subglacial bed form morphology controlled by ice speed and sediment thickness subglacial bed forms drumlins ribbed moraines and megascale glacial lineations are enigmatic repetitive flow parallel and flow transverse landforms common in glaciated landscapes their evolution and morphology are a potentially powerful constraint for ice sheet modeling but there is little consensus on bed form dynamics or formative mechanisms here we explore shallow sediment bed form dynamics via a simple model that iterates i down flow till flux ii pressure gradient driven till flux and iii entrainment and deposition of sediment under various boundary conditions replicas of subglacial bed forms readily emerge bed form dynamics mirror those in subaqueous and aeolian domains transitions between ribbed moraines and elongate flow parallel bed forms are associated with increasing ice speeds and declining sediment thickness these simulations provide quantitative flux estimates and suggest that widely observed transitions in shallow sediment subglacial bed forms e g ribbed moraines to drumlinoids to megascale glacial lineations are manifestations of subtle variations in ice velocity and sediment thickness

the late devensian 22 000 bp irish sea basin the sedimentary record of a collapsed ice sheet margin the late devensian 20 ka bp glacial geology of the irish sea basin 4000 km2 is an event stratigraphy recording the entry of marine waters into a glacio isostatically depressed basin and the rapid retreat of the irish sea glacier as a tidewater ice margin marine limits occur up to 140 m o d across much of the central basin the ice margin was uncoupled from its bed exposing a subglacially scoured topography to glaciomarine processes the irish sea glacier was a major drainage conduit of the last british ice sheet calving of the marine ice margin resulted in fast flow surging of ice streams recorded by drumlin fields around the northern basin margin and tunnel valleys rapid evacuation of the basin may have stranded large areas of dead ice in peripheral zones e g cheshire shropshire lowlands and initiated the collapse of the ice sheet thick wedges of ice contact glaciomarine sediments were deposited during ice retreat as morainal bank complexes by successive tidewater ice margins stabilized at pinning points around the irish sea coast where morainal banks occur on the seaward side of drumlin swarms there is a clear sequential relationship between rapid ice loss from calving ice margins the development of fast flowing ice streams drumlinization and the pumping of subglacial sediment to tidewater raised delta complexes are locally associated with marine limits along the high relief coastal margins of wales east central ireland and the lake district associated valley infill complexes record downslope resedimentation of heterogenous sediments into the marine environment during ice retreat co eval offshore deposits are represented by well stratified glaciomarine complexes that infill a subglacially scoured topography that shows networks of tunnel valleys glaciomarine mud drapes occur well to the south of the maximum limit of grounded ice in the basin e g north devon scilly islands southern ireland the age of these distal sediments previously mapped as pre devensian tills is constrained by amino acid ratios basin rebound following deglaciation was rapid with over 100 m recovery in 3 ka and was followed by a low marine still stand peat accumulating in offshore areas now as much as 55 m below sea level has been drowned by the postglacial eustatic rise in sea level the glacio sedimentary model identified in this paper involving rapid ice retreat and related sedimentation triggered by rising relative sea level suggests that isotatic downwarping is an important mechanism for deglaciating continental shelves

the instability theory of drumlin formation and its explanation of their varied composition and internal structure despite their importance in understanding glaciological processes and constraining large scale flow patterns in palaeo glaciology there is little consensus as to how drumlins are formed attempts to solve the drumlin problem often fail to address how they are created from an initially flat surface in the absence of obvious cores or obstacles this is a key strength of the instability theory which has been described in a suite of physically based mathematical models and proposes that the coupled flow of ice and till causes spontaneous formation of relief in the till surface encouragingly model predictions of bedform height and length are consistent with observations and furthermore the theory has been applied to a range of subglacial bedforms and not just drumlins however it has yet to confront the myriad observations relating to the composition and internal structure of drumlins and this could be seen as a major deficiency this paper is a first attempt to assess whether the instability theory is compatible with the incredible diversity of sediments and structures found within drumlins we summarise the underlying principles of the theory and then describe and attempt to explain the main types of drumlin composition e g bedrock till glaciofluvial sediments and combinations thereof contrary to a view which suggests that the presence of some sedimentary sequences e g horizontally stratified cores is inconsistent with the theory we suggest that one would actually expect a diverse range of constituents depending on the inheritance of sediments that pre date drumlin formation the duration and variability of ice flow and the balance between erosion and deposition till continuity at the ice bed interface we conclude that the instability theory is compatible with and potentially strengthened by what is known about drumlin composition and as such offers the most complete and promising solution to the drumlin problem to date

sheetfloods sheetwash sheetflow or abstract following mcgee s 1897 use of the term sheetflood there developed a variety of conflicting opinions and numerous terminologies relating to this process many of which are ill considered and poorly defined a classification system is devised on both a hydrologic and geomorphic basis which dispenses with many superfluous terms that have been used with respect to sheetfloods magnitude of a storm is expressed in terms of the amount of surface runoff generated from a given storm due to the surface conditions a sheetflood is defined as a sheet of unconfined flood water moving down a slope the frequency of a sheetflood is relatively low while its magnitude is relatively great sheetflow is defined as relatively high frequency low magnitude overland flow occurring in a continuous sheet and is restricted to laminar flow conditions sheetwash a term of geomorphic origin is considered to be redundant and is superseded by the more meaningful term rainwash defined as the washing action of rain on slopes

glaciotectonically induced water throughflow structures in a late pleistocene drumlin kanrawer county galway western ireland abstract during drumlinisation brecciated bedrock was sheared over the proximal end of a late pleistocene drumlin at kanrawer western ireland high effective water pressures generated by shearing at the ice substrate interface during this event created a marked proximal to distal hydrostatic gradient affecting about 60 of the diamict which forms most of the drumlin sediment pile this resulted in large scale sediment reorganisation by darcian through flow and closed conduit flow of expelled pore waters and subglacial meltwater proximally this is reflected in the development of integrated honeycomb gravel structures formed by glaciotectonic shunting and tortuous throughflow in the diamicts distally the honeycomb structures give way to dish and pillar structures which are connected to gravelly and laminated sandy channel fill facies which cut obliquely across the long axis of the drumlin this facies transition reflects throughflow of pore water rising through the sediment pile and expelled along nye type channels excavated on the distal drumlin surface these proximal to distal sediment transformations suggest that current theoretical models of drumlin formation do not adequately represent the continuum of subglacial processes which accompany drumlinisation

geometric sound propagation through an inhomogeneous and moving ocean scattering by small scale internal wave currents ray equations appropriate for ocean acoustic propagation through an inhomogeneous and moving ocean are put forth with applications to sound scattering by internal waves the result reveals the important role played by range dependent horizontal current shear the garrett munk internal wave model and observations of upper ocean shear and sound speed fluctuations suggest that inertial frequency upper ocean shear may play a comparable role to internal wave induced sound speed fluctuations as a source of upper ocean acoustic scattering

observations and models of inertial waves in the deep ocean the structure of the inertial peak in deep ocean kinetic energy spectra is studied here records were obtained from polymode arrays deployed in the western north atlantic ocean 40 w to 70 w 15 n to 42 n the results are interpreted both in terms of local sources and of turning point effects on internal waves generated at lower latitudes in most of the data there is a prominent inertial peak slightly above f however the peak height above the background continuum varies with depth and geographical environment three classes of environment and their corresponding spectra emerge from peak height variations class 1 is the 1500 m level near the mid atlantic ridge with the greatest peak height of 18 db class 2 includes a the upper ocean depth less than 2000 m b the deep ocean depth greater than 2000 m over rough topography and c the deep ocean underneath the gulf stream with intermediate peak height of 11 5 db class 3 is the deep ocean over smooth topography with the lowest peak height of 7 5 db near f the horizontal coherence scale is 0 60 km at depths from 200 m to 600 m and the vertical coherence scale is 0 200 m in the lower part of the main thermocline and 0 1000 m in the deep water the phase difference suggests a downward energy propagation in the lower thermocline and standing waves in the deep water a one turning point model is developed to describe inertial waves at mid latitudes based on the assumption that inertial waves are randomly generated at lower latitudes global generation where their frequency wave number spectrum is given by the model of garrett and munk 1972 1975 using the globally valid wave functions obtained by munk and phillips 1968 various frequency spectra near f are calculated numerically the model yields a prominent inertial peak of 7 db in the horizontal velocity spectrum but no peaks in the temperature spectrum the model is latitudinally dependent the frequency shift and bandwidth of the inertial peak decrease with latitude energy level near f is minimum at about 30 and higher at low and high latitudes the observations of class 3 can be well described by the model a low zonal wave number cutoff is required to produce the observed frequency shift of the inertial peak the differences between the global generation model and the observations of class 1 and class 2 are interpreted as the effects of local sources a locally forced model is developed based on the latitudinal modal decomposition of a localized source function asymptotic eigensolutions of laplace s tidal equation are therefore derived and used as a set of expansion functions the forcing is through a vertical velocity field specified at the top or bottom boundaries of the ocean for white noise forcing the horizontal velocity spectrum of the response has an inertial peak which diminishes in the far field with the forcing located at either the surface or the bottom several properties of the class 2 observations can be described qualitatively by a combination of the global and local models the reflection of inertial waves from a turbulent benthic boundary layer is studied by a slab model of given depth frictional effects are confined to the boundary layer and modeled by a quadratic drag law for given incident waves reflection coefficients are found to be greater than 0 9 for the long waves which contain most of the energy this result suggests that energy containing inertial waves can propagate over great distance as is required by the validity of the model of global generation

