practical application of the sequential balancing technique an example from the irish variscides the process of sequential restoration presented by cooper trayner 1986 is applied to a n s cross section through the irish variscides to demonstrate that this approach improves the accuracy of thin skinned structural modelling and allows the progressive deformation of a marginal belt to be examined the principles and methods of sequential restoration are illustrated the cross section is restored in four steps and gives a bulk shortening of 52 in this model the fold belt evolved as the basal decollement migrated northwards and considerable ductile shortening by folding and cleavage formation preceded faulting although faulting gradually became more important towards the north the section is divided into four structural domains based upon different deformation histories each is restored independently by removing the ductile strain before restoring the faulting from this example it is clear that many models derived from single step balancing especially those through ductile deformed areas should be re examined

short paper seismic reflection evidence for the location of the iapetus suture west of ireland a new deep seismic profile recorded west of ireland by the british institutions reflection profiling syndicate birps images prominent north dipping reflectors west of the shannon estuary qualitatively similar north dipping reflectors have been imaged on previous birps profiles recorded across the solway line both east and west of britain and have there been interpreted as structures related to the iapetus suture by analogy with these other north dipping reflectors and because the newly discovered reflectors lie close to previously hypothesized locations of the iapetus suture the structures seen on the new birps profile are here interpreted as representing part of the iapetus suture zone

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

three dimensional hydrostratigraphy of the orangeville moraine area southwestern ontario canada regional scale three dimensional modelling of quaternary sediments in the orangeville moraine area of southwestern ontario has been completed as part of the ontario geological survey groundwater in

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

borehole geophysical log signatures and stratigraphic assessment in a glacial basin southern ontario over the past two decades the geological survey of canada has used a standardized suite of slim hole geophysical tools to log 57 polyvinyl chloride cased boreholes drilled in the glacial sediments of southern ontario this article documents downhole tool responses natural gamma apparent conductivity magnetic susceptibility and seismic velocity in the context of mineralogical characteristics of the region and grain size data from 28 of the 57 boreholes characteristic geophysical properties and or patterns are identified within the units of a regional hydrostratigraphic framework in southern ontario the importance of a calibrated suite of tools is emphasized as stratigraphic units may have variable response from site to site the use of a high sensitivity magnetic susceptibility induction probe is shown to be an important tool in the log suite for lithostratigraphic interpretation and more broadly for provenance studies of source rock across the region ranges of compressional p and shear s

preservation of local high resolution data in a regional low resolution dataset a nested 3d modeling approach using an example from a quaternary glacial stratigraphy ontario canada abstract regional scale 3d models provide insight into general unit geometries and stratigraphic relationships however potentially significant local scale data and unit geometries are rarely preserved accurate modeling of local scale unit geometries and stratigraphic relationships is important for understanding aquifer aquitard connectivity estimation of aquifer volume and prediction of contaminant migration pathways this is particularly important in previously glaciated areas because of the high degree of lithological heterogeneity and stratigraphic variation in glaciogenic deposits this study presents a case study ontario canada that explores the impacts of an optimized model in which high resolution data are preserved in local scale models as golden blocks and non optimized regional scale 3d models on the predicted geometries and stratigraphic relationships of coarse and fine grained units lithofacies associations within a quaternary stratigraphy each of the regional scale models include high quality data outcrop and cored borehole logs and low quality data water well lithology logs however the optimized model was constructed by seamlessly integrating the golden blocks into the regional scale model using an algorithm producing a nested model whereas the non optimized model was built using a global modeling approach by way of comparison to unit geometries previously delineated in outcrops and surficial geology maps and hydraulic responses from pumping tests the results of this study indicate that coarse grained units which host significant municipal aquifers and fine grained units were more realistically and accurately represented in the optimized model as a result of the nested 3d modeling approach this case study demonstrates a relatively simple and effective method for preserving high quality data in a regional scale 3d model without sacrificing model accuracy in key areas of interest e g aquifer well fields

cleavage strain in the variscan fold belt county cork ireland estimated from stretched arsenopyrite rosettes abstract in south west ireland hydrothermally formed arsenopyrite crystals in a devonian mudstone have responded to variscan deformation by brittle extension fracture and fragment separation the interfragment gaps and terminal extension zones of each crystal are infilled with fibrous quartz stretches within the cleavage plane have been calculated by the various methods available most of which can be modified to incorporate terminal extension zones the strain reversal method is the most accurate currently available but still gives a minimum estimate of the overall strain the more direct hossain method which gives only slightly lower estimates with this data is more practical for field use a strain ellipse can be estimated from each crystal rosette composed of three laths assuming the original interlimb angles were all 60 and because actual rather than relative stretches are estimated this provides a lower bound to the area increase in the plane of cleavage based on the average of our calculated strain ellipses this area increase is at least 114 and implies an average shortening across the cleavage of at least 53 however several lines of evidence suggest that the cleavage deformation was more intense and more oblate than that calculated and we argue that a 300 area increase in the cleavage plane and 75 shortening across the cleavage are more realistic estimates of the true strain furthermore the along strike elongation indicated is at least 80 which may be regionally significant estimates of orogenic contraction derived from balanced section construction should therefore take into account the possibility of a substantial strike elongation and tectonic models that can accommodate such elongations need to be developed

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

the irish variscides a fold belt developed within a major surge zone an arcuate fold belt characterizes the irish variscides south of the dingle dungarvan line the folds are cross cut by numerous faults which tend to be sinistral in the west and dextral in the east clockwise transection of major fold axes by minor folds and by cleavage has been recorded in the east while anticlockwise transections are common in the west these and other features suggest that north south dextral and sinistral shearing occurred in the east and west respectively this is considered to be a consequence of the morphology of the munster basin which allowed the development of a surge zone in the centre of the fold belt the variscan sole thrust is believed to lie at the base of or within the old red sandstone however to the east thinning of the upper palaeozoic cover sequence forces the sole thrust to cut down laterally stratigraphic section into lower palaeozoic basement in the depocentre the sole thrust surged ahead by propagating along bedding surfaces or possibly a devonian carboniferous extensional detachment beneath the basin while to the east and offshore to the west its propagation was inhibited by the absence of easy decollement horizons within the basement consequently the tip line became strongly arcuate and the thrust developed into a surge zone the sole thrust propagated northwards by stick slip processes much of the deformation within the cover sequence occurred during sticking events particularly fold tightening and accommodation faulting

