tertiary flexural evolution of the beaufort mackenzie basin canada abstract seismic reflection data indicate that more than 14 16 km of clastic materials are preserved in the beaufort mackenzie basin these correspond to tectonic activity and sediment loading from the upper cretaceous to the present day to understand better how the basement movement responds to the thick sediment loading an inverse flexural model has been used to simulate the evolutionary process the results of the inverse flexural model simulation show that the basin subsidence developed dominantly as an elastic flexural response to sediment loading with a minor component due to lateral compressive stress the forward model results show that the late eocene to pleistocene subsidence of the basin included three stages with different rates of subsidence the koponoar kugmallit and iperk sequences represent products of two rapid subsidence stages whereas the mackenzie bay and akpak sequences reflect the products of a more stable and slower subsidence stage

fault death a perspective from actively deforming regions abstract patterns of activity on large faults in regions of distributed continental deformation can change quite rapidly in less than 1 ma such changes include faults becoming permanently or temporarily inactive spatial migration of activity between different faults or faults sets within a deforming region and faults apparently going through episodes of intense activity separated by long quiescent periods greater knowledge of these changes combined with our increasingly detailed understanding of the geometry of large faults have implications for a variety of structural and tectonic issues such as 1 the nature of the interaction between neighbouring faults 2 the link between the discontinuous deformation on faults in the seismogenic upper crust and the more distributed deformation in the aseismic lithosphere beneath and 3 the relation between the instantaneous kinematics and the cumulative finite deformation examples from the active extensional province of central greece are used to illustrate the nature and possible significance of changing fault patterns

threshold dominated fluvial styles in an arid zone mud aggregate river the uplands of fowlers creek australia abstract fowlers creek is a mud aggregate fluvial system floodplain muds dominate the river s deposits and consist of silt fine to very fine quartzose sand and clay up to 80 of the silts and clays are bound into sand and silt sized aggregates and comprise a substantial component 42 of the floodplain muds mud aggregate sediments behave like sands during transport and as a result muds can be deposited under conditions of greater flow velocity than would otherwise be the case newly deposited floodplain muds are loose and easily entrained but older floodplain muds are cohesive and the distribution of modern and older floodplain muds influences erosion patterns across fowlers creek in the lower order streams of the fowlers creek uplands alternate reaches of shallow rectangular channels and unchannelled floodplains collectively form discontinuous ephemeral streams these landform sequences consist of gullies coalescing downstream to arroyos which terminate in distributary intermediate floodouts at fowlers creek floodouts are preferentially located at tributary junctions reflecting their origin during very large floods at floodouts low slope and high vegetation density promote sheetflow infiltration and landform stability their efficiency in retaining runoff make floodouts drought refugia they are an important ecological element in this arid area the higher order channel of the mid uplands is a mobile low sinuosity single thread arroyo incised into wide muddy unstable floodplains fluvial processes are dominated by episodic flood driven channel avulsion and variability in stream energy and boundary resistance contributes to a non equilibrium fluvial style frequent reach scale channel relocation is accompanied by the burial of the abandoned channel in floodplain muds and both erosion and aggradation in downstream floodplains

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

a new scheme for the very deep geological disposal of high level radioactive waste a case is presented for reconsidering deep borehole disposal for high level radioactive waste hlw in the light of recent discoveries about the hydrogeology and hydrogeochemistry of the continental crust a new multi barrier concept is proposed for very deep elevated temperature disposal of vitrified hlw spent fuel and ex military and other fissile materials which involves melting and recrystallization of a granitic host rock utilizing the heat from the waste this scheme is fundamentally different from previously proposed borehole disposal scenarios in its key aspects particularly in utilizing only partial melting and a surviving container the perceived advantages of the scheme are discussed in general and in relation to previous deep disposal ideas particularly from the standpoint of safety it is concluded that the scheme offers a potentially safer and environmentally more acceptable option for the permanent disposal of high level waste

geologic site characterization for deep nuclear waste disposal in fractured rock based on 3d data visualization abstract three dimensional 3d visualization of data is an essential step in the development of descriptive hydrologic and rock mechanical models of fractured rock systems data includes geological properties rock mechanical properties and hydraulic responses the 3d visualization approach is applied to characterizing a hypothetical site for a high level nuclear waste repository located at 500 m depth in granitoids of the trans scandinavian igneous belt 1 6 1 8 ga old the study is based on site specific data at the aspo hard rock laboratory hrl on the southeastern coast of sweden the data are extensive and spatially complex and are based on surface information as well as a large number of boreholes penetrating up to 1000 m at this site fractures and fracture zones control both groundwater flow and mechanical stability structures are found within a 2 2 1 km deep model block representing the site 3d locations of fractures and fracture zones are hypothesized based on surface and subsurface geological and geophysical information including borehole radar this results in a 3d geological model structure of the site

petroleum seepages and impregnations in great britain abstract this paper documents 173 occurrences of surface petroleum seepages and impregnations in great britain a brief description is given of the seven main areas where they are found the descriptions range from pore fillings of bitumen to steady flows of oil and gas the principal areas in which seepages are found are as follows the orcadian basin of north east scotland the midland valley of scotland the flanks of the pennines the welsh borderlands cornwall the dorset coast of the wessex basin and the south eastern weald stratigraphically the seepages range from caledonide basement to holocene peat the principal sources for the petroleum are devonian oil shales orcadian basin carboniferous shales and coals midland valley pennines welsh borderlands devon and cornwall and the liassic wessex and wealden basins several anomalous seepages are recorded from welsh and scottish basement there are three clearly defined styles in which the seepages occur many are found where permeable carrier beds unconformably overlie impermeable basement around the margins of basins orcadian basin midlands carboniferous basin others are fault related especially in the wessex and wealden basins the seepages in the cornish tin mines are best explained as sourced from the culm series and drawn by the convective flow of connate waters into the thermal chimneys of the granite intrusions

