seismic reflection borehole and outcrop geometry of late wisconsin tills at a proposed landfill near toronto ontario the search for new landfill sites in the greater toronto area of southern ontario canada is producing a wealth of data regarding the subsurface stratigraphy and geometry of late wisconsin 25 ka

mapping bedrock beneath glacial till using cdp seismic reflection methods this paper is a case history demonstrating the applicability of the common depth point cdp seismic reflection method to image bedrock beneath glacial till in northwestern iowa reflections from the base of the 40 m thick glacial till are clearly observable on field files at around 45 to 50 ms two way traveltime and possess a dominant frequency of around 100 hz the bedrock reflection is confirmed by drill data the seismic data are of sufficient quality to detect local bedrock topographic changes and to interpret discontinuities along the till bedrock interface finite difference synthetic seismograms substantiate the interpreted reflections and the diffraction signatures from faults observed on the field files at some locations along the seismic line intra till reflections are apparent on the field files these intra till features are on the order of tens of meters in length along the line traverse and reflections from them are not enhanced by common depth point processing intra till reflections could be indicative of gravels or other alluvial materials that may serve as local aquifers

tunnel channels in southeast alberta canada evidence for catastrophic channelized drainage abstract tunnel channels in southeast alberta are attributed to erosion by channelized subglacial meltwater flows an anabranching tunnel channel network dissects the preglacial drainage divide of the ancestral milk river channel morphology and landform associations are used to evaluate competing hypotheses of tunnel channel formation mechanisms that invoke subaerial channel incision direct glacial erosion or steady state time transgressive erosion at the ice margin cannot explain convex up longitudinal channel profiles anabranching channel networks or confinement to the preglacial drainage divide results conclude that the tunnel channel network in southeast alberta represents late stage erosion by a channelised subglacial flow of catastrophic dimensions interpretations for this tunnel channel network are in agreement with conclusions obtained for the regional subglacial landform assemblage

glacial tunnel valleys in the eastern goldfields of western australia cut below the late paleozoic pilbara ice sheet abstract open cut gold mines near kalgoorlie in the eastern goldfields region of western australia expose a late paleozoic carboniferous permian network of glacially eroded valleys filled with tillite and shale valleys are incised into archean and proterozoic strata of the low relief western australian shield and buried by tertiary and quaternary strata subsurface data identify a poorly integrated network of narrow 200 m wide deeply incised 80 m and steep sided channels that connect broader sub basins up to 3 km wide lowermost infill strata within valleys consist of bouldery diamictites containing abundant striated clasts of far travelled lithologies and are identified as tillites deposited by a late paleozoic ice sheet pilbara ice sheet that covered the west australian shield tillites are blanketed by shales containing early permian palynomorphs the form and sedimentary infill of the buried valley network is comparable to subglacially cut tunnel valleys that formed below the wet based margins of pleistocene continental ice sheets and a similar origin is proposed for those at kalgoorlie the significance of this finding is that hitherto little was known of the late paleozoic gondwanan ice sheet that grew on the western australian shield and which contributed substantial volumes of sediment to adjacent sedimentary basins the preservation of a relict late paleozoic glacial topography near kalgoorlie indicates that post permian erosion of the region was minimal and cannot be reconciled with denudation models which propose uniform post permian uplift and deep stripping across the shield

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

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

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

shallow seismic reflection study of a glaciated valley shallow seismic reflection data were recorded along two long 1 6 km intersecting profiles in the glaciated suhre valley of northern switzerland appropriate choice of source and receiver parameters resulted in a high fold 36 48 data set with common midpoints every 1 25 m as for many shallow seismic reflection data sets upper portions of the shot gathers were contaminated with high amplitude source generated noise e g direct refracted guided surface and airwaves spectral balancing was effective in significantly increasing the strength of the reflected signals relative to the source generated noise and application of carefully selected top mutes ensured guided phases were not misprocessed and misinterpreted as reflections resultant processed sections were characterized by distributions of distinct seismic reflection patterns or facies that were bounded by quasi continuous reflection zones the uppermost reflection zone at 20 to 50 ms 15 to 40 m depth originated from a boundary between

sedimentologic evidence for outburst floods from the laurentide ice sheet margin in wisconsin usa implications for tunnel channel formation abstract clast supported boulder gravel in outwash fans along the glacial maximum margin of the laurentide ice sheet in wisconsin indicates the occurrence of outburst floods these sediments with clast intermediate axes of up to 2 m are located downstream of tunnel channels and were deposited shortly before cessation of glaciofluvial activity on each fan since tunnel channels with fans are widespread along the ice sheet margin in the western great lakes region these outburst floods were probably common paleodischarge estimates derived from the boulder deposits are poorly constrained but values of at least several hundred m3 s 1 are likely four potential water sources for the floods exist an extreme surface melt event an extreme precipitation event drainage of supraglacial lakes or drainage of stored subglacial meltwater we focus on the storage of subglacial meltwater behind the ice sheet margin as proglacial permafrost was present as ice advanced to its maximum extent and a frozen bed zone upstream from the margin probably impeded drainage through groundwater aquifers decay of this permafrost seal would have eventually allowed trapped water to drain through the tunnel channels we suggest that the 2 m boulders were entrained in an outburst of subglacial water that enlarged a pre existing channel cut by ablation derived flows

