenvalue analysis is weaker and pebble dip is more dispersed in the upper structureless horizon stronger fabrics in the lower platy horizon may be primary depositional fabrics which are destroyed by subglacial shearing to give weaker fabrics in the upper horizon alternatively upper horizon fabrics may be characteristic of all recently deposited lodgement tills with stronger fabrics developing at depth by dewatering and consolidation there is a general reduction in fabric strength and an increase in particle dip associated with the transition from melt out tills through undeformed and deformed lodgement tills to sediment flow deposits and ice slope colluvium there is however considerable overlap in the fabric strengths characteristic of sediment flow deposits and deformed lodgement tills fabric data from modern glacial sedimentary facies are used to assist in interpreting the mode of deposition of some quaternary glacial sediments relatively strong fabrics characteristic of melt out tills and undeformed lodgement tills are more likely to be diagnostic of genesis than weaker fabrics associated with deformed sediments

nature and origin of corrugated ground moraine of the des moines lobe story county iowa abstract corrugated ground moraine in story county iowa consists of low relief 1 2 m ridges which most commonly consist of late wisconsinan till the average spacing between ridges is about 105 m and they generally parallel the configuration of the bemis moraine the terminal moraine of the des moines glacial lobe till fabric pebble orientations and anisotropy of magnetic susceptibility facies variations and landform fabric analyzed at three sites in typical corrugated ground moraine landscape near ames in story county iowa indicate that at sites 1 and 2 the ridges consist of till probably deposited by lodgement at site 3 two ridges consist of till and massive to cross bedded sand till at site 3 is also probably lodgement till the nature of the till and related glaciofluvial sediments at each site suggests that the corrugation ridges formed in basal cracks or crevasses in the des moines lobe the cracks are postulated to have formed during extending flow of the des moines lobe as it advanced toward its terminal position retreat of the des moines lobe was rapid enough to preclude significant accumulations of supraglacial sediment upon corrugated ground moraine

residual strength of clays in landslides folded strata and the laboratory the post peak drop in drained shear strength of an overconsolidated clay may be considered as taking place in two stages first at relatively small displacements the strength decreases to the fully softened or critical state value owing to an increase in water content dilatancy second after much larger displacements the strength falls to the residual value owing to reorientation of platy clay minerals parallel to the direction of shearing if the clay fraction is less than about 25 the second stage scarcely comes into operation the clay behaves much like a sand or silt with angles of residual shearing resistance typically greater than 20 degrees conversely when the clay fraction is about 50 residual strength is controlled almost entirely by sliding friction of the clay minerals and further increase in clay fraction has little effect the angles of residual shearing resistance of the three most commonly occurring clay minerals are approximately 15 degrees for kaolinite 10 degrees for illite or clay mica and 5 degrees for montmorillonite when the clay fraction lies between 25 and 50 there is a transitional type of behaviour residual strength being dependent on the percentage of clay particles as well as on their nature the post peak drop in strength of a normally consolidated clay is due only to particle reorientation measurements of strength on natural shear surfaces agree within practical limits of variation with values derived from back analysis of reactivated landslides this field residual strength can be recovered by multiple reversal shear box tests on cut plane samples but in high clay fraction materials it is typically somewhat higher than the strength measured in ring shear tests residual strength is little affected by variation in the slow rates of displacement encountered in reactivated landslides and in the usual laboratory tests but at rates faster than about 100 mm min qualitative changes take place in the pattern of behaviour a substantial gain in strength is followed with increasing displacement by a fall to a minimum value in clays and low clay fraction silts this minimum is not less than the slow or static residual but in clayey silts with clay fractions around 15 25 according to tests currently in progress the minimum can be as low as one half of the static value author trrl

deformation of the batestown till of the lake michigan lobe laurentide ice sheet deep pervasive shear deformation of the bed to high strains 100 may have been primarily responsible for flow and sediment transport of the lake michigan lobe of the laurentide ice sheet to test this hypothesis we sampled at 0 2 m increments a basal till from one advance of the lobe batestown till along vertical profiles and measured fabrics due to both anisotropy of magnetic susceptibility and sand grain preferred orientation unlike past fabric studies interpretations were guided by results of laboratory experiments in which this till was deformed in simple shear to high strains fabric strengths indicate that more than half of the till sampled has a 5 probability of having been sheared to moderate strains 7 30 secular changes in fabric azimuth over the thickness of the till probably due to changing ice flow direction as the lobe receded indicate that the bed accreted with time and that the depth of deformation of the bed did not exceed a few decimeters orientations of principal magnetic susceptibilities show that the state of strain was commonly complex deviating from bed parallel simple shear deformation is inferred to have been focused in shallow temporally variable patches during till deposition from ice