diagenetic origin of basal anhydrite in the cretaceous maha sarakham salt khorat plateau ne thailand development of a diagenetic anhydrite bed at the base of the cretaceous maha sarakham saline formation the basal anhydrite member of the khorat plateau in north eastern thailand took place due to leaching and or pressure dissolution of salt at the contact between an underlying active sandstone aquifer system and an overlying massive halite dominated evaporite sequence basal evaporites composed of halite with intercalated anhydrite of the latter sequence are undergoing dissolution as a result of subsurface flushing with anhydrite produced as the insoluble residue the result is a 1ae1 m thick interval of nodular anhydrite displaying unique basin wide continuity observed textures petrographic features and chemical data from the anhydrite and associated authigenic minerals support the origin of the basal anhydrite member as an accumulation residue from the dissolution of the maha sarakham salts petrographically the anhydrite in this unit is made up of crystals that are blocky and recrystallized sheared generally elongated and broken and is bounded at the bottom by organic rich stylolite surfaces authigenic and euhedral dolomite and calcite crystals are associated with the anhydrite traces of pyrite galena and chalcopyrite are present along the stylolite surfaces suggesting supply of fresh water from the underlying sandstone at highly reducing conditions of burial the d 34 s of sulphate in the basal anhydrite averages 15 cdt and falls within the isotopic composition of the anhydrite in the cretaceous maha sarakham formation proper and the cretaceous values of marine evaporites measured d 18 o in dolomite range from 4ae37 to 14ae26 pdb suggesting a re equilibration of dolomite with basinal water depleted in 18 o and possible recrystallization of dolomite under relatively elevated temperatures the d 13 c however varies from 1ae57 to 2ae53 pdb suggesting a contribution of carbon from oxidation of organic matter this basal anhydrite bed similar to basinwide beds found at the bottom of many giant evaporite sequences has always been considered to be depositional here at the base of the maha sarakham formation we demonstrate that the anhydrite is diagenetic in origin and was formed by accumulation of original anhydrite by dissolution of interbedded halite from waters circulating though the underlying aquifer it represents an upsidedown caprock

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

the structural geology of boulby potash mine cleveland united kingdom abstract this is the first description of the structural geology of the upper permian potash and salt of the third zechstein cycle exposed in boulby mine the near seam lithologies are described briefly and the associated veins vein networks shear zones and slides are introduced it is argued that the boulby potash migrated from lower levels to those at which it is now found to fill fractures which developed in a grey anhydrite shale produced by the reduction and wetting of the lower parts of the carnallitic rotten marl continued introduction of chlorides of na and k into vein networks dispersed the shale as clasts of ever decreasing size in a sub horizontal zone of gneissose sylvinite which when it reached a significant thickness began to flow in an extraordinary manner hundreds of asymmetric sub horizontal lobes of gneissose sylvinite root to and repeat the first formed layer in structures which developed to various degrees in different parts of the mine these are shown to represent small scale movement cells e g 15 30 to 100m in axial section in which the movements integrate within a pattern of larger scale movement cells 15 200 to 400 m where the sylvinite was thickest these movement cells progressed to a stage which involved the complete circulation of a compound layer of sylvinite and grey shale continued circulation almost homogenised this compound layer to sylvinite in which the grey shale has been disrupted and dispersed to small aggregates and individual clay particles included within the chloride grains the circulation cells in the sylvinite are rectilinear rolls close to where thin but extensive sub horizontal lobes of boulby halite moved through and over them late in the structural history away from such salt lobes the small scale circulation cells are concentric within the boundaries of larger cells which have polygonal planforms after rejecting explanations invoking near surface slumping for the small scale circulation cells it is argued that they developed near their maximum depth of burial in late jurassic or cretaceous times the multiwavelength circulation pattern in the boulby potash is interpreted as due to thermal convection influenced by primary gravitational instabilities on a larger scale the sub horizontal lobes of boulby halite are assigned to a final laramide transcurrent reactivation of faults inherited from the basement during earlier phases of movement

displaced carbonates in the zechstein of the uk north sea abstract the cored zechstein 2 interval of amoco well 38 24 1 drilled on the southern margin of the mid north sea high contains deformed conglomeratic and brecciated dolostones which are interpreted as displaced distal shelf sediments redeposited in deeper water the displaced dolostones indicate disruption and sliding of consolidated rock down a slope accompanied by slumping of semi consolidated sediment minor collapse brecciation also occurred as a result of the partial removal of underlying evaporites such features have not hitherto been described from cored wells in the uk north sea nor have their petrophysical attributes been discussed the best reservoir properties occur in the slumped deposits showing plastic deformation brecciated beds have unpredictable and wide ranging porosity and permeability values which are attributable to variable anhydrite cementation and clasts with different textures and very variable fabric displaced zechstein carbonates on or at the foot of unstable palaeo slopes for example in the vicinity of the mid north sea high can develop moderately attractive reservoir characteristics and might in places be thick enough to be of economic importance

anatomy of mushroom shaped diapirs we produced more than 100 mushroom shaped diapirs in eight centrifuged models under acceleration of 1200 g the modelling results allow natural examples to be recognized mushroom diapirs have an overhanging bulb fringed by one or more skirts which can curl inward to form vortices capable of entraining cover material to various degrees most of the internal folds are downward facing the oldest buoyant unit occurs in the skirt and in the diapiric core younger buoyant units or their immediate cover are infolded in crescentic patterns external mushroom diapirs have skirts that infold cover material with greater maturity the skirts can curl inward to form an external vortex entraining cover in contrast internal mushroom diapirs have skirts confined entirely within the intrusion again with greater maturity these confined skirts may curl inward but they entrain only diapiric material internal mushroom structure results from toroidal circulation confined within the diapir where isoviscosity prevails external mushroom structure results where source and entrained cover have similar effective viscosities entrained inclusions of more permeable country rock threaten the integrity of a mined cavern by creating a plumbing system

