buried late quaternary channel systems in the danish north sea genesis and geological evolution abstract the danish north sea has been subject to extensive seismic surveying both 2d and 3d during the past four decades the 3d seismic data from the area has mainly been used for hydrocarbon exploration but in the last decade it has also been utilized for investigating the quaternary sediments and especially buried incised valleys the erosional and depositional processes involved in forming such valleys are important in paleolandscape reconstruction but are often difficult to determine since sediment deposited by flowing water displays ambiguous geometric structures in this study we combine the analysis of infill geometries from high resolution 2d seismic data with plan view morphology analysis from 3d seismic data and lithological data from borehole reports in order to constrain the origin of buried channels in the danish north sea this data integration makes differentiations of a subaerial vs subglacial origin more robust the study focusses on analyzing a proglacial to fluvial channel system which was initiated as a subglacial tunnel valley system during the last glacial maximum and subsequently evolved as a fluvial system after deglaciation the channel system was active in a marsh environment during a period of subaerial exposure in the late weichselian to early holocene during the holocene transgression the system was gradually flooded leaving estuarine tidal deposits in the underfilled channels once the area was completely flooded the channel system was buried under marine sands

genesis of streamlined landforms and flow history of the green bay lobe wisconsin usa abstract the distribution of streamlined landforms in southeastern wisconsin suggests that drumlins and flutes formed during several phases of the green bay lobe between 18 and 14 000 bp the largest group of drumlins formed during a still stand of the ice margin during the johnstown phase presumably about 18 16 000 bp flutes and smaller drumlins are superimposed on larger forms and larger drumlins are remolded this indicates that drumlin modification continued during retreat three fields containing smaller drumlins formed after ice re advanced a short distance or stabilized during the green lake rush lake and st anna phases about 16 14 000 bp the drumlin forming process included erosion and deformation of pre existing ice marginal and proglacial sediments drumlins associated with the johnstown phase show an increase in length up glacier this is probably the result of variations in flow velocity length of time of drumlin formation and sediment availability flutes and small drumlins are associated with retreat moraines and formed near a thin retreating ice margin ice surface profile reconstructions suggest that margins were relatively steep during the johnstown milton green lake and early rush lake phases during the lake mills and late rush lake phases when flutes were forming the ice surface slopes were lower and margins were retreating during retreat minor sublobes developed some perhaps resulting from surges into small proglacial lake basins thus steep ice margins and driving stresses on the order of 15 25 kpa were typical during drumlin formation the formation of flutes and the remolding of larger drumlins into smaller forms took place beneath gently sloping ice of retreating margins associated with lower driving stresses

the origin of deep buried channels of elsterian age in northwest germany buried channels cut to a depth of over 100m into the underlying substratum of north germany are almost exclusively of elsterian age evaluation of geophysical well logs in the hamburg area for the first time allows the construction of detailed longitudinal and transverse profiles it can be shown that an apparently chaotic infill known from borehole records in some cases is actually a regular sedimentary sequence fining from proximal to distal channel formation seems to have been the result of catastrophic meltwater release in the marginal parts of the ice sheet the channel infill reflects these episodic outbursts of meltwater during deglaciation the channel remnants became infilled with fine sediments the channels ceased to exist as morphologically detectable forms at the end of the holsteinian stage

overdeepened quaternary valleys in the eastern danish north sea morphology and origin abstract a system of 100 350 m deep overdeepened and buried valleys has been mapped in the eastern danish north sea individual valleys can be traced for several tens of kilometres and valleys generally begin and terminate abruptly a correlation to buried valleys onshore southwest denmark is possible the valleys post date glaciotectonic structures and are interpreted to be elsterian and possibly saalian in age the origin of valleys in the eastern danish north sea is explained by repeated cycles of sub glacial down cutting of meltwater channels and channel closure due to ice creep eventually producing deeply incised valleys glacial erosion may have been significant in cases where valleys are broad and shallow the origin of overdeepened quaternary valleys in general can be attributed to a combination of three end member processes i steady state drainage of meltwater ii catastrophic outbursts of meltwater and iii glacial erosion the main requirement for sub glacial valley formation is the presence of a melting ice sheet overlying a poorly consolidated substrate permeability variations of the substrate may produce subregional variations in the pattern of valley incision while pre glacial rivers faults or salt structures are of local importance only