surge zones in the moine thrust zone of nw scotland abstract the caledonian thrust zones of assynt show several examples of large fault bounded structures surge zones up to 8 km 2 in extent which have moved further than adjacent rocks extensional faults can be traced into strike slip faults and then to contractional imbricate faults there are also zones of extensional and contractional flow as shown by strained bioturbation marks in the cambrian pipe rock several other low angle extensional fault zones have been recognized along the length of the moine thrust zone notably in the kinlochewe district recognition of these extensional faults and local surge zones has solved several local problems such as the lack of continuity of the glencoul thrust and the out of sequence character of some of the large low angle faults though the thrust propagation direction was generally from east to west in the transport direction several of the eastern faults have been reactivated later and locally cut down as extensional faults the so called moine thrust shows extensional fault movement at several localities along its length the extensional structures and the surge zones suggest that body forces have been important in driving the faults rather than just a push from the rear the moines and moine thrust zone were presumably driven to the wnw by gravity spreading and thinning of the main scottish caledonides

a thin skinned thrust model for variscan pembrokeshire wales abstract the east and west coasts of pembrokeshire sw wales provide two sections through the variscan fold and thrust belt the evolution of these structures is interpreted in terms of a thin skinned tectonic model balanced cross sections are constructed for the high level imbricate sequences and these allow reasonably accurate estimates of shortening to be made basement control on structures developed in the upper carboniferous cover rocks is minimal though some thrust ramp positions may be determined by the location of earlier normal faults the thrust belt may be divided into two parts according to the depth to the decollement horizon in the north imbricate fans developed from a shallow level detachment shortening is heterogeneous and both thrust periodicity and fold style are partly determined by rheology cumulative tectonic displacement increases to the west across pembrokeshire resulting in a net clockwise rotation of about 40

contrasting sources for lead in cu polymetallic and zn pb mineralisation in ireland constraints from lead isotopes devonian red bed hosted deposits in sw ireland occur either as low grade stratiform disseminations and minor veinlets with bornite and djurleite or as major quartz veins with copper polymetallic deposits pb isotopic ratios vary according to the style of mineralisation for disseminated sulphides there is a considerable scatter of lead isotope compositions these appear to define a mixing line between a basement source and a more radiogenic component which may have been granite derived detrital minerals in the red beds for minor veinlets there is a more restricted distribution of lead isotopic data which could be interpreted in terms of fluid flow homogenising the wide spread of pb isotope ratios from the disseminated sulphides this is consistent with published sulphur isotope data major quartz veins show a clustered array of pb isotopic ratios and it is suggested that the bulk of the metals were derived after variscan compression from a basement source with a minor contribution from red beds lower carboniferous limestone hosted deposits form three groups cu ag hg as sb u epigenetic vein hosted deposits cu as zn pb ag co ni stratabound deposits and cu as pb ag mo co ni epigenetic vein deposits lead isotope compositions for the stratabound deposits have a very restricted range for each locality and plot close to the same mixing line as the carboniferous zn pb deposits of central ireland it is suggested that the lead in both areas has been derived from a similar basement source the model age of ca 350 10 ma for stratabound mineralisation is close to that of their host rocks lead isotope data for the epigenetic cu as vein deposits suggest a similar source but a younger age of 280 10 ma for mineralisation this 280 10 ma age has important implications for the timing of the variscan orogeny in sw ireland because chalcopyrite tennantite occurs along pressure solution cleavage planes that have been deformed by thrusting the emplacement of the mineralised tectonic ross island sheet west of killarney must post date 280 ma the difference in the ore assemblage between the limestone hosted cu polymetallic deposits of ireland and the penecontemporaneous zn pb deposits of central ireland may be because of differences in the underlying lithology in the sw up to 6 km of red beds underlie the limestone hosted stratabound cu polymetallic deposits whereas in central ireland the zn pb ore bearing carbonate rocks are underlain by lower palaeozoic greywackes with a minimal thickness of late devonian red beds however the isotopic evidence only indicates a major red bed source of metals at ardtully elsewhere unless the metals in the red beds originated from a basement rather than a granite source a significant red bed contribution to the metals seems unlikely an alternative possibility is that copper ores were derived from an extension of the ordovician island arc of se ireland

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

late mesozoic reactivation of variscan faults in the north celtic sea basin ireland abstract three major ene trending reactivated fault systems downthrow to the north west in the north western part of the north celtic sea basin they control the location of three mesozoic half grabens the location of the faults is determined by the underlying basement structures these are variscan in age although they exploit older caledonide trends comparison of the variscan structures identified in the area with regional structures suggests a series of imbricate thrust features these structures reactivated during the mesozoic controlled both the location and depositional histories of the half graben basins in the region

the role of pressure solution and intermicrolithon slip in the development of disjunctive cleavage domains a study from helvick head in the irish variscides the offset of sedimentary laminae bedding surfaces veins and other passive markers across disjunctive cleavage domains has commonly been attributed solely to pressure solution a detailed study of disjunctive cleavage within siltstones and sandstones from helvick head co waterford on the eastern margin of the irish variscides indicates that the cleavage domains were initially formed by pressure solution processes however the study also revealed features which cannot be explained by pressure solution alone the cleavage domains represent planes of weakness within otherwise relatively homogeneous rock during fold tightening they were rotated out of parallelism with the principal xy plane of the finite strain ellipsoid thus enabling shear movement between the microlithons it is considered that this intermicrolithon slip caused a rotation of the elongate quartz grains within the cleavage domains resulting in an asymmetry of their long axes with respect to the cleavage domain margin grain microfracturing and occasionally slickensides on cleavage surfaces intermicrolithon slip is unlikely to be a localized phenomenon but is probably widespread throughout the irish variscides and other orogenic belts

nature and origin of sediments infilling poorly defined buried bedrock valleys adjacent to the niagara escarpment southern ontario canada the buried paleozoic bedrock surface of southern ontario is dissected by an interconnected system of valleys these buried valleys are infilled with thick successions of glacial interglacial and fluvial sediments that contain a lengthy record of changing environmental conditions during the late quaternary detailed logging of over 500 m of sediment recovered from 11 continuously cored boreholes provides the basis for this study the boreholes were drilled within two poorly defined bedrock valleys located east of the niagara escarpment in southern ontario as part of a groundwater exploration program six distinct facies types were identified within the cores sand gravel fine grained sediment and sand rich mud rich and clast rich diamict textural characteristics of the cored sediments and vertical changes in facies types were used to identify six stratigraphic units su i through su vi within the valley infill deposits these units are interpreted to record fluvial or colluvial su i lacustrine su ii fluvial glaciofluvial or deltaic su iii subglacial su iv glaciofluvial su v and subglacial or ice marginal su vi conditions sediment characteristics and stratigraphic relationships allow tentative correlation with known surficial deposits analysis of the subsurface characteristics and geometries of this stacked succession of coarse and fine grained stratigraphic units also allows identification of the geometry of potential aquifers