approaches to the study of glacier bed deformation investigation of glacier bed conditions and subglacial processes is an obligatory requirement for the modelling and understanding of contemporary glacier behaviour past glacier behaviour glacier response to both spatial and temporal variations in bed material and glacier response to external forcing factors such as climate change the recognition of the importance of coupling between the glacier and its bed resulted in a paradigm shift in the subject and drew attention to the critical importance of subglacial deformation as opposed to simply subglacial deposition and sliding a number of methods have been adopted to measure glacier bed deformation ranging from direct observation and instrumentation the interpretation of macroscale and microscale sedimentary structures to the interpretation of glacier bedforms and glacier bed modelling this paper is concerned to describe the methods used by the two authors and co workers and attention is given to field methods the type and arrangement of sedimentary structures thin section micromorphology and microfabric and clast microfabric and glacier bedform morphology as evidence of subglacial deformation sedimentary structures macro and mesoscale are used to identify constructional deformation where deformational units are preserved one on top of another due to the formation of a thin deforming layer by low basal stresses and excavational deformation in which the deformational style is superimposed on pre existing styles due to increasing thicknesses of glacier deformation caused by increasing basal shear stresses thin section micromorphology has been used to define a range of glacial processes of which subglacial deformation is just one but has drawn attention to critical criteria such as discrete microscale shears orientated matrix kink structures and fractured grains as well as deformed intraclasts that can be used to recognise deformational processes more recently individual aggregates taking the form of rounded pebbles within till matrix have been shown to reflect rotational processes during glacial deformation and provide an additional mechanism to discrete shear within subglacial deformation and an explanation for the transport paths of subglacial materials finally measurements of sediment flow directions using clast macrofabric characteristics coupled with measurement of quantities of sediment transfered and detailed palaeoglacial properties have been used to determine the rates and scales of subglacial deformation associated with the formation of small scale glacier bedforms

the interpretation and classification of subglacially deformed materials abstract a general classification of subglacially deformed materials is proposed based on sediment properties and their relationship to styles of subglacial strain deformation till is defined as homogenized usually diamictic material formed by glacially induced shear of subsole materials three types are recognized type a formed by pervasive ductile deformation type b formed by brittle shear and comminution till produced by the reduction of void space by in situ crushing and abrasion the term glacitectonite is adopted for materials that have undergone subglacial shear but retain some of the structural characteristics of the parent material original structures may be truncated by glacitectonic fabric elements type a or distorted but not truncated type b the principles of strain and material response particle orientation mechanisms fabric development and drainage conditions are reviewed examples of deformation tills and glacitectonites from breidamerkurjokull iceland slettmarkbreen norway loch lomond scotland and east yorkshire england are used to illustrate the nature of subglacially deformed materials and their relationship to former subglacial strain and drainage conditions the geologic evidence suggests a continuum of bed strengths can be recognized in deforming substrata ranging from high strength low strain type b deformation till and comminution till to low strength high strain type a deformation till

subglacial till formation microscale processes within the subglacial shear zone abstract this was a study of subglacial deformation till genesis from a modern temperate glacier at skalafellsjokull iceland detailed microscale properties of till samples from scanning electron microscope sem and thin section analysis were examined from a glacial site with in situ subglacial process monitoring and an exposed subglacial surface in the foreland two lithofacies were examined a grey sandy till derived from the ash and basalt and a silty reddish brown till derived from oxidized paleosols and or tephra layers these also represented a clay content continuum from low 0 3 to high 22 3 the evolution from debris to subglacial till was investigated this included a reduction in grain size 21 for grey lithology 13 reddish brown lithology and reduction in rounding ra 32 for the grey lithology 26 for the reddish brown lithology and the quantification and analysis of the different grain erosion comminution processes in the resultant till it was shown that the microstructures within a till were dependent on shear strain and glaciological conditions deformation history the low clay content tills were dominated by linear structures lineations and boudins and anisotropic microfabric whilst the higher clay content tills were dominated by rotational structures turbates and plaster and isotropic microfabric these results are important in our understanding of the formation of both modern and quaternary tills and informs our reconstruction of past glacial dynamics

the experimental basis for interpreting particle and magnetic fabrics of sheared till particle fabrics of basal tills may allow testing of the bed deformation model of glacier flow which requires high bed shear strains 100 field studies however have not yielded a systematic relationship between shear strain magnitude and fabric development to isolate this relationship four basal tills and viscous putty were sheared in a ring shear device to strains as high as 714 fabric was characterized within a zone of shear deformation using the long axis orientations of fine gravel and sand particles and the anisotropy of magnetic susceptibility ams of small 5 8 cm3 intact samples results indicate that till particles rotate toward the plane of shearing with long axis orientations that become tightly clustered in the direction of shear 0 78 s1 0 94 for three dimensional data these strong steady state fabrics are attained at shear strains of 7 30 with no evidence of fabric weakening with further strain regardless of the specific till or particle size fraction under consideration these results do not support the jeffery model of particle rotation which correctly describes particle rotation in the viscous putty but not in the tills owing to fluid mechanical assumptions of the model that are violated in till the sensitivity of fabric development to shear strain magnitude indicates that for most till units where shear strain magnitude is poorly known attributing fabric variations to spatial differences in other variables such as till thickness or water content will be inherently speculative attributing fabric characteristics to particular basal till facies is uncertain because shear strain magnitude is unlikely to be closely correlated to till facies weak or spatially variable fabrics in the absence of post depositional disturbance or major deviations from unidirectional simple shear indicate that till has not been pervasively sheared to the high strains required by the bed deformation model strong flow parallel fabrics are a necessary but insufficient criterion for confirming the model copyright 2008 john wiley sons ltd