glaciohydraulic supercooling in former ice sheets glaciohydraulic supercooling has been observed at many modern glaciers and has been shown to generate thick zones of debrisladen basal ice which when exposed to melting can produce debris flow and melt out till deposits as well as hummocky terrain glaciohydraulic supercooling probably occurred in former continental ice sheets especially where ice flowed out of deep basins or up steep slopes evidence to support glaciohydraulic supercooling in former ice sheets however is limited but may include sedimentary sequences of melt out till and debris flow deposits and hummocky terrain along the distal rim of overdeepenings and crests of adverse bed slopes d 2005 elsevier b v all rights reserved

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

stabilizing feedbacks in glacier bed erosion glaciers often erode transport and deposit sediment much more rapidly than nonglacial environments1 with implications for the evolution of glaciated mountain belts and their associated sedimentary basins but modelling such glacial processes is difficult partly because stabilizing feedbacks similar to those operating in rivers2 3 have not been identified for glacial landscapes here we combine new and existing data of glacier morphology and the processes governing glacier evolution from diverse settings to reveal such stabilizing feedbacks we find that the long profiles of beds of highly erosive glaciers tend towards steady state angles opposed to and slightly more than 50 per cent steeper than the overlying ice air surface slopes and that additional subglacial deepening must be enabled by non glacial processes climatic or glaciological perturbations of the ice air surface slope can have large transient effects on glaciofluvial sediment flux and apparent glacial erosion rate

glaciohydraulic supercooling in iceland we present evidence of glaciohydraulic supercooling under jokulhlaup and ablation dominated conditions from two temperate icelandic glaciers observations show that freezing of sediment laden meltwater leads to intraglacial debris entrainment during normal and extreme hydrologic regimes intraglacial frazil ice propagation under normal ablation dominated conditions can trap copious volumes of sediment which forms anomalously thick sections of debris rich ice glaciohydraulic supercooling plays an important role in intraglacial debris entrainment and should be given more attention in models of basal ice development extreme jokulhlaup conditions can result in significant intraglacial sediment accretion by supercooling which may explain the concentration of englacial sediments deposited in heinrich layers in the north atlantic during the last glaciation

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

preliminary results of tritium analyses in basal ice matanuska glacier alaska u s a evidence for subglacial ice accretion

the last glacial maximum in the north sea basin micromorphological evidence of extensive glaciation despite a long history of investigation critical issues regarding the last glacial cycle in northwest europe remain unresolved one of these refers to the extent timing and dynamics of late devensian weichselian glaciation of the north sea basin and whether the british and scandinavian ice sheets were confluent at any time during this period this has been the result of the lack of the detailed sedimentological data required to reconstruct processes and environment of sediments recovered through coring this study presents the results of seismic sedimentological and micromorphological evidence used to reconstruct the depositional processes of regionally extensive seismic units across the north sea basin thin section micromorphology is used here to provide an effective means of discriminating between subglacial and glacimarine sediments from cored samples and deriving process based interpretations from sediment cores on the basis of micromorphology critical formations from the basin have been reinterpreted with consequent stratigraphic implications within the current stratigraphic understanding of the north sea basin a complex reconstruction is suggested with a minimum of three major glacial episodes inferred on at least two occasions during the weichselian devensian the british and scandinavian ice sheets were confluent in the central north sea whilst micromorphology can provide much greater confidence in the interpretation of late quaternary offshore stratigraphic sequences it is noted that a much better geochronology is required to resolve key stratigraphic issues between the onshore and offshore stratigraphic records copyright 2006 john wiley sons ltd

a m gurnell and m j clark eds 1987 glacio fluvial sediment transfer an alpine perspective chichester etc john wiley and sons ltd

