origins of the ice contact stratified ridges eskers of ireland abstract an extensive system of ridged landforms composed of ice contact stratified sediment was deposited in the central lowland of ireland during the most recent deglaciation 18 000 bp the ridges have been interpreted by others as deposits of an ice sheet drainage system i e eskers and as such have been used with other data to reconstruct the deglacial history the traditional deglaciation model shows systematic retreat of ice from south to north our study which involves an analysis of the ridged landforms using lithofacies sedimentary structures and paleocurrent data in conjunction with the geomorphology indicates that the pattern and nature of the ridges are not compatible with this model and support a new model of deglaciation warren 1991 in which the esker system fo med in an interlobate area during the simultaneous shrinking of two main glacial outflow centers an extensive lake system developed in the lowland between the two outflow centers ridges formed both perpendicular and parallel to ice margins involved both active and stagnating ice are both continuous and segmented beaded and were deposited in subaqueous and subaerial environments almost all of the ridges are associated with lacustrine sediments ice contact ridges are polygenetic and a genetic classification is proposed those that formed perpendicular to the ice margin are termed eskers those that formed parallel are termed moraines distinction between esker types continuous or beaded subglacial tunnel fills fluvial ice channel fills and subaqueous fans and moraine types s baqueous or subaerial is crucial to a reconstruction of the mode and pattern of deglaciation in the central irish lowlands

jokulhlaups a reassessment of floodwater flow through glaciers 1 in glaciated catchments glacier generated floods jokulhlaups put human activity at risk with large sporadic jokulhlaups accounting for most flood related fatalities and damage to infrastructure in studies of jokulhlaup hydrodynamics the view predominates that floodwater travels within a distinct conduit eroded into the underside of a glacier however some jokulhlaups produce subglacial responses wholly inconsistent with the conventional theory of drainage by focusing on icelandic jokulhlaups this article reassesses how floodwater flows through glaciers it is argued that two physically separable classes of jokulhlaup exist and that not all jokulhlaups are an upward extrapolation of processes inherent in events of lesser magnitude and smaller scale the hydraulic coupling of multiple nonlinear components to the flood circuit of a glacier can induce extreme responses including pressure impulses in subglacial drainage representing such complexity in mathematical form should be the basis for upcoming research as future modeling results may help to determine the glaciological processes behind heinrich events moreover such an approach would lead to more accurate predictive models of jokulhlaup timing and intensity

hydraulics of subglacial outburst floods new insights from the spring hutter formulation using a slightly modified form of the spring hutter equations glacial outburst floods are simulated from three classic sites hazard lake yukon canada summit lake british columbia canada and grimsvotn iceland in order to calibrate the hydraulic roughness associated with subglacial conduits previous work has suggested that the manning roughness of the conduits is remarkably high but the new calibration yields substantially lower values that are representative of those for natural streams and rivers the discrepancy can be traced to a poor assumption about the effectiveness of heat transfer at the conduit walls the simulations reveal behaviour that cannot be interred from simplified theories 1 during flood onset water pressure over much of the conduit can exceed the confining pressure of surrounding ice 2 local values of fluid potential gradient can differ substantially from the value averaged over the length of the conduit contradicting an assumption of simple theories 3 as the flood progresses the location of flow constrictions that effectively control the flood magnitude can jump rapidly over large distances 4 predicted water temperature at the conduit outlet exceeds that suggested by measurements of exit water temperature

theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products publisher elsevier articletitle theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products journaltitle earth science reviews articlelink http dx doi org 10 1016 j earscirev 2016 01 010 content type article copyright copyright 2016 elsevier b v all rights reserved

the sedimentary architecture of outburst flood eskers a comparison of ground penetrating radar data from bering glacier alaska and skeiðararjokull iceland we present ground penetrating radar gpr profiles that reveal the sedimentary architecture of an esker deposited during a surge associated outburst flood at the bering glacier alaska the wide up flow end of the esker contains a transition from large backset beds to large foreset beds interpreted to reflect composite macroform development in an enlarged part of the conduit by contrast the narrow down flow portion of the esker is dominated by plane beds interpreted to have been deposited where the conduit was constricted and the flow was faster a previously studied outburst esker at skeidararjokull iceland has a similar morphology and stratigraphic architecture this suggests that outburst floods generate distinct depositional signatures in eskers both in terms of morphology and sedimentary architecture identification of these distinct signatures in ancient eskers will help assess the paleohydraulic conditions under which ancient eskers formed and by extension the nature of meltwater drainage systems beneath the laurentide and eurasian ice sheets

looking through drumlins testing the application of ground penetrating radar ground penetrating radar gpr is becoming a commonly applied technique in geomorphology however its use in the study of subglacial bedforms has yet to be fully explored and exploited this paper presents the results of a gpr feasibility study conducted on a drumlinized terrain in cumbria uk where five drumlins were investigated using multiple radar antenna frequencies the site was selected for the presence of nearby bedrock outcrops suggesting a shallow drumlinized diamict bedrock contact and a permeable lithology despite the clayey sediment and unfavourable weather conditions a considerable penetration depth of 12m was achieved when using a 50mhz antenna with a separation of 1m trace spacing of 1m and 128 fold vertical stack results indicate that the drumlinized diamict is in direct erosional contact with the bedrock while the internal drumlin geometry is generally chaotic on the stoss side evidence of layering dipping downflow at an angle greater than the drumlin surface profile was found on the lee side the inter drumlin areas comprise 4m of infill sediment that masks part of the original drumlin profile overall this study indicates that gpr can be deployed successfully in the study of glacial bedform sedimentary architecture

the fezouata fossils of morocco an extraordinary record of marine life in the early ordovician the discovery of the fezouata biota in the latest tremadocian of southeastern morocco has significantly changed our understanding of the early phanerozoic radiation the shelly fossil record shows a well recognized pattern of macroevolutionary stasis between the cambrian explosion and the great ordovician biodiversification event but the rich soft bodied fezouata biota paints a different evolutionary picture the fezouata assemblage includes a considerable component of cambrian holdovers alongside a surprising number of crown group taxa previously unknown to have evolved by the early ordovician study of the fezouata biota is in its early stages and future discoveries will continue to enrich our view of the dynamics of the early phanerozoic radiation and of the nature of the fossil record supplementary material a complete faunal list is available at http www geolsoc org uk sup18843

subglacial drainage eskers and deforming beds beneath the laurentide and eurasian ice sheets glaciological theory predicts that the subglacial drainage network at the base of gently sloping ice sheets resting on deforming sediment should consist of many wide shallow probably braided canals distributed along the ice sediment interface rather than an arborescent network of relatively few large tunnels as would develop over a rigid substrate a corollary prediction examined here is that eskers which form in large subglacial tunnels should be rare where subglacial bed deformation occurred but they may be relatively common where the bed was rigid bed deformation would be most likely where subglacial till was relatively continuous fine grained and of low permeability that is in regions where till is derived primarily from underlying sedimentary bedrock but unlikely where discontinuous coarse grained high permeability till was derived from underlying crystalline bedrock the observed distribution of eskers in areas covered by the laurentide and eurasian british scandinavian and barents sea ice sheets during the last glaciation shows that most eskers occur over crystalline bedrock overlain by discontinuous high permeability till but are rare or absent over sedimentary bedrock overlain by fine grained low permeability till thus matching reasonably well our prediction glaciological theory and geologic evidence indicate that esker systems on a subcontinental scale are time transgressive sedimentological evidence for a canal drainage system appears to be present in fine grained tills where eskers are largely absent

time transgressive deposits of repeated depositional sequences within interlobate glaciofluvial esker sediments in koylio sw finland abstract evidence of conspicuous repeated seasonal to annual deposition of glaciofluvial and glaciolacustrine sequences within a structurally complex interlobate esker segment in sw finland is presented the time transgressive overlapping depositional sequences consist of deposits from two successive melt seasons including three vertically stacked lithofacies associations 1 massive to stratified coarse gravels summer deposits 2 trough and ripple cross stratified fine grained deposits autumn to winter deposits and 3 sandy stratified beds spring deposits the depositional environment of each lithofacies association involves a transition from subglacial or submarginal tunnel to a subaqueous re entrant environment which then passes to a proglacial glaciolacustrine environment the study also presents evidence of headward extension of subglacial tunnel deposits related to the rapid shifting of a tunnel expansion point during the increasing spring discharge which occupied the old tunnel exit this mode of annual deposition has not been reported previously in esker studies the good preservation of the rhythmic lithofacies associations is suggested as resulting from interlobate depositional conditions associated with rapidly decaying icestreams therefore the depositional model may provide a key to recognizing time transgressive interlobate eskers that form an important geomorphological and sedimentological record of meltwater activity during the last deglaciation of the fennoscandian and laurentide ice sheets the identification of time transgressive interlobate eskers and associated palaeo icestream behaviour is an essential step forward for more accurate models of ice sheet behaviour and palaeoclimatic reconstructions

