internal structure of aeolian dunes in abu dhabi determined using ground penetrating radar a ground penetrating radar survey of aeolian dunes in the al liwa area of abu dhabi reveals a variety of dipping reflectors which are interpreted as primary sedimentary structures the interpretation of the radar profiles has been confirmed by bulldozing trenches through the study area and comparing logged sections in the trenches with the radar profiles nnw sse orientated radar profiles approximately parallel to the prevailing wind direction show two sets of dipping reflectors which are interpreted as sets of cross stratification and second and third order bounding surfaces radar profiles orientated wsw ene across the prevailing wind direction are dominated by concave up reflectors which are interpreted as trough shaped scours and sets of trough cross stratification produced by oblique progradation of barchanoid dunes nested troughs with small sets of trough cross stratification within larger troughs may be due to reactivation following wind reversal or the superposition of small dunes on larger dunes and the fill of large dune troughs by smaller dunes convex upwards reflectors are interpreted as linear spurs on the convex portions of sinuous dunes or erosional remnants between troughs overall there is a tendency for the larger second order bounding surfaces to dip downwind which confirms brookfield s ideas of the relative migration paths of dunes and draa

the sedimentary structure of linear sand dunes linear sand dunes dunes that extend parallel to each other rather than in star like or crescentic forms are the most abundant type of desert sand dune1 but because their development and their internal structure are poorly understood they are rarely recognized in the rock record2 models of linear dune development2 3 4 5 6 have not been able to take into account the sub surface structure of existing dunes but have relied instead either on the extrapolation of short term measurements of winds and sediment transport or on observations of near surface internal sedimentary structures from such studies it has not been clear if linear dunes can migrate laterally2 7 8 here we present images produced by ground penetrating radar showing the three dimensional sedimentary structure of a linear dune in the namib sand sea where some of the world s largest linear dunes are situated these profiles show clear evidence for lateral migration in a linear dune moreover the migration of a sinuous crest line along the dune produces divergent sets of cross stratification which can become stacked as the dune height increases and large linear dunes can support superimposed dunes that produce stacked sets of trough cross stratification these clear structural signatures of linear dunes should facilitate their recognition in geological records

grain size controls on the morphology and internal geometry of river dominated deltas predictions of a delta s morphology facies and stratigraphy are typically derived from its relative wave tide and river energies with sediment type playing a lesser role here we test the hypothesis that all other factors being equal the topset of a relatively noncohesive sandy delta will have more active distributaries a less rugose shoreline morphology less topographic variation in its topset and less variability in foreset dip directions than a highly cohesive muddy delta as a consequence its stratigraphy will have greater clinoform dip magnitudes and clinoform concavity a greater percentage of channel facies and less rugose sand bodies than a highly cohesive muddy delta nine self formed deltas having different sediment grain sizes and critical shear stresses required for re entrainment of mud are simulated using deflt3d a 2d flow and sediment transport model model results indicate that sand dominated deltas are more fan shaped while mud dominated deltas are more birdsfoot in planform because the sand dominated deltas have more active distributaries and a smaller variance of topset elevations and thereby experience a more equitable distribution of sediment to their perimeters this results in a larger proportion of channel facies in sand dominated deltas and more uniformly distributed clinoform dip directions steeper dips and greater clinoform concavity these conclusions are consistent with data collected from the goose river delta a coarse grained fan delta prograding into goose bay labrador canada a reinterpretation of the kf 1 parasequence set of the cretaceous last chance delta a unit of the ferron sandstone near emery utah usa uses ferron grain size data clinoform dip data clinoform concavity and variance of dip directions to hindcast the delta s planform the kf 1 last chance delta is predicted to have been more like a fan delta in planform than a birdsfoot delta

theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products publisher elsevier articletitle theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products journaltitle earth science reviews articlelink http dx doi org 10 1016 j earscirev 2016 01 010 content type article copyright copyright 2016 elsevier b v all rights reserved

exact and approximate analytical solutions for unsteady fully developed turbulent flow in porous media and fractures for time dependent boundary conditions summary the problem of recharge of water to an initially dry semi infinite aquifer is investigated where the occurrence of turbulent flow is considered by applying a similarity transform an analytical solution is obtained by the adomian s decomposition method the integration constant of the power series is determined by examining the mathematical structure of the solution at the downstream part of the front it is demonstrated that for two specific cases the adomian s series are reduced to exact solutions the analytical solutions above derived on the basis of fully developed turbulent flow regime are compared to the water table profile computed on the basis of the forchheimer equation four specific cases in real world coordinated are investigated a the propagation of a linear traveling wave b injection of fluid at constant flow rate in a fracture or aquifer c flow induced by a sudden raise of the piezometric head d redistribution of a finite quantity of water in a porous medium potential applications of the above cited results are water body aquifers interaction and exploitation of hot dry rocks

scales and causes of heterogeneity in bars in a large multi channel river rio parana argentina to date published studies of alluvial bar architecture in large rivers have been restricted mostly to case studies of individual bars and single locations relatively little is known on how the depositional processes and sedimentary architecture of km scale bars vary within a multi km reach or over several 100s km downstream this study presents ground penetrating radar gpr and core data from 11 km scale bars from the rio parana argentina the investigated bars are located between 30 km upstream and 540 km downstream of the rio parana rio paraguay confluence where a significant volume of fine grained suspended sediment is introduced into the network bar scale cross stratified sets with lengths and widths up to 600 m and thicknesses up to 12 m enable the distinction of large river deposits from stacked deposits of smaller rivers but are only present in half the surface area of the bars up to 90 of bar scale sets are found on top of finer grained ripple laminated bar trough deposits bar scale sets make up as much as 58 of the volume of the deposits in small incipient mid channel bars but this proportion decreases significantly with increasing age and size of the bars contrary to what might be expected a significant proportion of the sedimentary structures found in the rio parana is similar in scale to those found in much smaller rivers in other words large river deposits are not always characterised by big structures that allow a simple interpretation of river scale however the large scale of the depositional units in big rivers causes small scale structures such as ripple sets to be grouped in thicker co sets which indicate river scale even when no obvious large scale sets are present the results also show that the composition of bars differs between the studied reaches upstream and downstream of the confluence with the rio paraguay relative to other controls on downstream fining the tributary input of fine grained suspended material from the rio paraguay causes a marked change in the composition of the bar deposits compared to the upstream reaches the sedimentary architecture of the downstream reaches in the top 5 m of mid channel bars shows i an increase in the abundance and thickness up to m scale of laterally extensive 100s of metres fine grained layers ii an increase in the percentage of deposits comprised of ripple sets to 40 in the upper bar deposits and iii an increase in bar trough deposits and a corresponding decrease in bar scale cross strata