seismological hazard assessment for a site in northern germany an area of low seismicity abstract potential sites of nuclear waste deposits in the federal republic of germany are situated in areas of low seismicity nevertheless seismic hazard assessment has to be performed for a very long time period in order to prove the facilities of the repositories able to withstand seismic induced loads even though there is a considerable debate whether or not it is possible to quantify the seismic risk in such an area a combination of deterministic and probabilistic methods is used to assess the seismic hazard for a site in northern germany fulfilling the standards of the german building code for nuclear power plants as an example the site of the former iron ore mine konrad is investigated the deterministic method is based on the assumption that the strongest earthquakes inside a tectonic region can happen everywhere there also near the selected site for the probabilistic method several models describing the seismicity in an area of 200 km around the site are used to show the influence of the variability in input parameters like the maximum intensity of each source region on the exceeding probability of the site intensity it can be shown that the seismic hazard of a site in an area of low seismicity is mainly caused by the effects of distant but strong source regions and the background seismicity for very low probabilities probabilistic evaluation has the advantage of quantifying the seismic risk but deterministic and probabilistic methods together seem a practical tool for mutual control of the results and to overcome the weakness of each approach alone the historical german earthquake catalog with an observation period of about 1200 years is the basis for the input data for a probabilistic model from a deep knowledge of geological development and structural geology the time history of the surrounding faults is developed indications were found that the nearest and most important fault was active at least 5 ma ago the combination of both seismicity and tectonics provides the basis for a long term prognostic with probabilities of exceedance in the order of 10 5 per year for the investigated site the following parameters were derived site intensity as a function of exceeding probability site acceleration strong motion duration site dependent response spectra for the surface and the underground inside the mine

recurrent fault activity in the grimsel test site gts switzerland revealed by rb sr k ar and tritium isotope techniques fault gouge and mylonite samples of prominent faults au 126 m and au 96 m occur at the grimsel test site underground research laboratory in the aar massif fractions the phengite green biotite plagioclase and k feldspar of the penetratively deformed host grimsel granodiorite form a rb sr isochron of 19 3 9 ma the deformation has a similar k ar age the rb sr data from the phengite and albitic plagioclase of the gouges and the mylonite samples suggest an earlier isotopic exchange 26 ma years ago the fault gouge and mylonite seems to have been formed during an early hydrothermal tectonic pulse of the alpine metamorphism however there may be small amounts of inherited 87 sr and a mixing line cannot be excluded the apparent k ar ages of the 18 o 2 h values from percolating waters in a parallel fault confirm the existence of recent fluid activities from

discontinuous ephemeral streams abstract many ephemeral streams in western north america flowed over smooth valley floors before transformation from shallow discontinuous channels into deep arroyos these inherently unstable streams of semiarid regions are sensitive to short term climatic changes and to human impacts because hillslopes supply abundant sediment to infrequent large streamflow events discontinuous ephemeral streams appear to be constantly changing as they alternate between two primary modes of operation either aggradation or degradation may become dominant attainment of equilibrium conditions is brief disequilibrium is promoted by channel entrenchment that causes the fall of local base level and by deposition of channel fans that causes the rise of local base level these opposing base level processes in adjacent reaches are maintained by self enhancing feedback mechanisms the threshold between erosion and deposition is crossed when aggradational or degradational reaches shift upstream or downstream extension of entrenched reaches into channel fans tends to create continuous arroyos upvalley migration of fan apexes tends to create depositional valley floors with few stream channels less than 100 years is required for arroyo cutting but more than 500 years is required for complete aggradation of entrenched stream channels and valley floors discontinuous ephemeral streams have a repetitive sequence of streamflow characteristics that is as distinctive as sequences of meander bends or braided gravel bars in perennial rivers the sequence changes from degradation to aggradation headcuts concentrate sheetflow a single trunk channel conveys flow to the apex of a channel fan braided distributary channels end in an area of diverging sheetflow and converging sheetflow drains to headcuts the sequence is repeated at intervals ranging from 15 m for small streams to more than 10 km for large streams lithologic controls on the response of discontinuous ephemeral streams include 1 amount and size of sediment yielded from hillslopes 2 infiltration capacity of valley floor alluvium that influences both the unit stream power available for upstream headcut migration and the attenuation of flashy streamflow events by riparian vegetation in sheetflow reaches and 3 cohesiveness of alluvium which affects headcut and streambank morphology and rates of arroyo extension downcutting and widening initiation of arroyo cutting may be too complex to be attributed to a single cause such as change in mean annual precipitation or grazing by livestock but is most likely associated with a decrease in density of protective plant cover on hillsides and along valley floors relatively larger unit stream power makes downstream reaches more susceptible to initial entrenchment during floods than headwaters reaches and favors persistent arroyos entrenchment continues until an equilibrium longitudinal profile is briefly attained then channel widening occurs streambanks are undercut and aggradation begins

patterns of isostatic land uplift during the holocene evidence from mainland scotland preliminary results of a lengthy and detailed investigation into middle and late holocene raised shorelines in mainland scotland indicate that three shorelines are sufficiently widely distributed to permit an examination of their extent and of the patterns of uplift which they indicate the shorelines are the storegga tsunami shoreline reached at c 7100 14c years bp at the culmination of the main postglacial transgression and the blairdrummond shoreline reached after the main postglacial shoreline at c 2000 4200 14c years bp at the time of a tsunami believed to have been caused by the second storegga submarine slide the main postglacial shoreline reached at c 5800 6850 14c years bp the main postglacial shoreline previously believed to be the highest holocene raised shoreline in scotland is now believed to be overlapped around the periphery of the uplifted area by the later blairdrummond shoreline isobase models based upon quadratic trend surface analysis of available comparable height dat