glacial structures in northern germany revealed by a high resolution reflection seismic survey in recent years the sediments filling pleistocene glacial structures have become of increasing importance to paleoclimate research climatic changes are documented by the deposition cycles in small closed trough or bowl like structures a 2 d high resolution shallow reflection seismic survey was conducted in 1996 over such a structure near tostedt in northern germany the objective was to obtain a more accurate picture of the structure and the underlying geology designed especially for shallow surveys a newly developed impulse source provided sufficient energy to observe reflectors as deep as 400 m signals are characterized by a high frequency content with maximum energy between 80 and 180 hz the migrated section shows a distinct reflection pattern revealing local glacial dynamics reflectors at depths of 30 40 and 45 m from inside the tostedt structure correlate well with three interstadials of the weichselian period weak reflections define the bottom of the structure with a maximum depth of 70 m the structure is imbedded in a much larger previously unexpected depression of similar shape the fill of this larger depression is seismically nearly transparent high amplitude reflections delineate its lower boundary with a maximum depth of 130 m two major reflectors at depths of 170 and 270 m correlate with lower miocene and middle oligocene units they indicate that subrosion which might have been expected from the presence of a nearby salt diapir is absent this confirms the purely glacial origin of the two bowl like structures

imaging the structural style of an active normal fault through multidisciplinary geophysical investigation a case study from the mw 6 1 2009 l aquila earthquake region central italy firb abruzzo high resolution analyses for assessing the seismic hazard and risk of the areas affected by the 2009 april 6 earthquake no rbap10zc8k 005

high resolution seismic traveltime tomography incorporating static corrections applied to a till covered bedrock environment a major obstacle in tomographic inversion is near surface velocity variations such shallow velocity variations need to be known and correctly accounted for to obtain images of deeper structures with high resolution and quality bedrock cover in many areas consists of unconsolidated sediments and glacial till to handle the problems associated with this cover we present a tomographic method that solves for the 3d velocity structure and receiver static corrections simultaneously we test the method on first arrival picks from deep seismic reflection data acquired in the mid late to 1980s in the siljan ring area central sweden to use this data set successfully one needs to handle a number of problems including time varying near surface velocities from data recorded in winter and summer several sources and receivers within each inversion cell varying thickness of the cover layer in each inversion cell and complex 3d geology simultaneous inversion for static corrections and velocity produces a much better image than standard tomography without statics the velocity model from the simultaneous inversion is superior to the velocity model produced using refraction statics obtained from standard reflection seismic processing prior to inversion best results using the simultaneous inversion are obtained when the initial top velocity layer is set to the near surface bedrock velocity rather than the velocity of the cover the resulting static calculations may in the future be compared to refraction static corrections in standard reflection seismic processing the preferred final model shows a good correlation with the mapped geology and the airborne magnetic map

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

imaging a shallow aquifer in temperate glacial sediments using seismic reflection profiling with dmo processing we acquired a seismic reflection profile to image a shallow 100 m aquifer system on a small island in puget sound north of seattle washington the aquifer system is comprised of temperate glacial sedimentary strata with the primary aquifer lying approximately 45 m below the surface we chose the site because there are water well boring descriptions available and a sea cliff parallels the profile location providing stratigraphic control a 20 lb 9 kg weight drop was used as an energy source to acquire a 30 fold common midpoint cmp reflection profile along a 400 m line analysis of the recorded wavefield was not straightforward because of complex stratigraphy and the presence of a laterally variable thin bed 5 15 m of high impedance lodgement till within a few meters of the surface we used finite difference modeling to determine that conventional cmp processing would provide a reasonable approximation for imaging strata in the primary target zone roughly 20 m below the high velocity till fo

shallow seismic reflection mapping of the overburden bedrock interface with the engineering seismograph some simple techniques where unconsolidated overburden exceeds 20 m in thickness the reflection method may be efficiently used with a 12 channel engineering seismograph to map topography on the overburden bedrock interface as well as possible structure within the overburden the two techniques we suggest are the simplest forms of reflection profiling which can be applied with a minimum amount of equipment which we suggest is a 12 channel enhancement seismograph a 12 geophone array and a hammer source these techniques require good transmission characteristics of the overburden as well as a sharp velocity discontinuity at the overburden bedrock interface for data processing and display a microcomputer is essential

structurally controlled hydrocarbon seeps on a glaciated continental margin hecate strait offshore british columbia abstract this paper describes structurally controlled gas seeps and their influence on seafloor morphology at sites within queen charlotte basin qcb a sedimentary basin situated offshore british columbia western canada numerous surficial and shallow sediment features related to both ice cover and shallow gas have been identified in high resolution multichannel seismic huntec deep towed seismic multibeam bathymetry and backscatter and piston core data that were acquired over a 10 km by 10 km grid in an eastern part of qcb pockmarks seafloor mounds and iceberg ploughmarks are visible in the multibeam data small scale folds vertical blank zones and acoustic turbidity are observed in the huntec data the multichannel seismic data show a faulted anticline phase reversed bright spots zones of low instantaneous frequency and velocity pull ups one seafloor mound lies directly above a shallow fault that offsets an asymmetric fold the remaining mounds lie either above or in close proximity to asymmetric folds that are faulted bright spots are observed along some fault traces and within a cut and fill sequence at two way travel times between 0 42 and 1 0 s velocity pull ups are observed in the multichannel data below gas expansion voids present in piston cores backscatter data show that seafloor mounds are highly reflective compared to the surrounding seafloor these observations indicate that gas has been migrating upward through the rock column and expelled into the shallow sediments the shallow gas has been biologically mediated to form carbonate sediments within and around seafloor mounds shallow gas has also deformed surficial sediments into positive relief mounds and negative relief pockmarks