flexural cantilever models of extensional subsidence in the munster basin sw ireland and old red sandstone fluvial dispersal systems

regional unconformities and the sedimentary architecture of the oak ridges moraine area southern ontario seismic stratigraphy geometry and sediment facies within the oak ridges moraine orm area of ontario record major structural elements and surfaces of the quaternary sedimentary sequence the derived stratigraphic architecture can be used to identify the key elements of a regional erosional surface represented by an unconformity in the subsurface and associated overlying channel sediments the erosional surface unconformity forms a distinct time datum in the quaternary sequence which provides an important aid to lithostratigraphic correlation the architecture also gives improved understanding of the effects of erosion on the late glacial landscape the surfaces of erosional drumlins and intervening troughs and the beds and banks of meltwater channels in the orm area define the regional unconformity highlighted by seismic profiles linked to continuously cored boreholes these features are attributed to regional scale subglacial meltwater flow events the sculpted surfaces which are analogous to water eroded forms the presence of boulder lags and coarse grained deposits on the regional erosional surface and the channels with undulating profiles provide the vital supporting evidence for a meltwater interpretation the inter regional extent of the unconformity is inferred from the coherence of regional paleoflows and the extent of drumlinized uplands tunnel channels and scoured bedrock terrain across 75 of the landscape from the orm area east and south to the finger lakes new york the implied magnitude of erosion suggests a pressing need for directed sedimentological study in those ocean basins that were probable depositional sites for flood deposits

evaluating the groundwater resource potential of the dundas buried bedrock valley southwestern ontario an integrated geological and hydrogeological case study population growth in the groundwater dependent municipalities of southwestern ontario has prompted interest in the exploration for new previously untapped groundwater resources in this study th

thermal effects and timing of thrusting in the moine thrust zone illite crystallinity studies in the foreland to the moine thrust belt show a widespread heating event which is interpreted as a thermal response to thrust sheet emplacement temperatures reached 275 c approximately 24 ma after the main thrusting events based on the relationships between syn thrusting intrusions and the thrust planes combined with radiometric dates from the moine sheet it is argued that movement on the moine thrust started between 440 and 430 ma ago contractional slip in the moine thrust belt ceased at about 425 ma ago and it is proposed that illite metamorphism attained temperatures of 275 c at about 408 ma ago

subglacial hydrology in north western germany during the last glaciation groundwater flow tunnel valleys and hydrological cycles abstract numerical modelling of groundwater dynamics in an overpressured system of subglacial aquifers and aquitards under the marginal portion of the scandinavian ice sheet in northwestern germany has been coupled with calculations of basal meltwater production rates and with field evidence of meltwater erosional features largely fine grained subglacial sediments with relatively low hydraulic conductivities had a capacity to drain only about 25 of all basal meltwater produced in the relevant subglacial catchment area the rest of the meltwater was evacuated through tunnel valleys in spontaneous outburst events subglacial drainage cycles are proposed in which each cycle begins with meltwater drainage through the substratum groundwater flow followed by water ponding at the ice bed interface and ends in catastrophic meltwater releases through tunnel valleys

structural controls on palaeozoic basin evolution and inversion in southwest wales the tectonic evolution of sw dyfed is presented in terms of a model of basin evolution and subsequent destruction by reactivation of the basin bounding faults in contractional mode movements along long lived palaeozoic extensional faults were reversed during variscan compression with the pre existing faults accommodating the contraction in addition to the development of a thin skinned thrust and fold system basin analysis of the pre variscan extensional system uses sedimentological and stratigraphical evidence to constrain the timing of movements of the basin bounding faults thus allowing division of the stratigraphy into depositional megasequences related to these extensional movements field examples on all scales illustrate active extension during the palaeozoic structural cross sections through sw dyfed demonstrate the interrelationship of the extensional and contractional deformations with palinspastic restorations of the cross sections illustrating the complexity of the pre variscan template

thrust surface geometry implications for thrust belt evolution and section balancing techniques abstract the application of modern concepts of thrust tectonics to orogenic belts has yielded many detailed restorable sections thrusts generally restore with a staircase geometry onto the undeformed stratigraphic template since the work of rich 1934 it has been generally assumed that thrusts have propagated with a staircase trajectory and that hangingwall structures result from the movement of the thrust sheet up footwall ramps however thrusts often cut through previously folded strata with a smooth trajectory smooth trajectory thrusts can be identified from footwall geometry and strain state once a smooth trajectory thrust has been identified it is insufficient to produce a balanced deformed section and its restored counterpart sequential balanced sections must be constructed reversing the deformation history the restoration of thrusts to a staircase trajectory has no mechanical significance as the thrust is merely acting as a marker if restored back beyond the time at which it propagated thrust ramps are not necessarily actual steps in the thrust surface they are often just zones where the thrust cuts through bedding in a smoothly curving trajectory ductile deformation in thrust belts can reduce bed length at depth thus removing the need for structures which are necessary to preserve constant bed lengths ductile deformation in the internal zones of orogenic belts renders the construction of balanced sections in these zones highly problematical