an approximate analytical solution for non darcy flow toward a well in fractured media estuarine suspended sediment is transported in a mixed nonuniform way under unsteady flows sediment of different grain sizes has different characteristics and transport behavior and has a different effect on the ecological system therefore classification and fractionization of the mixed sediment are required before the flux is estimated a fuzzy clustering approach is applied to the classification of suspended fine grained sediment in the changjiang estuary two populations are objectively found by considering the standard grain size distribution statistics of each cluster the critical grain size of approx 10 m in diameter is the size limit for cohesive sediments a grid with equal cell areas is used to estimate fractional sediment fluxes through an estuarine cross section since this type of grid introduces less statistical error in the flux calculation the sediment transport mechanism is analyzed

causes of rapid mixing at a junction of two large rivers rio parana and rio paraguay argentina 1 airborne and satellite observations show that when large rivers join they can take hundreds of kilometers to mix completely but on occasion may mix very rapidly application of established semitheoretical analyses shows that long mixing lengths should be expected the first measurements of mixing processes at a large river junction rio parana and rio paraguay argentina combined width 2 8 km are presented at two occasions first when they mix in 400 km and second when mixing is complete in only 8 km downstream of the junction for the case of slower mixing at a point surveys showed that mixing driven by turbulent shear associated with a near vertical shear layer was restricted to close to the junction to 0 272 multiples of the postconfluence width downstream transect surveys showed penetration of more turbid water from the rio paraguay underneath the rio parana but this was insufficient to promote more rapid mixing there was no clear channel scale circulation present and slow mixing was compounded by reverse topographic forcing on the mainstream rio parana side of the river this kept more turbid water on the rio paraguay side of the river close to the bed in the case of rapid mixing we found clear channel scale circulation the momentum ratio between the combining flows reinforced the effects of the discordance in bed height between the tributaries at the confluence and allowed penetration of more turbid rio paraguay water further across the channel width deeper within the flow the importance of the interaction between momentum ratio and bed morphology at channel junctions makes mixing rates at the confluence dependent upon basin scale hydrological response which is more likely to differ between large confluent rivers than small rivers as a result of the different climatic topographic zones that they may capture

reconstructing the architecture and sequence stratigraphy of the preserved fluvial record as a tool for reservoir development a reality check driven in part by the need for better information about fluvial systems for the purpose of nonmarine reservoir evaluation and development much valuable work is now being conducted on modern rivers and their deposits aided by such techniques as ground penetrating radar however studies of modern and recent systems cannot address the question of the long term preservability of the present day deposits only studies of the rock record itself can explore this issue two separate studies of ancient fluvial systems illustrate some of the problems a study of the hawkesbury sandstone triassic sydney basin australia highlighted the difficulty in interpreting the dimensions of large sand bodies from comparisons with a modern analog even when very large outcrops are available a seismic time slice study of pliocene pleistocene fluvial systems in the gulf of thailand revealed major changes in channel size and fluvial style over short vertical intervals braided and meandering systems meander belt widths 4 to 10 km 2 5 to 6 mi are separated by a few tens of meters of section or less and are interbedded with the deposits of much smaller rivers showing straight meandering and anastomosed patterns incised valleys and underfit streams are also present these variations can be interpreted in terms of a sequence model but they indicate the problems that could arise from the use of a single suite of dimensional variables as input into numerical reservoir heterogeneity and flow models most numerical simulation models make use of sets of equations relating such parameters as channel width depth and sinuosity but most such equations are generalized across the whole spectrum of fluvial styles and can be conditioned to the reality of individual reservoirs only with difficulty the application of the principles of sequence stratigraphy to fluvial deposits is rendered difficult by the complex response to allogenic forcing that characterizes fluvial systems episodes of aggradation and degradation that may be used to define sequences and their bounding unconformities in the stratigraphic record may be the result of the complex interplay of several allogenic mechanisms governing varying stream power and sediment supply mechanisms that may be operating at different time scales and may be out of phase with each other in developing practical solutions for reservoir development numerical modeling and simulation may provide generalized starting points for the analysis history matching commonly demonstrates inaccuracies in many initial models further progress may be made by direct study of the reservoir itself using three dimensional 3 d seismic and surveillance techniques there is a continuing role for the study of ancient analogs as providing a realistic database on the long term preservation styles of fluvial reservoir deposits

preservation of planar laminae due to migration of low relief bed waves over aggrading upper stage plane beds comparison of experimental data with theory experimental studies of the formation of planar laminae by migration of low relief bed waves over aggrading upper stage plane beds show that the average thickness of laminae at a point increases with both aggradation rate and the variance of the heights of bed waves passing that point in general the preserved laminae represent only a small proportion generally less than 50 of the height of the largest bed waves in the population the theory developed by paola borgman 1991 relating the probability density function of stratal thickness to that of bed wave height for the case of no net aggradation was adapted for aggrading conditions and shown to agree well with data these results suggest that the theory can be used to estimate from the distribution of thickness of planar laminae either the distribution of bed wave heights mean aggradation rate mean bed wave length or mean bed wave celerity provided the other parameters can be estimated

reconstructing random topography from preserved stratification one of the fundamental problems of interpreting the sedimentary record is reconstructing the original heights of palaeotopographical features such as bedforms or river channels this requires an understanding of the relationship between topography and set thickness but at present an exact theory exists only for periodic topography of uniform height the applicability of this simple theory is severely limited by the random variability characteristic of many sedimentary systems in this paper we develop an exact theory for the probability density function pdf of sets generated from topography of random height we focus on the limiting case of zero net deposition in order to provide a lower bound for the set thickness and derive an analytical set thickness pdf that is determined by one parameter of the pdf for topographical height this parameter measures the breadth of the tail of the topographical pdf the mean set thickness is 0 8225 for bedforms and 1 645 for river channels if the topographical height is gamma distributed the preservation ratio defined as mean thickness of preserved sets mean topographical height is 0 8225 r2 for bedforms and 1 645 r2 for river channels where r is the coefficient of variation standard deviation mean of the generating topography in a comparison with data from laboratory current ripples our analytical predictions compare well with observations of both mean set thickness and thickness distribution the preservation ratio for the ripples is about 0 5 whilst measured dune heights give a preservation ratio of about 0 12 depth data from two modern braided streams yield preservation ratios ranging from 0 4 to 0 75 as more data on the distributions of topographical height in modern environments become available calculations such as these should help provide reliable error bounds for quantitative topographical reconstruction

erosional origin of hummocky terrain in south central alberta canada we propose that hummocky terrain in south central alberta is the product of subglacial erosion rather than supraglacial letdown during ice disintegration a common view of hummock formation extensive exposures in hummocks contain sediments marking the history of laurentide ice advance and deglaciation in the region regardless of the genesis of those sediments units are abruptly truncated at the surface that represents the hummock and trough morphology indicating that the hummocks are the product of differential erosion subglacial sediments predating the erosion and subglacial eskers overlying the erosion surface suggest that the erosion was also subglacial hummock morphology lithostratigraphy correlated from hummock to hummock truncation at the land surface and widespread coarse boulder lags support a glaciofluvial origin for hummocky terrain in this region

a single late wisconsin laurentide glaciation edmonton area and southwestern alberta the absence of igneous and metamorphic clasts of canadian shield origin in the saskatchewan gravels and sands preglacial valley fills in seven active gravel pits studied over a 5 yr period coupled with 29 radiocarbon dates 21 300 42 910 yr b p suggest that laurentide glaciation prior to the late wisconsin did not reach the edmonton area on the basis of edmonton9s elevation 670 m above sea level asl compared to higher regional elevations to the south calgary 1060 m asl and maps of the retreat pattern of late wisconsin laurentide ice which we equate approximately with advance pattern it is likely that large areas of higher elevation in alberta south and west of edmonton only underwent the late wisconsin event not multiple glaciations