glaciohydraulic supercooling a freeze on mechanism to create stratified debris rich basal ice ii theory simple theory supports field observations lawson and others 1998 that subglacial water flow out of overdeepenings can cause accretion of layered debris bearing ice to the bases of glaciers the large meltwater flux into a temperate glacier at the onset of summer melting can cause rapid water flow through expanded basal cavities or other flow paths if that flow ascends a sufficiently steep slope out of an overdeepening the water will supercool as the pressure melting point rises and basal ice accretion will occur diurnal occasional or annual fluctuations in water discharge will cause variations in accretion rate debris content of accreted ice or subsequent diagenesis producing layers under appropriate conditions net accretion of debris bearing basal ice will allow debris fluxes that are significant in the glacier sediment budget

geophysical evidence of a late pleistocene glaciation and paleo ice stream on the atlantic continental shelf offshore massachusetts usa abstract interpretations of seismic reflection data collected offshore massachusetts usa reveal the first conclusive geophysical evidence of a pre wisconsinan glaciation that extended beyond the limits of the last glacial maximum lgm in the region the data image numerous glacial geomorphic features that define the extent of a paleo ice stream including 1 a regionally distributed erosion surface that forms a 50 km wide trough with steeply eroded sidewalls 4 18 and nearly 100 m in relief at the margins 2 a network of sub ice sheet meltwater channels and 3 a transparent glacigenic seismic unit the orientation of the paleo ice stream trough indicates that the ice stream flowed to the south southwest toward the shelf break this suggests that the ice stream formed further to the north where it appears that georges bank southeast of the gulf of maine usa redirected ice flow limited well data constrain the glacial erosion event up to 300 m below sea level to occur within the pleistocene the glacial event represents a time of larger ice volume on the northern atlantic continental shelf as compared to the lgm thus we suggest that the event corresponds to marine oxygen isotope stage 12 late pleistocene when the first major pleistocene shelf crossing glaciation began offshore southeastern canada these geophysical constraints on a late pleistocene glaciation offshore massachusetts have important implications for 1 models of the laurentide ice sheet as the geomorphic evidence of pre lgm ice streams are difficult to characterize yet account for most of the ice sheet s mass flux and 2 the pore water salinity pattern offshore new england as sedimentary basins near an ice sheet margin often contain large volumes of glacially emplaced freshwater

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

pleistocene subglacial tunnel valleys in the central north sea basin 3 d morphology and evolution four phases of cross cutting tunnel valleys imaged on 3 d seismic datasets are mapped within the middle late pleistocene succession of the central north sea basin witch ground area in plan the tunnel valleys form complex anastomosing networks with tributary valleys joining main valleys at high angles the valleys have widths ranging from 250 to 2300 m and base to shoulder relief varying between 30 and 155 m with irregular long axis profiles characteristic of erosion by water driven by glaciostatic pressures the youngest phase of tunnel valleys are smaller and have a thinner infill than the older generations the fill of the larger valleys comprises three seismic facies the lowermost of which has high amplitudes and is discontinuous the middle facies consists of wedge shaped packages of low angle dipping reflectors and is overlain by a facies characterised by sub horizontal reflectors which onlap the valley margins the seismic character and comparison with lithologies identified in other northwest european pleistocene tunnel valleys both onshore and offshore suggests that the lower two seismic facies are most likely sand and gravel dominated while the uppermost facies consists of glaciolacustrine and marine muds the 3 d morphology of the valley margins combined with the geometry of the infill packages suggest that episodic discharge of subglacial meltwater was responsible for incising the valleys and depositing at least some of the infill proglacial glaciofluvial deposits are inferred to account for some of the fill overlying the subglacial deposits glaciolacustrine and marine muds filled remaining valley topography as the ice sheet retreated the preserved valley margins are shown to be time transgressive erosion surfaces that record changes in geometry of the tunnel valley system as it evolved through time implying that valleys associated with each ice sheet advance retreat cycle were dynamic and probably long lived within the constraints of the existing stratigraphy the oldest tunnel valleys in the witch ground area of the central north sea are most likely to be marine isotope stage mis 12 elsterian ca 470 ka in age and the youngest pre mis 5e last interglacial ca 120 ka if each tunnel valley phase was formed during the retreat of a major ice sheet then four glaciations with ice coverage of the central north sea are recorded in the pre weichselian middle late pleistocene stratigraphy copyright 2006 john wiley sons ltd