drumlinized tunnel valleys in south central ontario abstract a network of late wisconsin valleys deeply incise thick successions up to 200 m of quaternary sediment and paleozoic bedrock across south central ontario the valleys have previously been interpreted as an integrated network of tunnel channels recording catastrophic releases of subglacial meltwater across the bed of the laurentide ice sheet lis recent geologic investigations in the western part of the valley network integrate information gained from surficial sediment and landform mapping continuously cored boreholes that penetrate the entire quaternary sediment succession in valleys as well as intervening uplands and both water borne and land based geophysical surveys together these data provide new insights and a refined interpretation of the genesis timing and paleoglaciological significance of these valleys valleys within the study area are some of the largest observed within the regional network up to 30 km long 7 km wide and 175 m deep they are oriented both north south and in a radial pattern varying from ne sw in the southern part of the study area to ese wnw in the north an undulating late wisconsin till sheet newmarket till caps the regional uplands and is commonly observed along the flanks and floors of the valleys all of which show evidence of a drumlinization phase that postdates excavation of most of the valleys at least two generations of valleys are interpreted from cross cutting relationships with abrupt heads and locally perched downflow ends of some valley forms radiocarbon age determinations of organic material from both below and above the newmarket till constrain the timing of valley excavation to between 28 05 and 12 81 14c kyr bp borehole and outcrop data within the valleys reveal four stratigraphic units su1 su4 that form the floor and sediment infill of the valleys the newmarket till su1 is commonly underlain by deformed substrates the till is absent in parts of some valleys and the lowermost parts of the valley fill commonly consist of coarse grained gravels su2 that pass upwards into sands su3 the bulk of the sediment infill consists of glaciolacustrine silt clay rhythmites su4 that pass upwards into organic and mollusk bearing nearshore and fluviodeltaic deposits that record drainage of proglacial lakes from the study area during deglaciation it is proposed that these features are a previously unrecognized type of tunnel valley that developed subglacially during late wisconsin ice cover meltwater surpluses would be generated at the ice bed interface due to the low transmissivity substrate in the study area with flow preferentially routed into lows on the bed downward incision into confined and pressurized aquifers likely facilitated rapid headward erosion of the valleys enhanced by piping waning meltwater flow velocities discharges allowed ice creep to re occupy newly formed valleys permitting renewed till deposition and subsequent drumlinization of the valleys the valleys also provided efficient routing for meltwater draining the retreating ice marginal zone during deglaciation and allowed localised sand and gravel deposition as eskers and ice proximal subaquatic fans in the oak ridges moraine and within proglacial lakes that developed during ice retreat however there is no evidence for regional scale catastrophic release of subglacial meltwater in the valley infills this integrated dataset represents one of the most complete characterizations of subglacial valleys in a terrestrial setting in north america

the variscan fold thrust belt of southeast ireland and its control on early mesozoic extension and deposition a method to predict the sherwood sandstone

a determination of the bulk rotational deformation resulting from displacements in discrete shear zones in the hercynian fold belt of south ireland abstract in the hercynian fold belt of south ireland near the town of ardmore county waterford units of openly folded upper palaeozoic sandstone and siltstone are traversed by closely spaced arrays of en echelon quartz veins these arrays mark the sites of shear zones which are bounded by weakly deformed wall rocks we used the attitude of cleavage and the sigmoidal shapes of veins to calculate the cumulative shear strain across each zone and then determined the bulk deformation suffered by the outcrop by tensor analysis with the aid of an off axis mohr circle construction we conclude that as a consequence of vein formation the rocks at this outcrop suffered significant bulk deformation with an 8 14 clockwise rotation

cataclasites along the saltville thrust u s a and their implications for thrust sheet emplacement abstract cataclasis and frictional wear are the primary bulk deformation mechanisms along steeply dipping portions of the saltville thrust in the southern appalachian foreland zone u s a fault character ranges from a single discrete sliding surface with negligible gouge to a zone of several discrete sliding surfaces or a zone up to 0 3 m thick of pervasive cataclasite marked fracturing occurs up to 20 m above the fault whereas minimal deformation is found in the footwall rocks hanging wall dolomites range from crush breccias less than 5 matrix to ultracataclasites with 90 matrix although cataclasites 50 70 matrix are predominant foliated cataclasites occur where dolomite is thrust over shale progressive development of cataclastic fabrics is due to comminution by fracturing and grinding along intersecting fractures continued frictional grinding results in complete disruption of the original fabric to produce cataclasite and minor ultracataclasite grain alignment occurs by rigid body rotation with subsequent local enhancement by pressure solution microstructural relations of the fault gouge suggest periodic fluctuations in fluid pressure where v ratio of fluid to overburden pressure probably ranged between 0 45 and 1 the saltville thrust sheet emplacement must have occurred in a caterpillar like fashion involving aseismic and seismic shear shear stresses accompanying fault motion as determined from dolomite twin lamellae are in the order of 65 mpa

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

buried ancestral drainage between lakes erie and ontario the bedrock topography for the eastern niagara peninsula of ontario and for adjacent areas of new york was reconstructed using more than 7 600 well records and geophysical data this topography reveals an elaborate system of buried channels dominated by three main passageways connecting lake erie to lake ontario these channels are the erigan crystal beach and st davids the erigan with a depth near 150 ft below lake erie water level precludes the existence of a lake in the erie basin north of the niagara escarpment the erigan channel bifurcates into three outlets that extend into lake ontario the extensive widths and differences in elevation of the outlet valleys suggest that the erigan channel was active at least three times elevations in two of the outlets indicate that water levels in the ancestral lake ontario were similar to those of the present day lake the third outlet requires water levels at least 30 ft lower than present conditions the crystal beach channel originates west of buffalo and trends north along part of the present day course of the niagara river at niagara falls it changes to a southwesterly direction to join the erigan channel near its southern end the crystal beach system received inflow from lake erie through at least 4 inlets with bed elevations only 10 30 ft below the present lake erie water level this suggests the existence of an ancestral lake erie similar to present day conditions limited retreat in the waterfall of the erigan channel indicates that during the active period of the crystal beach and erigan drainage systems only the lake erie watershed emptied through the niagara peninsula the upper great lakes may have drained through the buried laurentian valley between georgian bay and lake ontario drainage of the upper great lakes through the niagara peninsula may have first occurred through the st davids buried channel the limited retreat of the st davids waterfall suggests a history of only a few thousand years its channel above the niagara escarpment probably occupied part of the bedrock depression formed by the crystal beach drainage system but at a higher elevation

vitrinite reflectance data from the kinsale harbour old head of kinsale area southern ireland and its bearing on the interpretation of the munster basin vitrinite reflectance of the upper devonian carboniferous succession in the kinsale harbour old head of kinsale area ranges from 4 06 5 21 m and 6 21 7 82 max which is indicative of meta anthracite coal rank no correlation is apparent between vitrinite reflectance and stratigraphic position through c 2 km of section suggesting that near maximum temperatures were maintained until after variscan folding low reflectance values of 0 62 and 0 30 0 35 m at the local base of the jurassic succession in the north celtic sea basin well 48 30 1 and onshore at cloyne respectively indicate a major discontinuity in the maturation profile in the region comparison with high reflectance gradients from upper palaeozoic successions in the halen and turnhout boreholes in belgium and the munsterland no 1 borehole in west germany suggests that a thick 5 7 km cover of pre permo tnassic rocks must once have been present in the munster basin to account for the high maturation levels observed whether the cover was emplaced by silesian sedimentation or by late palaeozoic thrusting is unclear