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

when is a fault extinct existing regulatory definitions of time periods over which evidence should be searched to demonstrate whether a fault is or is not active may be unconservative in slowly deforming continental regions where recurrence intervals of surface faulting are in excess of 100 000 years self evidently no definition of an active fault can be satisfactory when founded on some extrinsic time period instead it must be derived from the properties of the system itself preferably through knowledge of fault movement recurrence intervals or alternatively and conservatively from an understanding of the time period over which the present conditions of regional stress and strain the current tectonic regime have prevailed borrowing from volcanological terminology out of the definition of the active fault emerges an antithesis the extinct fault time periods by which directly dated faults are designated as active or extinct have to be locally derived

high resolution seismic profile through a late cenozoic delta complex in the southern north sea abstract a digitally recorded and processed multichannel seismic profile between cromer england and sylt germany has provided the first continuous section through the miocene to middle pleistocene delta complex of the southern north sea basin the profile depicts the progressive westward expansion of deltaic sedimentation that eventually filled in this gently subsiding epicontinental basin significant basinward shifts in seismic facies occur above all but one of 14 seismic sequence boundaries that punctuate the deposition of up to 830 m of deltaic sediments on the profile these sequence boundaries and the morphology of prodelta delta front delta top and fluvio deltaic seismic facies record the interaction between subsidence cyclic fluctuation in sediment supply to and cyclic change of relative sea level within the basin two submarine slides were developed within delta front facies in the pliocene section and the profile has intersected a swarm of elsterian middle pleistocene glacial palaeovalleys that have been incised into the upper part of the delta complex

quantitative determination of tertiary sediment supply to the north sea basin sediment supply comprises a major family of regime variables that influence geomorphic surface grade spatial and temporal changes in total sediment supply and sediment texture gravel sand mud ratio will cause reconfiguration of depositional and erosional profiles potentially creating or influencing the sequence stratigraphic framework of the basin sequence grain volume which is defined as the volume of sedimentary grains in an individual stratigraphic sequence total sequence volume minus cement and porosity volume has been quantitatively determined for each of 16 genetic stratigraphic sequences in the north sea basin rates of total sediment supply to the basin in both time and space were then calculated sand grain volume and sand mud ratio were also calculated for each sequence these data define four principal episodes of tertiary sediment supply the most significant episode occurred in the late paleocene and was followed by secondary episodes in the eocene and oligocene a fourth neogene episode extends through the present all episodes correlate to source terrain tectonic pulses related to evolution of the north atlantic basin to intraplate stress changes associated with successive phases of the alpine orogeny or to the late cenozoic epeirogenic uplift of scandinavia the major episodes in turn contain secondary sequence to sequence variations that correspond to changes in spatial or temporal values of one or both of the supply regime variables again most changes closely reflect details within the histories of the principal tectonic phases the history of changing source area relief and resulting topographic grades and related changes in sediment yield into the basin was a principal control on north sea cenozoic sequence development source basin relief in turn was largely determined by regional tectonism

inversion tectonics of the variscan foreland of the british isles late westphalian inversion structures on the variscan foreland display wide variations in orientation that are the product of the underlying basement grain the orientation of the major basement lineaments in relation to the direction of maximum shortening also determined the severity of deformation of the inverted basins ne sw trending faults were oriented roughly perpendicular to the nw se to nnw sse direction of maximum shortening determined from thrust transport directions in the variscan orogen to the south as a consequence ne sw trending basins in the variscan foreland have been strongly inverted the silesian post rift fill has commonly been expelled from the basins and the syn rift sediments have been deformed by chevron folds dublin and bowland basins in contrast the n s and nw se trending faults were oriented more obliquely to the direction of maximum shortening and as a consequence display a significant component of oblique slip this oblique slip component is manifested as en echelon periclines and flower structures in general the n s and nw se trending basins were less strongly inverted and the silesian post rift fill has been retained

the recognition of reactivation during continental deformation reactivation involves the accommodation of geologically separable displacement events intervals 1 ma along pre existing structures the definition of a significant period of quiescence is central to this phenomenological definition and the duration of the interval chosen represents the resolution limit of reactivation criteria found in most ancient settings in neotectonic environments reactivation can be further defined as the accommodation of displacements along structures that formed prior to the onset of the current tectonic regime this mechanistic definition cannot always be applied to ancient settings due to the uncertainties in constraining relative plate motion vectors four sets of criteria may be used to recognize reactivation in the geological record stratigraphic structural geochronological and neotectonic some structural criteria may not be reliable if used in isolation to identify reactivated structures much of the previously published evidence cited to invoke structural inheritance is equivocal as it uses similarities in trend dip or three dimensional shape of structures numerous fault and shear zone processes can cause significant weakening both synchronously with deformation and in the long term and may be invoked to explain reactivation the collage of fault bounded blocks forming most continents therefore carries a long term architecture of inheritance which can explain much of the observed complexity of continental deformation zones

ice ages and nuclear waste isolation abstract the greatest natural threats to the integrity of the geological barriers to nuclear wastes isolated in cavities mined at depths between 400 and 800 m are likely during rapid retreats of future ice sheets the next major glacial retreat is expected at ca 70 ka well within the lifetime of high grade nuclear waste but it is not yet clear how long man s greenhouse effect may delay it this contribution discusses the potential problems posed to european waste isolation sites during erosion by ice and over pressurizing of meltwater and gasses in a lithosphere flexed by major ice sheets these depend on the target rocks and the location of the site with respect to the ice streams and margins of future ice sheets of particular size no sites are planned under the centres of future ice sheets in europe where end glacial earthquakes can be expected to reactivate major faults nor where ice can be expected to deepen and lengthen fjords along the atlantic coast sites in the alps may be vulnerable to radical changes in the patterns of glacial troughs the stability and geohydrology of sites in coastal areas beyond future ice margins are threatened by river gorges when sea level falls ca 125 m or in enclosed basins like the mediterranean ever lower the greatest problems are likely in lowland regions exposed by the rapid retreat of thick ice fronts where large lakes on or under thick warm based ice are dammed by more distal cold based ice groundwater in subhorizontal fractures dilated by glacial unloading may reach over pressures capable of hydraulically lifting megablocks of bedrock with fracture permeability and or the ice damming them so that less permeable substrates are susceptible to incisions eroded to depths of ca 360 at locations controlled mainly by ice topography kinematics and history