a fault controlled trough through a carbonate platform the middle cambrian house range embayment during the cambrian part of western north america was a rapidly subsiding passive continental margin a belt of carbonate deposition dominated the central shelf during middle cambrian time the belt was bounded by fine grained terrigenous sediments that accumulated in deeper water to the west and in a coastal setting to the east rapid and possibly repeated movement along a northeast southwest oriented fault produced a conspicuous eastward re entrant into the carbonate belt the house range embayment in nevada and utah during the middle middle cambrian the embayment was an asymmetrical trough that deepened and widened as it extended 400 km westward across the shelf toward the edge of the continent the trough axis lay near its abrupt southern margin which was a normal fault separating an area of continuous shallow water accumulation to the south from deep water deposition the northern flank of the embayment was initially a drowned platform that formed a southward sloping ramp facies within the embayment represent deposition in an anoxic basin on a deep ramp that was locally steepened and on a shallow low energy ramp the facies rapidly prograded basinward across the northern flank and through vertical accretion a broad carbonate platform was re established there in contrast to the northern flank a continuous sequence of deep water lithofacies accumulated throughout the late middle cambrian in the trough axis of the embayment this region seemingly subsided more rapidly than did that of the northern flank and it may have functioned as a sediment bypass zone although a source of periplatformal carbonates was nearby sedimentation rates in the trough were thus inadequate to re establish a shallow water depositional setting

venus global gravity and topography a new gravity field determination that has been produced combines both the pioneer venus orbiter pvo and the magellan doppler radio data comparisons between this estimate a spherical harmonic model of degree and order 21 and previous models show that significant improvements have been made results are displayed as gravity contours overlaying a topographic map we also calculate a new spherical harmonic model of topography based on magellan altimetry with pvo altimetry included where gaps exist in the magellan data this model is also of degree and order 21 so in conjunction with the gravity model bouguer and isostatic anomaly maps can be produced these results are very consistent with previous results but reveal more spatial resolution in the higher latitudes

a spreading drop model for plumes on venus many of the large scale plume related features on venus can be modeled by a buoyant viscous drop or plume head as it rises and spreads laterally below a free fluid surface the drop has arbitrary density and viscosity contrast and begins as a sphere below the surface of a fluid half space the boundary integral method is used to solve for the motion of the plume head and for the topography geoid and stress at the fluid surface as the plume approaches the surface stresses in the fluid above it cause it to spread and become thin below the surface during the spreading the surface swell above evolves through various stages whose morphologies resemble several different plume related features observed on venus when the plume head first approaches the surface a high broad topographic dome develops with a large geoid and radial extensional deformation patterns at later stages the topography subsides and becomes plateau like the geoid to topography ratio gtr decreases and the dominant stress pattern consists of a band of concentric extension surrounded by a band of concentric compression we find that a low viscosity model plume head viscosity that is 0 1 times the mantle viscosity produces maximum topography that is 20 lower and swell features which evolve faster than for an isoviscous plume we compare model results with both the large scale highland swells and smaller scale features such as coronae and novae the dome shaped highlands with large gtrs such as beta atla and western eistla regiones may be the result of early stage plume motion while the flatter highlands such as ovda and thetis regiones which have lower gtrs may be later stage features comparison of model results with gtr data indicates that the highlands result from plume heads with initial diameters of about 1000 km on a smaller scale an evolutionary sequence may begin with novae domes having radial extensional deformation followed by features with radial and concentric deformation such as arachnoids and end with coronae with mostly concentric deformation the model predicts that the highlands evolve on a timescale of order 10 ma and the smaller scale features evolve in a 100 ma timescale

architectural analysis of a volcaniclastic jokulhlaup deposit southern iceland sedimentary evidence for supercritical flow the 1918 eruption of the glacially capped katla volcano southern iceland generated a violent jokulhlaup or glacial outburst flood inundating a large area of m rdalssandur the proglacial outwash plain where it deposited ca 1 km3 of volcaniclastic sediment the character of the 1918 jokulhlaup is contentious having been variously categorized as a turbulent water flow a hyperconcentrated flow or as a debris flow based on localized outcrop analysis in this study outcrop based architectural analyses of the 1918 deposits reveal the presence of lenticular and tabular bedsets associated with deposition from quasi stationary antidunes and down current migrating antidunes and from regular based bedsets associated with transient chute and pool bedforms all of which are associated with turbulent transcritical to supercritical water flow conditions antidune wavelengths range from 24 to 96 m corresponding to flow velocities of 6 to 12 m sec 1 and average flow depths of 5 to 19 m this range of calculated flow velocities is in good agreement with estimates made from eyewitness accounts architectural analysis of the 1918 jokulhlaup deposits has led to an improved estimation of flow parameters and flow hydraulics associated with the 1918 jokulhlaup that could not have been achieved through localized outcrop analysis the observations presented here provide additional sedimentological and architectural criteria for the recognition of deposits associated with transcritical and supercritical water flow conditions the physical scale of sedimentary architectures associated with the migration of bedforms is largely dependent on the magnitude of the formative flow events or processes sedimentary analyses must therefore be undertaken at the appropriate physical scale if reliable interpretations regarding modes of deposition and formative flow hydraulics are to be made

evolution of the subglacial hydrologic system beneath the rapidly decaying cordilleran ice sheet caused by ice dammed lake drainage implications for meltwater induced ice acceleration abstract a positive correlation between ice dammed lake drainage and ice acceleration at antarctic ice sheets ais and land terminating sections of the greenland ice sheet gris has been implicated in enhanced ice sheet decay however the paucity of direct measurements at the ice sheet bed restricts our understanding of subglacial drainage system evolution in response to transient water inputs we present evidence that two meltwater corridors on the former bed of the thin 600 m at last glacial maximum over the interior plateaus of british columbia and rapidly decaying cordilleran ice sheet cis were generated subglacially in response to the drainage of an ice dammed lake and operated as canals tunnel channels geomorphological ground penetrating radar gpr and electrical resistivity tomography ert data reveal a simple event sequence that includes initial propagation of a broad at least 2 5 km wide floodwave inefficient drainage from an ice dammed lake over relatively short 3 24 km zones at the corridor heads that collapsed into efficient canals large up to 0 25 2 5 km wide channels incised down into the sediment bed and up into the ice downglacier canal formation on the southern fraser plateau involved synchronous along the full canal length system development including elements of headward erosion and plunge pool formation our data suggest that ice dammed lake drainage beneath a rapidly decaying thin ice mass that has an efficient antecedent drainage network is not conducive to large scale ice acceleration these data may aid better assessment of the role of ice dammed lake drainage on the dynamics of former as well as contemporary ice sheets

topographic controls on the surging behaviour of sabche glacier nepal 1967 to 2017 abstract using a combination of landsat pleiades and corona satellite imagery from 1967 to 2017 we map changes in the terminus position ice surface velocity and surface elevation of sabche glacier and report the first observations of surging behaviour in central nepal our observations show that sabche glacier surged four times over the last 50 years the three most recent surges occurred at 10 to 11 year cycles which is one of the shortest surge cycles ever recorded detailed analysis of the most recent surge 2012 onwards indicates that the glacier advanced 2 2 km and experienced maximum velocities of 1 6 0 10 m day 1 during this surge there was a surface elevation gain at the terminus of up to 90 6 19 m a 1 with a corresponding surface lowering of between 10 6 19 and 60 6 19 m a 1 3 km up glacier of the terminus this transfer of mass amounted to a volume of 2 7 107 0 1 107 m3a 1 sabche glacier is the first surge type glacier to be observed in the central himalayas but this is consistent with a previous global analysis which indicates that surge type glaciers should exist in the region we hypothesise that the surge is at least partially controlled by subglacial topography whereby a major subglacial overdeepening and constriction 3 km up glacier of the terminus provides resistance to glacier flow from the accumulation area to the ablation area this overdeepening appears to store mass until a threshold is crossed after which the glacier flows out of the subglacial depression and rapidly surges over a bedrock lip and down the valley thus whilst the surges are likely to be facilitated by subglacial processes e g changes in subglacial hydrology and or basal thermal regime the topographic setting of the glacier appears to be modulating both the timing and duration of each surge

a restudy of utahcaris orion euarthropoda from the spence shale middle cambrian utah usa utahcaris orion conway morris robison 1988 from the lower middle cambrian series 3 stage 5 spence shale member situated in utah usa is redescribed based on a restudy of the original material newly recognized features including ventral lateral eyes trunk appendages and a bulbous cephalic shield with subtriangular extension reinforce similarities with sanctacaris uncata from the middle cambrian burgess shale both are assigned to the new family sanctacarididae sanctacaridids represent the oldest chelicerates their ecology and that of their nearest non chelicerate relatives indicate that chelicerata were plesiomorphically predatory

interpreting flash flood palaeoflow parameters from antidunes and gravel lenses an example from montserrat west indies the wavelength of stationary water surface waves and their associated antidune bedforms are related to the mean velocity and depth of formative flow in sand bed flume experiments alexander et al 2001 found that lens structures were preserved during antidune growth and change and the dimension of the lenses were empirically related to antidune wavelength and thus could be used to estimate flow velocity and depth this study is the first to compare observations of formative flow conditions and resulting sedimentary structures in a natural setting testing the alexander et al 2001 relationship at a field scale trains of stationary and upstream migrating water surface waves were prevalent during the flash flood in october 2012 in the belham valley montserrat west indies wave positions and wavelengths were assessed at 900 s intervals through the daylight hours of the event within a monitored reach the wave data indicate flow depths up to 1 3 m and velocity up to 3 6 m s 1 sedimentary structures formed by antidune growth and change were preserved in the event deposit these structures include lenses of clast supported gravel and massive sand with varying internal architecture the lenses and associated low angle strata are comparable to sand bed structures formed from stationary and upstream migrating waves in flume experiments confirming the diagnostic value of these structures using mean lens length in the event deposit underestimated peak flow conditions during the flood and implied that the lenses were preserved during waning flow