bedrock jointing and geomorphology in southwestern ontario canada an example of tectonic predesign in southwestern ontario canada palaeozoic sedimentary strata are covered by pleistocene glacial sediments the topography of the buried bedrock surface has been mapped over an area of some 45 000 km2 using new computerised depth to bedrock data from more than 200 000 waterwells this information shows that arching of the underlying proterozoic basement canadian shield and differential erosion of carbonate and shale strata are first order regional controls on bedrock geomorphology these data also define a network of buried bedrock channels quantitative analysis of the trends of these channels reveals a statistical correspondence between channel orientation and regional bedrock joint sets formed in response to the mid continent stress field the orientations of bedrock channels and joints correspond regionally with mean trends at 153 and 164 respectively in addition two dominant orientations of postglacial river valleys and bedrock joints correspond with mean trends at 104 vs 104 and 54 vs 52 respectively our findings show that the erosional channelled topography cut on bedrock together with the postglacial drainage pattern cut into younger glacial sediments are related to the trend of bedrock joints and consequently can be described as tectonically predesigned in addition this study yields new insights into the form of the preglacial tertiary drainage system in southern ontario prior to pleistocene glaciation regardless of the precise age of this preglacial system and degree of subsequent glacial modification an important factor in its geomorphic development has been the mid continent stress field

geological and geophysical studies of buried valleys and their fills near elora and rockwood ontario buried bedrock valleys are of economic interest for groundwater and engineering studies and of scientific interest for the sedimentary sequences they may contain two buried valleys about 60 m deep north and northeast of guelph ontario are tributary to the major dundas valley southwest of guelph located initially from water well data their locations and geometry were refined with surface geophysics including very low frequency resistivity terrain conductivity resistivity transient electromagnetic gravity and seismic methods useful data are obtained roughly in proportion to cost cored and rotary holes in the two valleys were followed by geophysical downhole logging with normal electric spontaneous potential single point resistance calliper neutron epithermal neutron gamma gamma and natural gamma methods cores revealed multi till sequences in the upper parts of the holes and together with geophysical logs and their interpreted electrofacies indicate the presence of underlying unifo

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

structural evolution of the irish variscides the effects of the variscan orogeny on the upper palaeozoic succession are recognizable throughout central and southern ireland the deformation can be zoned using structural styles and trends which illustrate a northward decrease in deformation intensity coupled with increasing control by pre variscan structures in the basement the most intense deformation occurs to the south of the dingle dungarvan line where shortening approaches 50 as a result of layer parallel shortening followed by buckling and finally thrusting the deformation is considered to have developed above a shallow dipping sole thrust which has been located offshore by the swat seismic reflection profiles cross faults which are common throughout the orogen resulted from n s shear couples this is consistent with their origin as accommodation structures due to differential shortening along strike the original shape of the munster basin has exerted a control on the distribution and overall facing of the major structures the sole thrust extends to the north of the dingle dungarvan line and the structural zonation of the orogen is due to variations in the stratigraphic level of this thrust the leinster massif is considered to be essentially autochthonous slip on the sole thrust decays towards the massif which acted as a rigid passive block and was partially responsible for the arcuate trend of the orogen

erosion and infill of new york finger lakes implications for laurentide ice sheet deglaciation a comprehensive seismic reflection profile investigation of the new york finger lakes has revealed the extent of bedrock erosion and nature of sediment fill beneath the lakes bedrock deepens to the south where gorgelike profiles have been eroded to as much as 304 m below sea level and infilled with up to 275 m of late quaternary age 14 ka sediment we propose a working hypothesis that invokes ice streaming and pressurized subglacial meltwater and sediment to account for regional geomorphology this working hypothesis is consistent with recent concepts of the collapse of continental ice sheets via ice streaming and the marine isotopic record for a meltwater spike at 13 14 ka the finger lakes may provide a critical testing ground for deglaciation models of the southern laurentide ice sheet because they contain a thick well preserved and now seismically defined record of the deglaciation process

geological framework of the laurentian trough aquifer system southern ontario the laurentian trough lt a depression 100 km long 3000 km2 in area and 100 m deep at the base of the niagara escarpment extends from within georgian bay to lake ontario it has a complex er

an interpretation of the variscan structures in sw england the variscan structures in sw england apart from the high level structures in the culm synclinorium of n devon are recumbent with an overall gentle southward dip of cleavages thrusts and recumbent folds translation was towards the nnw the structures are interpreted on a thin skinned model with a basal decollement dipping gently s related to a s dipping subduction zone s of the lizard the granites are thought to have been generated to the s of their present position and injected northwards above the decollement zone

strains and folds within thrust sheets an analysis of the heilam sheet northwest scotland abstract the changes of shape and orientation of fossil worm burrows have been used to carry out a detailed strain analysis of the heilam thrust sheet in the moine thrust zone northwest scotland the strains have been factorised into components of layer parallel shear 1 layer normal shear 2 and layer parallel shortening or elongation variations in the 1 shear are largely lithologically controlled some strain is due to flexural processes as the thrusts and overlying beds change dip over thrust ramps or are tightened by later deformation while there may also be a component of drag which increases towards the overlying thrust this predating the shear due to flexure a contoured map of the values suggests that major folds within the thrust sheet developed as a result of stick or drag on the floor thrust prior to ramp climb this led to differential movement in the transport direction causing layer parallel shortening accompanied or followed by buckling of the layers a map showing layer normal shear strains 2 suggests that the heilam sheet forms a major surge zone the central part having moved further wnw than the northern and southern parts this has led to components of lateral shear along parts of the sheet especially in the north producing curvature of the imbricate faults

the variscan front and the midi fault between the channel and the meuse river abstract the ecors seismic profile allows a new insight into the deep geology of northern france various geological data such as ancient coal mines surface mapping and boreholes are used to interpret the seismic profile a new cross section directly superimposed on the ecors seismic profile cambrai dreux is compared with a previous one drawn along the meuse river through the ardenne massif the cylindrism and length balancing hypotheses are discussed by comparing the two cross sections the main geological results are a the emplacement of the dinant nappe is later than most of its internal deformation and b major folding and thrusting give an approximate 30 35 shortening without taking into account the internal deformation minor folding and cleavage and the unknown net translation vector of the dinant nappe 50 km at least possibly 150 km