subsidence and foundering of strata caused by the dissolution of permian gypsum in the ripon and bedale areas north yorkshire underground dissolution of thick gypsum beds in the edlington formation and roxby formation of the zechstein sequence in north yorkshire england has resulted in a 3 km wide and 100 km long belt of ground susceptible to foundering within this belt a large subsidence depression at snape mires near bedale was largely filled with lacustrine deposits in the later part of the late devensian and during the flandrian south of snape mires the nosterfield ripon bishop monkton area has suffered about 40 episodes of subsidence in the past 150 years and the presence of several hundred other subsidence hollows indicates considerable activity from the later part of the devensian onwards the linear and grid like arrangement of these subsidence hollows indicates collapse at intersections in a joint controlled cave system linear subsidence features at snape mires are also joint controlled the transition from anhydrite at depth to secondary gypsum near surface marks the down dip limit of the subsidence prone belt cavities are propagated upwards by roof collapse of caverns in the gypsum leading to the formation of breccia pipes choking of the pipes can reduce the surface expression of the underground collapse but the larger cavities are liable to produce pipes that reach the surface even at the eastern boundary of the 3 km wide belt described further subsidence in the ripon area is predicted and some suggestions for remedial measures are given

glacio seismotectonics ice sheets crustal deformation and seismicity abstract the last decade has witnessed a significant growth in our understanding of the past and continuing effects of ice sheets and glaciers on contemporary crustal deformation and seismicity this growth has been driven largely by the emergence of postglacial rebound models pgm constrained by new field observations that incorporate increasingly realistic rheological mechanical and glacial parameters in this paper we highlight some of these recent field based investigations and new pgms and examine their implications for understanding crustal deformation and seismicity during glaciation and following deglaciation the emerging glacial rebound models outlined in the paper support the view that both tectonic stresses and glacial rebound stresses are needed to explain the distribution and style of contemporary earthquake activity in former glaciated shields of eastern canada and fennoscandia however many of these models neglect important parameters such as topography lateral variations in lithospheric strength and tectonic strain built up during glaciation in glaciated mountainous terrains glacial erosion may directly modulate tectonic deformation by resetting the orogenic topography and thereby providing an additional compensatory uplift mechanism such effects are likely to be important both in tectonically active orogens and in the mountainous regions of glaciated shields in general the short term response to ice fluctuations is similar to the earth s response to fluctuations in water reservoirs with the subsequent increase or decrease in seismicity which depends on the pre existing stress state when the ice fluctuations occur on the spatial scale and magnitude of the late pleistocene glaciation and deglaciation however the viscoelastic response of the earth especially the mantle causes significant changes in crustal deformation and earthquake activity that is spatially extensive and temporally complex the regions of greatest ice thickness and the regions marginal to the late pleistocene ice sheets indicate the most dramatic evidence of earthquake faulting the mantle response to these large ice mass fluctuations and the change in mass between the oceans and land caused and continues to cause measureable crustal deformation at hundreds of kilometres from the ice margins for both tectonically active and cratonic regions palaeoseismic investigations of late pleistocene and holocene faults are an important tool in evaluating earthquake hazard the predicted response of the earth over this time however is very dependent on the model assumed for most regions far more carefully designed field observations are needed to constrain the existing rheological and ice models

the mont terri rock laboratory a new international research project in a mesozoic shale formation in switzerland abstract in many countries argillaceous formations are being considered as potential host rocks for repositories of radioactive waste therefore in 1995 several organisations decided to start an international research project in the reconnaissance gallery of the mont terri motorway tunnel in north western switzerland in a mesozoic shale formation the opalinus clay aalenian this project is under the patronage of the swiss national hydrological and geological survey the following organisations are also partners in the project andra and ipsn france bgr germany enresa spain nagra switzerland jnc and obayashi japan sckcen belgium the aims of the project are to analyse the hydrogeological geochemical and rock mechanical properties of an argillaceous formation the changes of these properties induced by the excavation of galleries and to evaluate and improve appropriate investigation techniques in january 1996 eight niches were excavated for the project and 15 experiments were started in boreholes of no more than 30 m in length in the winter of 1997 1998 a new research gallery was excavated to host further experiments different excavation drilling and investigation techniques have been tested and improved significant results of the different experiments are already available some of those dealing with hydrochemistry porewater pressure measurements and the evolution of the excavation disturbed zone edz are reported in this paper

glacial erosion of the wash and fen basin and the deposition of the chalky till of eastern england abstract it is widely accepted that the low lying area of eastern england comprising the wash and the fens was excavated by the anglian ice sheet moving first from the northwest and then from the north the ice moved rapidly across the deformable bed on the jurassic mudrocks and rode up over the chalk escarpment eroding the scarp face and setting back the lowered escarpment by several kilometres the eroded material was deposited beyond the basin as the chalky till of the southeast midlands and east anglia in this paper reconstruction of the form of the preglacial landscape of the jurassic and cretaceous outcrops allows estimation of the excavated volume of both mudrock 312 km 3 and of chalk 243 km 3 a total of about 555 km 3 this volume and the lithological mix matches closely the reconstructed volume estimate 363 km 3 of the now dissected chalky till around the margins of the glacial lobe the amount of erosion achieved by the anglian ice sheet amounts to 3 4000 bubnoffs b i e mm ka 1 even if we allow 20 ka for a complex glaciation this far exceeds rates of erosion recorded anywhere with the british isles today even if averaged over the 440 000 yr since that event the rate is 167 b far greater than the erosion achieved on similar rocks within the adjacent unglaciated thames basin estimated at 45 b