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

physical processes in subglacial lake whillans west antarctica inferences from sediment cores abstract the hydrologic system beneath the antarctic ice sheet is thought to influence both the dynamics and distribution of fast flowing ice streams which discharge most of the ice lost by the ice sheet despite considerable interest in understanding this subglacial network and its affect on ice flow in situ observations from the ice sheet bed are exceedingly rare here we describe the first sediment cores recovered from an active subglacial lake the lake known as subglacial lake whillans is part of a broader dynamic hydrologic network beneath the whillans ice stream in west antarctica even though floods pass through the lake the lake floor shows no evidence of erosion or deposition by flowing water by inference these floods must have insufficient energy to erode or transport significant volumes of sediment coarser than silt consequently water flow beneath the region is probably incapable of incising continuous channels into the bed and instead follows preexisting subglacial topography and surface slope sediment on the lake floor consists of till deposited during intermittent grounding of the ice stream following flood events the fabrics within the till are weaker than those thought to develop in thick deforming beds suggesting subglacial sediment fluxes across the ice plain are currently low and unlikely to have a large stabilizing effect on the ice stream s grounding zone

sedimentological and deformational criteria for discriminating subglaciofluvial deposits from subaqueous ice contact fan deposits a pleistocene example ireland a pit located near ballyhorsey 28 km south of dublin eastern ireland displays subglacially deposited glaciofluvial sediments passing upwards into proglacial subaqueous ice contact fan deposits the coexistence of these two different depositional environments at the same location will help with differentiation between two very similar and easily confused glacial lithofacies the lowermost sediments show aggrading subglacial deposits indicating a constrained accommodation space mainly controlled by the position of an overlying ice roof during ice bed decoupling these sediments are characterized by vertically stacked tills with large lenses of tabular to channelized sorted sediments the sorted sediments consist of fine grained laminated facies cross laminated sand and channelized gravels and are interpreted as subglaciofluvial sediments deposited within a subglacial de coupled space the subglaciofluvial sequence is characterized by glaciotectonic deformation structures within discrete beds triggered by fluid overpressure and shear stress during episodes of ice bed recoupling clastic dykes and folds the upper deposits correspond to the deposition of successive hyperpycnal flows in a proximal proglacial lake forming a thick sedimentary wedge erosively overlying the subglacial deposits gravel facies and large scale trough bedding sand are observed within this proximal wedge while normally graded sand beds with developed bedforms are observed further downflow the building of the prograding ice contact subaqueous fan implies an unrestricted accommodation space and is associated with deformation structures related to gravity destabilization during fan spreading normal faults this study facilitates the recognition of subglacial submarginal depositional environments formed in part during localized ice bed coupling episodes in the sedimentary record the sedimentary sequence exposed in ballyhorsey permits characterization of the temporal framework of meltwater production during deglaciation the impact on the subglacial drainage system and the consequences on the irish sea ice stream flow mechanisms

theoretical framework and diagnostic criteria for the identification of palaeo subglacial lakes abstract the antarctic ice sheet is underlain by numerous subglacial lakes which comprise a significant and active component of its hydrological network these lakes are widespread and occur at a range of scales under a variety of conditions at present much glaciological research is concerned with the role of modern subglacial lake systems in antarctica another approach to the exploration of subglacial lakes involves identification of the geological record of subglacial lakes that once existed beneath former ice sheets this is challenging both conceptually in identifying whether and where subglacial lakes may have formed and also distinguishing the signature of former subglacial lakes in the geological record in this work we provide a synthesis of subglacial lake types that have been identified or may theoretically exist beneath contemporary or palaeo ice sheets this includes a discussion of the formative mechanisms that could trigger onset of or drain subglacial lakes these concepts provide a framework for discussing the probability that subglacial lakes exist ed beneath other palaeo ice sheets indeed we conclude that the former mid latitude ice sheets are likely to have hosted subglacial lakes although the spatial distribution frequency and type of lakes may have differed from today s ice sheets and between palaeo ice sheets given this possibility we propose diagnostic criteria for identifying palaeo subglacial lakes in the geological record these criteria are derived from contemporary observations hydrological theory and process analogues and provide an observational template for detailed field investigations

late wisconsin subglacial megaflood paths in alberta evidence of late wisconsin subglacial megafloods includes fields of giant flutings drumlins tunnel channels and scoured bedrock tracts scoured tracts are marked by channeled scabland water eroded depressions s forms and postglacial development of solonetzic saline clay pan soils on cretaceous bedrock belts of hummocky terrain and small zones of moraine plateaus are remnants left by incomplete sheet flood erosion of initial subglacial sediment later modified by ice pressing among multiple subglacial discharges the largest was the livingstone lake event which transmitted laurentide water from the northwest territories and northern saskatchewan through alberta to montana and the mississippi drainage this flood was augmented by cordilleran water relayed under valley glacier piedmonts coalescent with south western laurentide ice the short duration megaflood probably occurred within the longer time interval of 18 15 ka

till stratigraphy and sedimentology at the margins of terrestrially terminating ice streams case study of the western canadian prairies and high plains complex glacigenic sediment sequences arranged in large arcuate assemblages and demarcating the lobate termini of fast ice flow corridors in western canada represent the terrestrial equivalents of the trough mouth fans of the marine palaeo ice stream record intensive sedimentological and stratigraphic investigations at a regional scale are employed in the interpretation of thick units or sequences of diamicton and two depositional scenarios are proposed first glacier marginal till thickening and the stacking of till wedges takes place in areas where an ice stream margin was stationary for short periods of time stacks of subglacial traction tills potentially record annual sub marginal incremental thickening at ice stream margins especially in areas where densely spaced recessional push moraines appear to record seasonal climatic forcing second proglacial lake and valley infilling with glacilacustrine rhythmites and mass flow diamictons is associated with the advection of sub marginal till into preglacial bedrock depressions and records ice stream marginal oscillations and debris influx into discontinuous proglacial lakes which act as subaqueous sediment sinks for materials that would otherwise have been used to construct sub marginal till wedges and moraines glacial overriding modifies the deposits produced in both scenarios through deformation and the dislocation and entrainment of bedrock and sediment rafts at the margins of bedrock valley walls resulting in the widespread development of glacitectonites and megablocks the occurrence of the thickest deposits as preglacial valley fills may be significant in groundwater siphoning away from subglacial deforming layers and the promotion of sticky spots