the geomorphology and sedimentology of eskers in north central ireland abstract eskers are common glacial landforms in ireland and form regionally extensive networks that reflect stages of ice retreat of this sector of the late devensian weichselian british irish ice sheet however the sedimentary structures and composition of these eskers are poorly known despite being able to offer valuable insight into subglacial hydrological processes during deglaciation this study presents detailed morphological and sedimentary evidence from six esker systems in north central ireland where adjacent and interacting ice domes were characterized by changes in ice bed thermal and hydrological properties as well as significant ice dome centre migration morphologically esker ridges in this area vary in length size and continuity with some esker systems ascending over hills thus crossing watersheds whereas other systems are confined to basin settings esker systems commonly appear to feed terminal outwash and glaciolacustrine deltas but several systems do not show a clear terminus the internal sediments within eskers vary from concentrically bedded sands and gravels formed within enclosed tunnels to interbedded sands gravels and diamictons occasionally with water induced soft sediment deformations that are likely formed in interlobate settings in contrast with some previous glacier models that assume that esker systems are isochronous and were formed in solely subglacial settings the esker systems described here are interpreted as being of different ages formed in variously subglacial to ice marginal settings and thus cannot be used as a single snapshot of the disposition of ice domes during stages of late devensian ice retreat

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

subglacial processes from drumlins in clew bay western ireland interpretation of subglacial processes and environments can be usefully informed by the stratigraphy and structures of sediments preserved within drumlins and the down ice variability of these sediments drumlins in clew bay western ireland were formed by westward late pleistocene ice flow onto the atlantic shelf but the depositional processes and environments of these sediments remain uncertain this study describes and interprets the drumlin stratigraphy and sediment properties and structures from two drumlins on the south side of clew bay drumlin sediments are dominated by massive to stratified diamicton deposited subglacially by deformation of flows of varying rheology folds shears and clastic dikes within the sediments suggest active subglacial water flow related to variations in ice flow and ice bed coupling the distal ends of the drumlins are dominated by stratified diamicton and gravel units with sandy interbeds these sediments reflect deposition in a leeside subglacial cavity formed coevally with bedform development a model is proposed that can account for sediment stratigraphic patterns and drumlin formation copyright 2015 john wiley sons ltd

constraints on the preservation of diamict facies melt out tills at the margins of stagnant glaciers abstract this paper reviews the formation and preservation of diamict facies melt out till generated in situ by the downwasting of stagnant ice at the margins of continental glaciers melt out tills are commonly envisaged as being the product of the top supraglacial or bottom subglacial melt of stagnant debris rich glacier ice but such deposits are relatively uncommon in modern glacier environments where they are thin laterally discontinuous and have a low preservation potential theoretical considerations of the distribution of englacial debris in continental ice sheets show that only the glacier margin is likely to contain the necessary volumes of englacial debris for melt out to be a significant diamict forming process however the application of thaw consolidation theory to the formation of such deposits shows that substantial porewater pressures and self sediment deformations can be induced during melt out especially in matrix rich debris the classic melt out mechanism widely invoked in the literature since 1875 and involving passive in situ aggregation from thawing englacial debris can be seen to operate under restricted conditions diamict facies produced by melt out are likely to form spatially disjunct and laterally discontinuous elements within complex and ice marginal stratigraphics the melt out process is not a mechanism whereby regionally extensive diamiet ite units can be deposited across glaciated basins

reconstructing the last irish ice sheet 2 a geomorphologically driven model of ice sheet growth retreat and dynamics the ice sheet that once covered ireland has a long history of investigation much prior work focussed on localised evidence based reconstructions and ice marginal dynamics and chronologies with le

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

seismic characterization of multiple fracture sets does orthotropy suffice as geophysicists rely increasingly on effective media theories to characterize naturally fractured reservoirs they become more and more interested in evaluating the accuracy of different theories estimating their limits of applicability and assessing their usefulness for practice with this in mind we compare two popular seismological theories of hudson and schoenberg with the theory of kachanov developed in the context of mechanics of materials by performing finite element simulations of effective media for models that contain several sets of nonintersecting circular vertical fractures embedded in otherwise isotropic host rock we examine the accuracy of these theories our numeric study reveals that predictions of both the first and second order hudson s theories are typically inferior to those of others especially when the fractures are dry while on average the theories of schoenberg and kachanov fit finite element computations with comparable accuracy the latter appears to be more useful

looking through drumlins testing the application of ground penetrating radar ground penetrating radar gpr is becoming a commonly applied technique in geomorphology however its use in the study of subglacial bedforms has yet to be fully explored and exploited this paper presents the results of a gpr feasibility study conducted on a drumlinized terrain in cumbria uk where five drumlins were investigated using multiple radar antenna frequencies the site was selected for the presence of nearby bedrock outcrops suggesting a shallow drumlinized diamict bedrock contact and a permeable lithology despite the clayey sediment and unfavourable weather conditions a considerable penetration depth of 12m was achieved when using a 50mhz antenna with a separation of 1m trace spacing of 1m and 128 fold vertical stack results indicate that the drumlinized diamict is in direct erosional contact with the bedrock while the internal drumlin geometry is generally chaotic on the stoss side evidence of layering dipping downflow at an angle greater than the drumlin surface profile was found on the lee side the inter drumlin areas comprise 4m of infill sediment that masks part of the original drumlin profile overall this study indicates that gpr can be deployed successfully in the study of glacial bedform sedimentary architecture

the composition and internal structure of drumlins complexity commonality and implications for a unifying theory of their formation investigation of drumlins is significant to both glaciology and palaeoglaciology but the sheer diversity of their composition and internal structure is often cited as a major obstacle towards a satisfactory unifying explanation of their formation this paper presents the first systematic survey of the vast literature on this subject with the aim of concisely summarising observations and identifying any emergent patterns or commonality that theories of drumlin formation should be able to explain results confirm that investigations are often limited by availability of suitable sediment exposures 40 of studies report data from 5 drumlins and 44 do not specify sample size although borehole data and geophysical techniques can alleviate this problem however it is clear that the constituents of drumlins are incredibly diverse in terms of their composition e g a range of lithologies clast shapes sizes and fabrics structure e g sediments that are sorted homogeneous surface conformable unconformable and evidence of deformation e g ranging from pervasive to non pervasive limited to absent despite this diversity our review leads us to suggest that drumlin composition can be simplified to five basic types i mainly bedrock ii part bedrock part till iii mainly till iv part till part sorted sediments and v mainly sorted sediments this is a potentially significant step in that it reduces the oft cited complexity of drumlin composition and provides a more realistic goal for theories or numerical models of drumlin formation to target the various types can occur within the same drumlin field which leaves us with two possible implications for drumlin formation 1 different types of drumlin are formed seeded by different processes despite being morphologically similar equifinality investigation of drumlin composition may therefore reveal diagnostic processes explanations for these different types of drumlin and we argue that bedrock drumlins are an example 2 a single process occurs across large parts of the ice bed interface to create drumlinised terrain in a variety of sediments investigation of drumlin composition may in this case simply reflect pre existing sediments but importantly the way in which the drumlin forming mechanism modifies is modified by them we argue that the latter simpler explanation applies to all other types of drumlin excluding purely bedrock forms and conclude that the diversity in drumlin composition is no obstacle to a single unifying theory