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

evidence for rapid sedimentation in a tunnel channel oak ridges moraine southern ontario canada abstract in south central ontario a late wisconsinan regional unconformity consisting of tunnel channels and drumlinized till crops out north of lake ontario the tunnel channels are locally infilled and the unconformity buried by sediment of the oak ridges moraine based on seismic reflection profiling and drillcore the tunnel channels are known to continue beneath the moraine detailed sedimentological analysis of 300 m of continuous core from two drillholes located 7 km apart in subparallel tunnel channels identified three facies the gravel facies is 17 m thick and occurs in only one of the cores it directly overlies the unconformity and may include a number of upward fining units seismic reflection data suggests the gravel forms stacked gravel mesoforms or quasi horizontal gravel layers bed load sheets deposited from fluidal flows along the deep channel axis the graded massive sand facies up to 37 m thick is the most common facies and consists of silty medium sand with minor coarse sand strata are reverse graded normal graded or massive with isolated silt intraclasts and evidence locally for dewatering this facies is interpreted to have been deposited from hyperconcentrated dispersions downflow of a hydraulic jump or major channel confluence the third facies consists of medium scale and small scale cross stratified sand medium scale cross stratification occurs predominantly in the lower 37 m of the core and is interbedded with the graded massive sand facies small scale cross stratified sand is progressively more common upward in core medium and small scale cross stratified sands was deposited respectively by subaqueous dunes and ripples formed in dilute fluid flows the complete succession is interpreted to have been deposited very rapidly within a subglacial tunnel channel before discharge ceased along the channel deposition followed closely and in part coincided with rapid expansion of the channel by erosion in a hydraulic jump or at a major channel confluence along the grounding line of a subglacial lake scour into unconsolidated sediment contributed to the sediment flux and quickly overloaded the flow with suspended sediment which in turn resulted in extremely high rates of sedimentation immediately downflow such depositional conditions support the notion that tunnel channels in the study area formed and or served as conduits for subglacial jokulhlaup discharges and that the laurentide ice sheet most probably did not generally drain by steady state processes but instead by short lived catastrophic events

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

acoustic architecture of glaciolacustrine sediments deformed during zonal stagnation of the laurentide ice sheet mazinaw lake ontario canada abstract in north america the last laurentide ice sheet retreated from much of the canadian shield by zonal stagnation masses of dead ice severed from the main ice sheet by emerging bedrock highs downwasted in situ within valleys and lake basins and were commonly buried by sediment consequently the flat sediment floors of many valleys and lakes are now pitted by steep sided enclosed depressions kettle basins that record the melt of stagnant ice blocks and collapse of sediment at mazinaw lake in eastern ontario canada high resolution seismic reflection magnetic and bathymetric surveys integrated with onland outcrop and hammer seismic investigations were conducted to identify the types of structural disturbance associated with the formation of kettle basins in glaciolacustrine sediments basins formed as a result of ice blocks being trapped within a regionally extensive proglacial lake glacial lake iroquois 12 500 to 11 400 years bp that flooded eastern ontario during deglaciation kettles occur within a thick 30 m succession of parallel high frequency acoustic facies consisting of rhythmically laminated varved iroquois silty clays iroquois strata underlying and surrounding kettle basins show large scale normal faults fractures rotational failures and incoherent chaotically bedded sediment formed by slumping and collapse mazinaw lake lies along part of the ottawa graben and while neotectonic earthquake activity cannot be entirely dismissed deformation is most likely to have occurred as a result of the rapid melt of buried ice blocks seismic data do not fully penetrate the entire basin sediment fill but the structure and topography of bedrock can be inferred from magnetometer data the location and shape of buried ice masses was closely controlled by the graben like form of the underlying bedrock surface

high resolution images of shallow aquifers a challenge in near surface seismology a near surface multifold high resolution seismic reflection experiment was conducted in the vicinity of the waste management area of a potash mine in western canada a buried channel was identified in the data and the stratigraphy of the quaternary infill of this structure was mapped the spatial extent of several prominent gravel sandy aquifers which represent the hydrogeologic framework of the region was outlined by the survey the seismic signatures also established the hydraulic independence of three major aquifers along the survey line the complex heterogeneous lithology of the surface cover limited effective elastic wave generation to surface sources this geologic framework also caused propagation of strong diverse coherent noise patterns which severely degraded reflected signal the suppression of those overhelming interfering events required the design of noise specific filters and their sequential multistep implementations results of forward modeling of background geologic information were c

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

low cost geophysical investigations of a paleochannel aquifer in the eastern goldfields western australia compressional p wave and shear s wave seismic reflection techniques were used to delineate the sand and gravel aquifer within a highly saline clay filled paleochannel in the eastern goldfields of western australia the seismic refraction and gravity methods were also used to investigate the paleochannel the unsaturated loose fine grained sand up to 10 m in depth at the surface is a major factor in degrading subsurface imaging the seismic processing needed to be precise with accurate static corrections and normal moveout corrections deconvolution enhanced the aquifer and other paleochannel reflectors p wave reflection and refraction layer depths had good correlation and showed a total of six boundaries 1 water table 2 change in velocity compaction in the paleochannel sediments 3 sand and gravel aquifer 4 red brown saprolite and green saprolite boundary 5 weathered bedrock and 6 unweathered bedrock p wave explosive and hammer sources were found to have similar signal characteri

on the formation of the tunnel valleys of the southern laurentide ice sheet abstract catastrophic releases of meltwater produced by basal melting and stored for decades in subglacial reservoirs at high pressure may have been responsible for eroding the broad deep tunnel valleys that are common along the margins of some lobes of the southern laurentide ice sheet we surmise that these releases began when the high water pressure was transmitted to the margin through the substrate the water pressure in the substrate at the margin would then have been significantly above the overburden pressure leading to sapping failure headward erosion of a conduit in the substrate piping could then tap the stored water resulting in the outburst in some situations development of a siphon may have lowered the reservoir below its overflow level thus tapping additional water following the flood the seal could have reformed and the reservoir refilled setting up conditions for another outburst order of magnitude calculations suggest that once emptied a subglacial reservoir could refill in a matter of decades the amount of water released during several outbursts appears to be sufficient to erode a tunnel valley we think that tunnel valleys are most likely to have formed in this way where and when the glacier margin was frozen to the bed and permafrost extended from the glacier forefield several kilometers back under the glacier as reservoirs would then have been larger and more common and the seal more robust and more likely to reform after an outburst