how glaciers entrain and transport basal sediment physical constraints abstract simple insights from the physics of ice water and sediment place constraints on the possible sediment transport behavior of glaciers and ice sheets because glaciers concentrate runoff streams generated by glaciers transport much sediment and may erode bedrock rapidly deforming glacier beds also can transport much sediment particularly in marginal regions rapid sediment entrainment producing thick debris rich basal zones may occur by regelation into subglacial materials and by freeze on from rising supercooled waters numerous other mechanisms may be important but primarily near ice margins especially those of advancing or fluctuating glaciers several sediment entrainment mechanisms may be active beneath a single glacier but one process is likely to be dominant at any place and time

high resolution seismic and resistivity profiling of a buried quaternary subglacial valley northern alberta canada high resolution seismic and resistivity profiles were acquired to assist in the delineation of a buried quaternary subglacial valley the existence of which was first discovered by drilling one aspect of this valley is the glacial sediments that host substantial methane at shallow depths 50 m or less geophysical well logs indicate that the glacial sediments in this area generally have lower compressional wave speeds v p 10 m than adjacent undisturbed cretaceous age rocks which have v p 2200 m s and r 300 m valley the reflection profile reveals a complex series of events consistent with heterogeneous deposition one major reflection dips eastward across the valley the reflection character changes abruptly on either side of this event with dipping clinoform reflections beneath and nearly horizontal reflections above these may be related to changes in deposition processes beneath a retreating ice sheet as has been suggested by earlier workers the buried valley s basal reflections are complex and possibly reflect glaciotectonic deformation or block plucking the near surface methane gas is within but at the edge of the valley the methane gas saturated zone displays anomalously high resistivities both in the well logs and in resistivity tomography

tunnel valley formation in northwest germany geology mechanisms of formation and subglacial bed conditions for the bornhoved tunnel valley the bornhoved tunnel valley consists of a relatively broad elongated depression filled primarily with tills and a narrow channel in the axial part filled with outwash glaciolacustrine sediments and tills the tunnel valley is at least 13 km long 222 m deep bottom at 191 m b s l and extends parallel to the axis of a peripheral sink between two permian salt diapirs in the substratum it is suggested that the initiation of the tunnel valley was controlled by a differential response of the bed materials to ice overriding on top of and between the salt structures the bornhoved tunnel valley is a polygenetic feature that resulted from different erosional processes throughout all three main glaciations in northern germany during the elsterian glaciation glaciotectonic squeezing occurred to be later replaced by subglacial meltwater erosion during the ice retreat and the holsteinian interglacial the valley was filled with fine grained glaciolacustrine and marine sediments which were subsequently removed and redeposited further down ice as a 200 m thick glaciotectonic melange by the first saalian advance the second saalian advance caused only minor changes and the tunnel valley was reactivated as a subglacial channel during the weichselian glaciation it is suggested that at the time of the first weichselian ice advance a large subglacial water reservoir developed in the area of the baltic sea basin and caused a rapid surge like ice movement as the ice sheet advanced out of the baltic sea basin drainage of the water reservoir was prevented by the ice toe overriding the permafrost on the saalian highlands during the ice retreat frozen ground was left beyond the ice margin and subglacial meltwater catastrophically drained through the tunnel valley with a discharge roughly estimated at about 3 75 103 m3 s it seems possible that drainage of the subglacial water collector through the bornhoved tunnel valley and other similar channels south of the baltic sea led to increased ice sheet basal friction and was one reason for successively smaller extents of the subsequent weichselian advances evidence against pervasively deforming bed materials in the vicinity of the bornhoved tunnel valley in the weichselian glaciation indicates that the valley served as an efficient drainage path for porewater from the subglacial sediments

