instantaneous end moraine and sediment wedge formation during the 1890 glacier surge of bruarjokull iceland abstract contemporary understanding of the behaviour of surging glaciers and ice streams is hampered by the lack of data on landsystem evolution and sedimentary environments this study concerns the ice marginal environment of the surge type bruarjokull in iceland the sediment distribution in the glacier forefield as well as the morphology sedimentology and tectonic architecture of the 1890 end moraine is investigated for highlighting the interaction between very dynamic ice and sediment landform associations as a result of substrate bedrock decoupling during the 1890 surge subglacial sediment was dislocated across the bedrock surface and deformed compressively leading to gradual substrate thickening and the formation of a sediment wedge in the marginal zone a drop in subglacial porewater pressure at the very end of the surge led to substrate bedrock coupling and a stress transfer up into the sediment sequence causing brittle deformation of the substrate simultaneously the glacier toe ploughed into the topmost part of the marginal sediment wedge initiating the moraine ridge construction fine grained and incompetent sediment deformed in ductile manner resulting in a narrow moraine dominated by rooted folds while coarse grained and competent sediment deformed in brittle fashion resulting in a wider moraine dominated by thrust blocks a new sequential model of subglacial and ice marginal processes operating during a glacier surge is proposed illustrating the stepwise formation of a marginal sediment wedge and an end moraine a twofold inseparable marginal end product that formed during the last days of the 1890 surge

fissure fill and tunnel fill sediments expressions of permafrost and increased hydrostatic pressure a spectacular network of fissure fillings and pipes tunnels cuts quaternary gravelly delta deposits northeast of myvatn precisely on the spreading axis of the north icelandic rift zone the delta was formed in an ice contact lake during deglaciation towards the end of the last glaciation subsequently the lake was drained and permafrost conditions developed in these poorly sorted gravel deposits hydrostatic pressure was transmitted from the adjacent glacier to the non frozen core of the delta beneath the discontinuous permafrost crust and the seasonally frozen active layer owing to increased hydrostatic pressure a network of subhorizontal to vertical fissures was opened along the taliks in these fissures free ground water flow and sediment transport were established tunnel erosion and probably also seepage erosion were associated with these fissures subsequently the fissures and tunnels were filled by laminated fine sediments interbedded with poorly sorted material resulting in the formation of fissure fill sediments and tunnel fill sediments

altitude profiles of localized d region density disturbances produced in lightning induced electron precipitation events a three dimensional model of very low frequency vlf radio wave propagation in the earth ionosphere waveguide in the presence of lower ionospheric disturbances is used to quantitatively interpret vlf signatures of lightning induced electron precipitation lep events observed in two previously reported cases one case is that of a 28 5 khz signal originating in puerto rico and propagating to a receiver in lake mistissini quebec the other case involves a 24 0 khz signal originating in cutler maine and received at stanford california in both cases high resolution measurements of the vlf signals were made to accurately document characteristic signatures of lep events inan et al 1988b 1990 the comparison of the model calculations with the data yields information about the altitude profiles of electron density of both the extra ionization produced by the lep events and of the ambient ionospheric d region the comparisons are carried out using generally accepted values of the spatial extent of the disturbed regions and the intensity of the particle flux constituting the lep burst

simultaneous disturbance of conjugate ionospheric regions in association with individual lightning flashes characteristic whistler associated amplitude perturbations of subionospheric vlf or lf signals trimpi events observed within one second of each other at palmer station antarctica and at arecibo puerto rico suggest that ionospheric regions in both northern and southern hemispheres are disturbed together in association with individual lightning flashes during a one hour period on march 21 1989 the onsets of 44 out of 47 perturbations measured on a 21 4 khz signal from maryland to arecibo occurred within l s of perturbation onsets measured on a 23 4 khz signal from hawaii to palmer station similar activity occurred before and after this period and on the preceding and following days the observations are consistent with the disturbance of geomagnetically conjugate ionospheric regions by multiple bounces between hemispheres of bursts of radiation belt electrons scattered in pitch angle by whistlers in the magnetosphere analysis of patterns of perturbations with corresponding whistler and lightning information from this period suggests that there were at least two distinct ionospheric disturbances in each hemisphere