effective elasticity of fractured rocks production related changes in subsurface stress are known to cause velocity variations that can be detected from time lapse seismic data and may provide valuable information for reservoir surveillance it has been long recognized that the stresses primarily influence the most compliant part of a rock by preferentially opening or closing the fractures walsh 1965 therefore examining wave propagation in fractured formations might give important clues about the stresses acting upon them as the length of seismic waves is usually much larger than that of the fractures deformation of cracked rocks can be treated as static

an effective medium inversion algorithm for gas hydrate quantification and its application to laboratory and borehole measurements of gas hydrate bearing sediments the presence of gas hydrate in marine sediments alters their physical properties in some circumstances gas hydrate may cement sediment grains together and dramatically increase the seismic p and s wave velocities of the composite medium hydrate may also form a load bearing structure within the sediment microstructure but with different seismic wave attenuation characteristics changing the attenuation behaviour of the composite here we introduce an inversion algorithm based on effective medium modelling to infer hydrate saturations from velocity and attenuation measurements on hydrate bearing sediments the velocity increase is modelled as extra binding developed by gas hydrate that strengthens the sediment microstructure the attenuation increase is modelled through a difference in fluid flow properties caused by different permeabilities in the sediment and hydrate microstructures we relate velocity and attenuation increases in hydrate bearing sediments to their hydrate content using an effective medium inversion algorithm based on the self consistent approximation sca differential effective medium dem theory and biot and squirt flow mechanisms of fluid flow the inversion algorithm is able to convert observations in compressional and shear wave velocities and attenuations to hydrate saturation in the sediment pore space we applied our algorithm to a data set from the mallik 2l 38 well mackenzie delta canada and to data from laboratory measurements on gas rich and water saturated sand samples predictions using our algorithm match the borehole data and water saturated laboratory data if the proportion of hydrate contributing to the load bearing structure increases with hydrate saturation the predictions match the gas rich laboratory data if that proportion decreases with hydrate saturation we attribute this difference to differences in hydrate formation mechanisms between the two environments

doppler sonar observations of internal waves the wavenumber frequency spectrum abstract in may 1980 an 18 day sequence of velocity profiles of the top 600 m of the sea was collected off the coast of southern california the measurements were obtained using a pair of doppler sonars mounted on the research platform flip from these data estimates of the wavenumber frequency spectrum of the oceanic internal wavefield are obtained the spectra are characterized by a series of ridges which occur at near internal and tidal frequencies as well as higher harmonics and sums of these fundamentals the ridges run parallel to the wavenumber axis there is a pronounced near inertial spectral peak the near inertial motions are dominated by a few identifiable wave groups there is a net downward energy propagation in the near inertial frequency band the vertical wavenumber dependence of a the spectrum is decidedly asymmetric in this region the asymmetry extends to five times the inertial frequency making much of the so called continuum asymmetric a high wavenumber cutoff at approximately 60

quaternary deposits and glacial stratigraphy in ireland quaternary stratigraphic studies in ireland have been based largely on lithostratigraphy the division of complex sedimentary sequences into the traditional framework of munsterian and midlandian events has created a series of stratigraphic scenarios which cannot be fully substantiated either by biostratigraphy or by facies basin analysis many of the current problems of glacial stratigraphy in ireland therefore arise directly from the field procedures used and the frameworks used to subdivide sedimentary sequences these problems are centred on the selection of type sites the subdivision of sedimentary sequences into unnatural divisions the concepts of direct and indirect stratigraphy classical models of climatically determined ice advance retreat cycles and the genetic inferences of the term till the conceptual basis of various stratigraphic schemes has changed little since the work of farrington 1934 1966 it is argued that a more realistic approach to description and assessment of glacigenic sediments should be based on basinal facies analysis and must include a documentation of the composition and geometry of three dimensional facies variations palaeoenvironmental interpretations of complex glacigenic sequences especially in coastal locations should take account of the influences of marine ice sheets on the sedimentary record in the past most sequences were interpreted in terms of glacioterrestrial models a large body of sedimentary evidence now indicates that shallow glaciomarine environments occurred along the margins of the last ice sheet during its maximum and subsequent retreat phases the locations and patterns of associated icemarginal systems indicate that the magnitude of and patterns of crustal deformation were greater and more complex than formerly realised thus the primary control on sea level variation at successive ice sheet margins was largely a function of glacioisostatic disequilibrium although complex glacial and glacially related sequences are known it is argued here that most of the multiple glacigenic sequences in ireland can be accommodated within the midlandian cold stage the basal units from certain sequences and erratic fans have been attributed to events earlier than the midlandian however it is difficult to compare these traces of earlier events from different sectors of the island due to poor dating correlation and the fact that direct and indirect stratigraphy is not adequately defined or separated the age of the gortian style interglacial sequences the implications of their floristic composition and their relationships to drift stratigraphy remains a major problem in stratigraphic constructions of the irish quaternary the stratigraphic significance of many till units must remain largely unknown until these matters are resolved at present it is not clear if the gortian sequences represent more than one interglacial a series of exposures in ulster provide evidence for major environmental changes during the midlandian cold stage these include an early stadial an early forest biozone 48 1 ka bp an early herb biozone 41 5 ka bp a middle interstadial phase of cold but non glacial aspect 30 5 ka bp a late stadial max 24 22 ka bp and a late glacial climatic oscillation the late midlandian stadial comprises a major glaciation that almost covered the island and a later more restricted phase when drumlins formed in the northern and western parts of the island max 17 ka bp

sandy high energy flood sedimentation some criteria for recognition with an example from the devonian of s w england abstract many modern sand deposits resulting from episodic floods in semi arid zones are dominated by parallel laminations but parallel laminated sands are not a major feature in perennial streams single flood events may deposit over 1 5 m of parallel laminated sand hence ancient parallel laminated sand dominated alluvial deposits may be the product of ephemeral flows the trentishoe formation hangman sandstone group middle devonian of north devon consists in part of laterally extensive beds of parallel laminated fine grained red sandstones that build multistorey sand bodies individual beds are little more than 1 m thick but may extend laterally for hundreds of metres a vertical sequence of erosion surface sparse silt clasts parallel laminations silt drape is often found suggesting high flow stage deposition of plane bedded sand followed by a very rapid waning of flow with silt fallout in the final stages of the flood this sequence bears marked similarity to certain modern ephemeral flow sands

seismic properties of rocks with distributions of small cracks summary hudson s theory of multiple scattering for the dynamic properties of cracked rock is applied to media containing cracks of varying orientation radius and aspect ratio angular variation of velocity and attenuation due to parallel and nearly parallel cracks and cracks with normals at a fixed angle to a given axis are presented anisotropy gradually decreases as the distribution of orientations is widened from exactly parallel to random increasing the aspect ratio of thin saturated cracks generally causes the elastic properties to approach those due to dry gas filled cracks for fluid filled cracks the line singularity where the birefringent shear waves have equal velocity moves closer to the symmetry axis as either the distribution of orientations is widened or the aspect ratio is increased calculated attenuations due to rayleigh scattering and viscous dissipation in crack filling fluids are many orders of magnitude less than reported attenuations in the earth viscous dissipation varies linearly with frequency and aspect ratio of thin fluid filled cracks a power law distribution of crack radii intended to represent that found in the earth s crust has no effect on the angular variations of seismic velocities or viscous attenuation but scattering attenuation is sensitive to the power law exponent lognormal distributions of aspect ratio also corresponding to observations in real rock affect velocity as well as attenuation velocities derived from the theory are compared with those measured in laboratory specimens with known geometries of cracks

