discriminating between subglacial and proglacial lake sediments an example from the danischer wohld peninsula northern germany subglacial lakes are common features of contemporary ice masses however they are rarely identified in the geological record this is due to the difficulty in discriminating between subglacial and proglacial lake sediments a proglacial origin is typically preferred as the simplest explanation we hypothesise that numerous deposits currently interpreted to record proglacial lake sedimentation may actually have a subglacial origin here we try and find ways of distinguishing proglacial from subglacial lake sediments by investigating three sites along the danischer wohld peninsula northern germany which have been interpreted to record both proglacial and subglacial sedimentation we identify two major phases of ice activity and associated lake formation during the late weichselian glaciation 1 proglacial lake formation at 23 ka in front of the advancing baltic ice stream this lake was subsequently overridden and the sediments glaciotectonised as ice continued to advance to its maximum extent 2 retreat of ice back into the baltic basin at 19 ka and formation of a proglacial lake that persisted for 4 ka we suggest that subglacial lake activity may have occurred at two of the sites between 23 and 19 ka this is based on the presence of aggrading sediment deposits characterised by stratified laminated diamictons and interbedded tabular to channelized sorted sediments the juxtaposition of relatively undeformed waterlain sediment and subglacial till absence of glaciotectonic thrusting and folding or of fining coarsening successions and the geomorphic association with tunnel valleys to the south of the study area the style of sedimentation and deformation provided the greatest insight into the discrimination of proglacially vs subglacially deposited glaciolacustrine sediments the luminescence signal palaeodose distributions also offers a potentially powerful means of fingerprinting sediment transport pathways of young glacial systems

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

microscale evidence of liquefaction and its potential triggers during soft bed deformation within subglacial traction tills published conceptual models argue that much of the forward motion of modern and ancient glaciers is accommodated by deformation of soft sediments within the underlying bed at a microscale this deformation results in the development of a range of ductile and brittle structures in water saturated sediments as they accommodate the stresses being applied by the overriding glacier detailed micromorphological studies of subglacial traction tills reveal that these polydeformed sediments may also contain evidence of having undergone repeated phases of liquefaction followed by solid state shear deformation this spatially and temporally restricted liquefaction of subglacial traction tills lowers the shear strength of the sediment and promotes the formation of transient mobile zones within the bed which accommodate the shear imposed by the overriding ice this process of soft bed sliding alternating with bed deformation facilitates glacier movement by way of stick slip events the various controls on the slip events have previously been identified as i the introduction of pressurised meltwater into the bed a process limited by the porosity and permeability of the till and ii pressurisation of porewater as a result of subglacial deformation to which we include iii episodic liquefaction of water saturated subglacial traction tills in response to glacier seismic activity icequakes which are increasingly being recognized as significant processes in modern glaciers and ice sheets as liquefaction operates only in materials already at very low values of effective stress its process form signatures are likely indicative of glacier sub marginal tills

a morphometric analysis of tunnel valleys in the eastern north sea based on 3d seismic data a 1250 km2 3d seismic volume is used to provide a detailed spatial and geometrical analysis of fifteen pleistocene tunnel valleys in the danish north sea all the valleys are buried they are up to 39 km long 3 4 km wide and up to 350 m deep the valleys are part of a vast tunnel valley province covering an area of some 0 5 million km2 of the formerly glaciated lowland areas of north west europe the valleys consist of non branching non anastomosing troughs they exhibit strongly undulating bottom profiles with numerous sub basins and thresholds and are characterised by adverse end slopes cross cutting relationships and theoretical considerations suggest the occurrence of seven major episodes of valley incision attributed to ice marginal oscillations within a few glacials calculations considering the valley end gradients and theoretical ice surface profiles suggest that the valleys were formed by pressurised subglacial meltwater erosion given a range of theoretical ice surface profiles the adverse end slopes are well beyond the supercooling threshold which suggests that the water was not in thermal equilibrium with the basal ice and that flow was concentrated in substantial conduits with sufficient mass and flux to maintain water temperature well above the freezing point copyright 2007 john wiley sons ltd

observation of the 2009 samoa tsunami by the neptune canada cabled observatory test data for an operational regional tsunami forecast model 1 as part of the neptune canada cabled seafloor observatory an array of six high precision bottom pressure recorders was installed in the late summer of 2009 at depths from 100 to 2600 m seaward of the southwest coast of vancouver island in the northeast pacific the instruments transmit 1 sec bottom pressure data at roughly 0 1 mm equivalent sea level height to the data management and archiving system dmas at the university of victoria on september 30 2009 the array recorded waves of 2 5 to 6 cm amplitude associated with the transoceanic tsunami generated by the mw 8 1 samoa earthquake in the south pacific these open ocean observations were uncontaminated by coastal effects demonstrating that neptune records from future tsunami events can be effectively used as realtime input to a regional numerical tsunami forecast model we validate this proposition by showing that wave forms simulated by the regional model using the leading train of waves of the 2009 event are in good agreement with observed tsunami records for both the shelf stations and nearby coastal tide gauges tsunami waves simulated by this model are also significantly more accurate for local regions than those determined by global scale tsunami models this ability to assimilate pristine open ocean data from the cabled observatory into an operational tsunami forecast model makes it possible to provide updated wave information that could help mitigate the impact of future tsunamis approaching the west coast of british columbia and northern washington state

experimental modeling of pressurized subglacial water flow implications for tunnel valley formation tunnel valleys are elongated hollows commonly found in formerly glaciated areas and interpreted as resulting from subglacial meltwater erosion beneath ice sheets over the past two decades the number of studies of terrestrial tunnel valleys has continuously increased and their existence has been hypothesized also on mars but their formation mechanisms remain poorly understood we introduce here an innovative experimental approach to examine erosion by circulation of pressurized meltwater within the substratum and at the silicon substratum interface we used a permeable substratum sand partially covered by a viscous impermeable and transparent lid silicon putty below which we applied a central injection of pure water low water pressures led to groundwater circulation in the substratum only while water pressures exceeding the sum of the glaciostatic and lithostatic pressures led to additional water circulation and formation of drainage landforms at the cap substratum interface the formation of these drainage landforms was monitored through time and their shapes were analyzed from digital elevation model obtained by stereo photogrammetry the experimental landforms include valleys that are similar to natural tunnel valleys in their spatial organization and in a number of diagnostic morphological criteria such as undulating longitudinal profiles and tunnel shapes these results are consistent with the hypothesis that overpressurized subglacial water circulation controls the formation of tunnel valleys

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

the signature of strain magnitude in tills associated with the vistula ice stream of the scandinavian ice sheet central poland abstract at oborki in central poland we examined a 3 m thick late weichselian basal till deposited by the vistula ice stream a multi proxy approach involving a study of grain size grain roundness crushing index petrographic composition till fabric and till micromorphology in a closely sampled vertical profile was applied to decipher till formation and ice movement mechanisms the till consists of a succession of four texturally and structurally different units generated by a combination of processes related to effective stresses exerted on the bed by active ice deformation intergranular advection clast ploughing and localized basal decoupling played important roles during till formation micromorphological analysis reveals that the most common s matrix microstructures are grain lineations grain stacks turbate structures till pellets and crushed grains microshears dominate whereas plasmic fabric is not well developed we use grain roundness crushing indices and microshear geometry to determine subglacial strain magnitude in the till matrix microfabric and macrofabric s 1 eigenvalues together with i l indexes indicate low shear strains in the order of 10 10 2 strain estimates vary depending on the particle sizes used suggesting a plastic mode of till deformation all proxies studied suggest complex and temporally variable subglacial conditions under the marginal part of the vistula ice stream probably caused by changes in till rheology related to fluctuating porewater pressures fast ice flow was facilitated by a combination of bed deformation and enhanced basal sliding on a thin water layer our results may also be representative for other terrestrially based pleistocene ice streams moving over soft beds