lateglacial ice dynamics of the cordilleran ice sheet in northern british columbia and southern yukon territory retreat pattern of the liard lobe reconstructed from the glacial landform record the liard lobe formed a part of the northeastern sector of the cordilleran ice sheet and drained ice from accumulation areas in the selwyn pelly cassiar and skeena mountains this study reconstru

the harricana glaciofluvial complex abitibi region quebec its genesis and implications for meltwater regime and ice sheet dynamics abstract interlobate moraines have been defined by their relationshio to adjacent landforms and sediments with minimal reference to any genesis suggested from their own geomorphology and sedimentology a case in point is the harricana interlobate moraine the harricana complex a relatively continuous linear accumulation of glaciofluvial sediments is investigated in terms of its geomorphology sedimentology stratigraphic context and landform associations for the portion between latitudes 48 n and 50 n the complex is narrower to the north than to the south in the narrower northern part better rounding and poorer preservation of less resistant clasts suggest more vigorous transport in a narrower conduit the inverse in the wider southern part favours less vigorous flows and higher deposition rates these inferences together with unidirectional paleoflows towards the ice margin and the relatively continuous upslope path of the southward widening complex favour synchronous formation of the harricana complex in a continuous closed conduit beneath an ice sheet which thinned southward subaqueous fans and grounding line deposits with fine gravel and sandy in phase wave structures in the south may have been deposited in subglacial cavities adjacent to the complex or later deposited in reentrants in calving ice fronts the basic building blocks of the harricana complex are gravel facies which indicate generally high energy but unsteady transport these facies are arranged into macroforms composite and oblique accretion avalanche bed oaab macroforms are attributed to deposition by unsteady and non uniform flows in conduit enlargements pseudoanticlinal macroforms were deposited in relatively narrow conduit segments of uniform width gravel sand couplets register flow unsteadiness perhaps related to variations in supraglacial metlwater supply the origin of the harricana complex as a mainly synchronous subglacial landform is linked to formation of adjacent streamlined bedforms and bedrock erosional marks by meltwater outburst floods the complex is located where the orientation of these subglacial bedforms indicate strong flow convergence post flood ice sheet collapse along this convergence zone initiated redirection of ice flow resulting in cross cutting striae later subglacial meltwater systems followed new hydraulic gradients toward this trough of thinner ice forming a major conduit and depositing the harricana glaciofluvial complex

variations in effective compensation depth across aphrodite terra venus aphrodite terra is the largest elevated terrain on venus and it serves as a focal point in current discussions of global tectonic style using the topography and gravity data acquired by the pioneer venus orbiter pvo the authors have estimated an effective depth of airy compensation for each of 75 orbital arcs that provide fairly uniform areal coverage of the entire province the most pronounced pattern that emerges is a general increase in compensation depth to the east the most rapid change occurs near 135 degree the average west of there is 70 km while the average to the east is 230 km superimposed on this larger trend are five distinctive regional patterns four well defined peaks and one interval of widely scattered and poorly constrained depths the maxima in compensation depth are well correlated with regional topographic highs while these observations are easily reconciled with the general notion that aphrodite is a region of crustal divergence the great depth of compensation is difficult to accord with the more specific suggestion that aphrodite is a terrestrial type divergent plate margin the alternative suggestion that aphrodite and the other equatorial highlands on venus are surface manifestations of hot upwelling mantle plumes ismore consistent both with the great depths of compensation and with the pattern of regional peaks and intervening troughs the broader scale increase in effective depth of compensation from west to east is more enigmatic but it might reflect an age progression of the plumes less

fundamental issues in the geology and geophysics of venus a number of important and currently unresolved issues in the global geology and geophysics of venus will be addressable with the radar imaging altimetry and gravity measurements now forthcoming from the magellan mission among these are the global volcanic flux and the rate of formation of new crust the global heat flux and its regional variations the relative importance of localized hot spots and linear centers of crustal spreading to crustal formation and tectonics and the planform of mantle convection on venus and the nature of the interactions among interior convective flow near surface deformation and magmatism

the fezouata shale lower ordovician anti atlas morocco a historical review abstract exceptionally preserved fossils yield crucial information about the evolution of life on earth the fezouata biota from the lower ordovician of morocco is a konservat lagerstatte of major importance and it is today considered as an ordovician burgess shale this biota was discovered only some 15 years ago but geological studies of the area date back to the beginning of the 20th century pioneering geological investigations lead to the discovery of ordovician strata in the anti atlas 1929 and ultimately resulted in their formal subdivision into four main stratigraphic units 1942 in the agdz area the presence of fossiliferous tremadocian lower ordovician strata was suspected as early as 1939 but only definitively confirmed in 1955 in the 1960s 1990s jacques destombes provided the first detailed biostratigraphic framework for the lower ordovician of the anti atlas and collected thousands of fossils that were subsequently described in a series of monographs in the early 2000s exceptionally preserved fossils were discovered in the fezouata shale tremadocian late floian in the central anti atlas by mohamed ou said ben moula this biota now known as the fezouata biota is of utmost importance for it demonstrates the extent in the fossil record of non biomineralising animals typical of the cambrian explosion into the ordovician during the great ordovician biodiversification although most components are still in need of formal descriptions a fairly good picture of the composition and organisation of this biota and how it contributes to our understanding of the early evolution of metazoan communities can now be depicted moreover recent studies have substantially clarified the biostratigraphical and palaeoenvironmental context of the fezouata shale and are now being followed up by detailed investigations of the taphonomy geochemistry and micropalaeontology of this unique konservat lagerstatte these efforts will soon greatly benefit from the recovery of fresh unweathered samples from drill cores

subaqueous gravel dunes abstract gravel dunes are rarely reported owing to comparative rarity of occurrence and generic confusion with antidunes and classes of gravel bars a compilation of data from the literature for sediments with a median size greater than 2 mm shows that dunes have been developed in the laboratory in median grain sizes up to 28 6 mm for field conditions there are data for grain sizes up to 60 mm equilibrium and non equilibrium gravel dunes range in length from less than 0 6 m to greater than 100 m heights range from less than 0 1 m to 16 m height and length data for the steepest three dimensional 3 d gravel dunes are consistent with the h l function reported by ashley 1990 for equilibrium dunes developed in sand for a broad range of dune length 3 d gravel dunes are steeper than two dimensional 2 d dunes but for l 100 m and h 8 m there are too few data to draw any conclusion ashley s h l function does not apply to a broad range of 2 d gravel dunes a separate well defined function is proposed to describe the h l relationship for 2 d gravel dunes of maximum steepness there are few data on the hydrodynamic development of gravel dunes as a guide gravel dunes develop for a range of froude numbers up to 0 75 given that the nondimensional mean bed shear stress exceeds circa 0 1 dunes reach their maximum height at 0 25 and have reduced in height when 0 3 froude numbers of 0 84 and above lead to crestal flattening dune diminution and eventual replacement by antidunes or upper stage plane beds however owing to lag effects transitional bedforms have been recorded for froude numbers near the critical value of 1 the development of gravel dune steepness from lower stage plane bed through to upper stage plane bed is well described in only one case by dinehart 1992a fredsoe s 1975 function for equilibrium dune steepness applies reasonably well to dinehart s steepest dunes allen s 1984 bedform existence field based on and a characteristic grain size of the bed sediment has been extended from 5 mm to include bedstocks as coarse as 33 mm

a decade of sedimentation in ice contact proglacial lakes bering glacier ak abstract bathymetric surveys during the 1991 2000 decade in two ice contact proglacial lakes on the eastern sector of bering piedmont lobe captured the buildup effects of the 1993 1995 surge following ice front advance of 1 0 1 5 km into tsivat and tsiu lakes the basins were significantly altered by surge related sedimentation including the impact of a subglacial outburst into tsivat lake the subsequent changes in basin shape size and morphology were monitored by six bathymetric surveys measured changes in water depth serve as a proxy for determining increments of sediment accumulation upwelling ice front vents fed by subglacial tunnels transported suspended fine sediment directly into the lake system the rate of suspension settling within both lakes varied from 0 6 to 1 2 m year 1 prior to the surge suspended load during surge years increased sixfold from 1 7 to 13 9 g l 1 accompanied by increased sediment accumulation of 2 2 3 1 m year 1 vent related aggradation and subsequent filling of tsivat lake caused sediment bypassing to tsiu lake where encroachment by delta growth contributed to a postsurge rate of bottomset accumulation of 3 0 m year 1 the total sediment influx from subglacial sources is represented by the sum of bathymetrically determined accumulation plus an estimated volume of sediment that remained suspended thus passing through the lake system total sediment flux along the eastern bering piedmont lobe from 1991 to 2000 is approximately 227 million cubic meters