three dimensional sedimentary architecture of a large mid channel sand braid bar jamuna river bangladesh the three dimensional subsurface alluvial architecture of a large approximately 3 km long 1 km wide 12 m high mid channel sand braid bar in the jamuna river bangladesh is described evolution of the bar and its depositional characteristics are assessed from a unique combination of ground penetrating radar surveys vibracoring and trenching that are allied to a series of bathymetric surveys taken during growth of the bar over a 29 month period this methodology permits identification of the formative processes of different packages of braid bar sedimentation and provides a facies model for deposition within the entire bar mid channel bar growth occurred in a region of flow expansion and was probably initiated by the stalling and amalgamation of large dunes these dunes created a bar core that grew by i propagation of a downstream accreting slipface ii vertical accretion through stacking of dunes on both bar stoss and top and iii lateral accretion on the bar margins during recession of the flood hydrograph braid bar sedimentation is dominated by four radar facies 1 large scale predominantly planar dipping reflections interpreted as cross stratification up to 8 m in height and greater than 100 m in width that is produced by the cross channel migration of bar margins 2 medium scale trough shaped and planar discontinuous reflections interpreted as cross stratification up to 4 m in height and 300 m wide that is deposited from large sinuous crested sand dunes 3 discontinuous reflections up to 2 m high and 30 m wide interpreted as small scale trough cross stratification that is the product of smaller sinuous crested dunes and 4 high amplitude undulating reflections interpreted as mud drapes deposited in regions of slow flow often in the bar tail region at low stage dune size decreases vertically within the bar in response to the progressively shallower flows on the emerging bar top later evolution of the bar as one anabranch channel became dominant created a 1 5 km extension to the bar tail with an 8 m high angle of repose bar margin slipface formed by flow transverse to the long axis of the bar seven styles of deposition can be defined that constitute the alluvial architecture bar margin slipface vertical accretion in channel bar top vertical accretion upstream accretion lateral accretion downstream accretion and low stage mud drapes a model of braid bar sedimentation is presented that shares many similarities with previous studies of smaller sand bed braid bars with the dominance of dune scale cross stratification the presence of large scale bar margin cross stratification and the occurrence of lateral vertical upstream and downstream accretion however the contribution of the bar margin facies to the preserved stratigraphy highlighted herein may have been underestimated in previous models of braided rivers in which the braid bars were migrating slowly this study suggests a scale invariance in several aspects of mid channel bar sedimentation in sand bed rivers and proposes a model of braid bar sedimentation that may be applied widely within studies of braided alluvial architecture

on the interpretation of subglacial till fabric abstract the modal distribution of stone long axis fabrics and their respective eigenvalues can be used to infer the genesis of subglacial till in this paper we offer a two axis diagram that compares fabric modality to fabric isotropy s3 s1 and addresses the problem of eigenvectors falling between the modes of some well developed till fabrics with low eigenvalues our simple five fold scheme of modality categories includes 1 unimodal clusters 2 spread unimodal 3 bimodal clusters 4 spread bimodal and 5 polymodal to girdle like fabrics and requires the analyst to study equal area lower hemisphere schmidt plots of the fabric data after assigning the fabric to a morality category isotropy is calculated and both results are plotted on the graph whic helps to separate two main fields of subglacial till 1 lodgement and subglacial meltout tills and 2 deformation fill on the basis of selected published fabrics from tills at modern glaciers as well as our own pleistocene till data lodgement and subglacial meltout tills tend to have unimodal or bimodal fabrics in contrast deformation tills and tills that experienced multiple processes tend to have polymodal to girdle like fabrics some overlap occurs between fields be cause of the complex nature of till formation i e because pure end member till facies are rare and most tills are hybrids we strongly recommend that schmidt plots be visually analyzed and used in conjunction with eigenvalues when studying till however fabric data alone is not enough multiple criteria including structural lithologic and stone morphologic data from the till must also be considered before drawing conclusions on till genesis furthermore if eigenvectors fall between fabric modes then they cannot be used to indicate former ice movement directions finally our new modality isotropy diagram may have wider applications

flow depth scaling in alluvial architecture and nonmarine sequence stratigraphy example from the castlegate sandstone central utah u s a changes in the proximity and arrangement of channel belt sandstone bodies in the rock record and associated unconformities in alluvial successions i e alluvial architecture and nonmarine sequence stratigraphy are often interpreted to reflect changes in basin boundary conditions including subsidence and sea level such interpretations do not often consider how the size of a depositional system influences alluvial architecture and sequence stratigraphy we investigate how paleoflow depth scaling affects fluvial stratigraphic interpretations using the castlegate sandstone campanian at its type section in central utah as an example using lidar imaging channel preservation and paleoflow depths are estimated from bar clinoforms throughout the unit and these data are compared to scour spacing scour continuity and mudstone content within the study interval within the castlegate sandstone bar clinoforms are routinely close to fully preserved i e rollover of the upper part of the bar is present and an overall increase in paleoflow depth up section is coincident with changes in other alluvial architecture characteristics including increases in scour surface spacing and mudstone abundance up section this suggests that alluvial architecture commonly ascribed to changes in accommodation may result primarily from changes in the scale and style of fluvial deposition rather than accommodation limited reworking and amalgamation additionally although sequence boundaries have previously been identified in the section no scour surfaces deeper or more extensive than what could be produced autogenically by large rivers typical of the castlegate system were found in the study interval this study demonstrates that estimates of paleoriver depth can enhance studies of alluvial architecture and provide a more robust basis for correlating and interpreting fluvial deposits

large scale structure of calamus river deposits nebraska usa revealed using ground penetrating radar the large scale i e bar scale structure of channel deposits of the braided low sinuosity calamus river nebraska is described using ground penetrating radar gpr profiles combined with vibracores basal erosion surfaces are generally overlain by medium scale trough cross stratified sets 3 25 cm thick very coarse to medium sands that are associated with relatively high amplitude discontinuous gpr reflectors overlying deposits are bioturbated small scale cross stratified sets 3 cm thick and vegetation rich fine to very fine sands that are associated with low amplitude discontinuous reflectors near surface peat and turf have no associated gpr reflectors in along stream profiles through braid and point bars most gpr reflectors dip downstream at up to 2 relative to the basal erosion surface but some reflectors in the upstream parts of bars are parallel to the basal erosion surface or dip upstream in cross stream profiles through bars gpr reflectors are either approximately parallel to bar surfaces or have low angle inclinations up to 6 towards cut banks of adjacent curved channels basal erosion surfaces become deeper towards cut banks of curved channels these structures can be explained by lateral and downstream growth of bars combined with vertical accretion convex upwards forms up to 0 5 m high several metres across and tens of metres long represent episodic accretion of unit bars scroll bars and bar heads stratal patterns in channel fills record a complicated history of erosion and deposition during filling including migration of relatively small bars a revised facies model for this type of sandy braided river has been constructed based on this new information on large scale bedding structure

decimeter scale in situ mapping of modern cross bedded dune deposits using parametric echo sounding a new method for linking river processes and their deposits 1 collecting high resolution data to quantify the sedimentary architecture within contemporary alluvial channels remains one of the outstanding challenges in fluvial sedimentology here we present data collected using a new geophysical method the parametric echo sounder pes which can meet this challenge from surveys over a field of sand dunes in the rio parana argentina we demonstrate the unique ability of pes to image the subsurface structure within active channels at a decimetric resolution these data reveal the bounding surfaces between bars and dunes as well as the foresets and reactivation surfaces within them this provides quantitative in situ data for recent work that suggests cross strata preserved by dunes may be more related to flow depths less than the commonly assumed bankfull level these surveys demonstrate that pes can provide hitherto unobtainable data from alluvial channels and presents significant opportunities for more detailed coupled studies of fluvial processes and their deposits