flow mechanism of the des moines lobe of the laurentide ice sheet rapid flow of the des moines lobe of the laurentide ice sheet may have been related to its unlithified substrate new reconstructions of the lobe based on moraine elevations sediment subsidence during moraine deposition and flow direction indicators indicate that the lobe may have been 3 times thicker than in previous reconstructions nevertheless implied basal shear stresses are 15 kpa so internal ice deformation was not significant instead the lobe likely moved by a combination of sliding plowing of particles through the bed surface and bed shear consolidation tests on basal till yield preconsolidation stresses of 125 300 kpa so effective normal stresses on the bed were small a model of sliding and plowing indicates that at such stresses most particles gripped by the ice may have plowed easily through the till bed resulting in too small a shear traction on the bed to shear it at depth consistent with this prediction measurements of orientations of clasts in basal till yield a weak fabric implying pervasive bed shear strain less than 2 although some stronger fabrics have been reported by others we infer tentatively that movement was principally at the bed surface by plowing

pt effects of pleistocene glacial periods on permafrost gas hydrate stability zones and reservoir of the mittelplate oil field northern germany abstract during the past two million years low surface temperatures as well as episodically advancing ice sheets from scandinavia acted on the subsurface pt regime of northern germany their likely effects on the petroleum system of schleswig holstein were investigated for the entire quaternary mean annual ground temperature magt was reconstructed at a resolution of 1000 years by calibrating oxygen isotope records from odp site 659 to the climate of northern germany of the past 120 kyr the resulting magt trend served as input to an ice sheet model and a permafrost model along a 2d section crossing the petroleum bearing south western part of schleswig holstein here advances and retreats of the scandinavian ice sheet during saalian and elsterian glaciation stages were reconstructed maximum ice thicknesses of up to 1700 m and up to 20 periods of regional permafrost in northern germany were reconstructed for the past 1 25 million years based on a basal heat flow of 50 mw m 2 permafrost thicknesses exceeded 100 m during most of these periods temporarily extending down to depths of more than 300 m favourable surface temperatures and long durations of cold periods provided favourable conditions for onshore gas hydrate stability zones at mittelplate implementing these glacial dynamics into 2d basin modelling petromod ies aachen germany of the mittelplate oil field reveals five phases of gas hydrate stability at depths down to 750 m the latest of these events occurred during the weichselian about 20 kyr ago the effect of the ice sheets on pore pressure in the subsurface strongly depends on the hydraulic boundary conditions at the ice base e g frozen vs temperate ice sheet base excess pore pressure in the reservoir of more than 10 mpa during ice overriding is possible and probable the calculated temperature effect of the pleistocene cooling on the mittelplate reservoir is in the range of 3 7 c even today temperature in the reservoir is still lowered by about 4 c in comparison to pre pleistocene times despite the fact that a significant influence of glacial effects on petroleum generation can be ruled out at mittelplate we state that pt effects in reservoirs related to glacial processes in formerly glaciated areas have been underestimated in the past

internal architecture of a till sheet tiskilwa formation north central illinois usa application of architectural element analysis thick till sheets deposited during the quaternary form significant aquitards in many areas of north america however the detailed sedimentary heterogeneity and architecture and depositional history of till units are not well understood this study utilizes architectural element analysis to delineate the internal sedimentary architecture of the tiskilwa formation exposed at two outcrop sections in north central illinois usa architectural element analysis facilitates systematic delineation of sedimentary architecture based on the nature of facies contacts and change in facies associations delineation of unit geometries and understanding of depositional processes at different scales of resolution making architectural element analysis suitable for the sedimentological analysis and palaeoenvironmental reconstruction of subglacial deposits eleven facies types are identified in this study including sand gravel and diamict facies that record a suite of subglacial depositional processes detailed analysis of facies contacts bounding surface hierarchy and change in facies associations allows the delineation of five architectural elements including coarse grained lens coarse grained sheet mixed zone diamict lens and diamict sheet elements the spatial arrangement and genetic interpretation of elements and their spatial relationship with fifth order bounding surfaces allows the delineation of five larger scale architectural units element associations which can be mapped in the local study area and record at least three stacked successions of meltwater accumulation and till deposition the results of this study can be utilized for architectural analysis of till sheets and provide insight to groundwater flow pathways through till in the study area and elsewhere

progressive ductile shearing during till accretion within the deforming bed of a palaeo ice stream abstract this paper presents the results of a detailed microstructural study of a thick till formed beneath the weichselian devensian odra palaeo ice stream west of roda wielkopolska poland this se flowing ice stream was one of a number of corridors of faster flowing ice which drained the scandinavian ice sheet in the baltic region macroscopically the massive laterally extensive till which formed the bed of this ice stream lacks any obvious evidence of glaciotectonism thrusting folding however microscale analysis reveals that bed deformation was dominated by foliation development recording progressive ductile shearing within a subhorizontal subglacial shear zone five successive generations of clast microfabric s1 to s5 have been identified defining a set of up ice and down ice dipping riedel shears as well as a subhorizontal shear foliation coplanar to the ice bed interface cross cutting relationships between the shear fabrics record temporal changes in the style of deformation during this progressive shear event kinematic indicators s c and ecc type fabrics within the till indicate a consistent se directed shear sense in agreement with the regional ice flow pattern a model of bed deformation involving incremental progressive simple shear during till accretion is proposed the relative age of this deformation was diachronous becoming progressively younger upwards compatible with subglacial shearing having accompanied till accretion at the top of the deforming bed variation in the relative intensity of the microfabrics records changes in the magnitude of the cumulative strain imposed on the till and the degree of coupling between the ice and underlying bed during fast ice flow

formation of mega scale glacial lineations on the dubawnt lake ice stream bed 2 sedimentology and stratigraphy mega scale glacial lineations msgls are highly elongate subglacial landforms produced beneath zones of fast flowing ice while qualitative data on their morphology have existed for several decades studies of their composition and sedimentology are comparatively rare sediment exposures along the course of the finnie river in nunavut northern canada provide a window into the internal stratigraphy and sedimentology of msgls formed by the dubawnt lake palaeo ice stream during regional deglaciation of the laurentide ice sheet stratigraphic sections record evidence for an initial advance of ice into the study area followed by ice sheet recession and deposition of glacifluvial and glacilacustrine outwash subsequently the dubawnt lake palaeo ice stream overrode and reworked this outwash subglacially forming an msgl till this till comprises a sandy red diamicton facies forming the core of the msgl ridges and containing variably deformed lenses stringers and rafts of outwash the sedimentology of this diamicton is consistent with an origin as a glacitectonite and hybrid till formed by a combination of non pervasive subglacial sediment deformation and lodgement facies variations from stratified to massive diamicton reflect in turn variations in strain and subglacial transport distance the occurrence of stratified glacifluvial sediments within these ridges and the well preserved nature of many of the sandy inclusions within the diamicton imply relatively short transport distances and incomplete mixing msgls under the dubawnt lake palaeo ice stream formed through a combination of subglacial erosion and deposition this included non pervasive subglacial sediment deformation and the reworking of pre existing sediment depocentres during streaming flow these results highlight the importance of sediment supply to msgl formation with the presence of abundant pre existing sediments which were subsequently overridden being critical to lineation formation

late cretaceous to recent tectonic evolution of the north german basin and the transition zone to the baltic shield southwest baltic sea abstract in this study we investigate the late cretaceous to recent tectonic evolution of the southwestern baltic sea based on a dense grid of seismic reflection profiles this area covers the baltic sea sector of the salt influenced north german basin and its transition to the salt free baltic shield across the tornquist zone the upper cretaceous to recent structural evolution is discussed by means of individual seismic sections and derived high resolution time structure maps of the main horizons i e the upper cretaceous tertiary and pleistocene the upper cretaceous and tertiary layers reveal numerous significant faults throughout the study area several of these faults propagate upwards across the unconsolidated pleistocene sediments and occasionally penetrate the surface the salt influenced north german basin reveals three major fault trends nw se n s and nne ssw several of these faults are located directly above basement sub salt faults and salt pillows the majority of these faults are trending n s to nne ssw and parallel the direction of the gluckstadt graben faults in the salt free tornquist zone we identify two major shallow fault trends which are nw se and ne sw the majority of these faults are located above basement faults following the direction of the tornquist zone we conclude that generally basement tectonics controls activation and trends of shallow faults if salt is present the ductile salt layer causes a lateral shift between the sub and supra salt faults major plate reorganisation related to the africa iberia europe convergence and the subsequent alpine orogeny caused reactivation of pre existing faults and vertical salt movement in the late cretaceous the change of stress orientation from ne sw to a nw se during neogene caused another phase of fault and salt tectonic reactivation we explain that the ice sheet loading and or present day stress field may have acted in combination causing the recent tectonics and upward extension of the faults