complex strain patterns developed at the frontal and lateral tips to shear zones and thrust zones abstract many of the complex strain patterns seen in shear zones and thrust zones such as variable fabric orientations refolded folds and fabrics together with folds with hinges almost parallel to the main transport direction can be explained in terms of differential movement within the shear zones these strains are developed at the frontal and lateral tips of the zones as they propagate examples are taken from the moine thrust zone of scotland which show variations in strains particularly at the lateral tips the form of differential movement described here may lead to complex strain paths and non plane strain ellipsoids and the spatial variations in finite strain may be used to delineate zones of extensional and compressional flow and differential movement in the shear zones or thrusts

geological applicability of critical wedge thrust belt models critical wedge models for foreland thrust belts although popular paradigms for thrust mechanics also predict definite progressive changes within the orogenic belt the most important constraint on critical wedge models is the need for deformation of the interior of the wedge when new sheets are accreted to its front well dated foreland thrust belts have sequential formation of new thrust sheets in the front of the belt thus they also require continued superimposed deformation within the wedge geologic mapping and fabric studies provide evidence of only minor later intra wedge shortening severely reducing the acceptability of an accreting critically tapered wedge model to apply critically tapered wedge models to foreland fold and thrust belts one must relax requirements for constant wedge strengths and basal sliding laws the wedge will need to strengthen after initial thrust emplacement and the fault zones will probably need to progressively weaken it is most likely that critical wedge models which apply well to accretionary wedges do not apply in simple ways to foreland fold and thrust belts

the structural evolution of the western irish variscides an example of obstacle tectonics abstract detailed mesostructural and strain analysis investigations across the killarney mallow fault i e the traditional variscan front in southwest ireland reveal that this structural line separates two distinct tectonic regimes north of the killarney mallow fault bulk shortening orthogonal to orogenic strike is estimated to be 12 all of which is accounted for by late stage buckling microscopic strain analysis reveals that there is only local development of a tectonic fabric south of the front bulk shortening is 40 due to combined layer parallel shortening lps buckling and faulting variscan deformation is presented as being essentially coaxial the regional finite strain pattern outlined above is thought to be primarily controlled by the combined effect of a buried basement obstacle in eastern iveragh and increased sedimentary pile thickness at the western end of the orogen

basement controlled faulting of paleozoic strata in southern ontario canada new evidence from geophysical lineament mapping abstract basement fault reactivation is now recognized as an important control on sedimentation and fault propagation in intracratonic basins in southern ontario the basement consists of complexly structured mid proterozoic ca 1 2 ga crystalline rocks and metasedimentary rocks that are overlain by up to 1500 m of paleozoic sedimentary strata reactivation of basement structures is suspected to control the location of paleozoic fault and fracture systems but evaluation has been hindered by a limited understanding of the regional structural characteristics of the buried basement new aeromagnetic and gravimetric lineament mapping presented in this paper better resolves the location of basement discontinuities and provides further evidence for basement controls on the distribution of paleozoic fault and fracture systems lineament mapping was facilitated by reprocessing and digital image enhancement micro levelling regional residual separation derivative filtering of existing regional gravity and aeromagnetic datasets reprocessed images identify new details of the structural fabric of the basement below southern ontario and delineate several previously unrecognized aeromagnetic and gravity lineaments and linear zones linear zones parallel the projected trends of mid proterozoic terrane boundaries identified by field mapping on the exposed shield to the north of the study area and are interpreted as zones of shearing and basement faulting mapped aeromagnetic and gravity lineaments show similar trends to paleozoic faults and fracture networks and broad zones of seismicity in southern ontario these new data support an inheritance model for paleozoic faulting involving repeated reactivation and upward propagation of basement faults and fractures into overlying cover strata

the palaeoenvironment and deformation of beaconites like burrows in the old red sandstone at gortnabinna sw ireland the gortnabinna area contains a number of well preserved deformed burrows within the fluviatile old red sandstone of s ireland detailed logging of the local succession enables subdivision into four facies facies 1 represents sheet flood deposits facies 2 ephemeral stream channels facies 3 marginal sheet flood deposits and facies 4 low energy settling of fines the burrows occur in facies 1 and 4 the sediments are now folded into a large upright westward plunging anticline the fold has a well developed axial planar cleavage and is extended along strike by listric normal faults the burrows in facies 1 were originally circular and orthogonal to bedding and reveal a n s layer parallel shortening strain of 2 1 followed by a shear strain of 1 48 during folding these burrows have a bimodal distribution of diameters with peaks in the ranges 50 80 and 110 170 mm the burrows of facies 4 are sub parallel to the bedding plane and are generally curved with a maximum width of 180 mm and lengths of up to 2 3 m these burrows display passive infill following abandonment the burrows are morphologically similar though distinct from beaconites barratti which has been described from elsewhere in the old red sandstone and are tentatively interpreted as lungfish burrows

thrust tectonics of s devon the tectonics of the area from start point to chudleigh sw england are shown to be dominated by northward verging thrust nappes two major thrusts are postulated the dartmouth antiform is considered to be a major thrust structure not a fold structure as previously interpreted the torquay limestones are allochthonous and part of a klippe of a major thrust nappe with a minimum displacement of 13 km northwards a progressive and diachronous sequence of deformation has been established with deformation starting in the s and moving n many of the folds and cleavages are probably related to the strains developed locally in thrust tips and may have no regional correlation more intense and complex deformation is found in the s compared to the n the diachroneity is also reflected in the k ar mineral ages in the s at start point southward verging backfolding is the last significant hercynian deformation phase folding the thrust nappes

the flexural slip mechanism abstract detailed field studies of turbiditic sequences from south georgia south atlantic north devon england and cardigan bay wales show that flexural slip occurs on discrete movement horizons between rock packets in which the beds have welded contacts stair stepping displacements of sedimentary dykes and of early quartz veins show that the movement horizons generally have a decimetre to metre spacing and are marked by bedding parallel quartz veins these veins are from 1 mm to several cm thick and can be used to identify movement horizons in the absence of displaced markers they consist of several sheets of quartz fibres which each carry a slickenfibre lineation and which together preserve a record of the displacements on an individual surface complex slickenfibre patterns and departures from ideal behaviour in which slip occurs orthogonal to the fold hinge probably result from changes in slip vector on the limbs of growing non cylindrical folds movement horizons show many of the features associated with large scale thrusting such as ramps duplexes and imbricate structures and the shear sense given by fibre steps on these surfaces and by duplexes within stratigraphically restricted packages changes across fold hinges chevron folds are thought to develop mainly by flexural flow in the early stages with flexural slip becoming dominant later as the beds become lithified new slip surfaces are generated as the dip of the fold limbs increases

evidence for large scale subglacial meltwater flood events in southern ontario and northern new york state flow line patterns constructed from the orientation of drumlin long axes are used to infer two major subglacial meltwater floods in southern ontario and northern new york state in this interpretation the flood waters of the first event the algonquin surged southward in the later ontarian event powerful floods streamed southwestward along the basins of lakes ontario and erie bedrock fluting upstream pointing noses on escarpments and tunnel channels give supporting evidence for the regional flood hypothesis regional distributions of drift thickness may be explained by this hypothesis