basal mechanics of ice stream b west antarctica 1 till mechanics data from laboratory geotechnical tests on till recovered from beneath ice stream b west antarctica at the upstream b camp hereinafter the upb till show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate these data support use of a coulomb plastic rheology in modeling of ice stream behavior and subglacial till deformation our testing program combined triaxial ring shear and confined uniaxial tests to investigate till strength and compressibility results show that the upb till follows closely coulomb s equation in which shear strength is a linear function of normal effective stress apparent cohesion near zero and internal friction angle equal to 24 till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress in general the behavior of the upb till is consistent with other experimental evidence regarding mechanical behavior of granular materials based on our laboratory results we formulate the compressible coulomb plastic till model in which there are three interrelated primary state variables shear strength void ratio and normal effective stress this model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings these simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics 1 occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records and 2 distributed till deformation to depths of 0 1 1 0 m beneath the ice base our laboratory and modeling results substantiate application of the compressible coulomb plastic model in simulations of the motion of ice stream b over its weak till bed

deformation around rigid particles the influence of slip at the particle matrix interface abstract theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent in natural rocks the boundary behaviour may cover the whole range from free slip to complete coherence examples where slip may be promoted include crack seal deformation during the development of pressure shadows core mantle structures where the mantle is composed of much weaker fine recrystallized material and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase analogue scale model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non slipping particle matrix interfaces when slip does occur particles rotate faster in pure shear and slower in simple shear than for non slip leading to the more rapid development of a shape preferred orientation the preferred orientation tends to be bimodal in simple shear flow the flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses dependent on the original particle orientation in multi particle experiments slip on the interfaces enhances strong strain localization in non linear viscous material to produce patterns very similar to natural s c fabrics

basal hydraulic system of a west antarctic ice stream constraints from borehole observations pressure and tracer measurements in boreholes drilled to the bottom of ice stream b west antarctica are used to obtain information about the basal water conduit system in which high water pressures are developed these high pressures presumably make possible the rapid movement of the ice stream pressure in the system is indicated by the borehole water level once connection to the conduit system is made on initial connection here also called breakthrough to the basal water system the water level drops in a few minutes to an initial depth in the range 96 117 m below the surface these water levels are near but mostly somewhat deeper than the flotation level of about 100m depth water level at which basal water pressure and ice overburden pressure are equal which is calculated from depth density profiles and is measured in one borehole the conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed the thickness of the gap has to be about 2 mm to account for the water level drop on breakthrough and about 4 mm to fit the results of a salt tracer experiment indicating downstream transport at a speed of 7 5 mm s 1 the above gap conduit model is however ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole and also by the large hole to hole variation in measured basal pressure which if present in a gap conduit system with 2 or 4 mm would result in unacceptably large local water fluxes an alternative model that avoids the e objections called the gap opening model involves opening a gap as injection proceeds starting with a thin film the injection of water under pressure lifts the ice mass a round the borehole creating a gap 3 or 4 mm wide at the ice bed interface evaluated quantitatively the gap opening model accounts for the volume of water that the basal watery system accepts on breakthrough which obviates the gap conduit model in order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits of which the most promising type is the canal conduit proposed theoretically by walder and fowler 1994 flat low conduits incised into the till 0 1 m deep and perhaps 1 m wide with a flat ice roof the basal water pressure data suggest that the canals are spaced 50 300 m apart much closer than r tunnel would be the deepest observed water level 117 m i the most likely to reflect the actual water pressure in the canals corresponding to a basal effective pressure of 1 6 bar in this interpretation the shallower water levels are affected by loss of hydraulic head in the narrow passageway s that connect along the bed from borehole to canal s once a borehole has frozen up and any passageways connecting with canals have become closed a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till given enough time for pressure equilibration this pressure varies considerably with time over the equivalent water level range from 100 to 113 m basal pressure sensors 500 m apart report uncorrelated variations whereas sensors in boreholes 25 m apart report mostly but not entirely well correlated variations of unknown origin in part of the record remarkable anticorrelated variations are interspersed with positively correlated one and there are rare abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m a diurnal pressure fluctuation intermittently observed may possibly be caused by the ocean tide in the ross sea the lack of any observed variation in ice stream motion when large percentagewise variations in basal effective pressure were occurring according to our data suggests that the observed pressure variations are sufficiently local and so randomly variable from place to place that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed

experiments on the dynamics and sedimentary products of glacier slip abstract experimental work in glacial geomorphology is focused almost entirely on processes in the thin shear zone at the beds of sliding glaciers where ice at its pressure melting temperature moves over either rigid rock or deformable till experiments with rotary shear devices illuminate constitutive behavior there central to the dynamics of fast moving glaciers and provide a foundation for interpreting the sedimentary record of glacier slip and associated sediment transport results from experiments designed to study ice sliding over a rigid wavy bed shear deformation of till and plowing of clasts across the surface of a till bed point to a common conclusion drag at the bed can decrease with increasing slip velocity thereby concentrating driving stress elsewhere and promoting rapid glacier flow this rate weakening behavior at glacier beds is in contrast to the viscous slip resistance assumed in ice sheet models and most efforts to determine distributions of basal drag from glacier surface velocities ring shear experiments in which various basal tills and more idealized materials are sheared to high strains provide quantitative insight into grain size evolution mixing at contacts between basal tills microstructure development particle fabric development and development of anisotropy of magnetic susceptibility preferred orientations of principal magnetic susceptibilities provide the most dependable and complete description of till shear patterns when applied to basal tills of the geologic record magnetic till fabrics measured along thick till sections and calibrated experimentally indicate that deformation of the bed by two lobes of the laurentide ice sheet was shallow