glacial sedimentary evidence supporting stick slip basal ice flow last glacial late devensian subglacial sediments in the southern sperrin mountains north central ireland comprise tabular bedrock rafts which are interbedded with diamicton till and brecciated bedrock and separated by glaciotectonic shears bedrock rafts and diamicton beds alternate laterally and vertically in the profile suggesting the ice bed interface was chequered with both high strength rock rafts and low strength diamicton patches during accumulation of the sediment pile the overall facies arrangement is interpreted qualitatively as reflecting stick slip basal ice flow and is related to the redistribution of free subglacial meltwater at the ice bed interface it is inferred that stick phases occur over bedrock rafts as meltwater migrates to lower pressure regions infilling shallow cavities on the surrounding diamicton surface slip occurs when porewater pressure increases and the meltwater layer backfills across bedrock raft surfaces causing ice bed uncoupling the presented field evidence suggests a stick slip rather than pervasive subglacial deformation model may be a better approximation for the flow of parts of the late devensian mid latitude ice sheets

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 melange at amsdorf central germany sediment erosion transport and deposition in a complex soft bedded subglacial system abstract glacial melange in the open cast mine at amsdorf central germany consists of several square meters of large sorted sediment blocks embedded in till the blocks are composed of largely intact to slightly deformed glaciofluvial and glaciolacustrine sand silt and clay initially deposited in a proglacial lake 2 3 km up ice and subsequently overridden by a glacier the blocks typically have cuboid to subrounded outlines are randomly distributed in the till and the contacts with the surrounding till are distinctly sharp underneath the melange are varved clays which exhibit strong deformations occasionally intervening with entirely undisturbed areas it is suggested that the blocks were entrained into debris rich basal ice by bulk freeze on when the glacier sole was lowered onto the bottom of an overridden lake after entrainment the blocks were transported englacially and re deposited with far traveled till matrix as a melt out till from stagnant ice the glacier moved mainly by sliding enhanced by low permeability varved clays in the substratum the glacier is believed to have been of a polythermal type these results show that bulk freeze on can lead to entrainment of soft sediment blocks at least 20 m2 in size and that these blocks can be englacially transported with little or no deformation for several kilometers and more the occurrence of deformed and undeformed clays under the till melange indicates a possible mosaic of coupled and decoupled ice the latter caused by a thin transient subglacial water film separating the bed from the glacier

micromorphological evidence for the role of pressurised water in the formation of large scale thrust block moraines in melasveit western iceland pressurised meltwater has a major impact on ice dynamics as well as on sedimentary and deformational processes occurring below in front of glaciers and ice sheets but its role in glaciotectonic processes is yet to be fully understood this study explores micro and macroscale structures developed within decollements in two thrust block moraines of late weichselian age in melasveit western iceland the aim is to investigate how pressurised subglacial meltwater can aid the dislocation and transport of large unfrozen and unlithified sediment blocks by glaciers a detailed model is constructed for the development of the thrust block moraines and the microscale processes occurring along their detachments during thrusting the detachments are characterized by relatively thin zones of crosscutting hydrofractures which reflect fluctuating water pressures during glaciotectonism little evidence of shearing is observed along the leading edges of the thrusts in both moraines this is supported by high water pressures along the detachments and indicates that the thrust blocks were initially decoupled from the underlying deposits as the thrust moraines evolved an increased amount of shear occurred in between events of sediment liquefaction hydrofracturing and fluid escape this was followed by progressive locking up of the detachments and eventual cessation in the accretion of the thrust blocks

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

impact of late cretaceous to neogene plate tectonics and quaternary ice loads on supra salt deposits at eastern gluckstadt graben north german basin

sedimentological and deformational criteria for discriminating subglaciofluvial deposits from subaqueous ice contact fan deposits a pleistocene example ireland a pit located near ballyhorsey 28 km south of dublin eastern ireland displays subglacially deposited glaciofluvial sediments passing upwards into proglacial subaqueous ice contact fan deposits the coexistence of these two different depositional environments at the same location will help with differentiation between two very similar and easily confused glacial lithofacies the lowermost sediments show aggrading subglacial deposits indicating a constrained accommodation space mainly controlled by the position of an overlying ice roof during ice bed decoupling these sediments are characterized by vertically stacked tills with large lenses of tabular to channelized sorted sediments the sorted sediments consist of fine grained laminated facies cross laminated sand and channelized gravels and are interpreted as subglaciofluvial sediments deposited within a subglacial de coupled space the subglaciofluvial sequence is characterized by glaciotectonic deformation structures within discrete beds triggered by fluid overpressure and shear stress during episodes of ice bed recoupling clastic dykes and folds the upper deposits correspond to the deposition of successive hyperpycnal flows in a proximal proglacial lake forming a thick sedimentary wedge erosively overlying the subglacial deposits gravel facies and large scale trough bedding sand are observed within this proximal wedge while normally graded sand beds with developed bedforms are observed further downflow the building of the prograding ice contact subaqueous fan implies an unrestricted accommodation space and is associated with deformation structures related to gravity destabilization during fan spreading normal faults this study facilitates the recognition of subglacial submarginal depositional environments formed in part during localized ice bed coupling episodes in the sedimentary record the sedimentary sequence exposed in ballyhorsey permits characterization of the temporal framework of meltwater production during deglaciation the impact on the subglacial drainage system and the consequences on the irish sea ice stream flow mechanisms

incremental subglacial meltwater sediment deposition and deformation associated with repeated ice bed decoupling a case study from the island of funen denmark abstract on the island of funen denmark sediments that are 4 7 m below the land surface consist of a 2 4 m thick sequence of stacked and folded glaciofluvial sediments overlain by a 2 3 m thick basal till the glaciofluvial sediments consist of tabular beds of planar stratified massive and cross stratified sand and gravel with localized channel fills of open framework gravel sand and gravel beds exhibit curvilinear sheath folding often leading to eye folds this folding is attributed to repeated glacier bed decoupling and recoupling with subglacial deposition of sand and gravel by meltwater occurring during the decoupling phase and deformation occurring during recoupling sand and gravel beds are typically separated by silty clay beds and laminations recording slackwater deposition within subglacial cavities folds frequently display attenuated upper limbs reflecting shearing associated with overlying ice movement on lower fold limbs only localized thrusting is observed suggesting water saturation of sands and gravels during deformation and relatively low strain rates folds are frequently truncated by overlying sand and gravel beds no evidence of shearing is found along these contacts which are interpreted as erosional surfaces thus we propose that the glaciofluvial sediment sequence records incremental step wise subglacial deposition and deformation rather than wholesale deformation following emplacement of the entire glaciofluvial sequence specifically we envisage that individual beds were deposited and folded subglacially with ice bed decoupling occurring during sediment deposition followed by recoupling and shearing of the upper bed surface by the ice base to produce folds with attenuated upper limbs incremental deformation and folding were facilitated by elevated porewater pressure within the sediments silty clay beds helped to maintain elevated porewater pressure by acting as local aquitards a general absence of till from the glaciofluvial sequence save for localized cm scale blocks suggests either non deposition or reworking into the glaciofluvial sediments deposition of tabular sand and gravel beds and open framework gravel filling scours within single beds suggest broad sheet like subglacial flows 10 100 m wide that become increasingly channelized we propose that the sedimentary sequence records a highly dynamic glaciohydraulic system characterized by repeated localized subglacial cavity development and storage and release of meltwater leading to glacier decoupling that may have facilitated fast flow of the baltic ice stream

