growth faults and fault related structures of prograding terrigenous clastic continental margins abstract growth faults are the structural manifestation of the inherent tensional stress regime that is focused at the upwardly convex margin of a prograding continental platform extension and faulting may be triggered by either gravitational gliding or spreading extensional faulting results in both lateral translation and subsidence of the down faulted sediment mass consequently outer platform and upper slope facies are expanded and disproportionately preserved relative to paralic or deep water facies specific growth fault geometries and patterns are related in part to the depositional history and style of the prograding margin unusually continuous and low angle master faults are proposed to result from a history of rapid progradation of a deltaic depocenter onto an older retrograditional slope growth fault zones function as major conduits for large scale circulation of both ground waters and hydrocarbon fluids within the sedimentary prism

decadal scale variations in ice flow along whillans ice stream and its tributaries west antarctica this is the publisher s version also available electronically from http www ingentaconnect com

analysis techniques for coherent airborne radar sounding application to west antarctic ice streams 1 analysis of coherent radar sounding echoes from polar ice sheets can provide information suitable for classifying the subglacial environment echoes from a general interface consist of both specularly reflected and diffusely scattered contributions specular reflection results from smooth uniform interfaces whereas diffuse scattering results from rough nonuniform interfaces and inhomogeneous media this article discusses how these phenomena are important to the acquisition and analysis of coherent radar sounding data reflection results are presented from airborne surveys conducted in 1987 over the downstream portions of whillans ice stream and ice stream c west antarctica additionally reflection and scattering analyses along with new results are presented for repeat profiles flown in 2001 over ice stream c analysis methods include using echo amplitudes to compute reflection coefficients which are used for inferring the dielectric properties of the subglacial material echo phase analysis provides the locations of dominant scattering centers which relate to reflection or scattering from the interface as well as provide interface roughness estimates comparison of low and high resolution imaging obtained from synthetic aperture radar techniques indicates a reflecting and or scattering interface combining the results from these independent analyses provides classification of the subglacial environment classified regions include smooth seawater smooth saturated sediments accreted marine ice rough bottom crevasses mixed conditions with partial subglacial water and dry frozen conditions

seismic measurements reveal a saturated porous layer beneath an active antarctic ice stream seismic reflection studies recently conducted on ice stream b part of the marine ice sheet of west antarctica show a metres thick layer immediately beneath the ice in which both compressional p and shear s wave speeds are very low these low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure from this and from arguments presented in an accompanying paper1 to the effect that the layer would be too weak to support the shear stress exerted by the overlying ice we conclude that the layer is deforming and that the ice stream probably moves principally by such deformation

miocene chronostratigraphy paleogeography and play framework of the burgos basin southern gulf of mexico this study characterizes miocene chronostratigraphy and plays in the burgos basin and adjacent south texas within an area of approximately 39 700 km2 15 300 mi2 onshore and offshore to the 500 m 1640 ft isobath using greater than 40 000 linear kilometers 25 000 mi of two dimensional seismic lines 115 onshore wells 9 offshore wells and paleontological data we established a correlation framework of 9 key surfaces upper oligocene to lower pliocene representing major probably third order sequence boundaries and maximum flooding surfaces throughout the basin five of the burgos miocene surfaces coincide with regional chronostratigraphic surfaces from the veracruz and laguna madre tuxpan basins thus establishing a consistent correlation framework throughout much of the mexican gulf coast basin twenty miocene plays are defined by four age divisions lower miocene middle miocene upper miocene 1 and upper miocene 2 and four paleogeographic settings unexpanded and expanded shelf proximal slope and distal slope because of proven high productivity in salt bounded basins in the northern gulf of mexico the onlap of strata onto diapirs in the eastern burgos salt province was evaluated as a fifth setting the paleogeographic provinces and onlap areas exhibit characteristic seismic facies stratal geometries and structural relations a characterization of each one of these being key to the overall play evaluation this play framework provides the means for continuing exploration of miocene strata and evaluation of key play elements reservoir presence and quality seal trap source and migration and timing in this structurally complex underexplored basin the relative importance of these play elements varies systematically for each play especially between the onshore shelf plays and the offshore deep water plays where fault complexity and stratigraphic variability are greater