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

the sedimentology and alluvial architecture of a large braid bar rio parana argentina abstract this paper presents 30 km of ground penetrating radar gpr data from a mid channel bar in the sixth largest river in the world the rio parana argentina gpr profiles with depth of penetration up to 12 m below the bar surface were collected from a sandy braid bar 3 km long by 1 km wide on a grid with a 200 to 400 m spacing two facies were found to dominate the sedimentary architecture the principal facies 83 of total facies comprises trough and planar cross strata related to the migration of dunes with the thickness of the cross strata decreasing towards the bar surface the second significant facies 15 is high angle generally 10 20 strata that typically form by accretion at the bar margins or bartail clay drapes the rio parana gpr surveys are compared with other gpr studies of sandy braid bars of different sizes from the south saskatchewan wisconsin and jamuna rivers the dominance of dune deposits is ubiquitous to all rivers with each also possessing a significant proportion of large scale high angle strata however two differences were found to exist between the deposits of these rivers 1 the compound bar deposits of smaller rivers contained greater proportions of the fills of cross bar channels which suggests a potential role for discharge as a factor in shaping the alluvial architecture through its impact on the frequency of sediment reworking over the bar tops and 2 the thickness of large scale high angle sets decreases with the age of the bar which suggests that the deposits of older bars may provide more useful geometrical analogues for interpreting ancient successions than smaller transient or recent bar forms that have undergone only limited modification

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

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

exploring former subglacial hodgson lake antarctica paper ii palaeolimnology direct exploration of subglacial lakes buried deep under the antarctic ice sheet has yet to be achieved however at retreating margins of the ice sheet there are a number of locations where former subglacial lakes are emerging from under the ice but remain perennially ice covered one of these lakes hodgson lake 72 00 549 s 068 27 708 w has emerged from under more than 297 465 m of glacial ice during the last few thousand years this paper presents data from a multidisciplinary investigation of the palaeolimnology of this lake through a study of a 3 8 m sediment core extracted at a depth of 93 4 m below the ice surface the core was dated using a combination of radiocarbon optically stimulated luminescence and relative palaeomagnetic intensity dating incorporated into a chronological model stratigraphic analyses included magnetic susceptibility clast provenance organic content carbonate composition siliceous microfossils isotope and biogeochemical markers based on the chronological model we provisionally assign a well defined magnetic polarity reversal event at ca 165 cm in the lake sediments to the mono lake excursion ca 30 34 ka whilst osl measurements suggest that material incorporated into the basal sediments might date to 93 9 ka four stratigraphic zones a d were identified in the sedimentological data the chronological model suggests that zones a c were deposited between marine isotope stages 5 2 and zone a during stage 1 the holocene the palaeolimnological record tracks changes in the subglacial depositional environment linked principally to changing glacier dynamics and mass transport and indirectly to climate change the sediment composition in zones a c consists of fine grained sediments together with sands gravels and small clasts there is no evidence of overriding glaciers being in contact with the bed reworking the stratigraphy or removing this sediment this suggests that the lake existed in a subglacial cavity beneath overriding lgm ice in zone d there is a transition to finer grained sediments characteristic of lower energy delivery coupled with a minor increase in the organic content attributed either to increases in allochthonous organic material being delivered from the deglaciating catchment a minor increase in within lake production or to an analytical artefact associated with an increase in the clay fraction evidence of biological activity is sparse total organic carbon varies from 0 2 to 0 6 and cannot be unequivocally linked to in situ biological activity as comparisons of 13c and c n values with local reference data suggest that much of it is derived from the incorporation of carbon in catchment soils and gravels and possibly old co2 in meteoric ice we use the data from this study to provide guidelines for the study of deep continental subglacial lakes including establishing sediment geochronologies determining the extent to which subglacial sediments might provide a record of glaciological and environmental change and a brief review of methods to use in the search for life

the fluid dynamics of river dunes a review and some future research directions 1 dunes are present in nearly all fluvial channels and are vital in predicting flow resistance sediment transport and deposition within many rivers progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field laboratory and numerical investigations progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune additionally the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood with the feedback of dune three dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features this review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes five areas are then highlighted and discussed as a possible focus for future research 1 the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field 2 the influence of three dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress 3 flow field modification resulting from bed form superimposition and amalgamation 4 the scale and topology of dune related turbulence and its interactions with sediment transport and the flow surface and 5 the influence of suspended sediment on the dune flow field and dune morphology

preservation of cross strata due to the migration of subaqueous dunes an experimental investigation this experimental investigation examined the controls on the geometry of cross sets formed by subaqueous dunes a range of steady unidirectional flow conditions spanning the field of dune existence was investigated andaggradation rate ranged from 0 mm s 1 to 0 014 mm s 1 data from an ultrasonic depth profiler consist of high resolution temporal and spatial series of bed profiles from which dune height and length migration rate and the depth of trough scour were measured cross set thickness and length were measured from sediment peels the size and shape of dunes from an equilibrium assemblage change continuously individual dunes commonly increase in height by trough scouring and occasionally by being caught up by the upstream dune both types of behaviour occur suddenly and irregularly in time and hence do not appear to depend on dunes further upstream however dune climbing or flattening is a typical response of dunes that disappear under the influence of the upstream dune all types of behaviour occur at any flow velocity or aggradation rate successive dune trough trajectories defined by dunes showing various behaviours affect the geometry of the preserved cross sets mean cross set thickness mean dune height averages 0 33 0 7 and mean cross set length mean dune length averages 0 49 0 08 and both show no systematic variation with aggradation rate or flow velocity mean cross set thickness mean cross set length tends to decrease with increasing flow velocity and froude number therefore allowing a qualitative estimation of flow conditions quantitative analysis of the temporal changes in the geometry and migration rate of individual dunes allows the development of a two dimensional stochastic model of dune migration and formation of cross sets computer realizations produced stacks of cross sets of comparable shape and thickness to laboratory flume observations indicating a good empirical understanding of the variability of dune trough trajectories however interactions among dunes and aggradation rates of the order of 10 2 mm s 1 should be considered in future improved models