till stratigraphy and sedimentology at the margins of terrestrially terminating ice streams case study of the western canadian prairies and high plains complex glacigenic sediment sequences arranged in large arcuate assemblages and demarcating the lobate termini of fast ice flow corridors in western canada represent the terrestrial equivalents of the trough mouth fans of the marine palaeo ice stream record intensive sedimentological and stratigraphic investigations at a regional scale are employed in the interpretation of thick units or sequences of diamicton and two depositional scenarios are proposed first glacier marginal till thickening and the stacking of till wedges takes place in areas where an ice stream margin was stationary for short periods of time stacks of subglacial traction tills potentially record annual sub marginal incremental thickening at ice stream margins especially in areas where densely spaced recessional push moraines appear to record seasonal climatic forcing second proglacial lake and valley infilling with glacilacustrine rhythmites and mass flow diamictons is associated with the advection of sub marginal till into preglacial bedrock depressions and records ice stream marginal oscillations and debris influx into discontinuous proglacial lakes which act as subaqueous sediment sinks for materials that would otherwise have been used to construct sub marginal till wedges and moraines glacial overriding modifies the deposits produced in both scenarios through deformation and the dislocation and entrainment of bedrock and sediment rafts at the margins of bedrock valley walls resulting in the widespread development of glacitectonites and megablocks the occurrence of the thickest deposits as preglacial valley fills may be significant in groundwater siphoning away from subglacial deforming layers and the promotion of sticky spots

the deforming bed characteristics of a stratified till assemblage in north east anglia uk investigating controls on sediment rheology and strain signatures the glacial coastal exposures of north norfolk are a type site for subglacial glaciotectonic deforming bed sediments this investigation of the lower stratified diamict within the north sea drift at west runton reveals two distinct lamina types type 1 laminae are the product of primary extensional glaciotectonism with ductile intergranular pervasive shear predominating over brittle shear type 2 laminae also exhibit structures that can be attributed to ductile intergranular pervasive shear and brittle shear but the lateral continuity of type 2 laminae and the presence of dropstone like structures supports a primary subaqueous origin with secondary subglacial deformation when coupled with micromorphological analysis these findings show that ductile viscous creep mechanisms control sedimentary architecture and that shear stratification in particular has the potential to affect the rheological properties of the sediment pile and the hydraulic routing of basal water ultimately influencing critical effective pressure fluctuations and the thresholds controlling the subglacial drainage system

micromorphological evidence of liquefaction injection and sediment deposition during basal sliding of glaciers the sliding of an ice mass over its bed represents one of the main mechanisms for the forward motion of glaciers and ice sheets facilitated by the periodic introduction of meltwater along the ice bed interface or regelation of the overriding ice in the geological record however physical evidence of this process having occurred beneath former ice masses is apparently limited this paper presents the results of a detailed micromorphological study of thinly stratified subglacial tills exposed at two sites i galmis in switzerland and ii plumpe farm near gretna in sw scotland the stratification within these tills comprises alternating layers of massive to weakly foliated diamicton and variably deformed folded faulted laminated silt and clay microtextural and microstructural evidence is interpreted in terms of repeated phases of basal sliding as the ice overrode a soft sediment bed elevated meltwater contents pressures encountered immediately prior to and during basal sliding promoted localised liquefaction within the bed decoupling of the ice mass from its bed enabled the injection of the liquefied diamicton along the ice bed interface and or into the laminated sediments immediately adjacent to this boundary the laminated silts and clays record the settling out of fines clay silt from meltwater trapped along the ice bed interface after an individual phase of basal sliding has ceased injection of till into the locally water saturated silts and clays resulted in partial liquefaction and incomplete mixing of these fine grained sediments with the diamicton density contrasts between the two liquefied sediments led to the development of a complex vinaigrette like texture comprising rounded to irregular till pebbles within a matrix of variably homogenised silty clay recoupling of the ice with its bed led to localised folding and thrusting within the laminated silts and clays hydrofracturing and injection of a network of sand filled veins and the imposition of a variably developed clast microfabric in the diamicton layers analysis of the clast microfabrics indicates that the intensity of these fabrics is highly variable reflecting the variation in the intensity of deformation imposed by the overriding ice

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

a major ice drainage pathway of the last british irish ice sheet the tyne gap northern england the tyne gap is a wide pass situated between the scottish southern uplands and the english pennines that connects western and eastern england it was a major ice flow drainage pathway of the last british irish ice sheet this study presents new glacial geomorphological and sedimentological data from the tyne gap region that has allowed detailed reconstructions of palaeo ice flow dynamics during the late devensian marine isotope stage 2 mapped lineations reveal a complex palimpsest pattern which shows that ice flow was subject to multiple switches in direction these are summarised into three major ice flow phases stage i was characterised by convergent lake district and scottish ice that flowed east through the tyne gap as a topographically controlled ice stream this ice stream was identified from glacial geomorphological evidence in the form of convergent bedforms streamlined subglacial bedforms and evidence for deformable bed conditions stage ii involved northerly migration of the solway firth ice divide back into the southern uplands causing the easterly flow of ice to be weakened and resulting in southeasterly flow of ice down the north tyne valley and stage iii was characterised by strong drawdown of ice into the irish sea ice basin thus starving the tyne gap of ice and causing progressive ice sheet retreat westwards back across the watershed prior to ice stagnation copyright 2009 john wiley sons ltd

sedimentological record of subglacial conditions and ice sheet dynamics of the vistula ice stream north central poland during the last glaciation abstract deposits of the vistula ice stream draining the scandinavian ice sheet during the last glaciation were investigated at four field sites in north central poland using micro and macroscale features the study reveals several till units with specific structural textural and lithological characteristics the individual till units are either macroscopically massive or bedded and the contacts between the units are either sharp or transitional the nature of the contacts with the underlying sediments ductile deformation structures largely undeformed clayey clasts tectonic lamination thin horizontal stringers of sorted sediments ploughing marks boulder pavements and striated upper surfaces of pebbles in the till indicate both bed deformation and enhanced basal sliding under high subglacial water pressure conditions it is suggested that the till is a hybrid deposit generated by some combination of lodgement deformation and ploughing punctuated by periods of basal decoupling the depth of deformation at any point in time was thinner up to several decimetres than the maximum till thickness c 2 5 m the ice sliding velocity estimations indicate velocities of less than 100 to over 2000 m yr 1 which suggests an unstable and highly dynamic ice lobe consistent with spatial variability of till characteristics sand wedges in the deposits beneath the till and the nature of the till bed interface indicate permafrost under the advancing ice sheet we suggest that under the increasing ice thickness a layer of thawed water saturated sediment formed on top of the still frozen ground due to inefficient drainage and contributed to ice streaming by promoting pervasive deformation and basal sliding

development of the topography controlled upper odra ice lobe scandinavian ice sheet in the fore mountain area of southern poland during the saalian glaciation abstract during the saalian drenthe glaciation the upper odra ice lobe constituted the most southern part of the scandinavian ice sheet in central europe the lobe was situated in an area with varied topography in the foreland of a mountain range the ice lobe had a complex form because it was subdivided into several smaller sublobes the primary factors that controlled the lobe were investigated it was found that the ice lobe development depended largely on the topography but the lithology of the ice sheet substratum was also highly important the topography controlled the main movement direction of the ice and caused a concentration of the ice flow in the axial part of the silesian lowland between subglacial morphological barriers which thereby formed a corridor of relatively fast flowing ice which was probably streaming and which supplied the lobe the topography also determined the ice flow within the lobe which strongly matched the valley system the presence of sediments with relatively low permeability at the base of the lobe the quaternary deposits are underlain by neogene clays caused high pore water pressure in the subglacial zone and triggered the basal movement of the ice sheet which promoted a rapid flow of the ice and favoured the development of the lobe the basal tills deposited from the active and passive ice and extant till patches without a fluvioglacial cover atop some parts of the studied area indicate spatially varying ice flow dynamics especially in the later stage of glaciation certain parts of the lobe underwent stagnation while in other parts the ice was probably still active the varied behaviour of the ice within the lobe was determined primarily by the topography and lithology of the substratum which produced spatially varied marginal compression and basal friction a model of the development of the lobe is proposed