geology and structure of beta regio venus results from arecibo radar imaging arecibo radar images of a portion of the equatorial region of venus provide the first high resolution 1 5 to 2 0 km synoptic coverage of beta regio within this area tessera a complex deformed terrain is identified as a major geologic unit with the largest region corresponding to a plateau on the east flank of the highland three models are proposed to explain the origin and evolution of beta regio and are identified as mantle plume passive crust mantle plume active crust and mantle plume crustal spreading the mantle plume passive crust model appears to be the most consistent with the geology in this region and suggests that a plume disrupts a preexisting region of tessera

controls on the location morphology and evolution of complex esker systems at decadal timescales breiðamerkurjokull southeast iceland this paper uses detailed mapping of eskers to address three questions which are important for reconstructing meltwater behaviour beneath contemporary and ancient ice masses what controls the morphology of simple and complex esker systems how do esker systems evolve through time and are esker patterns compatible with the groundwater controlled hydraulic spacing of esker tunnels esker crestlines and widths are mapped on the brei amerkurjokull foreland for eight time slices between 1945 and 2007 from high resolution 50 cm aerial photography permitting their long term morphological evolution to be analysed in a high level of detail we find that complex eskers develop where meltwater and sediment is abundant such that sediment clogs channels forming distributary eskers isolated eskers are simpler and smaller and reflect less abundant meltwater and sediment which is unable to clog channels eskers may take several decades to emerge from outwash deposits containing buried ice and can increase or decrease in size when ice surrounding and underlying them melts out it has been suggested that groundwater channel coupling dictates the spacing between eskers at brei amerkurjokull our results do not dispute this but suggest that the routing of sediment and meltwater through medial moraines is an additional important control on esker location and spacing these results may be used to better understand the processes surrounding esker formation in a variety of geographical settings enabling a more detailed understanding of the operation of meltwater drainage systems in sub marginal zones beneath glaciers and ice sheets this article is protected by copyright all rights reserved

ice sheet dynamics and subglacial meltwater regime inferred from form and sedimentology of glaciofluvial systems victoria island district of franklin northwest territories on victoria island tunnel channels eskers and associated fans and extended deposits together constitute channelized glaciofluvial systems flutes and drumlinoid ridges interpreted as residuals left by erosive catastrophic subglacial meltwater sheet flows lie adjacent to these systems one tunnel channel is described in detail it exhibits deep scours a discontinuous thalweg sculpted margins and flutes on the downflow side of one wall features indicative of complex flow and possibly several flow events the tunnel channel is interpreted as the product of erosion by catastrophic subglacial meltwater flow in a combined ice substrate r n channel esker sediments and morphology are used to infer details of the depositional environment and meltwater regime a continuous esker with fans and extended deposits records seasonally controlled discharge events through an r channel these features may also suggest a grounding line environment thin ice and localized ice floatation events less well conn

thermal conditions at the bed of the laurentide ice sheet in maine during deglaciation implications for esker formation the university of maine ice sheet model was used to study basal conditions during retreat of the laurentide ice sheet in maine within 150 km of the margin basal melt rates average 5m m a 1 during retreat they decline over the next 100 km so areas of frozen bed develop in northern maine during retreat by integrating the melt rate over the drainage area typically subtended by an esker we obtained a discharge at the margin of 1 2 m 3 s 1 while such a discharge could have moved the material in the katahdin esker it was likely too low to build the esker in the time available additional water from the glacier surface was required temperature gradients in the basal ice increase rapidly with distance from the margin by conducting upward into the ice all of the additional viscous heat produced by any perturbation that increases the depth of flow in a flat conduit in a distributed drainage system these gradients inhibit the formation of sharply arched conduits in which an esker can form this may explain why eskers commonly seem to form near the margin and are typically segmented with later segments overlapping onto earlier ones

icelandic jokulhlaup impacts implications for ice sheet hydrology sediment transfer and geomorphology abstract glaciers and ice sheets erode entrain and deposit massive quantities of debris fluxes of subglacial meltwater exert a fundamental control on ice dynamics and sediment transport budgets within many glacial systems outburst floods jokulhlaups constitute high magnitude high frequency fluxes of meltwater relative to normal ablation controlled discharge this paper presents a synthesis of research on recent icelandic jokulhlaups and their geomorphological and sedimentary impact we identify jokulhlaup impacts within subglacial englacial and proglacial settings and discuss their wider significance for ice sheet hydrology sediment transfer and geomorphology because jokulhlaups erode deposit and re work sediment simultaneously they usually cause significant glaciological and sedimentological impacts jokulhlaups that propagate as subglacial flood waves often produce widespread hydromechanical disruption at the glacier base recent icelandic jokulhlaups have been recognised as highly efficient agents of reworking subglacial sediment and glacial sediment entrainment models of jokulhlaup impact therefore need to encompass the sub and englacial environment in addition to the proglacial zone where research has traditionally been focussed most jokulhlaups transport sediment to proglacial sandar and often directly to oceans where preservation potential of the impact is greater proglacial jokulhlaup deposits form distinctive sedimentary assemblages coupled with suites of high energy erosional landforms this study of modern jokulhlaup processes and sedimentary products may be useful for the interpretation of meltwater processes associated with quaternary ice sheets

esker characteristics in terms of glacier physics katahdin esker system maine the characteristics of large subglacially formed eskers such as the katahdin system are closely related to two special peculiarities of water filled tunnels along the beds of ice sheets 1 the water pressure approximates the weight of the overlying ice and 2 in tunnels that descend and those that ascend less steeply than 1 7 times the ice surface gradient the walls melt producing a sharply arched tunnel cross section whereas in those that ascend more steeply they freeze producing a wide low one the first peculiarity primarily governs the paths of these eskers it causes the tunnels to follow the paths ordinary rivers would follow were the land tipped downglacier 11 times the local ice surface gradient the paths therefore trend in the general direction of the former ice flow but tend to deviate so as to follow valleys and to cross divides at the lowest passes as observed ice surface gradients calculated from path deviations at two localities on the katahdin esker system indicate relatively thin sluggish ice the surface of which lay 200 m below the summit of mount katahdin in agreement with independent geologic evidence the second peculiarity primarily governs the form composition and structure of these eskers strong melting causes a large inflow of basal ice and entrained debris to the tunnel and produces sharp crested eskers of poorly sorted poorly bedded sand gravel and boulders with lithologies like the adjacent till whereas weaker melting produces multiple crested ones of similar composition freezing precludes inflow and produces broad crested eskers of fairly well sorted well bedded more water worn coarse sand with few large clasts ice surface gradients calculated from transitions from the multiple crested type to small areas of broad crested type on the katahdin system agree closely with those computed from the paths at nearby localities an anomalously low gradient calculated from a transition to an area of broad crested type approximately twice as wide and long as the probable ice depth apparently confirms that as expected the basal ice was supported by water pressure over most if not all of the width of the esker

macroforms large bedforms and rhythmic sedimentary sequences in subglacial eskers south central ontario implications for esker genesis and meltwater regime abstract eskers of south central ontario were deposited in closed subglacial conduits which were continuous main conduits this interpretation is supported by the intimate association of eskers with an anastomosing network of tunnel channels relatively continuous esker ridges minimal post formational disturbance of esker sediments intercalation of till and stratified sand and gravel diapiric folding at an esker core low variability in palaeocurrent direction and upslope flow paths down esker trends in clast lithology roundness and sphericity indicate continuous conduits with sediment supply by subglacial deformation of adjacent material into the conduits melt out of sediment from the conduit walls and fluvial resedimentation esker ridge morphology is attributed to synchronous erosion transportation and deposition along the main conduits such that ridge discontinuities are primarily explained as zones of non deposition during esker formation composite pseudoanticlinal and oblique accretion avalanche bed macroforms are identified gravel facies within these macroforms were deposited from fluidal flows or hyperconcentrated dispersions progradation of macroforms or migration of constituent large bedforms through the main conduits may have temporarily blocked constricted portions of those conduits and acted as possible internal autogenic controls on sediment availability and flow resistance the location of macroforms within conduits was primarily controlled by conduit geometry and sediment availability and later by feedback between macroform and conduit geometries sand and gravel units alternate rhythmically in vertical section rhythmicity is interpreted as a response to episodic flood flows controlled by seasonal changes in water supply but not necessarily representing annual events fans beads anabranched reaches and extended hummocky deposits are intimately associated with the main esker ridges they are interpreted in terms of subglacial cavities and localized flotation zones connected to the main conduits during flood events in phase wave structures are the products of hydraulic jumps in hyperconcentrated flood flows at flow expansions into swells within the main conduits into cavities connected to the main conduits or at a grounding line esker ridges record only the most powerful flood events whereas fans and beads record flow events in finer detail being primarily depositional reservoirs laterally fining deposits record the waning stages of conduit operation