migration and internal architecture of marine dunes in the eastern english channel over 14 and 56 year intervals the influence of tides and decennial storms submarine dune dynamics are controlled by tidal currents and wind forces according to the relative in uence of these forces and the nature of dune sediment different bedform behaviors can be observed the footprint of the different hydrodynamic agents is recorded into the internal architecture of dunes this paper is concerned with bedforms that compose the thick sediment wedge located in the eastern english channel off the bay of somme this sedimentary archive constitutes an interesting feature to achieve a better understanding of seabed sediment dynamics and its timeline building stages the dynamics of large submarine dunes which are organized in elds are studied thanks to bathymetric and seismic data over the periods 1937 1993 and 1993 2007 dune morphology presents low lee and stoss side slopes on average 8 and 3 respectively and dune migration rate is not very high dune movements are in the direction of residual tidal currents i e toward the east with mean migration rates around 0 8 to 5 0 25 m yr 1 and up to 6 6 0 7 m yr 1 respectively at multi decennial and decennial time scales the dune internal archi tecture is complex with superimposed eastward prograding units displaying locally opposite progradation second order discon tinuities dip of 0 5 4 perpendicular to dune crests constitute dune master bedding by counting the number of second order re ectors between 1937 1993 and 1993 2007 the formation periodicity of these bounding surfaces is estimated to range from 4 to 18 years these time intervals coincide with the long term tidal cyclicities and also with the inter annual to decennial variability of storm activity in northern europe two theories were made to interpret the dune internal structures the second order surfaces are interpreted either as the depositional surfaces corresponding to the marks of weak energy periods weak tidal and storm action or as erosive surfaces due to an opposite direction of dune migration provoked temporarily by exceptional storms from the northeast

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

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

deposition in an esker bead and fan complex lanark ontario canada abstract a glaciofluvial landform complex comprising a single esker ridge proximally multiple ridges and beads in the middle section and a complex of ridges and fans distally is mapped and described we present detailed successions in the associated sediment showing sedimentary structure grain size and palaeocurrent estimates sediment in the esker is mainly coarse grained and varies from boulders with a fine sand matrix deposited at maximum discharges to diamicton interpreted as a product of ice pressing under low discharges early stage deposition in the esker at high flow was from suspension and bed load intermediate stage deposition was mainly from bed load and late stage deposition was from hyperconcentrated suspension the esker rests in a bedrock channel which functioned as a nye channel prior to esker formation the channel surface carries a variety of fluvial erosional marks short distributary ridges of the ridges and beads complex contain mainly cobbles and boulders but beads are mainly in sand with rapid down flow fining and change in sedimentary structure faulting and overfolding of bead sediment suggest a subglacial origin deposition of the esker fan complex was highly variable in space and time down fan sedimentary changes with transitions from gravel to cross laminated fine sand in a few tens of metres indicate rapid loss in competence fan location lateral to the esker indicates a broadening of the zone of subglacial meltwater with time deposition from suspension and steep sided scours are interpreted to result from deposition and erosion by hyperconcentrated flows a model of underflows with submerged hydraulic jumps spilling laterally from the esker tunnel into subglacial cavities explains the morphological and sedimentary characteristics of the fans detailed analysis of small scale fining upwards sequences in the fans and beads indicates deposition by a large number of flow events these events are explained by glacier hydraulics whereby periodic separation of the glacier from its bed caused water with a high suspended sediment concentration to spill out from the esker tunnel into adjacent subglacial cavities evolution of the subglacial drainage system indicates a complexity of subglacial sedimentation near to a grounding line which is not readily apparent from observations at modern glaciers terminating in relatively deep water

low velocity non darcian flow to a well fully penetrating a confined aquifer in the first kind of leaky aquifer system in this study we use a finite difference method to solve low velocity non darcian flow to a well in the first kind of leaky aquifer system flow in the confined aquifer is assumed to be darcian and horizontal whereas flow in the aquitard is assumed to be non darcian and vertical the threshold hydraulic gradient existence of non darcian flow in low permeability porous media is employed to describe the non darcian flow in the aquitard a numerical solution has been obtained by using a finite difference method this solution is compared with the previous solution for darcian flow case in leaky aquifer system the error has been analyzed the comparison of this study and darcian flow case hantush and jacob 1955 in leaky aquifer system indicates that the error is very small and can be neglected however the hydrogeological parameter calculation of leaky aquifer system is remarkably influenced by low velocity non darcian flow in aquitard for the inflection point method hantush 1956 the absolute values of estimated errors for coefficient of transmissibility of confined aquifer and vertical hydraulic conductivity of aquitard show negative relationship with the pumping rate for the type curve fitting method walton 1962 the estimated errors for coefficient of transmissibility and elastic drainable porosity of confined aquifer are very small under small pumping rate in general the estimated errors for coefficient of transmissibility and elastic drainable porosity of confined aquifer can be controlled under certain level through adjusting pumping rate the estimated error of vertical hydraulic conductivity of aquitard is quite large no matter which method is used even up to nearly 300

radar architecture and evolution of channel bars in wandering gravel bed rivers fraser and squamish rivers british columbia canada abstract the alluvial architecture and evolution of two kilometer scale compound bars in the wandering gravel bed fraser and squamish rivers british columbia canada are described integrating ground penetrating radar bathymetry and aerial photographs enables the internal architecture to be linked to the evolution of the gravelly barforms over the previous 50 years these linkages reveal the sedimentary mechanisms that formed the various architectural packages within compound bars and unit bars growth of compound bars is controlled by the accretion of unit bars onto discrete segments along their gravelly edges the attachment of unit bars deflects the thalweg to impinge on and erode specific portions of bars and channel banks bar growth leads to the stabilization of bars vegetation colonization of bar interiors and island formation it is the formation of islands along with channel avulsions that maintains channel division in wandering rivers seven styles of gravelly deposition are imaged in the alluvial architecture vertical accretion deposits formed from the deposition of gravelly bedload sheets are the most common strata a moderate abundance of slipface deposits preserves high relief bar margins lateral accretion dominates point bar deposits downstream accretion deposits govern some phases of down bar growth partial and complete channel fill and chute fill deposits are eroded into underlying sediments as are scour and fill deposits upstream accretion deposits are uncommon a depositional model of gravelly channel bars in the two rivers is presented and reveals that the architecture is made up of depositional styles similar to those observed in braiding river successions although the sedimentary packages are preserved in different proportions differences in braiding river and wandering river sedimentology largely reflect the relatively frequent migration of channels and bars in braiding rivers which preserve high proportions of channel fill and chute fill and confluence scour and fill deposits conversely the moderately stable island and channel network in wandering rivers limits channel shifting and consequently preserves a low number of channel fills moderate proportions of slipface strata and coherent patterns of sand deposition along bar tops provide evidence of comparatively uniform flow patterns in independent channel segments divided by islands these patterns of bar sedimentation and channel shifting preserved in the alluvial architecture appear to be signature characteristics of wandering rivers the occurrence of similar architecture in both the fraser and squamish rivers suggests that the model likely applies to most wandering gravel bed rivers