subglacial decoupling at the sediment bedrock interface a new mechanism for rapid flowing ice abstract in undetermined on millennial or even centennial time scales the activity of rapid flowing ice can affect climate variability and global sea level through release of meltwater into the ocean and positive feedback loops to the climate system at the surge type glacier bruarjokull an outlet of the vatnajokull ice cap eastern iceland extremely rapid ice flow was sustained by overpressurized water causing decoupling beneath a thick sediment sequence that was coupled to the glacier this newly discovered mechanism has far reaching consequences for our understanding of fast flowing ice and its integration with sediment discharge and meltwater release c 2006 elsevier ltd all rights reserved

ice fracturing during jokulhlaups implications for englacial floodwater routing and outlet development theoretical studies of glacial outburst floods jokulhlaups assume that i intraglacial floodwater is transported efficiently in isolated conduits ii intraglacial conduit enlargement operates proportionally to increasing discharge iii floodwater exits glaciers through pre existing ice marginal outlets and iv the morphology and positioning of outlets remains fixed during flooding direct field observations together with historical jokulhlaup accounts confirm that these theoretical assumptions are not always correct this paper presents new evidence for spatial and temporal changes in intraglacial floodwater routing during jokulhlaups secondly it identifies and explains the mechanisms controlling the position and morphology of supraglacial jokulhlaup outlets and finally it presents a conceptual model of the controls on supraglacial outbursts field observations are presented from two icelandic glaciers skei ararjokull and solheimajokull video footage and aerial photographs taken before during and after the skei ararjokull jokulhlaup and immediately after the solheimajokull jokulhlaup reveal changes in floodwater routing and the positioning and morphology of outlets field observations confirm that glaciers cannot transmit floodwater as efficiently as previously assumed rapid increases in jokulhlaup discharge generate basal hydraulic pressures in excess of ice overburden under these circumstances floodwater can be forced through the surface of glaciers leading to the development of a range of supraglacial outlets the rate of increase in hydraulic pressure strongly influences the type of supraglacial outlet that can develop steady increases in basal hydraulic pressure can retro feed pre existing englacial drainage whereas transient increases in pressure can generate hydraulic fracturing the position and morphology of supraglacial outlets provide important controls on the spatial and temporal impact of flooding the development of supraglacial jokulhlaup outlets provides a new mechanism for rapid englacial debris entrainment

formation and deformation of subglacial intrusive clastic sheets in the dwyka formation of northern natal south africa abstract an arenaceous diamictite rests on fissile silty mudstone and argillaceous diamictite and is overlain by a laminated muddy siltstone east of helpmekaar in the dundee district of northern natal the argillaceous deposits originated either in the subglacial or proglacial zone of a partly grounded marine ice sheet where sediment was supplied by debris laden floating ice meltwater plumes and turbidity currents the arenaceous diamictite is the product of melt out debris released close to the grounding line of the ice sheet where it was subjected to overriding by the ice and slumping and redeposition by sediment gravity flow the laminated siltstone represents sediment that was deposited in a low energy setting by turbidity currents and suspension settling downslope from a retreating g ounding line dispersed ice rafted clasts occur in these fine grained deposits the upper surface of the arenaceous diamictite displays regularly spaced linear undulations that are parallel to the former iceflow direction as are corrugations in discrete subhorizontal slip planes in its upper part dike like bodies of this diamictite are found in the underlying mudstone and argillaceous diamictite these dikes or wedge shaped sheets of sandy diamictite are nearly vertical in their upper parts but at depth curl to the southeast to form sill like bodies these intrusive diamictites record a novel and previously undescribed form of subglacial bed shear by repeated hydraulic injections which shuffled the upper layers of the substrate to the south southeast bed compaction and shear beneath the grounded ice is recorded by folds in steepened portions of the clastic intrusive sheets and by pinch and swell and folded pinch and swell of the clastic sills displacement of steep folded sheet elements along subhorizontal slicken sided slip planes indicate that bed shear continued along spaced planes of slip during the process of compaction the corrugated and planar slip surfaces and the formation of the listric or spoon shaped intrusions are all shown to be different expressions of bed shear induced by traction from the overriding ice flowing to the southeast when it was temporarily grounded the base of an ice sheet flowing from land into the sea undergoes flow acceleration and forms a ramp where it decouples from the high friction substrate the arenaceous diamictite mound most likely to have developed beneath this ramp is inferred to have a structural core of sediments that have been squeezed and sheared and have slipped from beneath the ice in the tidal grounding zone