glacial and glaciofluvial deposits in the interlobate areas of the scandinavian ice sheet abstract rapid flow within ice streams took place in a broad marginal zone of the scandinavian ice sheet during deglaciation ice streams and interlobate zones have been characterised by analysis of glacial flow patterns and examination of sedimentary features of interlobate glacial and glaciofluvial deposits reconstructed glaciodynamic systems are compared with ice streams of modem ice sheets wide interstream areas of inactive ice were left between some adjacent ice streams where they separated during the retreat of the margin these areas are characterised by morainic hummocks composed of both till and glaciofluvial sediment and sections show indications of subglacial deformation observations from fennoscandia suggest that these interstream areas were sites of net accumulation of glacial and glaciofluvial sediment during deglaciation glaciofluvial sediment was deposited in subglacial tunnels or cavities simultaneously with till deformation and deposition resulting from subglacial melting substantial glaciofluvial complexes were formed in narrow interlobate joints where adjacent ice lobes coalesced a case study made in southern finland shows converging flow patterns of two ice lobes and intervening glaciofluvial deposits concentration of meltwater in interlobate zones was a consequence of ice sheet configuration thinner ice in interlobate zones caused convergence of supraglacial and subglacial drainage the crevassed and strain softened ice may also have allowed surface meltwater to penetrate down to the ice bed interface close to the ice margin steep ice velocity gradients in interlobate zones would have increased subglacial melting rates and this environment was favourable for establishment of a subglacial drainage network and deposition of glaciofluvial sediment

controls on the sedimentary architecture of a single event englacial esker skeiðararjokull iceland eskers have been used to infer the dynamics and palaeo hydrology of large ice sheets yet there are few suitable contemporary analogues for esker sedimentation during quaternary glaciations consequently models of esker sedimentation have been derived from morpho sedimentary analyses of quaternary eskers but these lack rigorous constraints on their depositional timescales we identify the controls on the large scale sedimentary architecture of a large 700 m long 10 inclined reflections interpreted as backset accretion associated with large scale bedform development 4 down flow dipping high angle 15 inclined reflections interpreted as foreset accretion associated with macroform progradation 5 quasi hyperbolic reflections interpreted as zones of boulder clustering and 6 zones of enhanced noise and em signal attenuation bounded on the upper surface by a continuous reflection sub parallel to the ground surface interpreted as buried ice associated with the conduit channel base these data suggest that deposition took place within a non uniform englacial conduit which has resulted in major up flow to down flow variations in sedimentary architecture the complexities observed are a direct consequence of variations in sediment supply and the within event evolution of conduit geometry this study provides the first detailed insight into the large scale sedimentary architecture of a single event jokulhlaup proto esker

towards a morphogenetic classification of eskers implications for modelling ice sheet hydrology abstract validations of paleo ice sheet hydrological models have used esker spacing as a proxy for ice tunnel density changes in crest type cross sectional shape along esker ridges have typically been attributed to the effect of changing subglacial topography on hydro and ice dynamics and hence subglacial ice tunnel shape these claims assume that all eskers formed in subglacial ice tunnels and that all major subglacial ice tunnels produced a remnant esker we identify differences in geomorphic context sinuosity cross sectional shape and sedimentary architecture by analysing eskers formed at or near the margins of the last cordilleran ice sheet on british columbia s southern fraser plateau and propose a morphogenetic esker classification three morphogenetic types and 2 subtypes of eskers are classified based on differences in geomorphic context ridge length sinuosity cross sectional shape and sedimentary architecture using geophysical techniques and sedimentary exposures they largely record seasonal meltwater flows and glacial lake outburst floods glofs through sub en and supraglacial meltwater channels and ice walled canyons general principles extracted from these interpretations are 1 esker ridge crest type and sinuosity strongly reflect meltwater channel type eskers formed in subglacial conduits are likely to be round crested with low sinuosity except where controlled by ice structure or modified by surging and contain faults associated with flank collapse eskers formed near or at the ice surface are more likely to be sharp crested highly sinuous and contain numerous faults both under ridge crest lines and in areas of flank collapse 2 esker ridges containing numerous flat crested reaches formed directly on the land surface in ice walled canyons unroofed ice tunnels or in ice tunnels at atmospheric pressure and therefore likely record thin or dead ice 3 eskers containing macroforms exhibiting headward and downflow growth likely record flood scale flows possibly glofs where a lake can be inferred these conclusions suggest that esker crest type sinuosity and geomorphic context when understood along with sedimentary architecture largely reflect formational position with respect to the ice surface reconstructions of ice sheet hydrology need to account for variation in esker morphology because basing hydrodynamic inferences on the presence or absence of an esker alone ignores encoded differences in water source supply flow magnitude and frequency and conduit position

morphometry of a glacier linked esker in nw tempe terra mars and implications for sediment discharge dynamics of subglacial drainage abstract we present a systematic metre scale characterisation of the 3d morphometry of an esker on mars and the first attempt to reconstruct the multi stage dynamics of esker formation on mars eskers are sinuous ridges comprising sediment deposited by meltwater draining through ice confined tunnels within or beneath glaciers detailed morphometric insights into eskers on mars are important for i informing morphometric tests of whether sinuous ridges elsewhere on mars are eskers and ii informing modelling experiments which aim to reconstruct the glaciological and environmental controls on esker formation on mars we use a digital elevation model generated from high resolution imaging science experiment hirise images to characterise the height and width of an extremely rare esker associated with a late amazonian aged viscous flow feature debris covered glacier in nw tempe terra mars our measurements suggest that the nw tempe terra esker is a stacked formation comprising an underlying lower member ridge that is superposed by a narrower upper member ridge we used a novel morphometric approach to test whether the apparent stacking records two distinct esker deposition regimes either within the same drainage episode or within temporally separated drainage episodes this approach posits that esker crest morphology is controlled by primary esker formation processes and by extension that portions of eskers with similar crest morphologies should have similar morphometric relationships we predicted the morphometric relationships described by the constituent upper and lower member ridges based on reference relationships observed for morphologically similar portions of the esker where no evidence of stacking was observed our observations corresponded well with the predicted relationships supporting our stacked esker hypothesis we propose conceptual models which invoke spatial and temporal variations in sediment supply and meltwater discharge to explain the stacked morphology these models are informed by morpho sedimentary relationships observed along eskers on earth

large scale internal structure of the sanbongi fan towada volcano japan putting the theory to the test using gpr on volcaniclastic deposits abstract the towada caldera volcano is located between the prefectures of aomori and akita northeast honshu island japan the caldera today filled by a lake has produced 15 000 years ago a complex eruption emplacing an ignimbrite topped by the lake outburst flood deposit through which the present oirase river erodes this flood deposit has shaped the geomorphologic feature named sanbongi fan on which towada city extends this flood event hypothesis is mainly based on sedimentological and geomorphological evidences of floods mainly from outcrops retrieved from the sanbongi fan area because of the lack of extended outcrops typical of the japanese environment the present paper has therefore put the theory to the test using gpr ground penetrating radar radargram extending along a 640 m length the gpr used for the survey was a pulse ekko pro with 50 mhz antennas and the software reflex was used to process the data the radargrams have displayed a sole unit which the gpr could not penetrate it can be interpreted as the ignimbrite on top of this deposit a series of subhorizontal layers with the alternation between a backset and a foreset extends between 5 m and 3 m depth above 3 m the units are regular and subhorizontal the deposit is also characterized by the extensive presence of boulders which are located along three bands 1 on top of the ignimbrite 2 in the units deposited by the outburst flood between 3 and 5 m depths 3 and in the units close to the surface although part of these punctual elements are most certainly anthropogenic compared with the outcrops the present research confirms that the material located above the ignimbrite material have been deposited by the outburst flood creating large sheet patterns which have transported boulders these sheets display backset and foreset patterns depending on the position of the deposit indicating flow pulsation or surges under a high energy flow condition

drainage beneath ice sheets groundwater channel coupling and the origin of esker systems from former ice sheets abstract the nature of the drainage system beneath ice sheets is crucial to their dynamic behaviour but remains problematic an experimentally based theory of coupling between groundwater and major channel systems is applied to the esker systems in the area occupied the last ice sheet in europe which we regard as a fossil imprint of major longitudinal drainage channels we conclude that the large scale distribution and spacing of major eskers is consistent with the theory of groundwater control in which esker spacing is partly controlled by the transmissivity of the bed it is concluded that esker patterns reflect the large scale organisation of the subglacial drainage pattern in which channel development is coupled to groundwater flow and to the ice sheet s dynamic regime the theory is then used to deduce basal meltwater recharge rates and their spatial variability from esker spacing in an area in which the ice sheet was actively streaming during its final retreat patterns of palaeo groundwater flow and head distribution and the seasonally varying magnitude of discharge from stream tunnels at the retreating ice sheet margin major channel esker systems appear to have been stable at least over several hundred of years during the retreat of the ice sheet although major dynamic events are demonstrably associated with major shifts in the hydraulic regime modelling suggests that glaciation can stimulate deep groundwater circulation cells that are spatially linked to channel locations with groundwater flow predominantly transverse to ice flow that the circulation pattern has the potential to create large scale anomalies in groundwater chemistry and that the spacing of channels will change through the glacial cycle influencing water pressures in stream tunnels subglacial hydraulic gradients and effective pressure if the latter is reduced sufficiently it could trigger enhanced bed deformation thus coupling drainage to ice sheet movement it suggests the possibility of distinctive phases of sediment deformation and drumlin mobilisation during a glacial cycle