modeling the subglacial hydrology of the james lobe of the laurentide ice sheet to investigate the drainage conditions that might be expected to develop beneath soft bedded ice sheets we modeled the subglacial hydrology of the james lobe of the laurentide ice sheet from hudson bay to the missouri river simulations suggest the james lobe had little effect on regional groundwater flow because the poorly conductive upper cretaceous shale that occupies the upper layer of the bedrock would have functioned as a regional aquitard this implies that general northward groundwater flow out of the williston basin has likely persisted throughout the quaternary moreover the simulations indicate that the regional aquifer system could not have drained even the minimum amount of basal meltwater that might have been produced from at the glacier bed therefore excess drainage must have occurred by some sort of channelized drainage network at the ice till interface using a regional groundwater model to determine the hydraulic conductivity for an equivalent porous medium in a 1 m thick zone between the ice and underlying sediment and assuming conduit dimensions from previous theoretical work we use a theoretical karst aquifer analog as a heuristic approach to estimate the spacing of subglacial conduits that would have been required at the ice till interface to evacuate the minimum water flux results suggest that for conduits assumed to be on the order of a tenth of a meter deep and up to a meter wide inter conduit spacing must be on the order of tens hundreds of meters apart to maintain basal water pressures below the ice overburden pressure while evacuating the hypothesized minimum meltwater flux r 2007 elsevier ltd all rights reserved

the role of pore fluid overpressure in the substrates of advancing salt sheets ice glaciers and critical state wedges critical state wedges ice glaciers and salt sheets have many geometric and mechanical similarities each has a tapering geometry and moves along a basal detachment their motions result from the combined effects of internal deformation and basal sliding wedge deformation and geometry basal conditions and overpressure pore fluid pressure less hydrostatic pore fluid pressure development within the substrate interact with each other in this mechanically coupled system however the nature of this interaction is poorly understood in order to investigate this coupled system we have developed two dimensional poromechanical finite element models with porous fluid flow in sediments we have simulated the advance of a salt sheet wedge across poroelastic sediments in this study we emphasize that our results have applications beyond salt wedges to both critical state wedges and ice glaciers overpressure develops within the substrate over time during the advance of the wedge the magnitude of the overpressure influences the wedge geometry and the wedge advance rate lower overpressure results in a thicker and steeper wedge geometry and a slower advance rate while higher overpressure favors a thinner wider and more flattened wedge geometry and a faster advance rate this study provides key insights into the links between wedge geometry basal shear stress and overpressure in substrates

sedimentary record of a scandinavian ice sheet drainage system and till deposition over subglacial obstacles promoting basal sliding an example from southern poland abstract subglacial obstacles occurring in the path of advancing ice sheets generally generate higher longitudinal compression and higher frictional drag than a flat substrate however in the case of a soft sediment substratum they can have a very different effect on ice sheet behaviour this study concerns a substrate composed of very fine grained sediments with low permeability the relationship between subglacial obstacles and the overriding scandinavian ice sheet was studied in an area of southern poland where a small intervalley neogene clay ridge 40 m high was present based on sedimentological and structural analysis of subglacial till and gravelly sandy sediments the basal depositional processes and subglacial conditions and their influence on ice sheet behaviour were analysed the till and related deposits within the ridge reflecting high water pressure conditions and lack of glacitectonic deformations indicate that the clay ridge did not generate much resistance against the advancing ice sheet but instead favoured basal slip the impermeable substratum weakened the ice bed coupling and promoted ice detachment from the substratum gravelly sandy inclusions at the till clay contact indicate that during the first stage of ice sheet overriding a canal drainage system developed at the ice substrate interface varied geometry size and location of inclusions of sorted sediments suggest periodic instability of the canal system which could lead to its transformation from initially uniform to being composed of conduits of different sizes during later stages of ice sheet overriding a traction till was deposited and occasional drainage through a water film was sufficient to evacuate basal meltwater the resulting change in the character of subglacial drainage was probably related to variations in water pressure gradient during progressive ice sheet advance

glacial sediments and landforms of holderness eastern england a glacial depositional model for the north sea lobe of the british irish ice sheet abstract borehole records and exposures at coastal cliffs and inland quarries around holderness are used in a critical assessment of the glacial depositional environments that operated at the margin of the north sea lobe of the british irish ice sheet at the last glacial maximum four lithofacies associations lfa are recognized in the area lfas 1 and 4 are laminated to massive diamictons traditionally called the skipsea and withernsea tills members and display deformation structures indicative of stress from the ne attenuated rafts of soft bedrock lithologies ice bed separation canal fill features and pseudo stratification and clast macrofabric data predominantly indicate deformation only to low strains they are interpreted as subaqueous deposits subsequently deformed by glacier ice to create end products that lie on the glacitectonite continuum localized increases in strain signature are equated with the development of shear zones between onshore thickening wedges of deformed pre existing lake sediment lfa 2 comprises cross bedded clinoforms locally steepening into foresets at the core of linear hummocks along the central spine of holderness lfa 2a and rhythmically laminated silts and sands with dropstones lfa 2b these sediments are interpreted as coalescent glacilacustrine subaqueous outwash fans fining distally to lake bottom rhythmites and varves ice contact deposition is evident in compressional folding faulting and hydrofracture filling due to ice overriding and extensional faulting associated with localized ice melt out depressions produced by folding or sagging provided the accommodation space for the later deposition of lfa 2 and 3 through syntectonic sedimentation lfa 3 comprises flat lying beds of well to poorly sorted cross stratified sands and gravels interpreted as proximal proglacial braided outwash deposits laid down largely in ribbon sandar defined by the topography of abandoned ice contact glacilacustrine depo centres well developed ice wedge pseudomorphs in lfa 3 record permafrost conditions during ice sheet marginal recession indicating that north sea lobe oscillations may have been non climatic or surge related a depositional model is proposed in which advance and retreat phase tills glacitectonites and associated ice contact lake sediments are the geological imprint of a single glaciation the initial advance of the north sea lobe is recorded by a westerly thinning advance till lfa 1 later lfa 1 and 4 retreat tills are strictly glacitectonites and are thicker depositional units because later ice readvances encroached upon and cannibalized more substantial sequences of deglacial lake sediment and subaqueous ice contact fans lfa 2 similar regional till architectures will be manifest wherever the palaeogeography resulted in the onshore flow of ice and the concomitant production of glacitectonites from glacial lake sediments

the post glacial landscape evolution of the north german basin morphology neotectonics and crustal deformation the recent evolution of the north german basin ngb which is presently a low seismic area was partly affected by glacial loading and unloading of the ice masses major stresses acting within the ngb are induced by the north atlantic ridge push the ongoing alpine collision and the post glacial rebound of fennoscandia present day horizontal stresses within the ngb are directed generally nw se but fan and bend north of 52 n towards nne major basement faults are directed nw se minor faults ne sw and nne ssw and are clearly detectable in geomorphological and satellite lineaments furthermore the drainage pattern and the distribution of lakes in northern germany follow exactly block boundaries and hence mark zones of present day subsidence the understanding of the post glacial morphology and reactivation of faults requires a view into the very heterogeneous crust and upper mantle below the ngb the re adjustment of the individual fault blocks during post glacial relaxation of the lithosphere leads to differential crust dependent uplift and probably to the formation of urstrom valleys the urstrom valleys and terminal moraines in northern germany appear to parallel the major tectonic lineaments and lithospheric block boundaries the lithospheric memory is expressed in the post glacial landscape evolution of the ngb

temporal changes in subglacial meltwater activity field evidence from the late devensian in the north of ireland abstract temporal changes in meltwater abundance distribution and characteristics controlling subglacial processes and ice sheet dynamics can be inferred from subglacial sediment successions field evidence for changes in subglacial meltwater characteristics over time is presented from two sites doonan drummee near a former late weichselian devensian ice centre in the north of ireland on a macroscale both sites investigated show subglacial diamicton overlying glacially planated bedrock platforms in more detail primary sedimentary structures and facies variability show a complex relationship between depositional processes and meltwater characteristics at the ice bed interface ibi sedimentary evidence suggests sediment transport and deposition took place by low viscosity subglacial slurries mobile sediment meltwater admixtures which are part of a continuum between the processes of subglacial sediment deformation and subglacial meltwater flooding subtle changes in meltwater abundance and distribution at the ibi controlled slurry rheology mechanisms of particle support and detailed sediment depositional processes