review of subglacial hydro mechanical coupling trapridge glacier yukon territory canada abstract the interaction of basal processes with the subglacial drainage system is a critical issue in understanding glacier dynamics since the recognition that many glaciers and ice masses overlie soft sediments rather than hard bedrock much research has been undertaken to investigate how mechanical and hydrological conditions of a deformable substrate control the coupling at the ice bed interface and thus affect fast ice flow and glacier surging in research undertaken on trapridge glacier a small surge type glacier in the st elias mountains yukon territory canada we have combined extensive field investigations using novel measurement techniques and theoretical modelling to study hydro mechanical coupling processes measurements of subglacial water pressure indicate that the basal water system can be dramatically inhomogeneous both spatially and temporally since ice bed coupling is strongly influenced by subglacial water pressure the stresses at the bed are also markedly heterogeneous and are expected to form a patchwork distribution which mimics the pressure distribution of the basal water system this heterogeneity in the stress field at the ice bed interface introduces a pronounced variability to the basal motion mechanics as such basal sliding and subglacial sediment deformation are not steady and continuous processes instead the variability of the subglacial water system leads to a spatial and temporal interplay of increased ice bed coupling at low water pressures at one site or time with ice bed decoupling during rising water pressures at other sites or times thus on the one hand there is downglacier shear deformation of the bed and accumulation of elastic strain in ice and sediment while on the other hand there is enhanced slip sliding of the glacier and upglacier shear motion of the bed due to an elastic relaxation of the sediment

numerical modeling of subglacial sediment deformation implications for the behavior of the lake michigan lobe laurentide ice sheet we apply a numerical model of the late wisconsin circa 20 000 years b p lake michigan lobe lml laurentide ice sheet to investigate how fine grained subglacial sediment might influence lobe behavior particularly rapid millennial scale marginal oscillations observed in the geologic record over the canadian shield we assume a rigid bed hard bedded basal boundary condition in areas overlain by fine grained sediment soft bedded the base of the ice is coupled to a deformable sediment layer using a rate dependent stress strain law geotechnical tests of clay rich till deposited by the lml provide control for sediment rheologic parameters simulated cross sectional profiles are consistent with reconstructions from geologic evidence time dependent simulations suggest that a soft bedded lobe could have reached steady state in about 20 000 years or less in contrast to 50 000 to 60 000 years for an otherwise identical hard bedded lobe a soft bedded lobe with sediment viscosity at the experimentally determined value is about twice as responsive to millennial scale shifts in accumulation or ablation as a nonsliding hard bedded lobe but in both cases the response is slower than that indicated by the geologic record results suggest that while strong millennial scale changes in accumulation and ablation can produce responses in hard bedded or soft bedded ice that are consistent with the geologic record changes in subglacial sediment viscosity even relatively modest changes whether independent or in conjunction with climate change might more readily account for millennial and submillennial scale fluctuations of the lobe margin these observations do not exclude a role for sliding but they do provide some perspective from which to evaluate relative contributions of the various processes that influence lobe behavior

the 3d microscopic signature of strain within glacial sediments revealed using x ray computed microtomography abstract x ray computed microtomography ct a non destructive analytical technique was used to create volumetric three dimensional 3d models representing the internal composition and structure of undisturbed pro and subglacial soft sediment sample plugs for the purposes of identifying and analysing kinematic indicators the technique is introduced and a methodology is presented addressing specific issues relating to the investigation of unlithified polymineralic sediments six samples were selected based on their proximity to type brittle and ductile deformation structures or because of their perceived suitability for successful application of the technique analysis of a proglacial ideal specimen permitted the 3d geometry of a suite of micro faults and folds to be investigated and the strain history of the sample reconstructed the poor contrast achieved in scanning a diamicton of glaciomarine origin is attributable to overconsolidation under normal loading the sediment demonstrated to have undergone subsequent subglacial deformation another overconsolidated diamicton contains an extensive small scale the subglacial sample evidence indicates that complex polyphase brittle ductile deformation histories are common in such diamictic soft sediments and that the local micro scale environment composition structure shear forces and effective pressure controls rheology the sediment void ratio is a key indicator of strain three of the samples are tentatively placed at different points on the strain cycle for subglacially deforming soft sediment based on their void ratio characteristics and distribution it has been demonstrated that ct offers significant potential for elucidating glacial soft sediment kinematics the ability of the technique to both test a variety of hypotheses pertaining to mechanisms operating within the subglacial environment and evaluate efforts to replicate those processes under controlled laboratory conditions is therefore discussed along with solutions to the problems encountered within this project ct also permits the seamless linking of analytical techniques applied at the hand specimen mm micromorphological m mm scanning electron microscopy and x ray diffraction fluorescence nm m scales and the archiving duplication and dissemination of sample volumetric 3d data