monitoring ice sheet behavior from space satellite remote sensing has revolutionized ice sheet research a variety of instruments sensitive to different parts of the electromagnetic spectrum take what the human eye detects as a flat white desert and provide data sets rich in scientific information image based maps of ice sheets are becoming commonplace and have become an integral component of field work more than a pretty picture the digital character of the satellite data from these instruments has become fundamental to the production of elevation motion accumulation and reflectance data sets visible imagery shows the scientist a wealth of features that offer clues to the history and current behavior of the ice sheet radar and microwave imagery provide information from beneath the surface and have been used to estimate snow accumulation rates interferometry principles have recently been applied to measure surface topography and ice motion with unparalleled precision nonimaging instruments also keep a watchful eye monitoring the ice sheet for indications of growth or shrinkage further expansion of the uses of satellite data is anticipated in the future

along track focusing of airborne radar sounding data from west antarctica for improving basal reflection analysis and layer detection this paper presents focused synthetic aperture radar sar processing of airborne radar sounding data acquired with the high capability radar sounder system at 60 mhz the motivation is to improve basal reflection analysis for water detection and to improve layer detection and tracking the processing and reflection analyses are applied to data from kamb ice stream west antarctica the sar processor correlates the radar data with reference echoes from subsurface point targets the references are 1 d responses limited by the pulse nadir footprint or 2 d responses that include echo tails unfocused sar and incoherent integration are included for comparison echoes are accurately preserved from along track slopes up to about 0 5deg for unfocused sar 3deg for 1 d correlations and 10deg for 2 d correlations the noise clutter levels increase from unfocused sar to 1 d and 2 d correlations but additional gain compensates at the basal interface the basal echo signal to noise ratio improvement is typically about 5 db and up to 10 db for 2 d correlations in rough regions the increased noise degrades the clarity of internal layers in the 2 d correlations but detection of layers with slopes greater than 3deg is improved reflection coefficients are computed for basal water detection and the results are compared for the different processing methods there is a significant increase in the detected water from unfocused sar to 1 d correlations indicating that substantial basal water exists on moderately sloped interfaces very little additional water is detected from the 2 d correlations the results from incoherent integration are close to the focused sar results but the noise clutter levels are much greater

physical conditions at the base of a fast moving antarctic ice stream boreholes drilled to the bottom of ice stream b in the west antarctic ice sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1 6 bars of the ice overburden pressure these conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick the mechanics of ice streaming plays a role in the response of the ice sheet to climatic change

seismic reflection evidence for a deep subglacial trough beneath jakobshavns isbrae west greenland seismic reflection methods were used to determine the ice thickness and basal topography of jakobshavns isbrae a large fast moving ice stream outlet glacier in west greenland a method of data analysis was developed which involves the pointwise migration of data from a linear seismic array and a single explosive source the method yields the depth horizontal position and slope of the basal reflector a deep u shaped subglacial trough was found beneath the entire length of the well defined ice stream the trough is incised up to 1500 m into bedrock and its base lies 1200 1500 m below sea level for at least 70km inland center line ice thickness along most of the channel is about 2500 m or about 2 5 times that of the surrounding ice sheet this prominent bedrock trough was not apparent in existing radio echo sounding data reflection coefficients indicate that much of the basal interface is probably underlain by compacted non deforming sediment the large ice thickness coupled with relatively steep surface slopes leads to high basal shear stresses 200 300 kpa along the ice stream the large shear stresses and lack of a deformable bed imply that internal deformation plays a dominant role in the dynamics of jakobshavns isbrae

recommendations for a uniform chronostratigraphic designation system for phanerozoic depositional sequences many published sequence stratigraphic frameworks lack a systematic and consistent designation for both depositional sequences and key surfaces despite the original goal to provide a fully integrated stratigraphic architecture including diagnostic age information based on a system in use for more than 10 yr at exxonmobil we recommend methodologies for a chronostratigraphic designation system cds using more uniform and robust sequence stratigraphic designations after objectively defining important physical stratigraphic surfaces biostratigraphic and other age constraining information is used to designate surfaces and the packages of rocks they bound this leads to the establishment of a sequence chronostratigraphic framework for a local area of investigation outcrop section field region or basin only after demonstrating clear well documented ties to phanerozoic global coastal onlaps or cycle charts are these sequences and associated surfaces considered as global entities and designated as such higher frequency sequences and surfaces are also accommodated in this cds alternative designations for areas with limited or poor quality chronostratigraphic information are also discussed the cds has proven to have great use in all phanerozoic strata different tectonic settings and depositional environments especially when chronostratigraphic age constraints are robust we have used this system at regional and basinal scales in many geographic locations to help reduce uncertainty in identifying and correlating reservoirs sources and seal rocks predicting the local distribution and quality of reservoirs as well as seals within a producing field and near field wildcats is also facilitated by this system this system has demonstrated use in the correlation of outcrop sections within a basin or between basins rigorous use of the cds proposed here will permit meaningful regional and or interbasinal correlation that is difficult to carry out with the diverse systems currently in use this uniform designation scheme will also facilitate communication within a company and between institutions as well as among academic investigators