carbon isotope record of the early jurassic toarcian oceanic anoxic event from fossil wood and marine carbonate lusitanian basin portugal thetoarcianoceanicanoxicevent oae intheearlyjurassic 183maago wascharacterizedbywidespreadnear synchronous deposition of organic rich shales in marine settings as well as perturbations to several isotopic systems characteristically two positive carbon isotope excursions in a range of materials are separated by an abrupt negative shift carbon isotope profiles from toarcian fossil wood collected in england and denmark have previously been shown to exhibit this large drop 7 i n 13 c values interpreted as due to an injection of isotopically light co2 into the ocean atmosphere system however the global nature of this excursion has been challengedonthebasisofcarbon isotopedatafromnektonicmarinemolluscs belemnites whichexhibitheavierthanexpectedcarbonisotope values here we present new data principally from fossil wood and bulk carbonate collected at centimetre scale from a hemipelagic section at peniche coastal portugal this section is low in organic carbon average toc 0 5 and the samples should not have suffered significant diagenetic contamination by organic carbon of marine origin the carbon isotope profile based on wood shows two positive excursions separated by a large and abrupt negative excursion which parallels exactly the profile based on bulk carbonate samples from the same section albeit with approximately twice the amplitude 8 in wood versus 3 5 in carbonate these data indicate thatthe negative carbon isotope excursion affected the atmosphereand by implication the global ocean as well the difference in amplitude between terrestrial organic and marine carbonate curves can be explained by greater water availability in the terrestrial environment during the negative excursion for which there is independent evidence from marine osmiumisotope records and plausibly changes in atmospheric co2 content for which independent evidence is also available the peniche succession is also notable for the occurrence of re deposited sediments their lowest occurrence coincides with the base of the negative excursionandtheirhighestoccurrencecoincideswithitstop thus slopeinstabilityandsedimentsupplycouldhavebeenstronglylinked to the global environmental perturbation an association that may misleadingly simulate the effects of sea level fall

subglacial lake mcgregor south central alberta canada abstract it is proposed that a lake here named subglacial lake mcgregor existed beneath the laurentide ice sheet at or near the last glacial maximum the lake resided in the ancient buried mcgregor and tee pee preglacial valleys which are now mostly filled with glacigenic deposits the greatest thickness of sediment in the valleys is in the form of chaotically deposited lake beds that were laid down in a subaqueous environment by a number of process gravity flow water transport and suspension settling topographic sedimentary and stratigraphic evidence point to a subglacial not a proglacial origin for the beds during the early stages of lake existence ice movement was significant as there are numerous sets of shear planes in the sedimentary beds this indicates that the lake filled lake sedimentation and drained shearing of the beds by overlying ice when ice contacted the bed often thus early in its history the lake s was were ephemeral during the later stages of lake existence the lake was relatively stable with no rapid draining or influx of sediment gradual drainage of the lake resulted in lowering of the ice onto the lake beds resulting in subglacial till deposition drainage was not a single continuous event rather it was characterized by multiple phases of near total drainage till deposition followed by water accumulation lake sedimentation water accumulation events became successively less significant reflected by thinning of lake beds and thickening of till beds higher in the stratigraphic sequence since subglacial lake sedimentation appears to be restricted to the subglacial valleys it is suggested that the valleys acted as a large scale interconnected cavity system that both stored and transported water it is also suggested that these acted as the main routes of water flow beneath the laurentide ice sheet

applications of land based high resolution seismic reflection analysis to quaternary and geomorphic research abstract the availability in the last few years of low cost microcomputers and multichannel seismographs has made possible the application of high resolution digital seismic reflection profiling to quaternary problems seismic analysis of the subsurface can provide continuous two dimensional sections containing detailed structural and stratigraphic information a review of the general principles of shallow seismic reflection methods includes the nature of seismic sources the use of shotgun shells the application of the optimum offset method data processing and the issue of site dependency a brief overview is provided of seismic and sequence stratigraphy four case studies illustrate the application of reflection seismic profiling 1 champlain sea sediments late wisconsinan on the flank of the gatineau hills near quyon quebec 2 the definition of bedrock valleys beneath pleistocene deposits near shawville quebec 3 the fill of a linear depositional basin okanagan valley british columbia 4 the seismic signatures of topset and foreset beds slope failures and a distributary channel of the fraser river delta british columbia

hydrogeophysical imaging of alluvial aquifers electrostratigraphic units in the quaternary po alluvial plain italy the integration of surface geological and geomorphological information with borehole point data and geophysical e g geoelectrical images of the subsurface yields spatially consistent representations of alluvial aquifers heterogeneity at different scales from depositional systems to basin fills such an approach requires a conceptual framework to match the stratigraphic units with their evidence from ground based dc resistivity methods to effectively fill the gaps between sparse borehole data and to obtain valid representations of sedimentary heterogeneities such an approach is applied to characterize two sites of the quaternary aquifers of the central po plain italy which represent 1 the middle upper pleistocene braided to meandering river depositional systems sitting on southalpine crust and 2 their down current counterparts where they are involved by the latest uplift and deformation due to the tectonic activity of the apennine frontal thrusts electrical resistivity was considered as a proxy of the litho textural properties of hydrofacies and their major hierarchical association at depth and was interpreted in accordance with the depth decreasing resolution of ground based resistivity methods thus it was possible to identify the geophysical signature of hydrostratigraphic units through electrostratigraphic units i e sedimentary volumes identified by resistivity contrasts that spatially preserve the vertical polarity hydrostratigraphy and electrostratigraphy were then joined together through a site specific relationship between electrical resistivity and hydraulic conductivity which takes into account the prevailing process of current conduction the litho textural properties of hydrofacies and the groundwater electrical conductivity at the scales of aquifer systems and complexes this approach permitted to establish the conceptual framework to match hydrostratigraphy electrostratigraphy average hydrodynamic properties and distribution of heterogeneities