evidence of mid holocene storm surge deposits from morecambe bay northwest england a biostratigraphical approach abstract this paper presents a high resolution bio stratigraphic methodology employed to show mid holocene storm surge signatures from sedimentary sequences deposited under limnic or lagoonal conditions although this methodology is effective it is essential to find suitable sites where storm surge deposits are well preserved and have not been reworked subsequently by waves and tidal current a case study from morecambe bay has indentified nine storm events within 290 yr between 6030 70 and 5740 60 yr bp

the sedimentology of middle holocene tsunami facies in northern sutherland scotland uk abstract lagoonal sediments attributed to the main holocene marine transgression in strath halladale northern sutherland contain a complex coarser layer believed to have been deposited during the tsunami associated with the second storegga slide off south west norway the coarser sequence is dated at between 7590 50 and 7290 50 radiocarbon years bp 6507 6260 cal bc and 6228 6029 cal bc detailed stratigraphical analysis has determined a distinctive suite of sedimentary sub units within the coarser layer in marked contrast to the sediments which occur above and below a pronounced erosional unconformity with the underlying sediments is recorded with the base of the tsunami layer characterised by eroded material from the underlying peat the presence of a mixed diatom assemblage although fragmentary indicates a chaotic accumulation of the deposit with all habitats represented variations in particle size within the sequence disclose striking similarities with those from contemporary tsunami deposits the run up of the tsunami sediments is calculated at a minimum of 4 6 m this is the first occasion on which a deposit of the second storegga slide tsunami has been found outside the north sea basin and indicates that the area affected by the tsunami may have been larger than has been previously described

geometry and growth kinematics of salt pillows in the southern north sea abstract low amplitude salt pillow structures can potentially reveal important details of the physical behaviour of salt and overburden from their growth histories this is because the initial characteristic wavelengths are likely to be preserved beds are more easily traceable across the tops of the structures and original salt thicknesses can be accurately estimated for these reasons a detailed analysis of depth converted seismic data was undertaken to gain insight into the formation growth and spatial distribution of salt pillows in the southern north sea basin around quadrant 44 this area contains a permian zechstein salt interval which has undergone widespread halokinesis to the pillow stage since triassic times but with the main movement during the tertiary perhaps due to inversion the wavelength of the pillows decreases northwards from over 24 to 1 km and estimated original salt thickness also decreases northwards from 1300 to 800 m and there is a strong positive relationship between salt thickness and pillow wavelength pillows appear to be spaced at a characteristic wavelength as they are not directly related to faults and the wavelength varies smoothly through the area however there is a strong alignment of some pillows parallel with the underlying nw se fault trend suggesting that their axis of elongation was controlled by faulting average vertical growth rates of the pillows vary from 40 m ma in the north to less than 10 m ma in the central part of the area measured over vertical distances up to 1250 m most of the growth occurred in the tertiary although a few pillows initiated in the triassic in the north of the area the northern pillows appear to have slowed or even stopped growing during the jurassic and cretaceous and then restarted in the tertiary assuming rayleigh taylor instability controlled the initial pillow wavelengths and using the calculated original salt overburden thicknesses ratios the viscosity ratio between the overburden and the salt can be estimated across the area and was found to vary from 6000 to less than 100 the pillows which moved earliest had the lowest inferred viscosity ratios which is probably because the sediments were less lithified at the time of movement

inverse modelling of extension and denudation in the east irish sea and surrounding areas abstract epeirogenic uplift and denudation are difficult to quantify because unlike basin subsidence they do not leave an unambiguous record in recent years much research has been focussed on calculating regional denudation from sonic velocity measurements from vitrinite reflectance profiles and from fission track analyses here we show that inverse modelling of the remnant stratigraphy within extensional sedimentary basins yields estimates of denudation within a corridor which crosses the british isles for a variety of reasons these estimates are minima but they do broadly agree with those determined by inversion of vitrinite reflectance profiles our results also demonstrate that denudation estimates from fission track analyses are meaningful a combined stratigraphic and thermal indicator approach should now be used to map denudation across different continents

evidence from apatite fission track analysis for the post devonian burial and exhumation history of the northern highlands scotland abstract devonian and older rock samples from outcrops in the northern scottish highlands have undergone protracted cooling since they reached palaeotemperatures of 110 c or more in the late palaeozoic to early mesozoic the results not only suggest that the northern highlands region has experienced kilometre scale exhumation since the late palaeozoic but also that devonian and possibly permo carboniferous sedimentation was probably more extensive than current outcrop patterns would imply a permian outcrop sample from the minches basin reached a maximum palaeotemperature of 70 90 c prior to the onset of cooling in the early tertiary while data from devonian and older samples suggest an acceleration in the rate of cooling in the early tertiary the magnitude of early tertiary palaeotemperatures on the mainland adjacent to the inner moray firth imf indicate similar amounts of tertiary exhumation to those derived from compaction analyses for the imf however to the west the magnitude of tertiary cooling cannot be solely ascribed to exhumation and a contribution of heating due to hydrothermal effects and or elevated heat flow associated with the tertiary igneous complex may also need to be invoked

exploring the continental shelf for low geological risk nuclear waste repository sites using petroleum industry databases a uk case study abstract site selection for deep disposal of intermediate and high level radioactive waste in the uk to date has not taken full advantage of oil industry databases that cover offshore areas the shallow water area of the uk continental shelf is twice that of the onshore area and due to variations in regional geology many geological settings occur that are not found onshore the geology and hydrogeology of the uk continental shelf contrast in many respects with that of onshore areas the geology of the north sea in particular is constrained by over 3000 deep 2 km exploration wells 50 000 km 2 of 3d seismic data and several million line kilometres of 2d seismic data vastly more than the coverage of data constraining the deep geology onshore combined with detailed studies of oil and gas fields this data has led to a greater technical understanding of many aspects of offshore geology relative to that onshore these data represent an original investment of 20bn by oil and gas companies but are now in the public domain these databases are used here to compare onshore and offshore settings in terms of criteria such as structural geology fault activity and uplift subsidence clear differences emerge such as risk of seismicity exhumation and the variety of hydrogeological systems the distribution and geometry of candidate repository sites throughout the north sea salt structures shale basins sealed sandstone reservoirs and basement highs are illustrated several lines of petroleum related research have yielded lessons that may be useful in assessing deep repository locations such as migration of hydrocarbons through shale seals the exceptional combination of seismic data and well control means that offshore areas on the continental shelf and the north sea in particular can now be thoroughly explored in this context and contribute candidate sites into a nationwide portfolio of deep geological disposal options in the event of site selection being pursued in the uk an example framework for this process is presented here