landform modification by palaeo ice streams in east central ireland in eastern ireland subglacial bedforms including drumlins and rogen moraines were modified by headward erosion along two ice streams which had over lapping flow tracks the ice streams which hadtidewater termini are dated by geochrono metric and morphostratigraphic methods to 5 15 0 14 ckyr bp castleblaney ice stream and 13 8 14 ckyr bp armagh ice stream bedforms along ice stream tracks show a morphological continuum which reflects a down ice increase in the degree of modification by ice stream activity i e resulting in unmodified remoulded overprinted cross cut streamlined bedforms the down ice changes inbedform types are inferred to relate to changes in subglacial drainage and sediment transport mechanisms bedform and sedi mentary evidence suggest that discrete subglacial meltwater channels which developed up ice changed in a down ice direction to unchannelized flows which deepened towards the ice margin meltwater release from subglacial cavities and produced by strain heating at sheared ice stream margins probably helped support ice stream flow which ended as the volume of subglacial meltwater discharge decreased dated millennial scale cycles of ice activity may be related to instability at tidewater margins followed by complex thermal and hydraulic responses within the ice mass

t matrix approach to shale acoustics summary the t matrix approach of quantum scattering theory is used here to place many long wavelength equivalent medium approximations for porous composites polycrystals and cracked media on a common footing and to indicate their limitations but also to derive some new results based on two point statistics in this way we have obtained an insight into the difficult problem of elastic inclusions at finite concentrations which is of foremost relevance when estimating the effective material parameters of porous cracked shales involving stacks of more or less horizontally aligned clay platelets mixed together with more rounded silt minerals and with fluid filling the spaces a rather involved perturbative analysis of the effects of interactions between or structural correlations among the various inclusions minerals and cavities making up a real shale of hexagonal symmetry was performed in an attempt to obtain a better match between theoretical predictions based on a combination of coherent and optical potential approximations and experimental results recovered from ultrasonic wave speeds for the effective elastic stiffness tensor for the particular data set considered in this study the t matrix approach was able to match the data better than the approach of hornby et al but the match was not completely satisfactory further progress in theoretical shale modelling may come from a better knowledge of the elastic properties of pure clay minerals a more detailed knowledge of the microstructure of shales the incorporation of constraints obtained from comparisons between theoretical predictions and experimental results as well as a continuing development of the t matrix approach numerical results also for the effect of bedding parallel microcracks on the elasticity of such a real shale have value in illustrating the importance of taking into account the effects of spatial distribution when trying to deal with non dilute mixtures of highly contrasting material properties

melt out till in the edmonton area alberta canada till at two localities in the edmonton area alberta was formed mainly by passive melt out from glacier ice and not by lodgment by active ice or as a result of supraglacially derived flows this interpretation is based on the implications of in situ stratified glacial sediment below and within till and of large unlithified clasts of sediment derived from subtill deposits the interpretation is also supported by more general regional stratigraphic and structural observations such associated properties as subtill deformation a sharply planar till preglacial sediment contact sole marks and fabric support formation by melt out or illustrate local exceptions to this process regionally consistent preferred orientation of clasts is interpreted to represent preferred englacial orientation collective analysis of the principal eigenvectors of individual englacial fabrics and flow or resedimented till fabrics from a modem glacial environment and similar analysis for till at villeneuve near edmonton show the edmonton till to occupy the intermediate position in terms of orientation strength predicted by the melt out hypothesis in places preferred orientation is considered to have been disturbed by flow and diapirism induced by the process of thaw consolidation thus preferred clast orientation in till may show considerable deviation from a regional trend stratigraphy and sedimentology are used to classify the till and to relate individual exposures to regional environments of deposition

subglacial hydraulic conditions and their possible impact upon subglacial bed formation abstract in reviewing hypotheses of subglacial bed formation it becomes apparent that a close relationship must exist between the development of these bedforms and subglacial hydraulic conditions a speculative classification of subglacial temperate beds is presented subglacial beds are subdivided into h q and m beds h beds are typically hard beds in which upper interface meltwater activity dominates m beds are those in whichd soft mobile subglacial debris conditions prevail and in which advective pervasive f flow of saturated debris occurs in q beds both of the other bed types occur but are of a spatially and temporally transitory nature thus this latter bed type may be the most common beneath ice masses it is under an m bed state that many drumlin subglacial bedforms and associated forms are thought to develop discussion on the processes involved in mobile debris entrainment and the effect of changing rheological parameters are developed it is argued that such bedform initiation occurs due to anisotropic rheological variations within the mobile debris setting up a feedback effect such conditions are thought to be found most likely at the lower interface of a fast moving ice mass possibly typical of an ice stream in using empirical analyses of mobile debris conditions a limited set of relationships between subglacial meltwater flux rates basal ice velocity debris porosity and bed width are established with reference to mobile debris thicknesses using this relationship a set of possible parameter values that would allow bed formation to be achieved are presented the impact of bed formation under these conditions upon possible sedimentological facies and deformation structures is postulated the wider effect of m bedd conditionsd on subglacial sedimentology is introduced and the need to investigate subglacial hydraulic and bed conditions under past glaciations from the evidence of their glacial sediments and landforms emphasised

seismic wave propagation through material containing partially saturated cracks summary expressions for the wave speeds and attenuation of elastic waves through material containing aligned cracks are available in an approximation valid up to second order in the number density of cracks and for wavelengths that are long compared with the size of the cracks conditions within the crack may vary from dry stress free to fluid or solid filled the situation where cracks are partially filled with liquid and part with gas is very different from either extreme of dry or liquid filled cracks since the liquid is forced to flow laterally within a crack at the expense of the more compressible gas by tensions or compressions imposed normal to the plane of the crack expressions are derived here which enable the range of internal crack conditions to be extended to the case of partial saturation in the formulae for the propagation characteristics of waves of long wavelength through material containing such cracks

drumlins and catastrophic subglacial floods shaw j kvill d and rains b 1989 drumlins and catastrophic subglacial floods in j menzies and j rose editors subglacial bedforms drumlins rogen moraine and associated subglacial bedforms sediment geol 62 177 202 a number of lines of evidence are shown to be consistent with drumlin formation by catastrophic meltwater floods drumlins are associated with landforms related to such floods and landscapes produced by outburst floods in extragiacial areas contain drumlin like landforms some drumlins which contain mainly residual materials are compared to similar forms covering a wide range of scales and are interpreted to be scour remnant ridges produced by the action of horseshoe vortices on subgiacial materials these vortices are a product of flow separation in turbulent fluids and this interpretation implies drumlin formation by meltwater other drumlin calied cavity fill drumlins have distinctive shapes that are analogous with infilled erosional marks produced by turbulent separated flows these drumlins are interpreted as the infills of cavities eroded upwards into glacier beds by broad subgiacial meltwater sheetflows the sedimentology and internal structure of the cavity fill drumlins indicate rapid deposition from hyperconcentrated flow clast shape surface texture and lithology are consistent with the short distance of giaciofhivial transport predicted by the cavity fill hypothesis estimates of flow discharge 0 6 6 x 107 m3 s total volume of flow 84 1 103 km3 and flow duration 16 162 days are given for the catastrophic flood proposed to have produced the livingstone like saskatchewan drumlin field subgiacially derived floods are used to explain the sudden drainage of extraglacial lakes and major depositional events in proglacial lakes and it is suggested that they might have contributed to the lid of cold freshwater over the north atlantic held to have caused a late glacial cooling event the flood hypothesis requires extensive subgiacial lakes to have occurred beneath the laurentide and fennoscandian ice sheets which if they existed must have influenced the form and behaviour of these major bodies of ice

drumlins and the dynamics of the subglacial environment abstract this volume contains a selection of papers presented at the vii international drumlin symposium westport ireland in may 2009 this introduction to the volume first outlines the significance of drumlins as a key subglacial bedform gives a brief history of drumlin research and identifies the major themes of subglacial research namely mapping and morphometry sedimentary and ice bed processes and glacier dynamics it then summarises in turn the major findings of the papers in this volume linking them to each other and to the wider literature finally this introductory paper considers future developments in studies of drumlins and the subglacial environment