fast shearing of pre existing shear zones in soil the paper presents results from a laboratory investigation into the influence of fast rates of displacement on the residual strength of soil this influence was studied in the ring shear apparatus shear zones were formed by slow drained shearing and then tested at alternately fast and slow rates of displacement results from a wide range of natural soils are presented all soils show initially an increase in strength when resheared at fast rates of displacement the strength then tends to decrease with fast displacement to a minimum value the fast residual strength three types of rate effects on the residual strength are identified a positive rate effect in soils showing a fast residual strength higher than the slow residual a neutral rate effect in soils showing a constant fast residual strength equal to the slow residual irrespective of rate of displacement and a negative rate effect in soils showing a significant drop in fast residual strength below the slow residual when sheared at rates higher

magnetic fabrics in the basal ice of a surge type glacier 1 anisotropy of magnetic susceptibility ams has been shown to provide specific useful information regarding the kinematics of deformation within subglacially deformed sediments here we present results from debris rich basal glacier ice to examine deformation associated with glacier motion basal ice samples were collected from tunabreen a polythermal surge type glacier in svalbard the magnetic fabrics recorded show strong correlation with structures within the ice such as sheath folds and macroscopic stretching lineations thermomagnetic low temperature susceptibility varying field susceptibility and isothermal remanent magnetism acquisition experiments reveal that the debris rich basal ice samples have a susceptibility and anisotropy dominated by paramagnetic phases within the detrital sediment sediment grains entrained within the basal ice are inferred to have rotated into a preferential alignment during deformation associated with flow of the glacier an up glacier directed plunge of magnetic lineations and subtle deviation from bulk glacier flow at the margins highlight the importance of noncoaxial strain during surge propagation the results suggest that ams can be used as an ice petrofabric indicator for investigations of glacier deformation and interactions with the bed

syngenetic inclusions in diamond from the birim field ghana a deep peridotitic profile with a history of depletion and re enrichment diamonds and their syngenetic mineral inclusions from placer deposits akwatia mine along the birim river ghana were studied thus providing the first detailed data collection for the west african craton inclusion contents indicate an almost exclusively peridotitic diamond suite with the vast majority being part of the harzburgitic paragenesis chemically the akwatian diamond inclusions differ from those in our 1100 sample world wide data base mainly by shifts towards lower mg fe ratios for harzburgitic olivines and orthopyroxenes extremely high ni contents in both harzburgitic and lherzolitic olivines and a higher mean cr content in chromites the inconsistency between the low mg fe ratios and the highly refractory compatible trace element signature seems best to be explained by re fertilisation of a previously depleted source similar to the metasomatic re enrichment of deformed fe ti rich and hot peridotites discussed by harte 1983 geothermometry shows akwatian inclusions to be 140 190 c hotter than the peridotitic average 1050 c given by harris 1992 since garnet opx equilibria 1100 c 50 kbar to 1370 c 67 kbar indicate a typical shield geotherm 40 42 mw m2 these elevated temperatures imply an origin of the akwatian diamonds unusually deep for a peridotitic suite this is consistent with the presence of extraordinary amounts of silicate spinel component in chromite inclusions indicative of crystallisation under higher pressures than recorded for most peridotitic suites in addition one garnet showed the highest knorringite component 66 4 mol so far observed in an inclusion in diamond the same garnet also contained a minor enstatite solid solution component which indicates crystallisation at pressures just below 80 kbar akwatian diamond inclusions therefore represent the most complete cross section through peridotitic subcontinental lithospheric upper mantle so far observed down to a maximum depth between 200 240 km

theory of glacial erosion transport and deposition as a consequence of subglacial sediment deformation a theory of erosion transport and deposition of unlithified sediments by glaciers is presented it predicts the large scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle together with the resultant intensity of large scale lincations drumlins which will be incised in the landscape the theory also predicts the dispersal patterns of subglacial lithologies together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till it predicts major erosional discontinuities within tills and the formation of boulder pavements it suggests that the dominant proportion of the lowland tills produced by pleistocene mid latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine grained the theory is based on an analysis of glacier dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition

shape fabric development in rigid clast populations under pure shear the influence of no slip versus slip boundary conditions abstract shape fabrics of elliptical objects in rocks are usually assumed to develop by passive behavior of inclusions with respect to the surrounding material leading to shape based strain analysis methods belonging to the rf family a probability density function is derived for the orientational characteristics of populations of rigid ellipses deforming in a pure shear 2d deformation with both no slip and slip boundary conditions using maximum likelihood a numerical method is developed for estimating finite strain in natural populations deforming for both mechanisms application to a natural example indicates the importance of the slip mechanism in explaining clast shape fabrics in deformed sediments