on the pulling power of ice streams gravity wants to pull an ice sheet to the center of the earth but cannot because the earth s crust is in the way so ice is pushed out sideways instead or is it the ice sheet sees nothing preventing i t from spreading out except air which is much less massive than ice therefore does not ice rush forward to fill this relative vacuum does not the relative vacuum suck ice into i t because nature abhors a vacuum if so the ice sheet is not only pulled downward by gravity i t is also pulled outward by the relative vacuum this pulling outward will be most rapid where the ice sheet encounters least resistance the least resistance exists along the bed of ice streams where ice bed coupling is reduced by a basal water layer especially if the ice stream becomes afloat and the floating p art is relatively unconfined around its perimeter and unpinned to the sea floor ice streams are t herefore fast currents of ice that develop near the margins of an ice sheet where t hese conditions exist because of these condit ions ice streams pull ice out of ice sheets and have pulling power equal to the longitudinal gravitational pulling force multiplied by the ice stream velocity t hese boundary conditions beneath and beyond ice streams can be quantified by a basal buoyancy factor that provides a life cycle classification of ice streams into inception growth mature declining and terminal stages during which ice streams disintegrate the ice sheet surface profiles of ice streams are diagnostic of the stage in a life cycle and hence of the vitality of the ice sheet

the morphology of ice streams a b and c west antarctica and their environs airborne radar soundings of the ice sheet surface made in 1984 1985 together with elevations measured by oversnow traverses between 1957 and 1964 have been used to produce a surface elevation map of ice streams a b and c and much of the region around them the flight positions and elevations were tied to many ground control stations whose positions in three dimensions were fixed by satellite tracking ice thicknesses were also measured along the flight lines but have not yet been mapped the surfaces of active ice streams a and b exhibit a longitudinal ridgetrough topography of uncertain origin prominent surface valleys are associated with most but not all of their marginal shear zones there is a deep subglacial trough beneath the grid northeastern side of ice stream a that connects to the subglacial trough beneath reedy glacier the boundary between ice streams bl and b2 the two tributaries of ice stream b persists as a suture zone for 250 km downstream from their merger ice stream bl overlies a relatively deep subglacial trough that continued under the corresponding part of ice stream b until it disappears close to the grounding line between ice streams bl and b2 there is a complex zone containing several regions of undisturbed ice separated by bands of disturbed ice which suggest to us that rafts of ice are being incorporated into the ice streams the previously reported negative mass balance for the drainage system of ice stream b may arise from the expansion of the ice stream into the interior of the west antarctic ice sheet crevasse bands at some places on ridge ab together with high glacial velocities reported elsewhere also suggest this expansion close to the ross ice shelf ice stream b widens into a nearly flat area where the mean surface slope is only 3 5 10 4 and the surface elevations are only a few tens of meters above hydrostatic equilibrium inactive ice stream c differs from active ice streams a and b in surficial and basal characteristics no elongated ridges and troughs are observed instead the ice stream surface exhibits several terraces including some maxima and minima in elevation radar sounding reveals areas where basal echoes are strong and steady indicating subglacial water alternating with areas of weaker echoes with short fading lengths calculations indicate that the strong reflections require a water layer at least several centimeters to several meters thick depending on the electrical conductivity of the water due to reversals in the surface slope the hydrologic potential gradient changes sign several times along ice stream c which implies that subglacial water should collect in some locations

the archean proterozoic boundary in the kaapvaal province of southern africa abstract the kaapvaal province has long been considered unusual in that archean cover sequences developed over granite greenstone gneiss terranes 3060 25 ga whereas in other provinces the oldest cover sequences are generally 2500 ga however two unconformity bounded sequences 2 5 ga and of greater inter regional extent than the unconformably underlying archean sequences can now be recognized in the kaapvaal province the two basal unconformities of these sequences are candidates for the temporally defined 2 5 ga archean proterozoic boundary the older but geologically more discordant unconformity occurs at the base of the uppermost group of the ventersdorp supergroup the pniel group and in at least five other erosional remnants in central transvaal and adjacent botswana these other remnants include parts of the buffelsfontein and wolkberg groups which heretofore have been considered to be conformable protobasinal phases of the younger transvaal succession the pniel and the other remnants are truncated by the locally well known unconformity at the base of the transvaal succession on a world wide basis cratonization and the evolution of mineralogically mature cover sequences were and continue to be diachronous the kaapvaal province is unusual only in that it also contains archean cover sequences however with major conformities at about 2 5 ga and the presence of proterozoic unconformity bounded sequences the kaapvaal province seems little different than most other provinces that contain the archean proterozoic boundary