stratified flow deposits in a late pleistocene drumlin in northwest ireland abstract extensive stratified flow deposits are present within a drumlin exposed on the eastern shore of donegal bay northwest ireland the stratified deposits are located towards the central lee portion of the drumlin infilling a major depression within a basal diamicton association and are overlain by a sand rich diamicton association the stratified sequence consists of laminated silts cohesive diamictic muds stratified diamictons well sorted massive sands and clast supported gravels a discernible upward vertical facies transition is apparent within the stratified deposits from fines and matrix dominant sediments to well sorted clastic sediments these facies variations appear to reflect increased hydrodynamic activity and a prograding sequence the sediments are interpreted as a subglacial conduit infill formed at the point of debouchure into a glaciomarine environment the envisaged increase in water availability and flow energy may have resulted from progressive enlargement of the conduit by basal melting and scouring associated with the water sediment flux within the conduit the depression in the basal diamicton association appears to have resulted from scour at the base of the conduit

a geological view of drumlins in ireland facies modelling of the internal structure of drumlins has resulted in recognition of five major facies associations within the irish drumlin belt the facies associations are linked with a sequence of depositional events during the last deglacial phase ca 20 16 ka bp drumlin sediments were formed by basal till deposition subglacial fluvial lacustrine sedimentation sheet flow debris flow and subglacial cavity deposition subglacial deposition was initiated by the infilling and blocking of subglacial networks which caused a reduction in hydraulic transmissibility at the base of the ice sheet prior to drumlinisation drumlins then formed as subglacial erosional forms during surge type events when subglacial hydraulic transportational processes were active and sediment fluxes to the ice margins were high it is argued on geologic evidence that basal boundary conditions of this type coupled with high rates of ice wastage by calving at the ice margin contributed to rapid disintegration of the last ice sheet in western britain

semi automated extraction of longitudinal subglacial bedforms from digital terrain models two new methods abstract relict drumlin and mega scale glacial lineation positive relief longitudinal subglacial bedforms lsbs morphometry has been used as a proxy for paleo ice sheet dynamics lsb morphometric inventories have relied on manual mapping which is slow and subjective and thus potentially difficult to reproduce automated methods are faster and reproducible but previous methods for lsb semi automated mapping have not been highly successful here two new object based methods for the semi automated extraction of lsbs footprints from digital terrain models are compared in a test area in the puget lowland washington usa as segmentation procedures to create lsb candidate objects the normalized closed contour method relies on the contouring of a normalized local relief model addressing lsbs on slopes and the landform elements mask method relies on the classification of landform elements derived from the digital terrain model for identifying which lsb candidate objects correspond to lsbs both methods use the same lsb operational definition a ruleset encapsulating expert knowledge published morphometric data and the morphometric range of lsbs in the study area the normalized closed contour method was separately applied to four different local relief models two computed in moving windows and two hydrology based overall the normalized closed contour method outperformed the landform elements mask method the normalized closed contour method performed on a hydrological relief model from a multiple direction flow routing algorithm performed best for an assessment of its transferability the normalized closed contour method was evaluated on a second area the chautauqua drumlin field pennsylvania and new york usa where it performed better than in the puget lowland a broad comparison to previous methods suggests that the normalized relief closed contour method may be the most capable method to date but more development is required

exhumed subglacial landscape in uruguay erosional landforms depositional environments and paleo ice flow in the context of the late paleozoic gondwanan glaciation abstract a well exposed glacial surface sculpted on precambrian crystalline basement rocks occurs below the glacial succession of the san gregorio formation on the eastern border of the chaco parana basin in uruguay and was formed in the context of the late paleozoic gondwana ice age on the glacial surface are asymmetric parallel streamlined bedrock landforms interpreted as whalebacks the downglacier lee side faces of the whalebacks have gentle slopes dipping nnw with striated and sometimes polished surfaces on crystalline rocks these landforms are covered by 10 100 cm thick layers of tillites and shear laminated siltstones suggesting glacial abrasion produced mainly by subglacial till sliding the subglacial facies are ice molded and exhibit meso scale glacial lineations such as ridges and grooves up to 30 m long and 30 cm deep the subglacial association is directly overlain by proglacial fine grained facies rhythmites with dropstones indicating a subaqueous depositional environment following ice margin retreat the fine grained facies are erosively cut by a succession of sandstones with wave generated stratification resting on a basal conglomerate intraformational striated surfaces nne oriented were found on four distinct bedding planes within the sandstone package and interpreted as ice keel scour marks produced by floating ice the san gregorio deposits are partially confined in a wide and shallow subglacial trough and the stratigraphic succession is interpreted as the record of a glacial advance retreat cycle comparable to deglacial sequences from other late paleozoic localities the paleo ice flow to the nnw indicated by subglacial lineations is parallel to that verified in the southernmost parana basin located north of the study area suggesting a paleogeographic scenario in which glaciers advanced northward into a glaciomarine environment the proposed palaeogeography does not confirm the previous hypothesis of an ice center on the sul riograndense shield but instead it corroborates a south derived uruguayan ice lobe advancing to the north probably with provenance far afield in terranes of the present day southern african

vorticity measurement in a region of coastal ocean eddies by observation of near inertial oscillations continuously repeated hourly profiles of horizontal velocity have been collected with the cartesian diver profiler on each side of a front the location was roughly 100 km west of point buchon california in a region of the california current dominated by offshore coastal jets and strong eddies concurrent temperature records from the quasi lagrangian diver indicate that one set of 21 profiles was taken on the southerly side of the front with cold surface water while an immediately following set of 16 was collected on the other side for the 3 day collection period average advection of the diver was 2 m s to the wnw the horizontal kinetic energy levels of the profiles are the same for either side of the front and are 4 2 j m roughly twice the garrett munk model level but these also include subinertial baroclinic velocities cross spectral analysis of the repeated profiles is used to determine the dominant intrinsic rotation frequencies of horizontal velocity for vertical wavenumbers between 00625 and 019 cpm in the absence of local vorticity this would be expected to be the local inertial frequency f on the cold southerly side of the front most rotation frequencies were above f while most were below f on the warm side the mean frequencies were 1 18f and 0 78f the difference between these dominant intrinsic frequencies and f can be attributed to local relative vorticity these vorticity estimates are consistent with the westward advection of the diver being due to positions first south of then north of the central region of an offshore jet

elastic wave propagation in media with parallel fractures and aligned cracks a model of parallel slip interfaces simulates the behaviour of a fracture system composed of large closely spaced aligned joints the model admits any fracture system anisotropy triclinic the most general monoclinic orthorhombic or transversely isotropic and this is specified by the form of the 3 3 fracture system compliance matrix the fracture system may be embedded in an anisotropic elastic background with no restrictions on the type of anisotropy to compute the long wavelength equivalent moduli of the fractured medium requires at most the inversion of two 3 3 matrices when the fractures are assumed on average to have rotational symmetry transversely isotropic fracture system behaviour and the background is assumed isotropic the resulting equivalent medium is transversely isotropic and the effect of the additional compliance of the fracture system may be specified by two parameters in addition to the two isotropic parameters of the isotropic background dilute systems of flat aligned microcracks in an isotropic background yield an equivalent medium of the same form as that of the isotropic medium with large joints i e there are two additional parameters due to the presence of the microcracks which play roles in the stress strain relations of the equivalent medium identical to those played by the parameters due to the presence of large joints thus knowledge of the total of four parameters describing the anisotropy of such a fractured medium tells nothing of the size or concentration of the aligned fractures but does contain information as to the overall excess compliance due to the fracture system and its orientation as the aligned microcracks which were assumed to be ellipsoidal with very small aspect ratio are allowed to become non fiat i e have a growing aspect ratio the moduli of the equivalent medium begin to diverge from the standard form of the moduli for flat cracks the divergence is faster for higher crack densities but only becomes significant for microcracks of aspect ratios approaching 0 3

inverse graded units within till in drumlins near caledonia southern ontario examination of several drumlins in the caledonia drumlin field of southern ontario reveals inverse graded units occurring from the surface to near bedrock these units were found within a two till conformable sequence evidence indicates that the two tills on the basis of provenance and geochemical and geotechnical characteristics are subfacies of a single depositional phase fine subunits within each inverse graded unit have above average calcium and magnesium contents and exhibit pore wall cutan formation below average calcium and magnesium contents occur within coarse subunits and no cutans were observed in explaining this differentiation and the cause of inverse grading a hypothesis is suggested of pore water migrating contemporaneously with localized and episodic till lodging or deposition from a mobile basal tractive load for the drumlins revealing these inverse graded units it is thought that an accretion depositional model is most likely for formation from an initiating nucleal patch o