time transgressive tunnel valley formation indicated by infill sediment structure north sea the role of glaciohydraulic supercooling structure and lithology of the infill sediments from 16 subglacial buried tunnel valleys of pleistocene age in the north sea were analyzed using 3d seismic data and geophysical log data from five hydrocarbon exploration wells the infill sediments are characterized by three seismic facies facies i the volumetrically most important and structurally most distinct is composed of clinoform reflections downlapping axially up valley up ice facies ii is composed of near horizontal continuous and well layered reflections onlapping the clinoform reflections and the valley walls and facies iii is composed of clinoform reflections downlapping axially down valley down ice a model of formation of this sediment architecture is proposed in which valley incision and infill are causally linked it is proposed that facies i is related to glaciohydraulic supercooling of subglacial meltwater in the distal parts of tunnel valleys the valleys formed time transgressively during ice retreat whereby sediment eroded further up ice was re deposited along adverse subglacial slopes of the valleys close to the ice margin the formation of facies ii and iii is related to the deposition in proglacial basins during final deglaciation copyright 2008 john wiley sons ltd

glacial curvilineations new glacial landforms produced by longitudinal vortices in subglacial meltwater flows abstract the glacial landscape of the dobrzyn plateau in central poland contains a complex suite of enigmatic bedforms consisting of elongate and sinuous sediment ridges occurring within an anabranched network of tunnel channels terminating at a former ice margin marked by extensive glaciofluvial fans the term glacial curvilineations is proposed to describe these enigmatic bedforms that occur as fields of parallel sinuous ridges separated by troughs the regional pattern of glacial curvilineations replicates the morphology and pattern of tunnel channel margins and suggests a common genesis tunnel channels and glacial curvilineations are eroded in the interbedded diamictons and glaciofluvial sediments of the plateau we propose that tunnel channels and glacial curvilineations are the products of subglacial meltwater erosion curvilineations are erosional remnants produced by longitudinal vortices within the tunnel channel forming flows parallelism of curvilineations reflects spacing of turbulent structures within the subglacial flow tunnel channel margins also control the sinuosity of curvilineations by dictating both the path and curvature of longitudinal vortices arguments for meltwater erosion are strengthened by the presence of extensive glaciofluvial fans at tunnel channel termini they are the depositional counterparts to the extensive subglacial fluvial dissection of the plateau understanding curvilineation genesis may shed light on glacier hydrologic processes tunnel channels develop abruptly in the plateau surface suggesting localized point sources of meltwater such as drainage of subglacial and or supraglacial lakes the extent of tunnel channels and lateral continuity of glacial curvilineations suggests broad sheet like flows as wide as a few kilometres

estimation of near surface elastic parameters using multicomponent seismic data in multicomponent seismic reflection surveys surface and near surface effects can severely deteriorate the quality of reflection data such effects are more pronounced on shear wave than on compressional wave profiles amplitude anomalies statics and strong coherent source generated noise i e surface waves are often associated with inhomogeneous poorly compacted near surface sediments of the weathering layer the magnitude of such effects increases when sources and receivers are deployed at or near the surface in proximity to such inhomogeneities near surface effects can be investigated with respect to their depth of occurrence by burying seismic sources and or receivers at various depths below the inhomogeneous weathering layer in this context an experiment was conducted to collect multicomponent seismic field data on a borehole controlled test site in east texas data were recorded on three component surface and buried receivers using a full vector wavefield surface source although the geolo

sedimentary signatures of basal ice formation and their preservation in ice marginal sediments reconstruction of subglacial processes from sedimentological evidence is central to our understanding of glaciological conditions beneath former ice masses at svinafellsjokull southeast iceland we assess the extent to which two different processes of basal ice formation regelation and glaciohydraulic supercooling can be identified from ice marginal sediments our results indicate that the sedimentary characteristics of deposits produced by these two processes can be distinguished from one another and that it may be possible to recognise evidence of these processes in quaternary sediments and to reconstruct their spatial pervasiveness sediments derived from the melting of regelation basal ice have i a massive structure ii a sediment matrix 0 to 10 dominated by coarse sand and iii a higher proportion of angular clasts than supercool basal ice and associated sediments sediments derived from supercool basal ice i can be either massive or layered ii tend to have a silt dominated matrix and iii contain a slightly higher proportion of rounded and well rounded clasts than regelation basal ice and sediments previous studies indicate that the dominance of silt within supercool basal ice may be unique to this process and hence supercooling should leave a readily recognisable signature in the sedimentary record our results from svinafellsjokull lend support to that idea although we suggest that further work is required to assess whether silt dominance is a process signature diagnostic of supercooling and in particular the extent to which subglacial sediment supply determines the sedimentary character of basal ice facies and associated sediments