surface uplift uplift of rocks and exhumation of rocks uplift of the surface of mountain belts requires forces that are comparable in magnitude to those associated with plate motion and therefore determination of rates of surface uplift could provide important information on the dynamics of mountain ranges rates of uplift of the surfaces of mountain ranges have not however been quantified sufficiently well that they provide useful constraints on those processes many reports off surface uplift in mountain ranges are based on mistaking exhumation of rocks or uplift of rocks for surface uplift and provide no information whatsoever on the rates of surface uplift

estimates of yet to find impact crater population on earth abstract one hundred and seventy six craters on earth are currently accepted as resulting from bolide impact this list is growing at a rate of approximately two craters per year because of new discoveries and revised interpretations here we take the subset of craters most relevant to sedimentary basin exploration and utilization phanerozoic kilometre scale craters and tackle the question of how many remain to be found in phanerozoic strata on earth a surprising range in predictions of crater numbers is obtained from published crater population functions reaching two orders of magnitude variation for craters in the range 1 5 km diameter this range in predictions is largely due to variations in treatment of impactor breakup in the atmosphere also required is quantification of the total area and timespan of phanerozoic sediment currently preserved on earth an estimate of 8 87 10 15 km 2 a is derived here together with a relatively conservative crater population function this yields a prediction of 845 craters subtracting the 131 known phanerozoic craters larger than 1 km leaves a predicted population yet to find of 714 of which 228 are larger than 2 5 km uncertainties subsets of this population and spatial distribution are also considered

geological and tectonic investigations in the former morsleben salt mine germany as a basis for the safety assessment of a radioactive waste repository abstract the safety assessment of a repository for radioactive waste is based on the results of model simulations and scenario analysis to obtain a well founded geological basis for this extensive geological and structural studies have been carried out for the morsleben repository the morsleben repository for radioactive waste was constructed in zechstein strata the location of the werra formation z1 was located in boreholes and the stassfurt z2 leine z3 and aller z4 formations are exposed in the repository mine the zechstein salt has a total thickness in the western part of the salt body of 580 m in the eastern part it is only 380 m thick the morsleben repository is in the allertal zone salt structure the root zone of a deeply eroded salt body this is evidenced by the relatively small thickness of the zechstein salt as well as by the many blocks of the hauptanhydrit z3ha in the salt which are present mostly in the synclinal hinge zones i e in the deeper parts of the structure as a result of tectonic separation of ductile salt from competent anhydrite the tectonics of the structure are documented by polyphase folds characteristic of the deformation of ductile material the general trend of the fold axes is horizontal nw se parallel to the trend of the allertal zone the western part of the salt structure is characterised by high ne vergent isoclinal folds the z2 to z4 strata involved in the folding extend from the salt table top of the salt body down to the lowest synclinal trough in the eastern part the folds are open and symmetric with considerably smaller amplitude correspondingly z4 strata mostly occur just below the salt table z3 at the repository level and z2 below that the different styles of folding in the east and west are due to the structure of the rocks beneath the zechstein salt the boundary between the two types of folds lies above a major fault in the rocks below the zechstein salt separating a downthrown block in the west from an upthrown block in the east ne vergent isoclinal folds occur above the downthrown block where the major ascent of the salt occurred in contrast the folding in the upthrown eastern block has hardly been affected by the ascending salt hence the wide open symmetrical folds thus the structure of the rocks beneath the zechstein determined the folding style in the overlying salt the present day fold structures chiefly result from late cretaceous compression that affected the north german basin the salt body is therefore primarily a tectonic structure and is not halokinetic

geology and geophysics of the underground research laboratory site lac du bonnet batholith manitoba the lease area of the atomic energy of canada limited underground research laboratory covers 3 8 km2 and is located 2 5 km north of the south contact of the lac du bonnet batholith a shaft to 255 m and 130 boreholes up to 1100 m deep expose the third dimension the underlying granite is largely of two types i pink porphyritic which may be biotite rich gneissic and or xenolithic and ii grey homogeneous and equigranular composition layering including xenolith rich zones outlines domes along an antiform trending north northeast through the western part of the lease area the southeast dipping flank underlies the eastern half of the site including the shaft axes of folding trend 065 and 140 homogeneous grey granite being relatively fresh and unfractured is associated with a magnetic field that is about 100 nt higher and with a resistivity that is up to 5000 m higher than those of other units a pattern of highs in the magnetic field caused by the high magnetite content of some xenolith

regional patterns of tectonic stress in europe nearly 1500 stress orientation determinations are now available for europe the data come from earthquake focal mechanisms overcoring measurements well bore breakouts hydraulic fracturing measurements and young fault slip studies and sample the stress field from the surface to seismogenic depths three distinct regional patterns of maximum compressive horizontal stress shmax orientation can be defined from these data a consistent nw to nnw shmax stress orientation in western europe a wnw ese shmax orientation in scandinavia similar to western europe but with a larger variability of shmax orientations and a consistent e w shmax orientation and n s extension in the aegean sea and western anatolia the different stress fields can be attributed to plate driving forces acting on the boundaries of the eurasian plate locally modified by lithospheric properties in different regions on average the orientation of maximum stress in western europe is subparallel to the direction of relative plate motion between africa and europe and is rotated 17 clockwise from the direction of absolute plate motion the uniformly oriented stress field in western europe coincides with thin to medium lithospheric thickness approximately 50 90 km and high heat flow values 80 m w m2 in western europe a predominance of strike slip focal mechanisms implies that the intermediate principal stress is vertical the more irregular horizontal stress orientations in scandinavia coincide with thick continental lithosphere 110 170 km and low heat flow 50 m w m2 the cold thick lithosphere in this region may result in lower mean stresses associated with far field tectonic forces and allow the stress field to be more easily perturbed by local effects such as deglaciation flexure and topography the stress field of the aegean sea and western anatolia is consistent with n s extension in a back arc setting behind the hellenic trench subduction zone the stress field is influenced in places by regional geologic structures e g in the western alps where shmax directions show a slight tendency toward a radial stress pattern not all major geologic structures however appear to affect the shmax orientation e g in the vicinity of the rhine rift system horizontal stress orientations are continuous