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

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

ice marginal sedimentation and processes of diamicton deposition in large proglacial lakes lake erie ontario canada detailed studies of coastal cliff exposures through two end moraines form the basis for a model of ice marginal sedimentation in large ice contact glacier fed lakes the input to the ice marginal environment directly from the glacier included subglacial till and subaquatic flow tills the subaquatic flow till thinly bedded diamicton was deposited in an apron up to 1 km wide along the ice margin an upward gradient of pore water pressure immediately beyond the ice margin causing heaving and dilation of the sediments initiated debris flows of glacially derived debris subaquatic flow tills most of the stratified sediments in the ice marginal zone entered the lake by way of a large proglacial stream sedimentation was dominated by quasi or near continuous density underflows that resulted in the deposition of a sequence of thick rhythmites the glacier in the lake erie basin most likely behaved like an ice stream with its movement controlled predominantly by a deforming bed of glacial debris separa

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

the structure of mountain fronts commonly the part of a mountain front which is visible at the surface consists of foreland dipping thrust belt rocks elevated above their regional structural position and overlain more or less conformably by molasse several explanations for their geometry are possible 1 major detachments exist within or beneath the molasse resulting in transport of the foreland basin examples of this geometry come from the swiss molasse plain the southern pyrenees and the mackenzie mountains of canada 2 displacement is lost on major backthrusts beneath the frontal monocline examples cited here are the rockies of alberta the sulaiman ranges of pakistan the mackenzies and the andes in peru 3 thrust sheets travelled over an old land surface and syntectonic molasse contemporaneously offlaps the topographic high of the thrust front this phenomenon occurs along the alpine thrust front in haute provence 4 the frontal fold represents deformation above a large scale thrust tip no unequivocal example of tip line strain at this scale has been recorded although this type of deformation may occur in the brooks range of alaska in many areas mountain fronts show a combination of these idealized geometries

strain hardening and strain softening in fault zones from foreland thrusts the movement of thrust sheets is accomplished in part by deforming the rocks adjacent to thrust faults in foreland fold and thrust belts emplacement generates an array of mesoscopic faults in a thick layer along the base of a sheet ductile deformation of rocks in the layer occurs by displacing fault bounded blocks relative to one another in a kind of mesoscopic grain boundary sliding analyses of data from three southern appalachian thrust sheets indicate that this block sliding is first accompanied by strain hardening then by strain softening and it is accommodated by intra block fractures small faults and veins the density of accommodation features increases near thrusts and where these features become sufficiently numerous a layer of cataclastic rocks forms along the thrust we describe foliated cataclasites from two thrusts the microstructures of which indicate that the cataclasites exhibited extreme ductility interrupted occasionally by brittle processes deformation of the cataclasites was accompanied by strain softening strain softening deformation in fault arrays near thrusts is genetically related to strain softening deformation in cataclasites during the formation of the cataclasite rocks in the sheet are impelled to cross a strain energy hump after which they deform at either higher rates or lower stresses

deep structure of southern ireland a new geological synthesis using birps deep reflection profiling the birps deep seismic reflection data of wire west of ireland 1 1b from galway bay to the celtic sea are integrated with geological and other geophysical data to produce a synthesis of the fundamental tectonic character of the crust in and around southern ireland the iapetus suture forms a zone of north dipping reflectors which projects up between the shannon estuary and the dingle peninsula and is overlain by a late caledonian reflector with a shallow south dip across the iapetus suture zone the crust thickens southward and the lower crust becomes less reflective it is proposed that the lower palaeozoic volcano sedimentary rocks which form the leinster massif extend southwestward beneath the variscan province and at least 30 km offshore to the wire lines we argue that the weak south dipping reflectors in the upper and middle crust south of the iapetus suture represent an early caledonian fault system linked to a mid crustal detachment which formed during the south dipping subduction event recorded onshore by the ordovician volcano sedimentary successions these faults were selectively reactivated causing the predominant ene trend of younger regional tectonic features in and around southern ireland the three onshore upper palaeozoic basins close east of the wire lines we point out that the variscan front is only a map line across the british isles with no consistent tectonic character thus the south dipping reflector below the north margin of the north celtic sea basin on swat 4 generally referred to as the variscan front is correlated by us with the northern boundary of the rosslare complex thus making it the northern tectonic boundary of the caledonian basement block known as the rosslare monian terrane furthermore the north dipping south irish sea lineament on winch 4 can be correlated with the menai straits lineament and could thus be the southern boundary of this terrane the south dipping reflector on swat 2 also previously called the variscan front is correlated with the johnston benton fault system in southwest wales

reply inferred subglacial meltwater origin of lakes on the southern border of the canadian shield paleozoic rocks form an escarpment up to 35 m high at the border with the frontenac axis of the canadian shield in southeastern ontario the escarpment which lies nearly transverse to the flow direction of pleistocene ice and subglacial meltwater in the region is cut by a series of deep channels locally incised into the bedrock at the escarpment a subbottom acoustic survey of two of these channels and mapping of a third partly flooded channel revealed the shape of the bedrock surface beneath the water and glacial and postglacial sediments of lakes now occupying the channels most cross sections show a smooth walled dominant depression eroded up to 100 m below the land surface nearby and flanked by one or more secondary depressions the pattern changes considerably in detail making secondary depressions especially difficult to trace along the length of the lake the pattern is similar to that of subglacial fluvial erosion marks more than three orders of magnitude smaller found in bedrock throughout

palaeomagnetic studies of fold development and propagation in the pembrokeshire old red sandstone abstract the geometry of two folds in lower devonian old red sandstone in pembrokeshire deformed in the hercynian has been defined at intermediate stages during their evolution by this preliminary palaeomagnetic study this is possible because remagnetization appears to have occurred in discrete episodes during deformation and the shape can be reconstructed by restoring the fold limbs to an angle at which the remagnetization directions coincide the plunge of the folds is shown to vary considerably during evolution one fold having an early easterly plunge which changed to the present westerly plunge the other fold having an early plunge which was more steeply to the west than its present attitude clockwise rotation of the whole of southern pembrokeshire by 40 towards the end of or since the hercynian deformation is indicated by the palaeomagnetic data and a structural model for this is proposed