glacial tunnel valleys and quaternary history of the outer scotian shelf high resolution seismic reflection data indicate the presence of huge sub surface channel networks on the outer nova scotian continental shelf channel axes extend to over 450 m below present sea level b s l channel walls average 2 3 km in width mechanisms capable of producing such channels include fluvial submarine canyon or glacial erosion considerable debate has focused on the positions of the tertiary quaternary t q and pleistocene holocene p h boundaries1 4 in this region and the relationship of the t q boundary to the unconformity at the base of the channel networks uncertainty also surrounds the extent of pleistocene ice sheets on the south east canadian continental margin4 7 here we present a new stratigraphy for the sable island region fig 1 based on seismic profiles with lithologic and biostratigraphic control provided by two strategically placed boreholes this strati graphic analysis establishes the positions of the t q and p h boundaries outside the valleys at 51 m and 220 m b s l respectively our analysis also implies that the large channel systems are tunnel valleys cut by a sub ice meltwater process under a pre to early wisconsinan ice sheet which extended close to the shelf edge

practical techniques for 3d resistivity surveys and data inversion1 techniques to reduce the time needed to carry out 3d resistivity surveys with a moderate number 25 to 100 of electrodes and the computing time required to interpret the data have been developed the electrodes in a 3d survey are normally arranged in a square grid and the pole pole array is used to make the potential measurements the number of measurements required can be reduced to about one third of the maximum possible number without seriously degrading the resolution of the resulting inversion model by making measurements along the horizontal vertical and 45 diagonal rows of electrodes passing through the current electrode the smoothness constrained least squares inversion method is used for the data interpretation the computing time required by this technique can be greatly reduced by using a homogeneous half space as the starting model so that the jacobian matrix of partial derivatives can be calculated analytically a quasi newton updating method is then used to estimate the partial derivatives for subsequent iterations this inversion technique has been tested on synthetic and field data where a satisfactory model is obtained using a modest amount of computer time on an 80486dx2 66 microcomputer it takes about 20 minutes to invert the data from a 7 by 7 electrode survey grid using the techniques described below 3d resistivity surveys and data inversion can be carried out using commercially available field equipment and an inexpensive microcomputer

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

dimorphism and a new record of barroisiceras de grossouvre ammonoidea from the coniacian of bagh central india barroisiceras de grossouvre was hitherto unknown in india b onilahyense basse which elsewhere belongs exclusively to the coniacian has been recorded from the bryozoan limestone formation of central india this find as well as the presence of another time diagnostic coniacian ammonite placenticeras kaffrarium etheridge and two inoceramid bivalves inoceramus mytiloides incertus jimbo and i platyceramus mantelli barrois mercey in the underlying horizons help in resolving the long standing problem of the age of the bagh group b onilahyense shows wide intraspecific variability and overlaps morphometrically with the late turonian type species b haberfellneri hauer they may be conspecific the present species has been found to be sexually dimorphic a phenomenon previously not well recognised in the subfamily barroisiceratinae as in many late cretaceous ammonites dimorphism is manifest mainly by differences in relative adult sizes and in strength of ornamentation the microconch being sma

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

pliocene paleoenvironment evolution as interpreted from 3d seismic data in the southern north sea dutch offshore sector a high resolution 3d seismic survey from the dutch offshore sector has been interpreted and subsequently correlated with existing regional seismo stratigraphic concepts derived from conventional 2d seismic data sets the interpreted 13 seismic units have been related to a newly established chrono stratigraphic framework kuhlmann et al 2006a b chronostratigraphy of late neogene sediments in the southern north sea basin and paleoenvironmental interpretations palaeogeography palaeoclimatology palaeoecology 239 426 455 integrated chronostratigraphy of the pliocene pleistocene interval and its relation to the regional stratigraphical stages in the southern north sea region netherlands journal of geosciences geologie en mijnbouw 85 1 19 35 resulting in up dated age control for the seismic units the generation of amplitude maps time slices and isopach maps from the 3d seismic data enabled detailed spatial and temporal reconstruction regarding the paleoenvironmental and climatological development as depicted by kuhlmann et al 2006a chronostratigraphy of late neogene sediments in the southern north sea basin and paleoenvironmental interpretations palaeogeography palaeoclimatology palaeoecology 239 426 455 the lowermost seismic units s1 s4 comprise condensed middle miocene to piacencian sediments deposited under warm open marine conditions these sediments show a uniform seismic facies of low amplitude reflectors the boundary of seismic unit s4 s5 around 2 6 ma delineates a shift towards generally colder climate conditions that are connected to the onset of northern hemisphere glaciation seismic unit s5 includes alternations of warmer and colder periods during warmer periods bottom currents generated elongated structures 2 5 4 km long 300 500 m wide on the horizon display these layers show as well shallow gas accumulations with a more regional extent and are related to coarser grained sediments sealed by clayey sediments of the cold phases a homogenous seismic facies is characteristic for the colder periods within seismic units s6 and s7 internal fore set structures display the regressional trend related to the cooling of this interval seismic units s8 s11 display a pattern of unoriented lineaments which was probably caused by icebergs drifting into the north sea and souring the sea floor this finding is in accordance with the shallow marine arctic conditions of this interval within these units a bright spot of 3 by 7 km in size is interpreted to result from shallow gas accumulation related to a grounded iceberg this feature represents another more local area of gas accumulation that is closely related to iceberg activity within the study area the uppermost seismic units s12 and s13 belong to a fluvial paralic paleo environment that is characterised by aggradational seismic reflectors pleistocene glacial valleys were recognised in the upper 200 m below sea floor 2007 elsevier ltd all rights reserved