west antarctic ice streams draining into the ross ice shelf configuration and mass balance extensive airborne radar soundings closely tied to accurately located ground stations made over a portion of the west antarctic inland ice sheet and the ross ice shelf have enabled us to map the boundaries of the ice streams and their flow bands on the ross ice shelf the surfaces of the active ice streams a and b are heavily crevassed but there are no visible crevasses on ice stream c however the existence of numerous crevasses at a depth of about 35 m implies that ice stream c ceased to be active about 250 years ago there is a complex zone at the head of ice stream b that suggests that ice stream b is currently widening and advancing toward the interior of the ice sheet the grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance and the character of the radar echoes the inland ice is more extensive than previously mapped and includes a large weakly grounded area downstream of ice streams a and b where the surface elevations are barely above hydrostatic equilibrium crary ice rise extends 70 km upstream of the previously mapped position to relate surface elevation data from satellite observations to sea level three geoidal models were tested we conclude that the gem 10c model is the best of the three for this section of antarctica a new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams a and b but not on ice stream c whose surface contours show signs of stagnation ice stream c shows a strongly positive and ice stream b a significantly negative net balance rate ice streams a and d also show a significantly negative net flux whereas ice streams e and f do not the overall net balance of this part of the west antarctic inland ice is suggestively negative 23 15 km3 yr 1 from the measured flux into the ross ice shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0 12 0 03 m yr 1

large scale ice flow over a viscous basal sediment theory and application to ice stream b antarctica recent seismic studies of ice stream b antarctica and field analysis of mid latitude glacial deposits suggest that deformable basal sediments e g water saturated till are important in determining ice sheet flow if the ratio of till viscosity to effective ice viscosity is small vertical shear associated with horizontal flow is confined to the deforming bed alone ice flow over a deformable bed is thus akin to that of floating ice shelves because ice shelves flow over inviscid seawater for some antarctic ice streams and possibly for portions of the late wisconsin ice sheets in north america and eurasia basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity instead basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients comparison of the observed flow of ice stream b to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile these simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance while bed deformation is critical to ice stream existence sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate

surficial glaciology of jakobshavns isbrae west greenland part i surface morphology jakobshavns isbrae is a large fast moving ice stream outlet glacier in west greenland which ends at a floating calving front it drains about 6 5 of the area of the greenland ice sheet studies of its surface morphology are described in this paper the surface is relatively steep 0 01 0 03 and the thickness is large up to 2600 m along the center line clarke and echelmeyer 1989 indicating very high driving stresses 200 300 kpa the ice stream is about 6 km wide and 85 90 km long all of which is in an area of surface melting the base of the ice stream and of much of the drainage area is below sea level marginal crevasse zones have a width on the order of the width of the ice stream itself unique surficial features are ice blisters and lakes the latter have a sequence of ogive like features on their floating ice cover which can be used to determine velocity there is a pinning point near the terminus which may act as a stabilizing influence possibly playing a role in halting at least temporarily a recent retreat of the terminus ice thickness estimates at the terminus lead to a flux which is less than previously assumed by others e g bindschadler 1984 pelto and others 1989 when estimating jakobshavns isbrae s drainage basin to be nearly in balance the driving stresses on jakobshavns isbrae are an order of magnitude higher than those of the ice streams of west antarctica its crevasse patterns are much less localized its relatively unconfined terminus is more comparable to that of relatively unbuttressed ice streams such as pine island and thwaites glaciers than it is to other west antarctic ice streams which terminate in large confined ice shelves

velocity of ice streams b and c antarctica the essential difference between ice stream and inland ice and the cause of ice streaming are not known in order to study these problems velocities have been measured on and near ice streams b and c by repeated tracking of transit satellites the results confirm the difference in mode of flow between fast on ice stream b and slow on the interstream ice ridges also as expected ice stream c is nearly stagnant which with other evidence of formerly fast flow confirms its recent slowing and current thickening the drainage of ice stream b has a negative balance equivalent to thinning by 0 15 0 05 m yr 1 over the entire catchment the lower portion of the ice stream is thickening at about 2 m yr 1 but the overall behavior is thinning this general thinning is probably partly due to progressive extension of the ice stream into inland ice the transition from inland ice to ice stream seems to occur irregularly both spatially and in time with rafts of inland ice being carried into the ice stream and slowly incorporated