sedimentological investigations in the forefield of an icelandic surge type glacier implications for the surge mechanism abstract a sedimentological study in the forefield of hagafellsjokull vestari iceland has been employed to distinguish between pervasive sediment deformation and sliding as mechanisms of basal motion in surge ice retreat following a surge in 1979 80 has exposed a two layered quasi continuous till cover and a suite of streamlined bedforms including flutes and drumlins an integrated approach using particle size data clast morphology scanning electron microscopy and micromorphology suggests that comminution processes were confined to the uppermost 16 25 cm of the till the lower till layer remained relatively undisturbed during the surge advance in both drumlinised and non drumlinised areas micromorphological evidence of a clay depositing ice sediment water layer suggests that decoupling was important during the surge phase we suggest that the till did not weaken sufficiently for pervasive deformation to occur at depth rather the surge mechanism at hagafellsjokull vestari is thought to involve the transition of the quiescent phase basal water system which consists of a few large subglacial conduits emerging at the ice margin to a distributed or linked cavity system

deformation of till beneath ice stream b west antarctica the behaviour and possible instability of the west antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it model calculations show that most ice stream velocity arises at the bed1 2 and radar sounding has shown the bed to be wet3 but the basal boundary condition is not well understood seismic evidence from the upstream b camp upb on the siple coast of west antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick in which the water pressure is only 50 kpa less than the overburden pressure because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5 we presume that it is glacial till we propose here that deformation within the till is the primary mechanism by which the ice stream moves and we discuss implications of this hypothesis

till fabric patterns and significance particle response to subglacial stress abstract the analysis of particle arrangements within glacial sediments fabric has long been considered to reflect the behaviour of the ice that transported and deposited or deformed the sediment particles can develop specific fabric arrangements as a response to bulk sediment strain driven by effective stress at the bed of a glacier as such fabric analysis has been used to try and understand the complex relationship between glacier and ice sheet dynamics and the geological evidence of former glaciation developments have focused on the use of stereonet patterns and statistical measures vector and eigenvector analysis of clast fabric to infer the process of sediment deposition deformation and consequent ice flow directions and dynamics however as this study identifies fundamental processes of particle orientation march jeffery and taylor rotation mechanisms would suggest more complex explanations than have been previously reported this study reports the results of a detailed field and laboratory investigation of macro scale clast and microfabrics from a till sequence in central scotland the results suggest a systematic behaviour of different particle size fractions which may be interpreted in terms of the strain response of sediment three fabric patterns are identified that illustrate consistent progressively shifting and more complex particle behaviour it is suggested these orientation patterns indicate the degree to which subglacial sediment has undergone strain and the nature of the deforming bed erosional or accretionary it is concluded that this approach may yield fundamental information about the strain conditions underlying different ice masses providing critical data linking glaciological modelling with the sedimentary evidence of glaciation

fabric pattern in a basal till succession and its significance for reconstructing subglacial processes abstract fabric data from a 2 5 m thick basal till succession in kurzetnik poland consisting of three structurally distinct till units including a deformation till were examined to test whether different processes of deposition and deformation as indicated by till structure are reflected in the till fabric pattern surprisingly throughout the succession the till fabric is unidirectional and very highly clustered mean s1 0 876 and standard fabric interpretation methods including binary and triangular diagrams failed to differentiate between the three units and constrain the origin of the till facies it is shown that the a axis length as well as the ratio between a b and a c axes has no effect on the fabric strength and there is no correlation between the fabric strength and clast size within the 0 7 5 6 cm grain size range the bootstrapping method applied to test the statistical significance of variance shows that 1 some variations in the fabric strength within the same till facies are due to the sampling effect while other variations can be interpreted as true effects of strain and 2 fabric strength may vary significantly over very short distances within the same till facies very high fabric strength in the deformation till is inconsistent with a model of weak to moderate fabric strength resulting from penetrative deformation supporting the theory that subglacial deforming tills behave as coulomb plastic rather than viscous materials

glacitectonism subglacial and glacilacustrine processes during a neoproterozoic panglaciation north east svalbard

depositional controls on the magnetic characteristics of lodgement tills and other glacial diamict facies genetic interpretation of massive unstructured diamict and diamictite facies is a commonly encountered problem faced by sedimentologists data are presented concerning the magnetic characteristics namely anisotropy of magnetic susceptibility ams and natural remanent magnetism nrm of facies deposited by 1 lodgement processes at a glacier base lodgement till and 2 subaqueously by pelagic mud deposition and ice rafting rain out diamicts lodgement tills have an nrm that is distorted around the geomagnetic pole position to form a girdle approximately 90 in length either transverse or parallel to ice flow direction this distension appears to be the result of subglacial shear processes because other diamicts deposited passively by melt out below stagnant ice and modelled by a laboratory experiment show a nondeformed nrm clustering around the geomagnetic pole the ams data show that lodgement tills have only a weakly orientated magnetic microfabric glaciolacustrine rain out diamicts show