seismic architecture and sedimentology of a major grounding zone system deposited by the bjornoyrenna ice stream during late weichselian deglaciation abstract a 280 km wide sediment wedge in outer bjornoyrenna bear island trough south western barents sea has been investigated using 2d and 3d seismic data sediment gravity cores as well as regional swath and large scale bathymetry data the bathymetry data indicate a division into an up to 35 m high frontal wedge with large depressions and an upstream part characterized by mega scale glacial lineations msgl from seismic sections increasing erosion is demonstrated for the upstream part coinciding with the location of msgl whether the latter are depositional features postdating an extensive erosional event or formed by erosion remains inconclusive based on the distinct morphology and internal structures we infer that the system was deposited during a rapid readvance whereby the ice front pushed and bulldozed predominantly soft diluted proglacial sediments analyses in the eastern part of the sediment system reveal the existence of imbricated thrust sheets in the frontal part of the wedge this is suggested to imply upstream erosion of sedimentary rock and incorporation of thrusted blocks into the moraine forming a composite ridge locally we argue that observed large scale depressions are dead ice features in the marine environment it is envisioned that intense englacial thrusting may have developed into a decollement as the cold glacier snout got overrun by ice masses from the interior thereby enabling the inclusion of slabs of ice in the push moraine mass radiocarbon dates indicate that the sediment wedge was deposited around 17 090 cal yrs bp 14 530 14 c yrs bp and that the ice front probably remained stable until 16 580 cal yrs bp 13 835 14 c yrs bp

reflections on soft subglacial beds as a mosaic of deforming and stable spots abstract much discussion regarding subglacial processes is centred around the question of how soft beds respond to ice overriding in particular how widespread and penetrative is their deformation how does it affect the ice stability and what are the consequences for large scale sediment redistribution patterns contrasting views have been proposed which are difficult to verify and reconcile in the light of scarcity of agreed criteria to identify deforming beds here we suggest that soft beds under warm based glaciers can be envisaged as a mosaic of deforming and stable spots transient in time and space according to changes in the control parameters at the ice bed interface the subglacial mosaic model is consistent with field data showing heavily deformed beds interspersed with largely undisturbed beds it is suggested that deformation found in tills is a cumulative effect of strain within a relatively thin ephemeral deforming spots under the ice sole affecting the till as it accretes during ice overriding in accord with the coulomb plastic rheology the role of clast ploughing in bed deformation is emphasised

response of salt structures to ice sheet loading implications for ice marginal and subglacial processes abstract during the past decades the effect of glacioisostatic adjustment has received much attention however the response of salt structures to ice sheet loading and unloading is poorly understood our study aims to test conceptual models of the interaction between ice sheet loading and salt structures by finite element modelling the results are discussed with regard to their implications for ice marginal and subglacial processes our models consist of 2d plane strain cross sections which represent simplified geological cross sections from the central european basin system the model layers represent i sedimentary rocks of elastoplastic rheology ii a viscoelastic diapir and layer of salt and iii an elastoplastic basement on top of the model a temporarily variable pressure simulates the advance and retreat of an ice sheet the durations of the individual loading phases were defined to resemble the durations of the pleistocene ice advances in northern central europe the geometry and rheology of the model layers and the magnitude spatial distribution and timing of ice sheet loading were systematically varied to detect the controlling factors all simulations indicate that salt structures respond to ice sheet loading an ice advance towards the diapir causes salt flow from the source layer below the ice sheet towards the diapir resulting in an uplift of up to 4 m the diapir continues to rise as long as the load is applied to the source layer but not to the crest of the diapir when the diapir is transgressed by the ice sheet the diapir is pushed down up to 36 m as long as load is applied to the crest of the diapir during and after ice unloading large parts of the displacement are compensated by a reversal of the salt flow plastic deformation of the overburden is restricted to the area immediately above the salt diapir the displacements after unloading range between 3 1 and 2 7 m larger displacements are observed in models with deep rooted diapirs thicker ice sheets longer duration of the loading phase thicker salt source layers and lower viscosity of the salt the rise or fall of diapirs triggered or amplified by ice sheet loading are likely to affect glacigenic deformation erosion and deposition above the diapir and within the rim synclines ice load induced uplift in front of an ice sheet will provide favourable conditions for the formation of push moraines for example by creating a topographic obstacle and inclining potential detachments subglacial subsidence of salt structures will enhance erosion by providing a preferential drainage pathway and fracturing of the overburden of the salt structure and thereby contribute to the incision of tunnel valleys however the resulting displacements are probably too low to have a marked effect on the advance or retreat pattern of the ice sheets

subglacial conditions under the last ice sheet in northwest germany ice bed separation and enhanced basal sliding abstract theoretical reconstruction of ice thickness during the weichselian glaciation in a portion of northwest germany yields maximum thicknesses of about 280 m some 50 km into the ice sheet from the margin this corresponds to averaged driving stress at the glacier bed in the order of 17 kpa the increase in ice thickness away from the margin was non uniform caused by undulations in pre existing topography steady state subglacial water pressure heads were high up in the ice sheet reaching the flotation point in places hydraulic lifting of the ice sheet probably occurred causing non deforming bed conditions where transformation of basal shear stresses to soft subglacial sediments was prevented by a water film thin stringers of undeformed sorted sediments occurring in tills are attributed to basal meltwater washing during phases of ice bed separation the basal water film was over 5 mm thick using a quantitative approach coupled with field data we demonstrate that neither internal ice deformation nor bed deformation contributed much to the ice motion instead the rapid movement of the weichselian ice sheet was caused by basal sliding with ploughing the ice sheet was probably characterized by an unstable behaviour

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

meltwater discharge through the subglacial bed and its land forming consequences from numerical experiments in the polish lowland during the last glaciation numerical experiments suggest that the last glaciation severely affected the upper lithosphere groundwater system in nw poland primarily its flow pattern velocities and fluxes we have simulated subglacial groundwater flow in two and three spatial dimensions using finite difference codes for steady state and transient conditions the results show how profoundly the ice sheet modifies groundwater pressure heads beneath and some distance beyond the ice margin all model runs show water discharge at the ice forefield driven by ice sheet thickness modulated down ice decreasing hydraulic heads in relation to non glacial times the transient 3d model shows significant changes in the groundwater flow directions in a regionally extensive aquifer ca 90 m below the ice bed interface and up to 40 km in front of the glacier comparison with empirical data suggests that depending on the model run only between 5 and 24 of the meltwater formed at the ice sole drained through the bed as groundwater this is consistent with field observations documenting abundant occurrence of tunnel valleys indicating that the remaining portion of basal meltwater was evacuated through a channelized subglacial drainage system groundwater flow simulation suggests that in areas of very low hydraulic conductivity and adverse subglacial slopes water ponding at the ice sole was likely in these areas the relief shows distinct palaeo ice lobes indicating fast ice flow possibly triggered by the undrained water at the ice bed interface owing to the abundance of low permeability strata in the bed the simulated groundwater flow depth is less than ca 200 m copyright 2009 john wiley sons ltd

saginaw lobe tunnel channels laurentide ice sheet and their significance in south central michigan usa abstract a network of tunnel channels in southern michigan records substantial subglacial meltwater activity beneath the saginaw lobe of the laurentide ice sheet the channels are incompletely filled with outwash contain eskers and in many places crosscut and continue beyond upland ridges previously mapped as recessional moraines the presence of the tunnel channels and drumlins on these upland ridges indicate that the ridges are not recessional moraines instead outwash fans and minor ridges record deglaciation in the area a palimpsest relationship between buried bedrock valleys and tunnel channels records the presence of multiple generations of tunnel channels evidence for a subglacial meltwater sheetflow consists of sheets of boulder gravel in upland 15 km wide with a hummocky upper surface the hummocks transform down flow into erosional remnant drumlins possibly the result of flow acceleration on a negative slope tunnel channels at the distal end of the drumlin swarm suggest collapse of the sheetflow into a channelized flow the tunnel channels then end at the sturgis moraine at the heads of large outwash fans the observed geomorphic relationships between tunnel channels moraines and drumlins in south central michigan are applicable to glacial landform studies elsewhere and indicate the important role of meltwater in landscape evolution