controls on englacial sediment deposition during the november 1996 jokulhlaup skeiđararjokull iceland this paper presents sedimentary evidence for rapid englacial debris entrainment during jokulhlaups previous studies of jokulhlaup sedimentology have focused predominantly on proglacial impact rather than depositional processes within glaciers however observations of supraglacial floodwater outbursts suggest that englacial sediment emplacement is possible during jokulhlaups the november 1996 jokulhlaup from skei ararjokull iceland presented one of the first opportunities to examine englacial flood deposits in relation to former supraglacial outlets using observations from skei ararjokull this paper identifies and explains controls on the deposition of englacial flood sediments and presents a qualitative model for englacial jokulhlaup deposition englacial jokulhlaup deposits were contained within complex networks of upglacier dipping fractures simultaneous englacial deposition of fines and boulder sized sediment demonstrates that englacial fracture discharge had a high transport capacity fracture geometry was an important control on the architecture of englacial jokulhlaup deposits the occurrence of pervasively frozen flood deposits within skei ararjokull is attributed to freeze on by glaciohydraulic supercooling floodwater flowing subglacially or through upglacier dipping fractures would have supercooled as it was raised to the surface faster than its pressure melting point could increase as glaciostatic pressure decreased evidence for floodwater contact with the glacier bed is supported by the ubiquitous occurrence of sheared diamict rip ups and intra clasts of basal ice within jokulhlaup fractures deposited englacially some 200 350 m above the bed of skei ararjokull evidence for fluidal supercooled sediment accretion is apparent within stratified sands deposited englacially at exceptionally high angles of rest in the absence of post depositional disturbance such primary sediment structures cannot be explained unless sediment is progressively accreted to opposing fracture walls ice retreat from areas of former supraglacial outbursts revealed distinct ridges characterized by localized upwellings of sediment rich floodwater these deposits are an important addition to current models of englacial sedimentation and demonstrate the potential for post jokulhlaup landform development copyright 2001 john wiley sons ltd keywords jokulhlaup englacial debris entrainment glaciofluvial sediment hydrofracturing iceland

deposition in an esker bead and fan complex lanark ontario canada abstract a glaciofluvial landform complex comprising a single esker ridge proximally multiple ridges and beads in the middle section and a complex of ridges and fans distally is mapped and described we present detailed successions in the associated sediment showing sedimentary structure grain size and palaeocurrent estimates sediment in the esker is mainly coarse grained and varies from boulders with a fine sand matrix deposited at maximum discharges to diamicton interpreted as a product of ice pressing under low discharges early stage deposition in the esker at high flow was from suspension and bed load intermediate stage deposition was mainly from bed load and late stage deposition was from hyperconcentrated suspension the esker rests in a bedrock channel which functioned as a nye channel prior to esker formation the channel surface carries a variety of fluvial erosional marks short distributary ridges of the ridges and beads complex contain mainly cobbles and boulders but beads are mainly in sand with rapid down flow fining and change in sedimentary structure faulting and overfolding of bead sediment suggest a subglacial origin deposition of the esker fan complex was highly variable in space and time down fan sedimentary changes with transitions from gravel to cross laminated fine sand in a few tens of metres indicate rapid loss in competence fan location lateral to the esker indicates a broadening of the zone of subglacial meltwater with time deposition from suspension and steep sided scours are interpreted to result from deposition and erosion by hyperconcentrated flows a model of underflows with submerged hydraulic jumps spilling laterally from the esker tunnel into subglacial cavities explains the morphological and sedimentary characteristics of the fans detailed analysis of small scale fining upwards sequences in the fans and beads indicates deposition by a large number of flow events these events are explained by glacier hydraulics whereby periodic separation of the glacier from its bed caused water with a high suspended sediment concentration to spill out from the esker tunnel into adjacent subglacial cavities evolution of the subglacial drainage system indicates a complexity of subglacial sedimentation near to a grounding line which is not readily apparent from observations at modern glaciers terminating in relatively deep water

transient subglacial hydrology of a thin ice sheet insights from the chasm esker british columbia canada abstract glacier drainage systems are never steady state instead they fluctuate over time and space in response to variation in water input in order to improve numerical models of glacier hydrology it is important to fully understand the processes controlling subglacial drainage system evolution but contemporary ice sheet beds are typically inaccessible thus esker distribution morphology and sedimentology have been used to infer the dynamics and hydrology of former ice sheets however debate remains as to the processes responsible for esker formation and most theoretical investigations have assumed that they formed due to processes that operated beneath thick ice despite many field investigations to the contrary we investigate esker formation during the final stages of the thin inactive and rapidly decaying cordilleran ice sheet cis in interior british columbia canada a combination of geomorphological ground penetrating radar gpr and electrical resistivity tomography ert data suggest esker ridge sedimentary architecture is consistent with synchronous formation during a glacial lake outburst flood glof these data reveal esker ridge deposition most likely took place within an ice tunnel that evacuated late waning stage flow following erosion and partial fill of a broader meltwater corridor esker ridge sedimentary architecture reveals this depositional environment was dynamic reflecting complex interaction between ice thickness ice structure ice tunnel geometry flow conditions and sediment supply under these thin inactive ice conditions ice tunnel location was initially governed by structural weaknesses in the ice and or equipotential gradient because creep closure rates were low the ice tunnel evolved through feedbacks between conduit growth via frictional melting mechanical ice excavation and conduit closure due to sediment infilling rather than ice creep this resulted in a non uniform ice tunnel that enlarged in an upglacier direction and locally unroofed to form open channels during its final stages these data suggest that eskers may record unsteady rather than steady flows in a drainage network and so the assumptions often made when using eskers to inform hydrological components of ice sheet models may not be applicable to all ice sheet settings

converging ice streams an unreasonable hypothesis for deposition of the oak ridges moraine southern ontario the hypothesis that oak ridges moraine orm formed between converging ice streams requires critical testing available data e g digital elevation models maps seismic profiles continuous core

structural controls on englacial esker sedimentation skeiðararjokull iceland we have used ground penetrating radar gpr to observe englacial structural control upon the development of an esker formed during a high magnitude outburst flood jokulhlaup the surge type skei ararjokull an outlet glacier of the vatnajokull ice cap iceland is a frequent source of jokulhlaups the rising stage waters of the november 1996 jokulhlaup travelled through a dense network of interconnected fractures that perforated the margin of the glacier subsequent discharge focused upon a small number of conduit outlets recent ice marginal retreat has exposed a large englacial esker associated with one of these outlets we investigated structural controls on esker genesis in april 2006 by collecting 2 5 km of gpr profiles on the glacier surface up glacier of where the esker ridge has been exposed by meltout in lines closest to the exposed esker ridge we interpret areas of englacial horizons up to 30 m wide and 10 15 m high as an up glacier continuation of the esker sediments high amplitude dipping horizons define the base of esker materials across many lines similar dipping surfaces deeper in the profiles suggest that 1 the dipping surfaces beneath the esker are englacial tephera bands 2 floodwaters were initially discharged along structurally controlled englacial surfaces tephra bands 3 the rapid increase in discharge resulted in hydrofracturing 4 establishment of preferential flow paths resulted in conduit development along the tephra bands due to localized excavation of surrounding glacier ice and 5 sedimentation took place within the new accommodation space to form the englacial structure melting out to produce the esker

structural control of englacial drainage systems in himalayan debris covered glaciers englacial cave systems were mapped using speleological techniques in three debris covered glaciers in the khumbu himal nepal detailed three dimensional mapping of the cave systems and observations of relationships with structures in the surrounding ice show conduits formed by a mechanism directly analogous to speleogenesis in limestone karst the highest oldest parts of all passages developed along debris filled crevasse traces with hydraulic conductivity in the range 10 4 to 10 5 ms 1 conduits form when these hydraulically efficient pathways bridge between areas with different hydraulic potential they then evolve by grading through head ward migration of nick points and vertical incision to local base level often the surface of supraglacial lakes most supraglacial lakes on himalayan glaciers are perched above the elevation of the terminal stream and exist for a few years before draining through englacial conduits as a result near surface drainage evolution is frequently interrupted by base level fall and conduits may record multiple phases of incision conduits commonly migrate laterally during incision undermining higher levels of the ice and encouraging collapse voids can be created by fluvial processes and collapse of crevassed ice the oft noted resemblance of the surface morphology of debris covered glaciers to karst landscapes thus extends to the subsurface and karst hydrology provides a framework for understanding englacial drainage

tsivat basin conduit system persists through two surges bering piedmont glacier alaska the 1993 1995 surge of bering glacier alaska occurred in two distinct phases phase 1 of the surge began on the eastern sector in july 1993 and ended in july 1994 after a powerful outburst of subglacial meltwater into tsivat lake basin on the north side of weeping peat island within days jokulhlaup discharge built a 1 5 km 2 delta of ice blocks 25 30 m buried in outwash by late october 1994 discharge temporarily shifted to a vent on weeping peat island where a second smaller outburst dissected the island and built two new sandar during phase 2 which began in spring 1995 and ended within five months continuous discharge issued from several vents along the ice front on weeping peat island before returning to the tsivat basin surge related changes include a five to six fold increase in meltwater turbidity the redirection of supercooled water in two ice contact lakes and an increase in the rate of glaciolacustrine sedimentation u s geological survey aerial photos by austin post show large ice blocks in braided channels indicating excessive subglacial discharge in a similar position adjacent to weeping peat island during the 1966 1967 surge during the subsequent three decades of retreat the location of ice marginal subglacial discharge vents remained aligned on a linear trend that describes the position of a persistent subglacial conduit system the presence of a major conduit system possibly stabilized by subglacial bedrock topography is suggested by 1 high level subglacial meltwater venting along the northern side of weeping peat island during the 1966 1967 surge 2 persistent low level discharge between surges and 3 the recurrence of localizing meltwater outbursts associated with both phases of the 1993 1995 surge