propagation of sound through a spicy ocean the sofar overture using a closely sampled 1000 km hydrographic section in the eastern north pacific the sound speed finestructure is separated into two component fields i isopyncal tilt dominated by internal waves the traditional view and ii spicy cold fresh to hot salty millifronts associated with upper ocean stirring numerical transmission experiments show significant scatter within the mixed layer from the spicy fronts this scattered energy arrives near the start of the sofar sequence and is superimposed on a triplication of the channel dispersion at the transition from reflected to upper ocean refracted energy this sofar overture is totally different from the finale which has been prominent for over 50 years the critical dependence of the overture on mixed layer processes suggests a scheme for acoustically monitoring the upper oceans at surface conjugate depths 3 to 5 km offering some advantages over in situ monitoring

physical environment of drumlin formation review of published descriptions of drumlin fields suggests that the following conditions are important to drumlin growth 1 compressive longitudinal and possibly extending transverse strain rates in the ice 2 thin ice such as occurs near the glacier margin and 3 high pore water pressure in the subglacial sediments most drumlin fields display all of these and no fields of well developed drumlins were found that did not on the oilier hand the lithology of drumlin forming sediment appears not to be important in promoting drumlin growth since it varied widely nor are the lithology and large scale topography of the bed

do subglacial bedforms comprise a size and shape continuum understanding the evolution of the ice bed interface is fundamentally important for gaining insight into the dynamics of ice masses and how subglacial landforms are created however the formation of the suite of landforms generated at this boundary subglacial bedforms is a contentious issue that is yet to be fully resolved bedforms formed in aeolian fluvial and marine environments either belong to separate morphological populations or are thought to represent a continuum of forms generated by the same governing processes for subglacial bedforms a size and shape continuum has been hypothesised yet it has not been fully tested here we analyse the largest data set of subglacial bedform size and shape measurements ever collated 96 900 bedforms our results show that flutes form a distinct population of narrow bedforms however no clear distinction was found between drumlins and megascale glacial lineations msgls which form a continuum of subglacial lineations a continuum of subglacial ribs also exists with no clear size or shape distinctions indicating separate populations furthermore an underreported class of bedform with no clear orientation to ice flow quasi circular bedforms overlaps with the ribbed and lineation continua and typically occurs in spatial transition zones between the two potentially merging these three bedform types into a larger continuum

elastic properties of hydrate bearing sediments using effective medium theory accurate and detailed models of the seismic velocity structure of gas hydrate bearing sediments may be determined by careful analysis of controlled source seismic data however interpretation of these velocities in terms of hydrate saturation of the pore space has hitherto relied on semiempirical formulas and or simple effective medium theory we develop a rigorous theoretical scheme to relate the seismic properties of a clay rich hydrate bearing sediment to its porosity mineralogy microstructural features and hydrate saturation we consider separately the two possible end members for the distribution of hydrate in the pore space 1 hydrates are unconnected and located in the pore voids without appreciable grain contact and 2 connected hydrates are forming cement binding around the grains the scheme is transversely isotropic to allow for anisotropy due to alignment of clay platelets and is based on a combination of a self consistent approximation a differential effective medium theory and a method of smoothing for crystalline aggregates we have applied the scheme to lithological and seismic velocity data from ocean drilling program site 995 on the blake ridge southeastern u s continental margin to make estimates of the hydrate saturation it was found that the hydrates are probably unconnected and their volume concentration varies between approx 0 at 100 m below the seabed and approx 9 at 400 m depth just above the bottom simulating reflector if the clay platelet orientation distribution resembles the function we have used

lithofacies types and vertical profile models an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences abstract increased knowledge of modern glacial depositional environments has resulted in rapidly evolving classifications of glacial tills these are based to a large degree on theoretical considerations of likely depositional processes the classifications are sophisticated and more advanced than the establishment of simple field criteria whereby individual till facies can be identified in quaternary and pre quaternary successions this situation is compounded in many quaternary terrains by the continued description of tills in terms of laboratory derived analytical data only reflecting a traditional interest in stratigraphic correlation rather than reconstruction of depositional environment detailed sedimentological logging of lithofacies is rarely undertaken there is thus considerable confusion as to what is being described or sampled when analytical data are presented for many pleistocene tills the same remarks apply to pre pleistocene tillites a lithofacies code is presented here for the rapid description and visual appraisal of field sequences or drill cores containing unconsolidated diamicts or lithified diamictites the term till is not used as it has a strict genetic definition referring to direct aggregation and deposition by glacier ice use of a four part code in conjunction with codes already published for fluvial sediments allows fundamental field properties to be depicted independent of genetic terminology and provides a firm basis for subsequent environmental interpretation and analytical work the value of this approach is illustrated by comparing a representative suite of vertical profiles of diamict assemblages deposited by modern grounded glaciers with a classic late pleistocene glacigenic sequence at scarborough bluffs ontario

basin scale patterns of subglacial sediment mobility implications for glaciological inversion modelling abstract patterns of glacial bedforms and their internal sediments are described from the omagh basin north central ireland here drumlins and ribbed moraines were variously formed preserved and remoulded during several late devensian ice flow stages and their patterns can be used as input into glaciological inversion models more detailed examination of the morphology and internal sediments of these bedforms shows a complex pattern of reworking driven by local scale basal thermal and hydrological regime sediment lithofacies associations and patterns of erratic carriage can be used to better evaluate ice bed conditions and basal processes than bedforms alone the sedimentary evidence suggests that bedforms were unlikely to have been formed during a single glacial episode as is implicit in glaciological inversion models and more likely formed during several episodes that cannot be resolved by crosscutting or superimposed bedform relationships alone

mechanical properties of organic matter in shales mapped at the nanometer scale abstract the mechanical properties of organic matter strongly affect the way shales deform and fracture however the way organic matter responds to mechanical stresses is poorly understood representing a critical obstacle to assessing oil and gas production in shale formations little is known about the mechanical properties of organic matter in fine grained rocks primarily because it often occupies tiny nanometer scale voids between the mineral grains which cannot be accessed using standard mechanical testing techniques here we use a new atomic force microscopy technique peakforce qnm to map the mechanical properties of organic and inorganic components at the nanometer scale we find that the method is able to distinguish between different phases such as pyrite quartz clays and organic matter moreover within the organic component young s modulus values ranged from 0 to 25 gpa in 3 different samples all of which come from thermally mature type ii iii source rocks in the dry gas window a modal value of 15 16 gpa was measured with additional peaks measured at 10 gpa in addition the maps suggest that some porous organic macerals possess a soft core surrounded by a harder outer shell 50 100 nm thick thus our results demonstrate that the method represents a powerful new petrographic tool with which to characterize the mechanical properties of organic rich sedimentary rocks

drumlin sedimentology in a hard bed lowland setting connemara western ireland implications for subglacial bedform generation in areas of sparse till cover cores of coastal drumlins in connemara contain stratified diamictons that interdigitate with gravelly clinoforms and finer grained rhythmites the diamictons are interpreted as subaqueous mud apron deposits delivered by subglacial till advection to continuously failing subaqueous ice contact fans whose strata were being syn depositionally over steepened by glacitectonic deformation the localized nature of the stratified sediments reflects the emergence of subglacial deforming tills and meltwater deposits in a glacilacustrine environment to produce interdigitated mass flow diamictons and grounding line fans wedges these depo centres became glacitectonized and subglacially streamlined during glacier overriding and hence regional drumlin sedimentology reflects the varying degrees of inheritance of pre existing glacigenic deposits and suggests that drumlin production relates more to the position of localized sediment accumulations at the glacier bed than full depth till deformation processes e g instability mechanisms within the same drumlin field till cored drumlins give way down ice to stratified cored drumlins with till caps and then to stratified drumlins this zonation is compatible with the increased lateral variability in drumlin composition that would arise from the occurrence of linear assemblages of glacifluvial esker and subaqueous grounding line sediments in an otherwise marginal thickening till sheet