seismic attributes for monitoring of a shallow heated heavy oil reservoir a case study in production geophysics detecting the zones of production or constraining the in situ conditions within a reservoir are often of greater importance than obtaining highly resolved seismic structural images standard seismic data processing distorts the signal and limits the potential for extracting additional information especially for shallow targets an alternative shift stack procedure is applied in the processing of a shallow 12 fold 1 m common midpoint cmp spacing reflection profile acquired over a heated athabasca heavy oil sand reservoir the shift stack involves summing of cmp traces which have been flattened to an appropriate reference event simple modeling confirms that the prestack waveforms are better preserved by this process amplitude and frequency attributes are extracted from the reflection profile amplitudes of a continuous reservoir event vary by 600 over 35 m intervals along the profile bright spots correlate with heated regions apparent frequencies as measured by the instantaneous frequency and by short time window power spectral estimates of the subreservoir event are 20 30 hz lower in these same regions these diminished apparent frequencies most probably result from interference of the subreservoir reflection with events related to structural changes within the reservoir a complete interpretation of the results has not been attempted as knowledge of the in situ conditions is incomplete however changes in the seismic response at the well locations suggest that these attributes are useful in detection and mapping of heated zones the shiftstack procedure may also be useful in environmental and geotechnical applications

late quaternary depositional history of the reuss delta switzerland constraints from high resolution seismic reflection and georadar surveys glacial erosion has caused overdeepening of many alpine valleys after retreat of the ice they were filled with heterogeneous deposits of glacial lacustrine and fluvial sediments a typical example of such a valley segment and its infill is the reuss delta on the southern shore of lake lucerne in switzerland to obtain a detailed three dimensional image of this valley segment the eth institute of geophysics has acquired several two dimensional high resolution seismic and georadar profiles and conducted a three dimensional georadar survey interpretations of these geophysical data were constrained by a geological core extracted from a borehole 300 m deep near the investigation site the seismic profiles imaged ca 600 m of sediment infill above bedrock based on their reflection characteristics five different deposition units were distinguished these units were interpreted as a succession of clay silt at the base followed by different sand units with variable but generally increasing amounts of gravel this succession represented a prograding delta that filled the southern part of lake lucerne the latest fluvial development of the region is best represented by the georadar data in particular the three dimensional georadar data set provides a detailed view of an ancient braided river channel copyright 2002 john wiley sons ltd

the late quaternary stratigraphic record northwest of montreal regional ice sheet dynamics ice stream activity and early deglacial events the quaternary sediments of previously unstudied buried valleys and sections near montreal are analyzed and other sites are revisited to further develop the stratigraphic framework of the st lawrence lowland and to establish regional glacial and deglacial models the southwest trending buried valleys were investigated by stratigraphic drilling and high resolution seismic profiling the quaternary succession consists from base to top of proximal glaciolacustrine sediments two superposed till sheets argenteuil and oka tills of inferred late wisconsinan age and champlain sea sediments the glacial sediments of this sequence record an ice advance toward south argenteuil till followed by an abrupt ice flow shift toward the southwest oka till compositional and geomorphic data indicate that oka till is ubiquitous and is associated with a regional set of glacial landforms the analysis of a regional digital elevation model in combination with published ice flow indicators shows convergent flow pattern

incipient tunnel channels abstract glaciated terrains in east central alberta and south central michigan contain channels that have hummocks and transverse ridges separating depressions along their floors this association imparts a linked pothole appearance similar channels are often interpreted as tunnel channels or subaerial channels partly filled with sediment from a subsequent glacial advance a stagnating ice roof or slumped sediment from the channel margins however the truncation of sedimentary packages in the channel walls and intrachannel hummocks indicates that they are erosional landforms cut into glacial sediments till bedrock or gravel eskers overlie and are found within a few channels indicating that these channels formed before the final stagnation that produced the eskers these two characteristics combined with the observation that many channels have convex up long profiles indicate that the channels were eroded by pressurized subglacial water because the formative mechanisms for this type of channel are not clear and modern environments that could produce this type of landform are inaccessible we draw on several morphologic analogues to propose mechanisms for channel erosion we conclude that the erosion of these linked pothole channels incipient tunnel channels was the product of the complex interaction between complex turbulent flow structures and various scales of roughness elements

large near surface velocity gradients on shallow seismic reflection data extreme velocity gradients occasionally present within near surface materials can inhibit optimal common midpoint cmp stacking of near surface reflection arrivals for example abrupt increases in velocity are observed routinely at the bedrock surface and at the boundary between the vadose and the saturated zone when a rapid increase in near surface velocity is found nmo correction artifacts manifested on cmp gathers as sample reversion sample compression or duplication of reflection wavelets can reduce s n ratio on stacked data or can stack coherently elimination of these nonstretch related artifacts using conventional nmo stretch muting requires near vertically incident reflection arrivals and allowable stretch ratios as small as 5 in some shallow environments radical allowable stretch mutes are not a feasible means to subdue these artifacts if high amplitude coherent noise on near offset traces inhibits identification and digital enhancement of shallow reflections on most shallow seismic refl