fast flow of the lake michigan lobe evidence from sediment landform assemblages in southwestern michigan usa abstract the dynamics of the late wisconsin lake michigan lobe of the laurentide ice sheet as interpreted by sediment landform assemblages along its southeastern margin are consistent with fast flow driven by high subglacial pore pressures a major advance of the lake michigan lobe terminated at the kalamazoo moraine where proglacial glaciotectonic activity marginal stagnation and release of subglacial meltwater marked the extent of the advance a large portion of the valparaiso moraine west of the kalamazoo moraine is actually a drumlinized till capped upland plain rather than an ice marginal landform assemblage lacustrine sediment underlying the surficial diamicton in the valparaiso upland is glaciotectonically deformed to a depth of 10 m the deformed interval consists of folded sandy and silty lacustrine sediment which is in places sharply truncated by the overlying diamicton decoupling of the glacier from its bed is suggested by a discontinous thin bed of sand at the base of the diamicton and by the sharp truncation of deformed lacustrine sediment the association of drumlins proglacial and subglacial deformation marginal stagnation and major ice marginal outwash fan deposition are indicative of fast flow perhaps in a surge these characteristics are consistent with a terrestrial ice stream model for the lake michigan lobe

melt out till and ribbed moraine formation a case study from south sweden abstract hummocky moraine with dispersed agglomerations of ribbed moraine here named asnen type ribbed moraine forms a 20 40 km wide zone over smaland south sweden terminated to the north by a sharp boundary to streamlined terrain the hummocky ribbed moraine zone can be geomorphologically linked to the subaqueous goteborg moraine formed at an oscillation stand still phase during the deglaciation of the swedish west coast based on detailed sedimentological and structural investigations of ribbed moraine ridges it is concluded that diamict sequences and associated sorted sediment were deposited due to passive melt out from stagnant debris rich ice with synsedimentary deposition of sorted sediments preferentially at a melting ice bed interface to accommodate for the geomorphological expression such an interpretation further implies that debris rich ice formed in an intermediate adfreezing zone between ice at the pressure melting point and a frontal frozen zone at deglaciation basal debris rich ice was stacked into transversally arranged zones controlled moraine forming ribbed moraine embryos the active phase of ridge formation the stacked sequences of debris rich ice eventually melted out beneath a stable and melt retarding supraglacial ablation complex to form the final moraine ridges the passive phase of ridge formation in areas with no stable supraglacial ablation complex the resulting landform after final de icing was a hummocky moraine landscape internal composition of moraine hummocks suggests that most of them are composed of sediment gravity flow sediments intercalated with stream deposited sediments all resting on a platform of subglacial melt out till de icing of the zone now occupied by hummocky and ribbed moraine took a considerable time deglacial 14 c age differences from lake basins on either side of the geomorphic boundary to the streamlined terrain indicate a separation between active and stagnant ice along that boundary and that it took another 200 300 yr for the stagnant ice zone to melt during the final formation of present day landforms

a ploughing model for the origin of weak tills beneath ice streams a qualitative treatment abstract glaciological studies of west antarctic ice streams have shown that weak sub ice stream tills provide the basal lubrication that makes fast ice streaming possible under low driving stresses given the significant current interest in time dependent ice stream behavior there is a clear need for a conceptual model of weak sub ice stream tills that treats in a simple but physically correct way the coupling between evolution of till properties and ice stream dynamics as a possible alternative to the previous viscous bed model we propose a ploughing model that is consistent with the experimentally determined coulomb plastic rheology of sub ice stream till in the ploughing model the till is a several meters thick layer of sedimentary material that is disturbed and transported by ploughing that occurs during sliding of a bumpy ice base the thickness of the till layer is determined in the ploughing model by the amplitude of the largest roughness elements ice keels there is no direct proof for the existence of ice bumps and ice keels beneath the modern west antarctic ice streams but bedforms e g megalineations and bundle structures left behind by pleistocene ice streams strongly support our assumption that an ice stream base is irregular generation of new till material occurs when ice keels protrude through the existing till layer and erode the top of the sub till preglacial sediments based on a single tethered stake measurement of engelhardt and kamb j glaciol 44 1998 223 made at the upb camp ice stream b west antarctica fig 1 we estimate that the till flux due to sliding with ploughing is there 3 yr 1 per meter width to balance the estimated till flux in the upb area substrata erosion by ice keels would have to take place at a high but not unreasonable non dimensional rate of 4 assuming 1 contact area in the case of the west antarctic ice streams erosion of sub till materials by ice keels may be particularly fast and unimpeded because these ice streams are overriding unlithified preglacial tertiary sediments the most significant implication of the proposed ploughing model is that it permits treating basal resistance to ice motion as being velocity independent plastic till rheology while allowing subglacial transport of till as in the viscous bed model models of ice streams with a plastic bed exhibit a greater potential for unstable behavior than models of ice streams with viscous beds

evidence for abundant subglacial meltwater beneath the paleo ice sheet in pine island bay antarctica marine geological and geophysical data collected from the continental shelf in pine island bay antarctica reveal a complex paleo subglacial drainage system controlled by bedrock topography and subglacial meltwater discharge significant amounts of freely flowing meltwater existed beneath former ice sheets in pine island bay subglacial drainage is characterized by descriptions of glacial landforms imaged on the sea floor and sedimentary deposits collected in piston cores bedrock geology is characterized using seismic data large scale landforms on the shelf include channels and cavities incised into impermeable crystalline bedrock there is a transition from randomly oriented channels on the inner shelf to a dendritic pattern of elongate channels on the middle shelf on the outer shelf a change in basal conditions occurs where sedimentary deposits bury crystalline bedrock no evidence for flowing meltwater exists on sedimentary substrates instead meltwater formed a the ice sediment contact was incorporated into the sediments contributing to development of a deforming lied which was sampled in piston cores characterization of subglacial meltwater processes that occurred in the past may aid in understanding the role meltwater plays in stability of the west antarctic ice sheet today

successive subglacial depositional processes as interpreted from basal tills in the lower vistula valley n poland abstract three basal till facies from the lower vistula valley were examined the lowest facies a sandy diamicton with characteristic sand inclusions forming detached and attenuated folds is overlain by a bedded till characterized by alternating diamictons and sorted sediment layers the uppermost till facies is a homogeneous diamicton the three till facies must have been formed by complex subglacial sedimentary processes during the first late weichselian ice advance the lowest till facies is interpreted as a deformation till and accumulated during the initial stage of the ice advance the middle facies represents a stagnation phase during the initial ice advance and was deposited during recurrent periods of subglacial melt out followed by meltwater sedimentation the upper till facies was deposited by direct subglacial melt out during a stage of stagnant ice it is suggested that bed deformation and temporarily enhanced basal sliding have been caused by ice streaming at the time of the ice sheet advance and just before its stagnation

subglacial conditions and scandinavian ice sheet dynamics at the coarse grained substratum of the fore mountain area of southern poland abstract the fore mountain areas of southern poland are locally composed of the coarse grained sediments of alluvial fans which created unusual conditions under the advancing scandinavian ice sheet during the elsterian glaciation this highly permeable substratum potentially enabled rapid outflow of meltwater from the ice sheet base thereby reducing the water pressure and strongly influencing the ice sheet dynamics the subglacial conditions and the relationship between the ice sheet behaviour and its coarse grained substratum were studied at the foreland of the western carpathian mountains the sedimentological and structural analysis of the till and related sediments that were deposited above the alluvial gravel of the fore mountain fans are presented the study indicates that despite the high permeability of the coarse grained substratum it did not slow the ice sheet movement conversely the ice sheet moved mainly due to basal slip and locally shallow deformations this was a consequence of very high basal water pressure which resulted largely from the presence of permafrost that restricted subglacial groundwater outflow in addition the ice sheet substratum was inclined opposite to the direction of its movement increasing the pressure of the subglacial water numerous subhorizontal sandy laminae within the till indicate that the meltwater from the ice sheet base was drained by a water film along the ice bed interface the water escape structures within the till and subtill sediments indicate the occasional instability of the ice sheet hydrological system and suggest that the meltwater was periodically stored in the ice sheet base temporal changes occurring in the ice sheet hydrological system might indicate variations in the ice sheet behaviour i e phases of relatively fast ice flow and phases of ice stagnation the latter were probably correlated with the freezing of the ice margin to its base the study shows how the coarse grained substratum could potentially influence ice sheet behaviour and provides a broader look on the factors controlling the dynamics of the pleistocene ice sheets which soft sediment substrata in many areas were texturally varied