changes in jokulhlaup sizes in grimsvotn vatnajokull iceland 1934 91 deduced from in situ measurements of subglacial lake volume a record of volumes of jokulhlaups from the subglacial grimsvotn lake vatnajokull iceland has been derived for the period 1934 91 the change in lake volume during jokulhlaups is estimated from the lake area ice cover thickness and the drop in lake level the jokulhlaup volumes have decreased gradually during this period of low volcanic activity and declining geothermal power the two jokulhlaups in the 1930s each discharged about 4 5 km peak discharge 25 30 10 3 m 3 s 1 in the 1980s jokulhlaup volumes were 0 6 1 2 km 3 peak discharge 2 10 3 m 3 s 1 the lake level required to trigger a jokulhlaup has risen as an ice dam east of the lake has thickened water flow in a jokulhlaup ceases when the base of a floating ice shelf covering grimsvotn settles to about 1160 m a s l apparently the jokulhlaups are cut off when the base of the ice shelf collapses on to a subglacial ridge bordering the lake on its eastern side the decline in melting rates has resulted in a positive mass balance of the 160 170 km grimsvotn ice drainage basin comparison of maps shows that the average positive mass balance rate was 0 12 km 3 a 1 25 of the total accumulation in the period 1946 87 a gradually increasing positive mass balance has prevailed since 1954 reaching 0 23 km 3 a 1 in 1976 86 48 of total accumulation

on the formation of large subglacial lakes the role of subglacial water storage beneath continental ice sheets is investigated primarily for a deformable bed subglacial ponding is shown to occur under most regions of warm based ice sheets and large subglacial lakes can become established for example in the hudson bay basin the formation of large lakes depends upon the fact that the ice surface gradient is reduced once subglacial ponding occurs and upon the feedback between the reduced ice surface gradient and increased subglacial ponding subglacial ponding likely played a large role in determining the ice sheet topography during late deglaciation and in speeding up the deglaciation process

laurentide glaciation in west central alberta a single late wisconsinan event the grande prairie region in west central alberta shows evidence of late wisconsinan laurentide glaciation in the form of a widespread till containing abundant erratic clasts derived from the canadian shield two sections located on the smoky river at watino and on the simonette river 80 km to the southwest expose sediment lying stratigraphically below the till a similar sequence is exposed in both localities 5 10 m of coarse quartzite gravel overlain by a thick sequence of sand and silt the gravel contains no laurentide erratics and is thus preglacial the contact between the basal gravel and the overlying sand and silt is sharp and conformable 14c dates from the base of the simonette section and throughout the watino section suggest a middle wisconsinan age thus the sediment exposed is interpreted as a conformable middle wisconsinan sequence the sedimentary sequences were deposited in a moderate to low energy braided stream system flowing towards the north the general fining up trend in the

the sedimentary history of the inner alpine inn valley austria extending the baumkirchen type section further back in time with new drilling the baumkirchen clay pit near innsbruck western austria is a well known site in alpine quaternary stratigraphy lacustrine sediments from the last glacial cycle from within the alps provide a unique opportunity to investigate the regional palaeoclimate recent drilling has extended the known sequence to a total length of at least 250 m consisting of almost entirely well laminated clayey silt luminescence dating identified two lake sequences separated by a hiatus of ca 7000 15 000 years lake phase 1 spans the period ca 77 55 ka i e from about marine isotope stage mis 5 4 to the mis 4 3 transition lake phase 2 extends from mid to late mis 3 between ca 45 and 33 ka down core x ray fluorescence core scanning confirmed the presence of the lake phases in the sediment composition suggesting different sediment sources and or transport mechanisms during these two intervals a unique section of exotic angular silt matrix supported gravel at the top of lake phase 1 is interpreted as ice rafted debris luminescence dating constrains this layer to ca 55 ka thus providing the first evidence of a late mis 4 or early mis 3 ice advance confined to the interior of the eastern alps a conceptual model of the sedimentary history of the valley is presented

rhythmic till sedimentation evidence for repeated hydraulic lifting of a stagnant ice mass the youngest sedimentary facies exposed in erosional hummocky terrain in south central alberta canada is composed of alternating beds of massive silty clay diamicton and sorted sand and or silt diamicton beds are similar in thickness 5 20 cm both laterally and vertically within the sequence and sedimentary structures and consistent clast orientations support a subglacial origin for both the diamicton and the sorted sediment alternation between diamicton and sorted strata document rhythmic sedimentary sequences deposition of melt out till followed by current deposition laterally extensive sorted beds indicate that the currents responsible for sand and silt deposition must have flowed as a sheet this flow was shallow no more than 20 cm indicated by scours below boulders that were suspended from the base of glacial ice sheet flow requires that the water was stored beneath the ice prior to drainage consequently some of the diamicton may have been deposited by rain out through the shallow stored water and hence each diamicton bed may represent a combination of direct melt out onto the bed and rain out through the water column inferred cycles of water accumulation and drainage may have been annual

loess in the cypress hills alberta canada loess deposits in the cypress hills were derived from a single local source located adjacent to the southwestern margin of the plateau the distributions of sand clay and heavy minerals the changes in thickness of the deposits and the orientation of the clasts all indicate that the direction of transport was from southwest to northeast the changes in the textural composition of the loess are reflected in variations in the sorting with the best sorted material being located in intermediate positions the loess is depleted in feldspar relative to the source sediments this depletion is due to the combined effects of weathering of the feldspar to hydrous mica clays and aeolian sorting the platy clay mineral grains would be less likely to be incorporated into the aeolian load than the spherical and bladed angular quartz grains and the partially weathered feldspar grains would be similarly preferentially winnowed the variations noted indicate that loess deposits formed from a single local source will

deglaciation of the eurasian ice sheet complex the eurasian ice sheet complex eisc was the third largest ice mass during the last glacial maximum with a span of over 4500 km and responsible for around 20 m of eustatic sea level lowering whilst recent terrestrial and marine empirical insights have improved understanding of the chronology pattern and rates of retreat of this vast ice sheet a concerted attempt to model the deglaciation of the eisc honouring these new constraints is conspicuously lacking here we apply a first order thermomechanical ice sheet model validated against a diverse suite of empirical data to investigate the retreat of the eisc after 23 ka bp directly extending the work of patton et al 2016 who modelled the build up to its maximum extent retreat of the ice sheet complex was highly asynchronous reflecting contrasting regional sensitivities to climate forcing oceanic influence and internal dynamics most rapid retreat was experienced across the barents sea sector after 17 8 ka bp when this marine based ice sheet disintegrated at a rate of 670 gigatonnes per year gt a 1 through enhanced calving and interior dynamic thinning driven by oceanic atmospheric warming and exacerbated by eustatic sea level rise from 14 9 to 12 9 ka bp the eisc lost on average 750 gt a 1 peaking at rates 3000 gt a 1 roughly equally partitioned between surface melt and dynamic losses and potentially contributing up to 2 5 m to global sea level rise during meltwater pulse 1a independent glacio isostatic modelling constrained by an extensive inventory of relative sea level change corroborates our ice sheet loading history of the barents sea sector subglacial conditions were predominately temperate during deglaciation with over 6000 subglacial lakes predicted along with an extensive subglacial drainage network moreover the maximum eisc and its isostatic footprint had a profound impact on the proglacial hydrological network forming the fleuve manche mega catchment which had an area of 2 5 106 km2 and drained the present day vistula elbe rhine and thames rivers through the seine estuary during the bolling allerod oscillation after c 14 6 ka bp two major proglacial lakes formed in the baltic and white seas buffering meltwater pulses from eastern fennoscandia through to the younger dryas when these massive proglacial freshwater lakes flooded into the north atlantic ocean deglaciation temporarily abated during the younger dryas stadial at 12 9 ka bp when remnant ice across svalbard franz josef land novaya zemlya fennoscandia and scotland experienced a short lived but dynamic re advance the final stage of deglaciation converged on present day ice cover around the scandes mountains and the barents sea by 8 7 ka bp although the phase lagged isostatic recovery still continues today