the sedimentology of a late pleistocene drumlin near kingscourt ireland abstract an exposure in a late pleistocene drumlin near kingscourt ireland provides a good insight into some of the processes that give rise to such subglacial bedforms the drumlin is located on a ridge cored by red sandstone of carboniferous namurian age which rises to 150 m above m s l the drumlin itself is about 380 m long by 170 m wide and is 20 m in height it is orientated vvnw ese ice flow direction in the area as inferred from general drumlin orientation striae and erratic dispersal was nw se the drumlin is composed of diamicton with four constituent facies containing large scale up to 20 m long and 1 5 m high slabs of the sandstone bedrock which have been displaced tens of metres by ice dragging the thin diamicton matrix is sand rich with few clasts greater than pebble size it contains green sandstone erratics from west and northwest of the study site along with clasts of weathered namurian sandstone which have been sheared from local bedrock the diamicton attains a maximum thickness of 5 4 m the slabs are confined to the basal 2 5 m of the diamicton thus the drumlin is essentially rock cored parts of the matrix are interpreted as injection sediments that have been squeezed into fractures and voids in the bedrock and between rock slabs under high porewater pressures when the ice began to displace fractured substrate deformation structures at a variety of scales are seen in the matrix deformation erosion and deposition were all important in the formation of the drumlin much of the upper part of the diamicton has been sheared subsequent to initial deposition by ice action the sheared units are uppermost in the stratigraphic sequence the shearing is the last glacial process apparent in the drumlin sediment and may have been contemporaneous with drumlinisation both the squeezing process which deposited the injections within the matrix and the deflection in flow of ice were influenced by the obstructing bedrock ridge which in this case is also responsible for the anomalous orientation of the feature

a new vlf method for studying burst precipitation near the plasmapause vlf signals in the 2 4 khz range transmitted from siple station antarctica l 4 3 and received at various antarctic locations have been used to detect the occurrence of burst precipitation of electrons e 50 kev into the nighttime lower ionosphere the receiving stations each 1400 km from siple were located to the north at palmer l 2 3 to the east at halley l 4 3 and to the south at south pole 74 rapid changes in the received phase and amplitude of the siple signal trimpi events were observed in conjunction with the reception of one hop whistlers in one case involving propagation paths lying poleward of the plasmapause amplitude decreases by 20 in 2 s decaying in 7 s were recorded simultaneously at halley and south pole post facto analysis of the south pole records using a new digital processing scheme showed corresponding fast phase advances of 15 s in a case of phase perturbations at palmer the siple signal path crossed the plasmapause projection and the associated whistlers propagated in the outer plasmasphere phase measurements appear to be a particularly sensitive means of detecting burst precipitation activity under experimental conditions of the kind described the length and spatial distribution of the signal paths provide a basis for studying the occurrence and approximate location of burst precipitation in regions outside the observing range of most instruments used for detection of precipitation direction finding on correlated whistler events as well as dispersion analysis may be used to increase the spatial resolution of the method

dike filled extensional structures in cenozoic deposits of the kleszczow graben central poland abstract the origin and development of the cenozoic kleszczow graben were strongly influenced by a pre existing fault pattern within the permian mesozoic bedrock dike filled fractures penetrating neogene quaternary deposits analysed in the opencast belchatow browncoal mine follow extensional structural patterns parallel to the axes of the local anticlinal forms established or rejuvenated in the quaternary these structures are particularly well developed along the fold crests the spacing and structural features of the fractures and normal faults are explained in terms of the tensile stress operating in beds subjected to folding the main localities of the quaternary folds precisely align with segments of two deep seated reverse faults the kleszczow kodr b fault and the gomunice piaski fault both are nw se orientated dislocations along a hinge zone of the laramide ekinsko anticline subcropping the cenozoic graben infill in the open mine the origin of the quaternary folds as fault overlying folds is attributed to a young block uplift on the ekinsko anticline area which is well recorded by a strong reduction in thickness of the cenozoic sediments the quaternary succession is reduced in thickness twice as much as the neogene the successive stages of quaternary reactivation of the ekinsko anticline during the cromerian through the late saalian with diminishing intensity are believed to be correlatable with the sequence of glacial rebound events coupled with mobility of the deep rooted fault zone