episodic basin wide hydrofracturing of overpressured early cenozoic mudrock sequences in the north sea basin a deformational fabric associated with the early cenozoic mudrock dominated sequences of the north sea is described seismic data show that the mudrock sequences are pervasively faulted over a large area of the basin axis in the central and northern north sea mapping the faults using regional two and three dimensional data shows that the faults are minor extensional structures arranged in stratigraphically bound tiers in the deep water sequences and bounded by regional condensed sequences seals the geometry of the faulting is complex faults are organized into cellular networks comprising polygonal prismatic and pyramidal forms a gravitational mechanism of downslope collapse faulting is rejected on the grounds that the distribution of the deformed sequences extends over large areas of formerly horizontal basin plain a mechanism involving the repeated development of basin wide overpressured compartments is proposed in which the cellular structure is related to deformation patterns associated with a density inversion between the overpressured units and the overlying seal hydrofracturing is invoked to account for the almost random strike distribution of the faulting the region affected by the pervasive fluid expulsion faulting encompasses almost half of the north sea basin basin wide collapse of the overpressured compartments could have resulted in a geologically rapid flux of enormous volumes of pore fluids this mechanism has implications for concepts of steady state compaction heat flow in basins hydrocarbon transport and the genesis of lead zinc ore deposits

assessing deep seated dissolution subsidence hazards at radioactive waste repository sites in bedded salt abstract deep seated salt dissolution and associated subsidence have occurred in many salt bearing sedimentary basins because of its capacity for brea hing the hydrologic integrity of a thick salt unit the dissolution subsidence process is a potential hazard requiring thorough assessment at proposed radioative waste repository sites in bedded salt in order to develop a better understanding of this potential hazard a combination of field studies and theoretical analyses are used to delineate the physical processes that control dissolution and subsidence this information is used in turn to develop strategies for assessing this hazard at any given bedded salt repository site a generalized hydrogeological model of dissolution has been developed consisting of a salt unit separated from an underlying aquifer by a low permeability unit this model suggests that local salt removal rates can vary over many orders of magnitude from microns per year to meters per year depending on the hydrogeologic conditions beneath the salt unit salt deformation is strongly rate sensitive so the rate of salt removal strongly influences the structural form of subsidence low salt removal rates produce predominantly ductile subsidence which is characterized by the gradual formation of a braod shallow depression that is narrower and deeper in successively lower horizons on the other hand high salt removal rates produce predominantly brittle subsidence which is characterized by the formation of a steep walled chimney filled with down dropped brecciated rock ductile subsidence depressions and brittle subsidence chimneys most likely represent the endpoints of a continuous range of structural forms assessing potential dissolution subsidence hazards at a given repository site begins with the identification and characterization of existing dissolution subsidence features in the site area studies of existing features should be complemented by an evaluation of the potential for undetected or future dissolution activity based on the deep hydrogeologic conditions at the site potential salt removal rates predicted by this analysis are then used as an analysis of the structural character and timing of potential subsidence

structural trends in central britain from images of gravity and aeromagnetic fields digital image processing techniques have been used to analyse the regional gravity and aeromagnetic datasets in central britain colour and shaded relief images have been generated which convey information on both anomaly amplitude as colour and anomaly gradient as relief and highlight structural trends lineaments and textural contrasts not easily discernible on standard contour maps we have identified some of the more important features and have tried to relate these to the evolution of the crust in each region on a broad scale the images emphasize caledonoid ene trends to the north of the solway line and both tornquist se and caledonoid ne to ene trends to the south the pattern of magnetic anomalies over central england seems to define the extent of the shallow late precambrian early palaeozoic basement of the midlands microcraton and delineates crustal elements and characteristic trends within it magnetic lineaments and the grain of gravity anomalies trending in northeasterly and northwesterly directions on either side of the microcraton seem to relate to structures which originated during the evolution of the welsh and eastern english caledonides respectively where lower palaeozoic and precambrian rocks form the concealed basement as in most of england the gravity lineaments tend to reflect the influence of basement fractures on the subsequent pattern of sedimentation whereas many of the features on the magnetic images are more directly related to structures within the basement itself

the origin of major incisions within the pleistocene deposits of the north sea a seismic survey to resolve the geometry of major incisions within the upper pleistocene has been run over an area of 1600 km2 in the mid north sea twelve separate incisions have been identified ranging up to 5 km wide and 25 km long with incised depths from 100 to 350 m deposits categorised into four recurring types infill the incisions to form a flat sea bed the surveyed features are comparable to others widely found beneath the northern continental shelves and lowlands of europe asia and america such incisions have a similar range of sizes and shapes were cut during at least three separate periods in the pleistocene and are down cut into substrates of various lithologies ranging from precambrian to pleistocene in age the major incisions are believed to be the product of a singular mechanism the outburst of intra ice sheet lakes forming jokulhlaup plunge pools except for late stage hollow infills which have interglacial temperate deposits the incision infills are periglacial the former deposits are similar to the sediments accumulating in the present day bathymetric enclosed deeps such as the lambay deep and devil s hole found on continental shelves it is suggested that the enclosed deeps are large kettle holes which resulted from the thawing of ice formerly buried in the incision infills