sediment deformation beneath glaciers and its coupling to the subglacial hydraulic system abstract the extent and style of shear deformation in sediments beneath modern glaciers and the geological evidence for such deformation in deposited sediments are reviewed new evidence is presented from beneath a modern glacier of the spatial and temporal patterns of water pressure fluctuation and of time dependent patterns of deformation in sediments it is concluded that in most experimental sites beneath soft bedded modern glaciers deformation is a significant or major contributor to glacier movement and the resultant discharge of till is large enough to make sediment deformation a major till forming process particular modes of deformation facilitate incorporation of underlying material into the till whilst the capacity of a deforming till to absorb strain can protect the underlying strata from deformation leading to the commonly found relationship where till overlies other strata with a sharp planar interface it is argued that the almost ubiquitous occurrence of drumlins on the beds of former ice sheets is a reflection of the widespread occurrence of sediment deformation beneath them with important implications for the coupling of ice sheet flow and bed properties it is argued that the mechanical behaviour of the subglacial system is not simply determined by till properties but largely controlled by the subglacial water pressure regime determined by the nature of subglacial drainage results of field experiments show how the nature of the basal hydraulic system can play a vital role in controlling the coupling between the glacier and till deformation processes they show that rapid glacier advances can produce undrained loading of sediments that effective pressure may increase either upwards or downwards in a till according to the direction of drainage and that interstitial water pressures in subglacial sediments can show large and rapid variations producing strong variations in the rate and distribution of strain and in the partitioning of basal movement between sliding and deformation

microscopic evidence of subglacial deformation abstract subglacial deformation is by now a well established process however recognition of this process in pleistocene tills is not always clear studies of thin sections of pleistocene tills show that there are a variety of microscopic features caused by deformation this paper gives a review of microscopic evidence for subglacial deformation and starts with a case where deformation could be established macroscopically then traces the microscopic features through planar and rotational movements which are described and separated accordingly attention is also given to features caused by crushing of grains and sediment dewatering these studies indicate that almost all basal tills show evidence of subglacial deformation rather than sedimentary derivation this implies that if we want to understand subglacial processes such tills should be described in tectonic rather than sedimentary terminology

laboratory study of the frictional rheology of sheared till 1 deformation of till produces power law creep for low strain at stresses high enough to cause permanent deformation but below the shear strength experiments were conducted on till a mixed size granular material from matanuska glacier alaska and the scioto ohio lobe of the laurentide ice sheet caesar till we deformed till in double direct shear under fixed shear velocity or shear stress creep normal stress ranged from 50 kpa to 5 mpa at shearing rates ranging from 1 to 300 m s for 1 cm thick samples creep was induced via small step perturbations in stress fabric development within till layers was investigated by varying shear strain prior to creep tests in velocity controlled experiments till deforms as a nearly coulomb plastic material with slight velocity strengthening corresponding to a stress exponent n 60 creep experiments conducted well below the shear strength indicate lower n values increasing as shear stress increases with increasing initial strain and inferred fabric development the creep strain rate decreases while n increases experiments at a normal stress of 1 mpa and no initial strain show n 6 8 increasing to n 17 5 at higher shear strains however strain rate was still decreasing and thus these values represent maximum estimates our data show that in the absence of dilatant hardening till exhibits rate sensitivity at strain of order 1 or less at low strains n likely depends on consolidation state permeability and dilation deformation is nearly rate insensitive coulomb plastic at shear stress near the shear strength or at high strain

random variation of fabric eigenvalues implications for the use of a axis fabric data to differentiate till facies fabric shape based on the relative values of three eigenvalues calculated from fabric data has been used to differentiate sediment facies and infer deformation histories the eigenvalues are based on samples drawn from parent populations and as such are subject to statistical variance due to sampling effects in this paper the degree of statistical variance in fabric data for two types of subglacial till from breidamerkurjokull iceland is investigated using bootstrapping techniques in which empirical confidence regions are built up by repeated resampling of the original data the experiments show that for each till type the observed between sample variability in the fabrics lies within the boundaries associated with random variations indicating that the observed range of fabric shapes within each till type is likely to be entirely the product of sampling effects differences in fabric shape between till types are generally greater than that associated with random variations indicating that their fabric shape characteristics as measured by eigenvalues are significantly different nevertheless the results suggest that great care should be exercised when using a axis fabric data to differentiate sedimentary facies or to infer subtle differences in physical processes or conditions copyright 2001 john wiley sons ltd

subglacial till formation sedimentary characteristics and classification we review the major subglacial till forming processes as presently understood by glacial researchers and define the parameters within which tills are produced and reconcile them with sedimentary end members processes of deformation flow sliding lodgement and ploughing coexist at the base of temperate glacier ice and act to mobilize and transport sediment and deposit it as various end members ranging from glacitectonically folded and faulted stratified material to texturally homogeneous diamicton the dominance of any one subglacial process varies both spatially and temporally giving rise to the possibility that a till or complex till sequence contains a superimposed signature of former transportation deposition at the ice bed interface we recommend that while glacial geologists and geomorphologists should be able to recognize the sedimentary imprints of various subglacial processes genetic fingerprinting of subglacial tills should be less process specific and till classification must reflect the range of products encompassed by the subglacial till production continuum glacial geologists can presently unequivocally identify a glacitectonite rock or sediment that has been deformed by subglacial shearing deformation but retains some of the structural characteristics of the parent material b subglacial traction till sediment deposited by a glacier sole either sliding over and or deforming its bed the sediment having been released directly from the ice by pressure melting and or liberated from the substrate and then disaggregated and completely or largely homogenised by shearing and support the theoretical case for c melt out till sediment released by the melting of stagnant or slowly moving debris rich glacier ice and directly deposited without subsequent transport or deformation because observations on contemporary glaciers reveal that their beds are most likely to be mosaics of deformation and sliding and warm based and cold based conditions the patterns of which change temporally and spatially it is extremely unlikely that subglacial till end members in the geological record will be anything but hybrids produced by the range of processes operative in the subglacial traction zone