subglacial water escape structures and till structures slettjokull iceland beyond slettjokull the north part of the ice cap m rdalsjokull south iceland an undulating till plain is dissected by meltwater streams throughout a transect from the ice margin to the 1900 ice marginal moraine a distance of about 1 2 km finely laminated wedge shaped water escape structures wes are present originating in the till and disappearing down ice into the underlying sands and fine gravels a second set of much smaller and deformed black wes occurring within the till has been produced by upward injection into the till as a response to stiffening of the till after dewatering by the downward developing wes at present the toe of the glacier is frozen to its bed and the frozen bed material is more than 2 m thick during the relatively cold period around the turn of the century and until the late 1920s a similar submarginal body of permafrost must have existed the sequence reveals that subglacial meltwater attributed to drainage following the 1918 katla eruption was blocked by a frozen glacier toe causing the formation of water escape structures subsequently glacier flow deformed the substratum and produced the threefold division of the till bed the study has important implications for theories envisaging subglacial floods because it demonstrates that it is very difficult for temperate glaciers with a temporary frozen bed in the toe region to retain subglacial meltwater copyright 1999 john wiley sons ltd

the origin of a giant downward directed clastic dyke in a kame belchatow mine central poland a kame near czubata gora in the pleistocene overburden of the belchatow open cast browncoal mine in central poland shows several types of deformations a number of diapirs occur but also a structure that shows all characteristics of a diapir but consists of sediments that apparently intruded the underlying rather than the overlying sediments analysis of this structure shows that it consists of material that was originally deposited as a fine grained mainly silty glacilacustrine sediment seismic activity triggered the material to fluidise resulting in a structureless mass part of the fluidised material flowed down into a fissure that had formed in the underlying sediments probably due to tectonics in the active graben thus forming a clastic dyke more of the overlying silty material was later pressed down into a dyke with the shape of a downward directed diapir

under pressure clastic dykes in glacial settings abstract clastic dykes are widespread in glacial settings and we provide examples from switzerland patagonia iceland and the antarctic ranging in age from tertiary to recent on the basis of these examples we establish the general characteristics of clastic dykes and proceed to establish the direction of propagation on sedimentological grounds micromorphological analysis reveals that sedimentation in the dykes is ruled by pressure gradients and that ordinary sedimentological rules do not apply clastic dykes act like safety valves in the subglacial hydraulic system their development depends on specific subglacial conditions like water volume and pressure the nature of the bed sediments or bedrock and the hydraulic properties of the bed a number of scenarios for clastic dyke development are presented as there is not just one set of conditions under which they form development of clastic dykes affects glaciodynamics like velocity and surging and landform formation like till thickness and bedrock disruption