ice wedge casts in late wisconsinan glaciofluvial deposits southern ontario canada ice wedge casts in late wisconsinan glaciofluvial sand and gravel deposits in southern ontario taper downwards to form a v or funnel shaped morphology ranging from 1 to over 3 m deep and from 1 0 to 2 5 m wide at the top pressure derived primary structures caused by repeated thermal contraction and expansion include upturned strata realigned stones and small folds in the enclosing sediments although paleotemperature is difficult to estimate the presence of ice wedge casts testifies to the former presence of permafrost under prevailing periglacial conditions over this area the regional till stratigraphy constrains the timing of ice wedge growth to ca 15 000 to ca 13 000 bp at this time newly exposed terrains would have occurred in an interlobate area where severe climate created favourable conditions for permafrost to develop due to the close proximity to the receding ice inactive areas in braided rivers such as raised topographic surfaces and sand filled channels with limited vegetation cov

buried bedrock valleys and glacial and subglacial meltwater erosion in southern ontario canada morphometric features from a recently compiled bedrock topography map by the ontario geological survey suggest a glacial erosion origin for the buried large bedrock valleys and troughs in southern ontario the bedrock valleys at milverton wingham and mount forest are tunnel valleys resulting from subglacial meltwater erosion beneath the huron ice lobe probably during or shortly after the late wisconsinan glacial maximum diagnostic features for this interpretation include abrupt valley beginning and termination uneven longitudinal valley profiles and up slope gradients the dundas bedrock valley is the western extension of the lake ontario basin no comparable bedrock valleys were found to connect it to the milverton valley for a joint drainage system as previously suggested the laurentian bedrock trough is the southeastward extension of the georgian bay basin both developed along shale bedrock between the precambrian shield highlands and the niagara escarpment resulting from long term mechanical weathering associated with pleistocene glacial erosion this bedrock low has a floor that exceeds 50 km in width and is 26 m and more below the current water level of georgian bay it could drain georgian bay should the drift cover be removed there is no evidence to suggest that a preglacial river channel if it existed is still preserved in the floor of the laurentian trough as previously suggested the framework for an intensely glacially sculpted bedrock surface differs from the previous view for simple modification of a preglacial landscape and is therefore important in regional subsurface geological mapping and groundwater studies

illinoian to late wisconsinan stratigraphy at woodbridge ontario near woodbridge northwest of toronto ontario a 15 metre high railroad cut and associated borrow pit first excavated in 1962 exposed a multiple till sequence and intervening fossiliferous sediments work over the next 35 years revealed that illinoian york till early wisconsinan sunnybrook till and late wisconsinan humber till halton till and wildfield till are interbedded with fossiliferous sediments equivalent to the sangamonian don formation early wisconsinan scarborough formation 50 ka bp and middle wisconsinan thorncliffe formation 45 ka bp a complex periglacial record displays multistage fossil frost wedges indicating intervals of severe climate in late illinoian and early wisconsinan time cored boreholes indicate deep gravel below and a till on ordovician shale bedrock georgian bay formation vertebrates molluscs ostracodes insects and plants diatoms wood seeds pollen indicate mostly cool conditions boreal to tundra for interstadial sediments interglacial conditions a

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

mechanisms of thrust propagation some examples and implications for the analysis of overthrust terranes abstract evidence is presented for the existence of three fundamental mechanisms by which thrust sheets move the mechanisms are 1 failure of a stiff layer to form ramp flat geometry imbricated thrust sheets 2 detachment of a layer by folding decollement thrust sheets 3 differential layer parallel shortening lps thrust sheets the mechanisms are independent but may operate interactively to form families of hybrid thrust sheets since lps thrust sheets have not been previously described or documented data from the new york plateau deformed almost exclusively by differential lps is presented to demonstrate the physical characteristics of this thrust mechanism information from the plateau indicates that finite strain behavior closely reflects the geometric boundary conditions and is independent of temperature and depth of burial finite strain data are used to construct a set of iso strain maps in both the deformed and undeformed states the iso strain maps are in turn used to determine the displacement field for the thrust sheet the displacement field allows visualization of the effects of both lagrangian and eulerian transformations on an initially orthogonal grid utilizing sections from a number of overthrust belts it is shown that the three mechanisms occur universally although in various proportions lps is regionally developed throughout the central and northern appalachians the presence of the lps thrust sheets probably accounts for the failure of structural cross sections to bed length balance in these blind thrust terranes finally a series of examples drawn from the field as well as the literature are used to illustrate both the characteristics of hybrid thrust sheet families as well as to indicate how knowledge of their behavior can be utilized in developing strategies for the construction and balancing of structural cross sections

a model for the development of the irish variscides a clear correlation exists in the southern irish variscides between extensional structures which controlled late palaeozoic sedimentation in the munster basin and contractional thrusts and folds of end carboniferous age this correlation is interpreted to indicate that the extensional structural framework of the basin including a basal low angle detachment and an array of superjacent normal faults was reused during transpressive inversion of the basin to produce the fold thrust belt the extensional framework which in turn was to some extent derived by reactivation of caledonian structures therefore exerted a fundamental control on the geometry and style of contractional deformation it is not evident that the detachment had a discrete link to the fold belt in south wales or the main variscan orogen in europe

cleavage geometry and the development of the church bay anticline co cork ireland abstract the church bay anticline is one of a series of e w trending hercyman folds in southern ireland the fold has a crudely axial planar cleavage which was initiated early in the buckling history the cleavage was deformed subsequently by a number of discrete bedding parallel shear zones as the fold continued to amplify this has resulted in a characteristically sigmoidal cleavage geometry in the fold profile plane the cleavage also displays kink bands and sharp angular changes across bedding planes in the fold hinge as a result of shearing within these sheared zones the cleavage is intensified indicating that conditions for cleavage formation were maintained throughout the development of the fold no new cleavage has formed despite the rotation of the cleavage during the deformation history

stratigraphic analysis of late wisconsin and holocene glaciolacustrine deposits exposed along the nottawasaga river southern ontario canada analysis of 56 outcrop exposures in cut banks along the nottawasaga river in southern simcoe county ontario canada has led to the identification of eight stratigraphic units su1 su8 that repre