buried and open tunnel valleys in denmark erosion beneath multiple ice sheets abstract tunnel valleys are large elongate and irregular depressions cut beneath the margin of former ice sheets they are generally believed to play a substantial role for the entire hydraulic system beneath ice sheets and thus also for ice sheet behaviour their origin however remains poorly understood examination of newly collected danish hydrogeophysical and lithological data has revealed numerous systems of buried valleys 50 350 m deep and 0 5 4 km wide these valleys are similar to the open tunnel valleys found in the present day landscape infills comprise a variety of sediments often with subglacial clay till and meltwater deposits the incised substratum consists of limestone heavy paleogene clay sand and silt the tunnel valleys have preferred orientations and can be divided into several generations each ascribed individual ice advances or ice sheets occurring during multiple glaciations the processes forming the valleys appear to prefer pre existing open and buried valleys for the renewed erosion thus old subglacial erosion pathways have been re used several times and some of the present tunnel valleys may have been established in the middle or early quaternary or even earlier this valley re use effect causes apparently anastomosed valley systems to emerge in the data images the valleys were mainly eroded by meltwater supposed to have drained from subglacially stored reservoirs probably behind a frozen margin the water was most likely released in repeated jokulhlaups and flowed in relatively small channels on the floors of the tunnel valleys which gradually became ice filled with the lowering of the bed selective linear glacial erosion is also believed to have contributed to the erosion this however was most pronounced for the widest valleys and is supposed to have played a secondary role

glaciohydraulic supercooling a freeze on mechanism to create stratified debris rich basal ice i field evidence debris laden ice accretes to the base of matanuska glacier alaska u s a from water that supercools while flowing in a distributed drainage system up the adverse slope of an overdeepening frazil ice grows in the water column and forms aggregates while other ice grows on the glacier sole or on substrate materials sediment is trapped by this growing ice forming stratified debris laden basal ice growth rates of 0 1 m a 1 of debris rich basal ice are possible the large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings including heinrich events and the till sheets south of the laurentian great lakes

evidence for late middle pleistocene glaciation of the british margin of the southern north sea the timing and extent of late middle pleistocene glaciations in england and the southern north sea are controversial topics the recent trent valley palaeolithic project uncovered evidence for a post anglian pre devensian glaciation that affected much of central and eastern england the wragby till of lincolnshire is associated with this glacial event attributed here to marine oxygen isotope stage mis 8 coeval glacigenic deposits in the middle trent suggest that both western and eastern lobes of mis 8 ice reached the derby area these various deposits have been assigned previously to mis 12 10 or 6 although the last can be excluded for the wragby till which is overlain by trent terrace deposits assigned to mis 7 from biostratigraphy and amino acid dating the disposition of these glacigenic deposits within the landscape particularly in relation to terrace deposits of the ancestral river trent and the absence of mis 11 and 9 deposits within the footprint of the glaciation also provide compelling evidence at its maximum extent in eastern england the mis 8 ice reached the peterborough area identifying its extension or otherwise into areas such as north west norfolk and the west midlands requires further work

the history of quaternary sedimentation in the uk sector of the north sea basin reconnaissance seismic and sampling surveys have shown that up to 500 m of lower and early middle pleistocene deltaic sediments are preserved in the uk sector of the north sea the distributions of delta related facies record the progressive expansion of peripheral delta systems northwards across the southern north sea during the early pleistocene a late middle pleistocene unconformity is present throughout the uk sector and sedimentation above the unconformity was dominated by glacial erosional and depositional processes a complex system of deeply incised valleys was eroded during each of three regional middle and upper pleistocene glaciations and the valleys have been filled or partially filled by glaciolacustrine or glaciomarine and interglacial marine sediments the valleys of the earliest elsterian glaciation are up to 400 m deep in the southern north sea the saalian and weichselian valleys are commonly more than 100 m deep they are eroded through their contemporary tills nearshore but occur well beyond the limits of these tills offshore the origins of these valleys are attributed to a combination of fluvial subglacial and periglacial processes