geology and formation mechanism of late quaternary shallow biogenic gas reservoirs in the hangzhou bay area eastern china late quaternary shallow biogenic gas reservoirs have recently been discovered and exploited in the coastal hangzhou bay area northern zhejiang province eastern china the river in this area strongly incised the underlying old beds during a period of glacial maximum which resulted in the formation of the qiantangjiang and the taihu incised valleys these incised valleys were filled with fluvial sediments and buried by marine sediments during the postglacial period late quaternary strata of the incised valley area are composed of four sedimentary facies in ascending order fluvial floor facies iv flood plain facies iii sublittoral marine bay facies ii and estuarine facies i all commercial gas fields occur in flood plain sand bodies of incised valleys the bodies are buried 3060 m 98197 ft deep and are 3 07 0 m 9 823 ft thick with a maximum thickness of more than 10 m 33 ft they are surrounded by impermeable clays rapid deposition of overlying sublittoral marine bay sediments supplied not only abundant gas sources but also good preservation conditions the main hydrocarbon sources are dark gray clays of the flood plain facies and gray muds of the sublittoral marine bay facies sediments of both facies have organic carbon content generally more than 0 4 shallow biogenic gas fields and deep gas fields require vastly different drilling and completion techniques drilling and completion costs are much lower for the biogenic gas fields quaternary incised valleys and flood plains other than hongzhou bay in coastal areas of eastern china are promising targets of further exploration for shallow biogenic gas

subglacial tunnel channels porcupine hills southwest alberta canada abstract at some time close to the last glacial maximum lgm high energy subglacial laurentide meltwater flows eroded a series of discontinuous tunnel channels into the northeastern flanks of the porcupine hills and adjacent parts of the high plains near nanton stavely and claresholm discrete channel segments kilometers long up to about 1 km wide and 100 m deep were carved into paleocene sandstone and shale of the porcupine hills formation floors of pine boneyard and crocodile channels all occur at elevations between 1050 and 1175 m a s l and share the characteristic of strongly convex up long profiles intrachannel drainage divides on each channel floor are tens of meters above the water entry and exit points formative flows therefore must have been pressurized in the subglacial nye channels prominent scour holes at some major bends in the channels now host ephemeral ponds or lakes during the channel erosion the overlying laurentide ice surface was probably close to its local lgm maximum elevation of ca 1400 1500 m a s l misfit modern streams now drain in opposite directions within the tunnel channels and there are only minor local distal accumulations of sediment derived from the tunnel channel erosion

muting the noise cone in near surface reflection data an example from southeastern kansas a 300 m near surface seismic reflection profile was collected in southeastern kansas to locate a fault s associated with a recognized stratigraphic offset on either side of a region of unexposed bedrock a substantial increase in the s n ratio of the final stacked section was achieved by muting all data arriving in time after the airwave methods of applying traditional seismic data processing techniques to near surface data 200 ms of data or less often differ notably from hydrocarbon exploration scale processing 3 4 s of data or more the example of noise cone muting used is contrary to normal exploration scale seismic data processing philosophy which is to include all data containing signal the noise cone mute applied to the data removed more than one third of the total data volume some of which contains signal in this case however the severe muting resulted in a higher s n ratio in the final stacked section even though some signal could be identified within the muted data this example supports the suggestion that nontraditional techniques sometimes need to be considered when processing near surface seismic data

shallow seismic surveying of an alpine rock glacier to map the internal structure and lower boundary of an alpine rock glacier we recorded three shallow seismic profiles and drilled four 70 m deep holes through to the underlying bedrock although analysis of the seismic data using standard reflection processing schemes did not yield conclusive results because of the dominantly low frequency returned signals and the presence of strong source generated noise tomographic inversions of first arrival times were successful in mapping several critical subsurface features a thin low velocity layer of loose boulders air voids and snow was found to extend across the entire surveyed area below this layer two distinct velocity regimes superimposed on a general increase in velocity with depth were identified a broad regime of high velocities was interpreted to contain boulders with numerous ice filled voids whereas an adjacent regime of relatively low velocities was explained in terms of boulders with air and water filled voids this latter region of degrad

electrical resistivity ground imaging ergi a new tool for mapping the lithology and geometry of channel belts and valley fills efforts to map the lithology and geometry of sand and gravel channel belts and valley fills are limited by an inability to easily obtain information about the shallow subsurface until recently boreholes were the only method available to obtain this information however borehole programmes are costly time consuming and always leave in doubt the stratigraphic connection between and beyond the boreholes although standard shallow geophysical techniques such as ground penetrating radar gpr and shallow seismic can rapidly obtain subsurface data with high horizontal resolution they only function well under select conditions electrical resistivity ground imaging ergi is a recently developed shallow geophysical technique that rapidly produces highresolution profiles of the shallow subsurface under most field conditions ergi uses measurements of the ground s resistance to an electrical current to develop a two dimensional model of the shallow subsurface 200 m called an ergi profile ergi measurements work equally well in resistive sediments

tunnel channels and associated landforms south central ontario their implications for ice sheet hydrology the peterborough trenton drumlin field and associated meltwater erosion marks s forms were interpreted as the products of erosion by turbulent separated flows within a catastrophically released subglacial meltwater sheet the algonquin event recent theoretical modelling of ice sheet hydrology suggests progressive channelization of meltwater during the collapse of such a sheet inductive testing of this hypothesis using observations in south central ontario forms the crux of this paper a geometric model of the interaction of rough ice base and bed surfaces gives inferred meltwater flow patterns such a rough interactive geometry is suggested as the bed is drumlinized and flows responsible for drumlins may be expected to have eroded similar forms into the ice base three landforms in south central ontario namely tunnel channels megachannels and late stage sheet flow scours are described and interpreted as evidence of progressive channelization and flow diversion processes governed by the geometri