subglacial sediment proglacial lake level and topographic controls on ice extent and lobe geometries during the last glacial maximum in nw russia abstract investigations in sections along the rivers severnaya dvina and vaga and morphological mapping based on a new digital elevation model and landsat imagery allow for a reinterpretation of the extent of the scandinavian ice sheet during the last glacial maximum lgm the reconstruction provides a much better link between stratigraphical and morphological expression of glaciation than previous proposals most striking is the configuration of long low gradient ice lobes ice streams extending for some 300 400 km up the wide and smooth river valleys their extremely low surface gradients are evidenced by glacial trimlines that formed along the sides of the ice lobes in contact with the gentle valley slopes in the main valleys end moraines marking terminal positions are present whereas drumlins are rare in the peripheral areas of the ice sheet but found in some tributary valleys at somewhat higher elevations large drumlin fields however are found farther up ice the glacial sediment succession is composed of waterlain sediments and tills the sediments provide evidence of distal ice damming glacier overriding and retreat and finally distally glaciolacustrine sedimentation and lake drainage the diamicton unit associated with the lgm shows evidence of basal coupling although more striking is the abundant evidence for ice bed decoupling these include in situ waterlain sediments within tills and clastic sills running along the till substrate contact showing lift off these features indicate that the weight of the glacier lobes was to a large extent carried by pore water pressure in subglacial sediments and that subglacial shear and erosion were moderate thus fast ice flow in the lobes is envisaged the balance between glacier weight and pore water pressure was probably maintained over time by the buoyancy effect of glaciers advancing into proglacial lakes subglacial lakes may have formed by capture of proglacial lakes during glacial advance the combination of low gradient ice and decoupled beds indicate that glacier advance and extent was largely controlled by lake levels and topographic thresholds this is taken to indicate that steeper gradient ice much farther upstream was providing the gravitational push a presumably abrupt change in ice surface profile is supported by the mapped distribution of glacial bedforms the model presented herein potentially has wide applicability for large parts of northern russia as the topographic setting and glacier advance in lakes across fine grained low permeable sediments provide very similar conditions

sedimentary and structural evolution of a relict subglacial to subaerial drainage system and its hydrogeological implications an example from anglesey north wales uk subglacial drainage systems exert a major control on basal sliding rates and glacier dynamics however comparatively few studies have examined the sedimentary record of subglacial drainage this is due to the paucity of modern analogues the limited recognition and preservation of upper flow regime deposits within the geological record and the difficulty of distinguishing subglacial meltwater deposits from other meltwater sediments e g glacier outburst flood deposits within this study the sedimentological and structural evolution of a subglacial to subaerial ice marginal proglacial drainage system is examined particular emphasis is placed upon the genetic development and preservation of upper flow regime bedforms and specifically recognising them within a subglacial meltwater context facies are attributed to subglacial meltwater activity and record sedimentation within a confined but progressively enlargening subglacial channel system produced under dune to upper flow regime conditions bedforms include rare large scale sinusoidal bedding with syn depositional deformation produced by current induced traction and shearing within the channel margins subglacial sedimentation culminated with the abrupt change to a more ephemeral drainage regime indicating channel abandonment or a seasonal drainage regime retreat of the ice margin led to the establishment of subaerial drainage with phases of sheet flow punctuated by channel incision and anastomosing channel development under diurnal ablation related seasonal discharge the presence of extensive hydrofracture networks demonstrate that proglacial groundwater levels fluctuated markedly and this may have influenced later overriding of the site by an ice stream

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

subglacial deformation and water pressure cycles as a key for understanding ice stream dynamics evidence from the late ordovician succession of the djado basin niger subglacial deformation is crucial to reconstructing glacier dynamics sediments associated with the late ordovician ice sheet in the djado basin niger exhibit detailed structures of the subglacial shear zone three main types of subglacial shear zones ssz are discriminated the lowermost ssz developed on sandstones displays riedel macrostructures and cataclastic microstructures these resulted from brittle deformation associated with strong glacier bed coupling and low pore water pressure where they developed on a clay rich bed the overlying ssz display s c to s c fabrics sheath folds and dewatering structures these features indicate high ductile shear strain and water overpressure on fine grained sand beds the ssz exhibit homogenized sand units with sand stringers interpreted as fluidized sliding beds the succession of subglacial deformation processes depends on fluid pressure behavior in relation to subglacial sediment permeability fluid overpressure allows subglacial sediment shear strength and ice bed coupling to be lowered leading to ice streaming

large scale reorganization and sedimentation of terrestrial ice streams during late wisconsinan laurentide ice sheet deglaciation glacial geomorphological mapping of shuttle radar topography mission srtm data from the western canadian prairies demonstrates that during the last late wisconsinan deglaciation of the laurentide ice sheet terrestrial ice streams underwent a major reorganization of their flow configuration this reorganization involved a 90 shift in flow direction and was accompanied by a corresponding increase in the influence of topography on streaming flow ice streams included both topographically confined and pure ice streams that flowed independent of topography streaming flow is recorded by suites of highly elongate 60 km long subglacial bedforms bounded sharply at their lateral margins by prominent moraines initial streaming flow was unconfined by topography but was replaced progressively and crosscut by younger topographically confined flows flow reorganization is inferred to have been caused by temporal and spatial variations in the interaction between frozen and thawed bed conditions with thinning and shutdown of one ice stream ultimately triggering initiation of others this highlights the role of internal glaciodynamically driven reorganization in triggering streaming flow within large ice sheets and shows that large scale flow reorganization can occur over the time scale of a single deglaciation in terrestrial ice streams

the pleistocene end moraine fans controls on their sedimentation and location abstract this paper presents a model of end moraine sedimentation based on data from pleistocene ice sheet margins according to this model end moraines quite often constitute alluvial fans or coalesced fans formed at the ice margin by redeposition of supraglacial material three types of end moraine fan have been recognized those composed mainly of mass flow deposits type a equivalent to dry alluvial fan in nonglacial settings those containing both mass flow and waterlain sediments type b and those that contain mainly sorted waterlain material type c equivalent to wet alluvial fan in nonglacial settings for each type of fan several sedimentary subenvironments differentiated by distance from the ice front are identified the diagnostic processes on the end moraine fans are cohesive and cohesionless debris flows or grain flows types a and b or hyperconcentrated flows and sheetflows types b and c the type b fan seems to represent the most typical sequence in the end moraine zones but the type c fan is also quite common at the pleistocene ice sheet margins our studies also suggest that the terminal supraglacial drainage controls the type of end moraine fan sedimentary subenvironment moraine types a b c whereas subglacial hydrology and ice stream regime which determine the mobility of the ice front and larger scale conditions of sediment supply to the ice margin control the location and size of the end moraine fan

soft sediment deformation of late pleistocene sediments along the southwestern coast of the baltic sea ne germany a 1 460 m long profile of a late glacial subglacial glacio fluvial glacio limnic and glacio deltaic sequence exposed at a cliff section on usedom island sw baltic sea coast is described the sequence is up to 31 m thick and shows sedimentary structures typical of a glacial setting soft sediment deformation is encountered and is associated with changes in lithology these deformations include liquefaction slumping and faulting as the most plausible cause earthquake induced shaking is discussed the associated neotectonic activity is seen as a consequence of the postglacial isostatic crustal rebound as the deglaciation earthquake ratio diminishes with time and as the rebound is phasing out no large earthquakes are anticipated for northern germany although in conclusion the lithosphere of the north german basin has to be regarded as weakened by repeated ice loading and deloading