irish ice sheet dynamics during deglaciation of the central irish midlands evidence of ice streaming and surging from airborne lidar abstract high resolution digital terrain models dtms generated from airborne lidar data and supplemented by field evidence are used to map glacial landform assemblages dating from the last glaciation midlandian glaciation oi stages 2 3 in the central irish midlands the dtms reveal previously unrecognised low amplitude landforms including crevasse squeeze ridges and mega scale glacial lineations overprinted by conduit fills leading to ice marginal subaqueous deposits we interpret this landform assemblage as evidence for surging behaviour during ice recession the data indicate that two separate phases of accelerated ice flow were followed by ice sheet stagnation during overall deglaciation the second surge event was followed by a subglacial outburst flood forming an intricate esker and crevasse fill network the data provide the first clear evidence that ice flow direction was eastward along the eastern watershed of the shannon river basin at odds with previous models and raise the possibility that an ice stream existed in this area our work demonstrates the potential for airborne lidar surveys to produce detailed paleoglaciological reconstructions and to enhance our understanding of complex palaeo ice sheet dynamics

the morphology and evolution of coronae on venus coronae on venus are large circular to ovoidal surface features that have distinctive tectonic volcanic and topographic expressions they range in diameter from less than 200 km to at least 1000 km new data from the magellan spacecraft have shown coronae to be among the dominant tectonic forms on the planet and have revealed their morphology in unprecedented detail typical coronae are dominated by concentric tectonic features and have a raised rim a central region higher than the surounding plains but in many instances lower than the rim and commonly a peripheral depression or moat some coronae also show significant amounts of radial tectonic structure and in most cases this predates the concentric features in addition there are other features on venus recognized for the first time in magellan data that consist of domical rises with intense radial tectonic patterns and little or no concentric structure all of these features commonly are associated with moderate to large quantities of volcanism in fact some radially fractured domes have undergone so much volcanism that volcanic construction appears to have played a significant role in establishing their topography we explore a model of corona formation that links these forms into a genetic sequence the model begins with the ascent of a mantle diapir upward mantle flow driven by its ascent forces the lithosphere above the diapir upward producing a gentle dome with a radiating pattern of extensional fractures as the diapir impinges on the underside of the lithosphere it flattens and spreads transforming the uplift to a more flat topped shape in this flattened near surface configuration the diapir can cool rapidly with the resultant loss of buoyancy the raised plateau can relax to form a central sag a raised rim and a depressed moat concentric tectonic features develop primarily during the latter stages of corona formation and hence are best preserved on mature coronae volcanism takes place during all phases of the uplift and may diminish as the relaxation occurs our analyses to date suggest that this scenario is broadly consistent with many of the coronae on venus however there is enormous diversity in corona morphology and features are present that require substantial deviations from this simple model in particular some circular depressions appear corona like in synthetic aperature radar images but may in fact be large calderas some of the variations observed in corona morphology may ultimately be interpretable in terms of variations in the behavior of individual diapirs and in the local properties of the venusian lithosphere

gravel antidunes in the tropical burdekin river queensland australia the geological record is punctuated by the deposits of extreme event phenomena the identification and interpretation of which are hindered by a lack of data on contemporary examples it is impossible to directly observe sedimentary bedforms and grain fabrics forming under natural particle transporting high velocity currents and therefore their characteristics are poorly documented the deposits of such flows are exposed however in the dry bed of the burdekin river queensland australia following tropical cyclone induced floods long wave length up to 19 m gravel antidunes develop during short days high discharge flows in the upper burdekin river maximum recorded discharge near the study reach over 25 600 m s in february 1927 flood water levels fall quickly metres in a day and flow is diverted away from raised areas of the river bed into subchannels exposing many of the high stage bedforms with little reworking by falling stage currents gravel bedforms were observed on the dry river bed after the moderate flows of february 1994 max 7700 m s and january 1996 max 3200 m s the bedforms had wave lengths in the range 8 19 m amplitudes of up to 1 m with steeper stoss than lee faces and crest lines generally transverse to local peak discharge flow direction the gravel fabric and size sorting change systematically up the stoss and down the lee faces the antidune deposits form erosive based lenses of sandy gravel with low angle downstream dipping lamination and generally steep upstream dipping a b planes the internal form and fabric of the antidune gravel lenses are distinctly different from those of dune lee gravel lenses the erosive based lenses of low angle cross bedded gravel with steep upstream dipping a b planes are relatively easy to recognize and may be diagnostic of downstream migrating antidunes the antidune gravel lenses are associated with thick to 1 m high angle cross bed sets ancient antidune gravel lenses may be diagnostic of episodic high discharge conditions and particularly when they are associated with high angle cross bedded gravelly sand they may be useful for palaeoenvironmental interpretation

ground penetrating radar for high resolution mapping of soil and rock stratigraphy ground penetrating radar is a technique which offers a new way of viewing shallow soil and rock conditions the need to better understanding overburden conditions for activities such as geochemical sampling geotechnical investigations and placer exploration as well as the factors controlling groundwater flow has generated an increasing demand for techniques which can image the subsurface with higher resolution than previously possible the areas of application for ground penetrating radar are diverse the method has been used successfully to map ice thickness water depth in lakes bedrock depth soil stratigraphy and water table depth it is also used to delineate rock fabric detect voids and identify karst features the effective application of the radar for the high resolution definition of soil stratigraphy and fractures in bedrock is highlighted the basic principles and practices involved in acquiring high quality radar data in the field are illustrated by selected case histories one example demonstrates how radar has been used to map the bedrock and delineate soil horizons to a depth of more than 20 m two case histories show how radar has been used to map fractures and changes of rock type to 40 m range from inside a mine another case history demonstrates how radar has also been used to detect and map the extent of groundwater contamination the corroboration of the radar results by borehole investigations demonstrates the power and utility of the high resolution radar method as an aid for interpolation and extrapolation of the information obtained with conventional coring programmes with the advent of new instrumentation and field procedures the routine application of the radar method is becoming economically viable and the method will see expanded use in the future

unstable behavior of the laurentide ice sheet over deforming sediment and its implications for climate change geologic records of fluctuations of the laurentide ice sheet margin following the most recent glacial maximum ca 20 000 14c yr b p identify fundamental differences in ice sheet behavior depending on subglacial bed conditions rapid and irregular icemargin fluctuations occurred only over areas of deforming sediment indicating nonclimatic forcing controlled by the inherent instability of coupled ice sheet deforming sediment dynamics in contrast largely uninterrupted ice margin retreat with no evidence of significant readvance occurred over rigid bed areas indicating stable behavior unstable ice sheet behavior was most pronounced from 15 000 until 10 000 14c yr b p by which time most of the ice margin had retreated onto a rigid bed unstable ice sheet behavior would have been an integral component in controlling variable fluxes of icebergs and meltwater as well as meltwater routing to the north atlantic thus affecting thermohaline circulation the abrupt climate oscillations in the north atlantic region that ended at 10 000 14c yr b p may thus have their origin in the inherently unstable behavior of the laurentide ice sheet overriding deforming sediment

sequence stratigraphy cyclic facies and lagerstatten in the middle cambrian wheeler and marjum formations great basin utah abstract recurrent taphofacies including conservation lagerstatten are identified within a spectrum of facies in fourth order sequences in the middle cambrian wheeler and marjum formations of the drum mountains and house range west central utah these sequences are 3 20 m thick and commence with sharply based compact oncolitic oolitic or pelletal pack and grainstones with sharply defined corroded and mineralized upper contacts that record drowning discontinuities and early transgressive systems tracts tsts overlying intervals of calcareous shale and thin bedded wacke to packstones with abundant disarticulated polymerid and agnostoid trilobites represent late tsts these are commonly followed by lavender gray mudstones rich in sponge spicules and comminuted fossil debris that reflect condensed maximum flooding zones the overlying early highstand hst intervals of black fissile shales are typically barren except for indistinct circular carbonized algae but in rare instances include soft bodied animal remains a combination of lower dysoxic anoxic conditions with a fluctuating oxycline and relatively rapid episodic influx of fine grained detrital sediment favored repeated burial and preservation of abundant organic detritus and rarely soft bodied animals interbedded dark gray shales include abundant articulated agnostoid trilobites and diminutive polymerids e g jenkinsonia overlying platy calcareous bedding planes covered with articulated bodies and molts of the polymerid elrathia indicate rapid blanketing of undisturbed seafloors by calcareous mud layers these beds grade upward successively into interbedded sparsely fossiliferous platy to flaggy shale and thin pale gray weathering calcisiltites and burrow mottled to nodular limestones recording late hst to falling stage fsst carbonate shedding thin calcisiltites include fossil debris and articulated larger polymerid trilobites and the eocrinoid gogia preserved by obrutionary deposits the repeated recurrence of these patterns provides the rudiments of a predictive model that not only explains the differing modes of preservation but may also aid in prospecting for new lagerstatten