rock as a critical point system and the inherent implausibility of reliable earthquake prediction summary the statistical mechanics of critical point thermodynamic systems provide a powerful conceptual tool with which to examine the plausibility of reliable earthquake prediction the issue turns on how rock is heterogeneous if the correlation range of a spatially fluctuating rock property is bounded by a characteristic length then the spatial average of that property over dimensions greater than may be regarded as defining an effective continuum value for that property ideas about earthquake preparation in rock with an effective continuum mechanical strength can plausibly focus on failure that is initiated at small anomalous zones and grows to instability with access to a continuous volume of strain energy the dimensions and temporal evolution of the growing anomaly can then be expected to relate to the size location and timing of the impending seismic event borehole log evidence indicates however that rock heterogeneity correlation lengths are unbounded and hence that the effective continuum approximation for rock must be applied with discretion an infinite correlation length indicates a statistical mechanical critical point system in which the correlation length is related to a singularity in a physical property in thermodynamic critical systems the singularity occurs at the critical system temperature tc 1 t tc in rock the singularity occurs at the critical percolation probability pc 1 p pc statistical physics descriptions of critical point spatial correlations are based on nearest neighbour inter actions at every scale length nearest neighbour interactions at one scale say that of large failure structures the loci of large earthquakes involve minimal coupling to structures at other scales in particular small scale failure structures the loci of small earthquakes the mechanics of nearest neighbour interactions plausibly form a fundamental constraint on the role of small scale failure or related nucleating activity as a prelude to large scale failure in light of 1 the evidence against rock being an effective medium notionally supporting localized precursory activity that reliably grows to large scale mechanical failure and 2 the history of failure to identify reliable precursors to major earthquakes the onus should now be on advocates of earthquake prediction schemes to evaluate their claims for precursory activity in terms of rock as a critical point system

distinguishing oil and water layers by interpreting acoustic logging data with changing well diameters during surveys water layers may interfere with the detection of oil layers in order to distinguish between oil and water layers research on the properties of well diameters and oil and water layers and their relation to acoustic logging rules is essential using hudson s crack theory we simulated oil and water layers with different well diameters or crack parameters angle and number density we found that when the well radius increases from 0 03 m to 0 05 m the variation ratio of compressional wave amplitude for the oil layer is less than that for the water layer the difference of stoneley wave amplitude between the crack parameters angle and number density is greater in the case of the water layer than in the case of the oil layer the response sensitivity of wave energy is greater for the water layer than that for the oil layer when the well radius increases from 0 05 m to 0 14 m the maximum excitation intensity for oil layer is greater than that for the water layer we conclude that the propagation of an elastic wave is affected by medium composition and well diameter and the influence has certain regularity these results can guide further reservoir logging field exploration work

sedimentation in a drumlin lee side subglacial wave cavity northwest ireland abstract lee side stratification sequences in a late pleistocene drumlin in northwest ireland are used to assess sedimentation patterns in subglacial wave cavities formed on the lee sides of drumlins three major subglacial cavity sub environments are identified 1 high energy divergent trough like closed conduit characterised by channel fill pebbly sand facies 2 low energy closed conduit characterised by alternating sand silts 3 trough margin dominated by stratified diamicts facies arrangements suggest two major transient states reflecting variations in subglacial meltwater flux the active phase showed meltwater flow directed across the drumlin apex suggesting complete uncoupling from the glacier bed supercavitation as flow velocities waned pebbly sand facies were deposited by in situ freezing of within conduit sediment admixtures the later quiescent phase was characterised by meltwater flow directed transverse to ice flow cavity shrinkage and concomitant deposition of subglacial sheetwash and cohesive sediment flows along cavity margins cavity closure resulted in complete coupling at the ice substrate interface with localised glaciotectonic loading on underlying lee side stratification sequences and concomitant till deposition these late stage sedimentation events at shinnan hill produced a very complex polygene endform which cannot be simply attributed to one formative process more realistically they reflect the influence of a rather complex and very dynamic subglacial system subject to complex spatial and temporal variations in subglacial erosion deposition and meltwater flux at the time of drumlinisation

drumlin evolution and ice sheet oscillations along the ne atlantic margin donegal bay western ireland abstract satellite imagery of donegal bay northwestern ireland reveals two streamlined subglacial bedform sets produced during the late devensian glaciation ca 22 000 15 000 yrs b p the first bedform set trends northeast southwest and records fast ice flow from inland ice domes onto the eastern atlantic continental shelf the second bedform set trending east west crosscuts and partially reorientates the first morphological and sedimentary evidence show that the second set corresponds to the last phase of fast ice flow drumlinization in northwestern ireland morphological characteristics of the donegal bay drumlin field and sedimentary characteristics of the mullinasole drumlin donegal bay support a two stage interpretation of drumlin evolution these are the following 1 deposition of glaciomarine mud and diamict facies at a tidewater glacier margin stratified diamicts record debris flow events and sediment reworking this facies sequence is erosionally truncated 2 deposition of subglacial diamict and sand facies recording ice readvance and drumlinization drumlinization sediment streamlining reflects ice mass balance destabilization episodic fast ice flow and ice marginal oscillation and may be correlated with millennial time scale climate changes in the circum north atlantic

effective elasticity of fractured rocks a snapshot of the work in progress exploration and development of naturally fractured reservoirs rely on understanding and interpretation of certain signatures associated with seismic waves propagating through cracked rocks this understanding comes primarily from the effective media theories that predict an overall elastic behavior of a solid containing many inhomogeneities cracks in particular whose sizes are too small to be seen individually by the waves to model seismic responses of fractured formations a geophysicist typically has a choice between the effective media schemes of hudson and schoenberg while the two predictions usually deviate slightly for liquid filled cracks the differences are significant when the fractures are dry explaining the origin of these differences and selecting a more accurate scheme is the first goal of this tutorial our second more challenging task is to prove that simply adding the compliance contributions of cracks as if they were isolated and noninteracting remains sufficiently accurate even for fractures that grossly violate the basic theoretical assumption of penny shaped cracks real fractures have notoriously irregular shapes might be partially closed and often form interconnected networks yet these details of fracture microgeometry turn out to be unimportant for the effective elasticity given a typical noise level in seismic data no closed form theory exists for irregular fracture shapes however take into account finite element simulations on so called digital rocks demonstrate which features of the crack geometry have to be taken into account because they influence propagation of long compared to the size of fractures seismic waves and which features can be ignored

overall properties of heterogeneous material summary small scale heterogeneity in crustal structure can be very complex and difficult to describe in detail and yet at the same time can be very important for the description of for instance tectonic stress and porosity statistical properties of such heterogeneity can be derived from the properties of waves of relatively large wavelength as they propagate through the crust the differences between measured wavelengths and attenuation coefficients and those of compact rock and the variations of these quantities in space and time provide in principle a means of determining quantities like the density and orientations of microfractures and the nature of crack infill material the theoretical basis for inferences of this kind is the concept of the effective or equivalent material based on an averaging process taken over the microstructure a number of methods have been used to calculate the properties of the effective medium several of which are described here

propagation of low mode internal waves through the ocean abstract the baroclinic tides play a significant role in the energy budget of the abyssal ocean although the basic principles of generation and propagation are known a clear understanding of these phenomena in the complex ocean environment is only now emerging to advance this effort a ray model is developed that quantifies the effects of spatially variable topography stratification and planetary vorticity on the horizontal propagation of internal gravity modes the objective is to identify baroclinic shoals where wave energy is spatially concentrated and enhanced dissipation might be expected the model is then extended to investigate the propagation of internal waves through a barotropic mesoscale current field the refraction of tidally generated internal waves at the hawaiian ridge is examined using an ensemble of mesoscale background realizations derived from weekly ocean topography experiment topex poseidon altimetric measurements the path of mode 1 is only slightly affected by typical cur

characteristics and origins of lee side stratification sequences in late pleistocene drumlins northern ireland spindle and parabolic shaped drumlins examined at fifty five localities in northern ireland possess stratification sequences on their lee side flanks these forms lack the distinctive steep stoss and tapering lee ends of classical drumlins and tend to occur in linear zones transverse to late pleistocene ice flow in most cases 90 per cent the stratified deposits infill embayments excavated in the lee side of barkhanoid forms and in the remainder 10 per cent they are superimposed on the lee side of whaleback forms the stratification sequences developed as a result of sedimentation in interconnected subglacial water filled cavities and are unlike remanie proglacial sediments moulded by ice into drumlin form stratigraphic evidence indicates that the lee side sequences developed during drumlin streamlining which supports the view that subglacial hydraulic processes played an important role in drumlin formation