glacigenic clast fabrics genetic fingerprint or wishful thinking the interpretation of glacigenic diamictons is a subjective process for which quantitative support is frequently sought from parameters such as clast shape and fabric it has been widely suggested that different glacigenic diamicton facies possess distinct clast fabric signatures this paper examines this concept using a data set of 111 clast fabrics and a synthesis of published results eigenvalues are calculated and compared for a variety sedimentary facies it is concluded that clast fabric alone is not able to discriminate between different glacigenic facies and it is argued that clast fabric offers little quantitative support in the interpretation of glacigenic sediments it is suggested therefore that although clast fabric may continue to have a role as an indicator of relative strain at specific sites its use in the discrimination of glacigenic facies is limited consequently we should be much more selective in undertaking such analyses in the future copyright 1999 john wiley sons ltd

flow mechanism of glaciers on soft beds subhourly measurements of bed deformation bed shear strength subglacial water pressure and surface speed at storglaciaren a glacier in northern sweden showed that the shear strain rates of the bed decrease during periods of high water pressure and surface speed high water pressures appear to be accompanied by a reduction in the coupling of ice with the bed that is sufficient to reduce or eliminate shearing the instability of large ice masses may result from similar decoupling rather than from pervasive bed deformation as has been commonly thought

basal till fabric and deposition at burroughs glacier glacier bay alaska basal till and debris rich ice in the basal transport zone of burroughs glacier southeast alaska were studied to develop a model for till deposition and to acquire new pebble fabric measurements from till at a modern glacier studies of till deposition and fabric measurements from modern glacial deposits are important for studying the genesis of pleistocene and pre pleistocene glacial sediment basal till at burroughs glacier is deposited at the ice margin by the melting of basal debris rich ice that stagnated progressively from the bottom upward beneath moving ice during deglaciation the debris rich ice is typically 1 3 m thick and has very high sediment content ranging from 60 to 70 by volume the long axes of pebbles in the debris rich ice and in other parts of the basal transport zone of the glacier are typically well oriented parallel to ice flow the average s 1 eigenvalue for nine fabrics measured in the basal transport zone is 0 8092 0 067 the basal till is massive matrix supported silty sandy diamicton striated lodged boulders and crag and tails are common on the till surface s 1 eigenvalues calculated from 27 pebble fabrics measured in the till range from 0 4930 no fabric to 0 8261 the average value is 0 6757 0 092 the range of values is nearly identical to that reported for fabrics from undeformed lodgment till by dowdeswell and sharp 1986 although the final process of till deposition at burroughs glacier is passive melt out from stagnant debris rich ice the nature and pebble fabric of the till are inherited from active ice processes specifically the progressive stagnation of debris rich ice beneath active ice evidence of differential movement shear within the debris rich ice during deposition is reflected in a change in fabric azimuth with depth in the till beveled bullet boulders and shear surfaces these features are typically thought to be characteristic of lodgment till or brittle deformation of subglacial sediment the melt out process is passive because of the very high debris content of the debris rich ice if the same sequence of till deposition occurred in older glacial deposits and the debris content of the basal ice was high the final process of melt out would not be reflected in properties of the sediment

deformation till in the great lakes region implications for rapid flow along the south central margin of the laurentide ice sheet structural and lithologic data indicate that while deposited under actively moving ice considerable portions of three muddy calcareous subglacial tills in the great lakes region probably experienced some component of ductile deformation viscous till flow and ductile shear are invoked to explain a combination of features such as recumbent isoclinal folds unlithified sediment clasts mixed ostracode shells reversed stone lee ends girdle and transverse fabrics irregular stone pavements fine striae following stone curves and inconsistent stone striae and a axes deforming fine textured subglacial till is considered as a subhorizontal shear zone rheologically layered with associated structures in descending order ductile e g isoclinal folds brittle ductile e g fissility and brittle e g till wedges rheology would be controlled mainly by till pore water content matrix texture and stone content spatial and temporal superposition of rheologies and subglacial processes probably occur

determining glacier flow direction from till fabrics abstract field and experimental data indicate that fabrics of sheared basal tills based on particle orientations and on anisotropy of magnetic susceptibility are inclined relative to the macroscopic plane of shear as a result if this plane is tilted with a transverse component one of the longest standing axioms of glacial geology that particles in basal till are preferentially oriented parallel to the direction of shear is not strictly true the resultant correction to the shearing direction increases with the transverse and longitudinal components of shear plane inclination and with the magnitude of the fabric inclination relative to the shear plane azimuthal corrections calculated herein range from being negligible to a few tens of degrees for reasonable ranges of shear plane tilt the past success of fabric azimuths as indicators of flow direction despite the absence of this correction likely reflects the clustering of shear planes about the horizontal in basal tills however on the flanks of accreting subglacial bedforms shear plane attitudes may well conform to the local bedform slope as illustrated with hypothetical fabrics of drumlins and flutes azimuthal corrections can in that case be sufficiently large and systematic to seriously affect interpretations of bedform genesis