deglacial meltwater drainage and glaciodynamics inferences from laurentide eskers canada abstract this paper evaluates current knowledge of laurentide eskers in canada in the light of developments in glacier hydrology and glacial sedimentology questions regarding the morpho sedimentary relations of eskers the synchroneity and operation of r channel systems the role of supraglacial meltwater input and proglacial water bodies the controls on esker pattern and the glaciodynamic condition of the ice sheet at the time of esker formation are discussed a morphologic classification of eskers is proposed five types of eskers are identified and investigated type i eskers likely formed in extensive synchronous dendritic r channel networks under regionally stagnant ice that terminated in standing water type ii eskers likely formed in short subaqueously terminating r channels or reentrants close to an ice front or grounding line that may have actively retreated during esker sedimentation type iii eskers plausibly formed in short r channels that drained either to interior lakes in or tunnel channels under regionally stagnant ice type iv eskers may have formed as time transgressive segments in short subaerially terminating r channels or reentrants that developed close to the ice margin as the ice front underwent stagnation zone retreat or downwasted and backwasted regionally stagnant ice however formation in synchronous r channels cannot be discounted on the basis of reported observations type v eskers may have formed in h channels that terminated subaerially the spatial distribution of these esker types is discussed the factors that determined laurentide r channel pattern and operation were likely a complex combination of i supraglacial meltwater discharge ii the number and location of sink holes iii the ice surface slope thickness and velocity and iv the permeability topography and rigidity of the bed these factors cause and respond to changes in ice dynamics and thermal regime over the glacial cycle

controls on jokulhlaup transported buried ice melt out at skeiðararsandur iceland implications for the evolution of ice marginal environments abstract high magnitude jokulhlaups glacier margin position and ice thickness have been identified as key controls on sandur evolution existing models however have focused primarily on observations made during short windows of time and often do not account for the subsequent modification of proglacial landsystems by repeated jokulhlaups or post depositional modification due to melt out over decadal time scales digital elevation models dems were used to reconstruct the development of large depressions on skei ararsandur an outwash plain in southeast iceland these depressions measure up to 1 km in width and up to 13 m in depth and are associated with ice bodies up to 1 km in length and up to 150 m in height emplaced during a high magnitude jokulhlaup in 1903 and subsequently buried by jokulhlaups in 1913 and 1922 the continued melting of the har askri a ice bodies over a century following their emplacement together with subsequent repeated burial by high magnitude jokulhlaups demonstrates that jokulhlaups may continue to serve as important controls on sandur evolution on a decadal to centennial timescale 101 102 years the har askri a depressions developed only following the retreat of the glacier margin after 1945 which highlights the controls of margin position on the evolution of the sandur margin position and thickness of the glacier profile was seen to affect not only the distribution and thickness of sediment emplaced during jokulhaups but also the rate and pattern of melt in the decades following the decoupling of the margin from the sandur the jokulhlaup landsystem model signatures identified at this site may provide a useful analogue for interpreting landforms and strata emplaced by glacier margin fluctuations jokulhlaups and melt out generated by retreating continental pleistocene ice sheets

holographic measurements of surface topography in laboratory models of mantle hotspots summary the evolution of surface topography produced by the rise of a buoyant droplet or diapir towards the free surface of a very viscous fluid in laboratory experiments is monitored using holographic interferometry such experiments enable us to investigate implications for surface topography of one possible unsteady model for intraplate hotspots the arrival of mantle thermals or diapirs near the base of the earth s lithosphere our model is possibly of most direct relevance to the interaction with the earth s surface and lithosphere of large spherical caps that are expected to rise at the head of new mantle plumes to previous experimental results for the axial height of topography olson nam we add further information on the height shape and width of the surface swell and on the evolution of the diapir itself when the ambient fluid has uniform density and viscosity no lithosphere surface topography is determined by the diameter density anomaly and depth of the droplet as the diapir approaches the surface a broad axisymmetric surface swell appears and initially increases in height while decreasing in width when the leading edge of the diapir is 0 2 diapir diameters below the surface the height passes through a maximum and the width through a minimum the swell then proceeds to subside and increase in width as the diapir spreads beneath the surface in separate experiments the lithosphere is modelled by a discrete surface layer of more viscous fluid whose thickness and viscosity contrast with the mantle are treated as independent parameters effects of lithosphere buoyancy relative to the mantle a property which may influence continental hotspot swells are also studied within the parameter range used the maximum swell height is independent of the lithosphere viscosity contrast but decreases with increasing lithosphere thickness and with decreasing lithosphere density surface uplift produced by the rise of two consecutive diapirs is shown to be more complex comparison of results with the characteristics of hotspot swells is attempted and should assist in determining the nature of hotspot plumes

the role of sediment supply in esker formation and ice tunnel evolution abstract meltwater is an important part of the glacier system as it can directly influence ice sheet dynamics although it is important that ice sheet models incorporate accurate information about subglacial meltwater processes the relative inaccessibility of contemporary ice sheet beds makes direct investigation challenging former ice sheet beds contain a wealth of meltwater landforms such as eskers that if accurately interpreted can provide detailed insight into the hydrology of former ice sheets eskers are the casts of ice walled channels and are a common landform within the footprint of the last laurentide and cordilleran ice sheets in south western alberta esker distribution suggests that both water and sediment supply may have been important controls the longest esker ridge segments are located within meltwater valleys partially filled by glaciofluvial sediments whereas the shortest esker ridge segments are located in areas dominated by clast poor till through detailed esker ridge planform and crest type mapping and near surface geophysics we reveal morpho sedimentary relationships that suggest esker sedimentation was dynamic but that esker distribution and architecture were primarily governed by sediment supply through comparison of these data with data from eskers elsewhere we suggest three formative scenarios 1 where sediment supply and flow powers were high coarse sediment loads result in rapid deposition and rates of thermo mechanical ice tunnel growth is exceeded by the rate of ice tunnel closure due to sediment infilling high sedimentation rates reduce ice tunnel cross sectional area cause an increase in meltwater flow velocity and force ice tunnel growth thus ice tunnel growth is fastest where sedimentation rate is highest this positive feedback results in a non uniform ice tunnel geometry and favours macroform development and non uniform ridge geometry 2 where sediment supply is limited but flow power high the rate of sedimentation is less than the rate of thermo mechanical ice tunnel growth here the ice tunnel enlarges faster than it fills with sediment and its evolution is independent of sedimentation resulting in more uniform ice tunnel geometry in these cases esker architecture is dominated by extensive vertical accretion of tabular units and ridge geometry is more uniform 3 where sediment is truly supply limited the sedimentation rate is negligible regardless of water supply and like scenario 2 ice tunnel growth is independent of sediment deposition forming a relatively uniform ice tunnel or eroding the bed because meltwater flows transport few gravel clasts the ice tunnel is not completely filled with gravel and instead saturated and pressurized diamicton or bedrock if deformable from beneath the surrounding ice is squeezed into the relatively low pressure ice tunnel during waning flow or after ice tunnel shutdown resulting in deformation of limited gravels deposited within the ice tunnel and a landform cored with diamicton or deformed bedrock and with a relatively uniform ridge geometry our data demonstrate that an esker map is a minimum map of ice walled channel location and that continued detailed investigation of morpho sedimentary relationships is essential to gaining a complete picture of esker forming processes validating the morpho sedimentary relationships identified in south western alberta and other areas with a larger data set may allow improved remote predictive esker mapping over larger areas and inferences to be made about spatial and temporal variations in esker depositional environments and ice tunnel evolution

coldspots and hotspots global tectonics and mantle dynamics of venus magellan radar images and altimetry data and pioneer venus gravity data of major highlands and lowlands on venus are examined with the objective of relating these tectonic and volcanic landforms to convection within venus mantle two roughly circular lowlands atalanta and lavinia planitiae are bowl shaped depressions which contain compressional tectonic features and lack hotspot related features such as coronae and shield volcanoes they are identified as surface expressions of young mantle coldspots or regions of mantle downwelling highlands can be divided into two distinct groups volcanic rises and plateau shaped highlands volcanic rises are domical circular to elongate regions characterized by volcanic construction extensional tectonism large positive gravity and geoid anomalies centered on the highlands and large apparent depths of compensation volcanic rises include beta bell atla and western eistla regiones and are identified as surface expressions of large mantle hotspots or plumes plateau shaped highlands are steep sided elevated regions that range from circular alpha regio to elongate or irregular ovda regio tellus regio in shape their surfaces are dominated by complex ridged terrain rather than the shield volcanoes and flows that dominate volcanic rises most plateau shaped highlands contain compressional structures some of which commonly lie along their margins and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises plateau shaped highlands which include those mentioned above and western ishtar terra thetis regio and phoebe regio are identified as surface expressions of mantle coldspots or regions of downwelling the highland coldspot features are elevated as a consequence of crustal thickening crustal thickening is absent or minor at lowland coldspots the importance of mantle downwelling in the tectonic deformation of venus suggests that its mantle like the mantle of earth is strongly heated from within the major venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet s core