deposits of the sandy braided south saskatchewan river implications for the use of modern analogs in reconstructing channel dimensions in reservoir characterization estimation of the dimensions of fluvial geobodies from core data is a notoriously difficult problem in reservoir modeling to try and improve such estimates and hence reduce uncertainty in geomodels data on dunes unit bars cross bar channels and compound bars and their associated deposits are presented herein from the sand bed braided south saskatchewan river canada these data are used to test models that relate the scale of the formative bed forms to the dimensions of the preserved deposits and therefore provide an insight as to how such deposits may be preserved over geologic time the preservation of bed form geometry is quantified by comparing the alluvial architecture above and below the maximum erosion depth of the modern channel deposits this comparison shows that there is no significant difference in the mean set thickness of dune cross strata above and below the basal erosion surface of the contemporary channel thus suggesting that dimensional relationships between dune deposits and the formative bed form dimensions are likely to be valid from both recent and older deposits the data show that estimates of mean bankfull flow depth derived from dune unit bar and cross bar channel deposits are all very similar thus the use of all these metrics together can provide a useful check that all components and scales of the alluvial architecture have been identified correctly when building reservoir models the data also highlight several practical issues with identifying and applying data relating to cross strata for example the deposits of unit bars were found to be severely truncated in length and width with only approximately 10 of the mean bar form length remaining and thus making identification in section difficult for similar reasons the deposits of compound bars were found to be especially difficult to recognize and hence estimates of channel depth based on this method may be problematic where only core data are available i e no outcrop data exist formative flow depths are suggested to be best reconstructed using cross strata formed by dunes however theoretical relationships between the distribution of set thicknesses and formative dune height are found to result in slight overestimates of the latter and hence mean bankfull flow depths derived from these measurements this article illustrates that the preservation of fluvial cross strata and thus the paleohydraulic inferences that can be drawn from them are a function of the ratio of the size and migration rate of bed forms and the time scale of aggradation and channel migration these factors must thus be considered when deciding on appropriate length thickness ratios for the purposes of object based modeling in reservoir characterization

dunes in the world s big rivers are characterized by low angle lee side slopes and a complex shape dunes form critical agents of bedload transport in all of the world s big rivers and constitute appreciable sources of bed roughness and flow resistance dunes also generate stratification that is the most common depositional feature of ancient riverine sediments however current models of dune dynamics and stratification are conditioned by bedform geometries observed in small rivers and laboratory experiments for these dunes the downstream lee side is often assumed to be simple in shape and sloping at the angle of repose here we show using a unique compilation of high resolution bathymetry from a range of large rivers that dunes are instead characterized predominantly by low angle lee side slopes 10 complex lee side shapes with the steepest portion near the base of the lee side slope and a height that is often only 10 of the local flow depth this radically different shape of river dunes demands that such geometries are incorporated into predictions of flow resistance water levels and flood risk and calls for rethinking of dune scaling relationships when reconstructing palaeoflow depths and a fundamental reappraisal of the character and origin of low angle cross stratification within interpretations of ancient alluvial sediments dunes in the world s big rivers are dominated by lee side slopes with angles of less than 10 according to a bedform analysis of high resolution bathymetric datasets

large barchanoid dunes in the amazon river and the rock record implications for interpreting large river systems abstract the interpretation of large river deposits from the rock record is hampered by the scarcity of direct observations of active large river systems that is particularly true for deep channel environments where tens of meters deep flows dominate these conditions are extremely different from what is found in smaller systems from which current facies models were derived mbes and shallow seismic surveys in a selected area of the upper amazonas river in northern brazil revealed the presence of large compound barchanoid dunes along the channel thalweg the dunes are characterized by v shaped concave downstream crest lines and convex up longitudinal profiles hundreds of meters wide up to 300 m in wavelength and several meters high based on the morphology of compound dunes expected preserved sedimentary structures are broad large scale low angle concave up and downstream cross strata passing laterally and downstream to inclined cosets examples of such structures from large river deposits in the rock record are described in the silurian serra grande group and the cretaceous sao sebastiao and marizal formations in northeastern brazil as well as in triassic hawkesburry sandstone in southeastern australia and the plio pleistocene ica formation in the western amazon all these sedimentary structures are found near channel base surfaces and are somewhat coarser than the overlying fluvial deposits favoring the interpretation of thalweg depositional settings the recognition of large barchanoid dunes as bedforms restricted to river thalwegs and probably to large river systems brings the possibility of establishing new criteria for the interpretation of fluvial system scale in the rock record sedimentary structures compatible with the morphological characteristics of these bedforms seem to be relatively common in large river deposits given their initial recognition in five different fluvial successions in brazil and australia potentially enabling substantial improvements in facies models for large rivers

approximate analytical solution for non darcian flow toward a partially penetrating well in a confined aquifer summary in this study non darcian flow to a partially penetrating well in a confined aquifer was investigated the flow in the horizontal direction was assumed to be non darcian while the flow in the vertical direction was assumed to be darcian the izbash equation was employed to describe the non darcian flow in the horizontal direction of the aquifer we used a linearization procedure to approximate the non linear term in the governing equation enabling the mathematical model to be solved using a combination of laplace and fourier cosine transforms approximate analytical solutions for the drawdown were obtained and the impacts of different parameters on the drawdown were analyzed the results indicated that a larger power index n in the izbash equation leads to a larger drawdown at early times while a larger n results in a smaller drawdown at late times the drawdowns along the vertical direction z are symmetric if the well screen is located in the center of the aquifer and the drawdown at the center of the aquifer is the largest along the vertical direction for this case the length of the well screen w has little impact on the drawdown at early times while a larger length of the well screen results in a smaller drawdown at late times the drawdown increases with k r at early times while it decreases as k r increases at late times in which k r is the apparent radial hydraulic conductivity a sensitivity analysis of the parameters i e the specific storage s s w n and k r indicated that the drawdown is not sensitive to them at early times while it is very sensitive to these parameters at late times especially to the power index n