sedimentary record of a late saalian jokulhlaup case study in siedlątkow outcrop central poland abstract the aim of the research conducted in an outcrop of late saalian sediments in siedl tkow central poland was to reconstruct extreme meltwater flows during a jokulhlaup on the basis of the geological record a combination of field and laboratory lithological methods was used which included lithofacies analysis analyses of grain size distribution shape and roundness of gravelly clasts examination of sediments made it possible to recognise palaeohydrological sedimentation and deformation processes during and immediately after the flood in particular the simultaneous operation of water flows in four paths in the immediate foreland of the ice front was reconstructed mechanisms of transport and sedimentation of deposits were determined characteristic features of the analysed jokulhlaup deposits include erosional and deformational contact with the substrate a high content of very coarse sediments poorly sorted with predominant massive clast supported gravels and boulders the presence of rip up clasts inverse grading in the lower part of the profile normal grading in the upper part and similarities in clast shape and roundness and the ratio of local rocks to northern origin rocks when compared to the surrounding saalian till the documented features allow for the following phases of jokulhlaup to be distinguished initial rising high stage and waning stage during the reconstructed event hyperconcentrated flow was dominant whereas fluvial flow and debris flow were of lesser importance the release of meltwater in the marginal part of the ice sheet came from the depression of the kolo basin to the north of the analysed sites complex deformation processes which occurred in the flood deposits and their substratum were also interpreted the analysed jokulhlaup event resembles some minor glacial floods in the ice contact zone of present day icelandic glaciers further research in marginal zones of pleistocene ice sheets should account for their occurrence as the analysed example indicates that jokulhlaups might have had considerable influence on the course of sedimentation processes in the marginal zones of ice lobes of pleistocene ice sheets as well as in the marginal valleys and sandars

broadband receiver response from dual streamer data and applications in deep reflection seismology a method to process dual streamer data with under and over configuration is presented the method combines the results of dephase sum and dephase subtraction methods in the dephase methods the response of one streamer is time shifted so that the primary arrivals on both streamers are aligned and these responses are then summed or subtracted the method provides a broad spectral response from dual streamer data and increases the signal to noise ratio by a factor of 1 5 testing was done on synthetic data and then applied to a real data set collected by the british institutions reflection profiling syndicate birps its application to a deep seismic reflection data set from the british isles shows that the reflections from the lower crust contain frequencies up to 80 hz suggesting that some of the lower crustal reflectors may have sharp boundaries and could be 20 30 m thick

reconstructing the dynamics of the north west european pleistocene ice sheets abstract the flow lines of the north west european pleistocene ice sheets can be reconstructed with the aid of fabric measurements and glaciotectonic investigations from this there is evidence for a more extensive saalian glaciation of the north sea area than previously assumed with regard to the weichselian different glaciological models can be tested against the field evidence deformable bed and instantaneous glaciation may be partially responsible for the observed phenomena however while at the end of the weichselian there was fast moving ice largely following the course of the baltic sea depression during the weichselian maximum conventional radial ice flow as envisaged by hughes et al 1981 was dominant directional evidence suggests that fast moving baltic ice was present during the late stages of the older and younger saalian glaciations however possibly as a result of isostatic effects it occupied a more southerly position than during the weichselian

the pre weichselian glaciations of north west europe abstract in north west europe no evidence has been found for glaciations older than the elsterian the elsterian seems to consist of at least two separate ice advances with a minor interval in between during the elsterian in the area south of the baltic sea over 400 m deep channels were cut into the underlying substratum by a combination of glacial erosion and meltwater activity the channels were active until the late elsterian and were subsequently filled with meltwater deposits and glaciolacustrine silt and clay during the saalian no comparative channels were being formed three different ice advances can be distinguished within the saalian the stratigraphical correlation of which is to be discussed no convincing evidence was found for any interglacial within the saalian sequence there are good reasons to assume that the ice advances were only separated from each other by minor ice free intervals