3 d high resolution reflection seismic imaging of unconsolidated glacial and glaciolacustrine sediments processing and interpretation shallow 3 d seismic reflection techniques have been used to map glacial deposits in a swiss mountain valley a dense distribution of source and receiver positions resulted in a small subsurface sampling of 1 5 m 1 5 m and a high fold of 40 common processing operations that included pseudotrue amplitude scaling deconvolution and band pass filtering successfully enhanced shallow reflections relative to source generated noise careful top muting helped avoid erroneous stacking of direct and guided waves azimuth dependent velocity analyses proved to be unnecessary three dimensional 3 d migration of the stacked data yielded the final high resolution images of the shallow subsurface 15 170 m because most reflections and diffractions were migrated to their correct subsurface locations confident interpretations of 3 d structures were possible time slices and cross sections along arbitrary directions proved to be powerful analysis tools even small scale features 20 m wide such as subglacial cha

the origin and glaciodynamic significance of sandstone ridge networks from the hirnantian glaciation of the djado basin niger the djado basin niger was located beneath the inner part of the late ordovician ice sheet the felar felar formation consists mainly of glaciomarine deposits associated with the major ice sheet recession within the glaciation and is bounded by two glacial unconformities structures corresponding to sandstone ridges are found within the felar felar formation sandstone ridges are several metres high about 10 m wide and hundreds of metres long these structures are organized in extensive anastomosed to subpolygonal networks the association of sandstone ridge networks with the later glacial unconformity and with other glacial evidence suggests sub glacial conditions for their origin sandstone ridge sedimentological characteristics indicate that sandstone ridges result from the scouring of the felar felar formation by sub glacial turbulent and pressurized meltwater then subglacial cavities were infilled with sand derived from glacial abrasion sandstone ridge networks are comparable with tunnel channels and document unusual drainage structures of the inner part of the palaeo ice sheet

the meltwater hypothesis for subglacial bedforms this paper presents an historical and in places informal account of the meltwater hypothesis which invokes enormous outburst floods for the formation of subglacial bedforms it begins with a brief discussion of the difficulties of determining processes of formation for landforms which are not seen in formation analogy provides a solution to these difficulties analogy between erosional marks at the bases of turbidites and drumlins which were the starting point for this hypothesis rests on the idea that inverted erosional marks at the ice bed are subsequently infilled to form drumlins field tests on the sedimentology architecture and landform associations of drumlins in the livingstone lake drumlin field are outlined before more extensive work on bedrock erosional forms and flood routes is introduced bedrock erosional forms played a central part in establishing the hypothesis since their form and ornamentation are confidently interpreted as fluvial their form and genesis are discussed mainly with reference to sites at french river and wilton creek ontario though some remarkable bedrock erosional forms in antarctica support their regional extent initially in the meltwater hypothesis drumlins were thought to be cavity fills and erosional drumlins were recognized later this development is shown to be central to the realization that drumlin composition may be inferred from drumlin form the scale of drumlin fields measured at about 10 3 km 2 and the magnitude of the inferred floods require that the flood events were regional regional scale flood tracts in ontario quebec alberta and the northwest territories extending over 1000 km in length and several hundred kilometers in width support this suggestion floods had they occurred would have caused rapid rates of sea level rise and may have changed climate through their effects on ocean stratification and sea surface temperatures the meltwater hypothesis covers a range of bedforms besides drumlins and bedrock erosional marksffluting rogen moraine hummocky terrain and transverse ridges recent work shows how these forms are best explained by the meltwater hypothesis the roles of water storage and release which underpin the theory of the meltwater hypothesis remain poorly understood r 2002 published by elsevier science ltd

cenozoic evolution of the eastern danish north sea this paper provides a review of recent high resolution and conventional seismic investigations in the eastern danish north sea and describes their implications for the development of the eastern north sea basin the results comprise detailed time structure maps of four major unconformities in the eastern danish north sea the top chalk surface mid paleocene near top oligocene the mid miocene unconformity and base quaternary the maps show that the eastern danish north sea has been affected by faulting and salt diapirism throughout the cenozoic carbonate mounds erosional valleys and pockmark or karst like structures were identified at the top of the upper cretaceous danian chalk group strike parallel erosional features and depositional geometries observed at near top oligocene and at the mid miocene unconformity indicate that these major sequence boundaries can be attributed to large scale lateral changes in sediment supply directions increases in sediment flux to the southeastern north sea at the eocene oligocene transition and in the post middle miocene appear to correlate with similar events world wide and with long term 18o increases indicating forcing by global factors i e eustasy and climate stratal geometries observed on the seismic data indicate that the so called neogene uplift of the eastern danish north sea may have been hundreds of metres less than previously suggested it is argued that late cenozoic uplift of the basin margin and of mountain peaks in southern norway may have been caused entirely by isostatic uplift of the crust in response to accelerated late cenozoic denudation and dissection of topography created in the paleogene the late cenozoic periods of accelerated denudation and incision rates were most likely driven by climatic deterioration and long term eustatic lowering rather than active late cenozoic tectonics the cause of which is conjectural a series of shallow thrust structures and an associated system of deep buried valleys were mapped thrust faulting most likely occurred in response to gravitational loading at the margin of an advancing ice sheet and it was followed by deep incision due to subglacial melt water erosion probably during the elsterian glaciation