late wisconsinan sub glacial clastic intrusive sheets along lake erie bluffs at bradtville ontario canada abstract numerous clastic intrusive sheets a few decimetres to more than 16 m long 1 120 cm thick and extending one to more than 25 m laterally occur along a 350 m long section of the late wisconsinan catfish creek drift in the lake erie bluffs at bradtville southwestern ontario most of them are downglacier dipping dikes the largest one terminating in the underlying middle wisconsinan tyrconnell formation most dikes strike nne ssw at right angles to the local direction of glacier movement during the deposition of catfish creek drift the tops of some of them are truncated or displaced downglacier by shearing the main concentration of clastic intrusive sheets is on the upglacier side of a glaciotectonically folded anticline of tyrconnell formation clays and silts underlying the catfish creek drift the host sediments are catfish creek till gravel sand and silt and tyrconnell formation silt and clay most intrusive sheets particularly the small to medium ones consist of massive to crudely laminated sand and silt intruded from below by a dewatering process the largest dike reflects in its composition mainly the adjoining or higher lying host sediment materials and its main part was formed by downward infilling or by gravity flows into an open fracture the large dike is flanked by small laminated silty sand sheets and several small apophyses some of them injected downward and sideways others upward by dewatering the clastic intrusive sheets were formed under a moving glacier the erie lobe probably both at the beginning and towards the end of the deposition of catfish creek till their location and position was predetermined by glaciotectonically induced listric planar structures and zones of weakness mainly tension fractures that strike transverse to glacier movement and dip downglacier and also by confinement of pore water in a permeable sediment wedge between the less pervious tyrconnell formation and massive catfish creek basal till

telescopic imaging of sprites telescopic images of sprites show a wide variety of generally vertical but also slanted fine structure including branching tree like shapes and well defined but isolated columns with transverse spatial scales ranging from tens of meters to a few hundred meters at 60 85 km altitude simultaneous analysis of radio atmospheric and lightning data indicates that specific columnar regions are selectively excited by successive discharges

characteristics of localized ionospheric disturbances inferred from vlf measurements at two closely spaced receivers the very low frequency vlf npm signal from hawaii was recorded at two closely spaced 50 km receivers located at palmer and faraday stations on the antarctic peninsula measurements of characteristic amplitude and phase signatures of lightning induced electron precipitation lep events were made on three different days in march 1992 both amplitude and for the first time phase measurements are quantitatively interpreted using a three dimensional model of vlf propagation in the earth ionosphere waveguide in the presence of lower ionospheric disturbances this is the first time such a study has been undertaken with mirrored precipitation differences between the amplitude and phase changes at the two sites are accounted for by the location of the lep ionospheric disturbance transverse to the vlf signal propagation paths the change in these differences is explained by the horizontal movement of the disturbance region and therefore the causative whistler duct footprint across the transmitter receiver paths trends in the amplitude and phase changes on a timescale of order 1 hour are found to be encompassed by the modeling of the passage of the day night terminator along the paths

what causes till thickness to change over distance answers from mýrdalsjokull iceland abstract many explanations have been provided for variations of the spatial distribution and thickness of till sheets this paper gives new insight into the architecture of a stratigraphically distinct till sheet as a function of the type of substratum and preadvance topography at a scale of 10 km this emphasises the sensitivity of the subglacial system to changes in the basal drainage conditions the study area is the forefield of slettjokull at the northern margin of the m rdalsjokull ice cap central south iceland here detailed lithostratigraphical and sedimentological investigations including mapping of the thickness for two till units sediment logging clast fabric and geotechnical measurements provide a basis for interpretations the results show that the thickness of a stratigraphically distinct till sheet is directly correlated to the type of substratum where the substratum consists of sorted sediments the till is thin conversely the till is thick where the substratum consists of till overlying sorted sediments a sedimentological model is presented in which till thickness is tied to the variable hydraulic conditions experienced in a deforming subglacial bed

recovery signatures and occurrence properties of lightning associated subionospheric vlf perturbations subionospheric vlf signatures of lightning associated ionospheric disturbances observed at multiple sites in the central united states are used to investigate the occurrence properties and recovery signatures of characteristic perturbations exhibiting rapid onset followed by slow recoveries the two different types of events so called early fast and lightning induced electron precipitation lep inan et al 1996a are distinguished by the lack of presence of a few hundred millisecond delay between vlf event onsets and causative lightning discharges respectively analysis of recovery signatures of the two types of events indicate subtle but distinct differences in the recovery rates a majority of early fast events were found to exhibit a more rapid initial recovery to preevent levels during the first 20 s of recovery when compared to lep events this experimental evidence indicates that the physical nature of the ionospheric disturbance involved in the two classes of events are different consistent with a recent theoretical suggestion inan et al 1996c the occurrence properties of early fast events observed by the holographic array for ionospheric lighting hail receivers indicate that ionospheric disturbances act primarily as forward scatterers