sedimentation at the margins of a late pleistocene ice lobe terminating in shallow marine environments dundalk bay eastern ireland late pleistocene morainic sequences around dundalk bay eastern ireland were deposited in a variety of shallow glaciomarine environments at the margins of a grounded ice lobe the deposits are essentially ice proximal delta fan and apron sequences and are divided into two lithofacies associations lithofacies association 1 occurs as a series of morainal banks formed at the southern margin of the ice lobe in a body of water open to influences from the irish sea the morainal banks consist mainly of diamictic muds deposited from turbid plumes and by ice rafting with minor occurrences of turbidites cross bedded gravels subaqueous outwash and massive boulder gravels high density debris flows lithofacies association 2 was deposited in a narrow arm of the sea at the north eastern margin of the ice lobe the deposits consist mainly of a series of coalescing ice proximal gilbert type fan deltas which are interbedded distally with tabular and lens shaped subaqueous deposits the latter are mainly ice rafted diamictons debris flow deposits and subaqueous sands and gravels both lithofacies associations are draped by diamictons formed by a combination of rain out debris flow and traction current activity at a few localities the upper parts of the sequence have been sheared by minor oscillations of the ice sheet margin these sequences form part of an extensive belt of glaciomarine deposits which border the drumlin swarms of east central ireland lithostratigraphic variability is partially related to the arrival of large volumes of debris at the ice lobe margin when the main lowland ice sheet surged during drumlin formation complex depositional continua of this type lack any major erosional breaks and should not be used either as climatic proxies or for stratigraphic correlations

reply to comments on crustal anisotropy from local observations of shear wave splitting in west bohemia czech republic by g h r bokelmann and j kawahara can the hudson crack model describe behavior of real cracks

ocean sound channel ray path perturbations from internal wave shear and strain the relative importance of internal wave strain and internal wave shear on perturbation of acoustic ray trajectories in the ocean is analyzed previous estimates based on the garrett munk internal wave spectral model are updated using data from recent field studies of internal waves estimates of the ratio of the rms shear effect to the rms strain effect based on data from the upper kilometer of ocean are as high as 0 25 0 4 exceeding the estimates of 0 08 0 17 stemming from the model increased strength of three phenomena that have shear to strain ratios higher than the internal wave average can cause this effect these are near inertial waves internal tides and vortical modes

sedimentology of an ice contact glaciomarine delta carey valley northern ireland an extensive late pleistocene delta body related to a water plane at 100 m above sea level is exposed in the upper carey valley northern ireland exposures along the modern river show a lowermost facies association of massive diamict and poorly sorted gravel deposited as sediment gravity flows overlain by a gravelly foreset and topset facies association recording the progradation of a large glacier fed delta diamict facies were emplaced as subaqueous debris flows associated gravels are chaotically bedded and were deposited by the downslope slumping of poorly sorted boulder gravel their subtle grading characteristics can be linked with a variety of mass flow processes sediment gravity flow facies originated upvalley from rapidly deposited sediments which accumulated near the exit of a narrow subglacial channel below an ice margin fronting in water subsequent ice retreat allowed the growth of a gilbert type delta regional evidence shows that the delta top records high relative sea levels in response to isostatic downwarping around the periphery of the last british ice sheet the sedimentology and structure of the carey valley infill may be typical of high level glaciomarine deltas preserved in topographic lows around the coastline of north britain

using the size and position of drumlins to understand how they grow interact and evolve drumlins are subglacial bedforms streamlined in the direction of ice flow common in deglaciated landscapes they have been widely studied providing rich information on their internal geology size shape and spacing in contrast with bedform investigations elsewhere in geomorphology aeolian and fluvial dunes and ripples for example most drumlin studies derive observations from relict and thus static features this has made it difficult to gain information and insights about their evolution over time which likely hampers our understanding of the process es of drumlin formation here we take a morphological approach studying drumlin size and spacing metrics unlike previous studies which have focussed on databases derived from entire ice sheet beds we adopt a space for time substitution approach using individual drumlin flow sets distributed in space as proxies for different development times periods framed and assisted by insights from aeolian and fluvial geomorphology we use our metric data to explore possible scenarios of drumlin growth evolution and interaction we study the metrics of the size and spacing of 36 222 drumlins distributed amongst 71 flow sets left behind by the former british irish ice sheet and ask whether behaviour common to other bedform phenomena can be derived through statistical analysis through characterising and analysing the shape of the probability distribution functions of size and spacing metrics for each flow set we argue that drumlins grow and potentially migrate as they evolve leading to pattern coarsening furthermore our findings add support to the notion that no upper limit to drumlin size exists and to the idea that perpetual coarsening could occur if given sufficient time we propose that the framework of process and patterning commonly applied to non glacial bedforms is potentially powerful for understanding drumlin formation and for deciphering glacial landscapes

emergent drumlins and their clones from till dilatancy to flow instabilities subglacial bedforms are a range of landforms 10 105 m in length shaped mostly in glacial sediments and are abundant on ice sheet beds numerous explanations for their generation especially of drumlins have been proposed rather than viewing them as resulting from erosion or deposition directly by ice smalley and unwin 1968 proposed that both might occur simultaneously if beneath the glacier a layer of lubricating sediment existed and flowed conducting geomorphic work erosion deposition shaping of its own this idea appears to have steered the field leading perhaps to final resolution of the centuries old problem of drumlin genesis here i trace how the idea evolved leading to the conclusion that subglacial bedforms are emergent phenomena arising from self organisation in the coupled flow of ice sediment and water the ubiquity and patterning of bedforms appears to be well explained by a naturally arising flow instability in the subglacial system and which has been demonstrated by analytical and numerical modelling a problem for the instability theory is an apparent mismatch between predicted and observed sedimentological properties of bedforms a distinction is made between emergent drumlins drumlin clones and obstacle drumlins which helps explain some apparent contradictions and a conceptual framework is erected that might form a basis for confrontation of the theory with the wealth of observational evidence that exists

subglacial meltwater origin and subaerial meltwater modifications of drumlins near morley alberta canada abstract drumlins in the bow valley west of calgary alberta are interpreted as the product of subglacial and subaerial meltwater erosion on the sides of the bow valley highly elongated streamlined first generation drumlins are aligned in an en echelon fashion and are not significantly modified by post glacial processes in the valley bottom scalloped stubby irregularly spaced second generation drumlins are found in close association with valley fill the drumlins are composed of bedrock and diamicton and some are overlain by a thin gravel veneer stratified gravel is found on the leeward flanks of some second generation drumlins meltwater erosional forms s forms found on the limestone bedrock up flow of the drumlins and crescentic furrows around their proximal ends indicate that subglacial meltwater beneath the bow valley ice was probably the main erosive agent responsible for the shaping of the drumlins the second generation drumlins are believed to be the remnants of the first generation drumlins that were dissected and reworked by postglacial subaerial meltwater flow s meltwater drainage from ice dammed glacial lake kananaskis and possibly other glacially dammed tributary valley lakes resulted in the deposition of megaripples 3 5 m high 40 m wavelength 8 km down flow from the drumlins and reverse eddy deposits on the flanks of the drumlins post glacial bow river incision and terrace development has continued to further sculpt these forms

anisotropic approximations for mudrocks a seismic laboratory study an experimental technique for determining the in situ elastic properties of mudrocks with horizontal alignments in the microstructure is used to study the accuracy of a set of three nested scalar anisotropic approximations for transversely isotropic ti media each subsequent approximation adds one more velocity parameter and includes the previous as a special case these approximations are convenient and robust because of their close relationship to standard geophysical measurements there exists no good theory to predict the effects of an imposed stress on the elasticity of mudrocks in this study the tensor of elastic moduli of a single test specimen of mudrock subjected to an anisotropic stress field is determined from ultrasonic group velocity measurements involving pointlike transducers the mechanical characterization performed at constant pore pressure is accompanied by detailed microscopic observations and analysis the method was used to obtain accurate elastic constants for five well defi

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