basal mechanics of ice stream b west antarctica 2 undrained plastic bed model based on the results of our studies of the physical conditions beneath ice stream b we formulate a new analytical ice stream model the undrained plastic bed model henceforth the upb model mathematically the upb model is represented by a non linear system of four coupled equations which express the relationships among ice sliding velocity till strength water storage in till and basal melt rate we examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant less than hundreds of years temporal variability is introduced into the upb model only by the direct dependence of till void ratio changes e t on the basal melting rate m r since till strength b e and ice stream velocity u b b change as long as till void ratio varies the first condition for ice stream stability is that of constant till water storage 0 the second condition for ice stream stability arises from the feedback between ice stream velocity till strength and the basal melting rate which depends on shear heating m r u b b this is the weak till condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress b n 1 1 d the salient feature of the upb model is its ability to produce two thermo mechanically controlled equilibrium states one with a strong bed and slow ice velocities ice sheet mode and one with a weak bed and fast ice velocities ice stream mode this bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in west antarctica basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes the upb model demonstrates that ice streams may be prone to thermally triggered instabilities during which small perturbations in the basal thermal energy balance grow leading to generation or elimination of the basal conditions which cause ice streaming

impact of regional variation in detrital mineral composition on reservoir quality in deep to ultradeep lower miocene sandstones western gulf of mexico abstract future exploration in lower miocene sandstones in the gulf of mexico will focus increasingly at depths greater than 4 5 km and reservoir quality will be a critical risk factor in these deep to ultradeep reservoirs the goal of this study was to understand the variation in reservoir quality of lower miocene sandstones across the western gulf of mexico to do this we examined regional variation in detrital mineral composition diagenesis and reservoir quality in five areas 1 louisiana 2 upper texas coast 3 lower texas coast 4 burgos basin mexico and 5 veracruz basin mexico using petrographic and petrophysical data from depths of 0 9 7 2 km there are strong variations in mineralogical composition within the study area lower miocene sandstones from offshore louisiana have an average composition of quartz 86 feldspar 12 and rock fragments 2 q 86 f 12 r 2 feldspar and rock fragment content increase southward as source areas shifted to include volcanic and carbonate rocks composition of samples from offshore texas ranges from q 67 f 24 r 9 in the upper texas coast to q 58 f 24 r 19 in the lower texas coast lower miocene sandstones from the onshore burgos basin northern mexico have an average composition of q 54 f 22 r 23 whereas sandstones from the veracruz basin southern mexico contain the highest proportion of rock fragments q 33 f 12 r 55 main diagenetic events in quartz rich lower miocene sandstones in louisiana were mechanical compaction and precipitation of quartz cement compactional porosity loss increased to the south with increasing rock fragment content calcite is the most abundant cement in the south and is strongly related to reservoir quality loss at moderate burial depths the best reservoir quality occurs in quartz rich sandstones in louisiana and decreases with increasing lithic content in texas and mexico porosity is higher in louisiana and upper texas than in lower texas and mexico at all depths and temperatures but at depths 5 km and temperatures 175 c porosity differences are lessened the lower miocene sandstone trend in the western gulf of mexico from louisiana to mexico is an example of the importance of variation in detrital mineralogy as a control on diagenesis and reservoir quality

fast glacier flow ice streams surging and tidewater glaciers fast flowing glaciers are unusual but important jakobshavns glacier in western greenland illustrates this point figure 1 considered to be the world s fastest glacier its maximum observed flow rate measured in midsummer near the floating terminus is 8360 m yr 1 lingle et al 1981 except for a tidal influence near the terminus the flow is believed to be fairly steady bindschadler 1984

till beneath ice stream b 3 till deformation evidence and implications most of the velocity of ice stream b near the upstream b camp upb west antarctica appears to arise from deformation of a seismically detected subglacial till layer that averages 6 m thick available evidence indicates that the entire thickness of this till layer is deforming and is eroding subjacent bedrock into flutes parallel to ice flow and hundreds of meters across the resulting till flux beneath upb is equivalent to an average erosion rate of about 0 4 mm yr 1 in the catchment area and suggests that till deltas tens of kilometers long have been deposited at the grounding line during the holocene such deltas should be characterized by partial ice till decoupling across a water film and by a small ice air surface slope they may have been discovered by recent geophysical work

rheological nonlinearity and flow instability in the deforming bed mechanism of ice stream motion contrary to what has recently been assumed in modeling the proposed deforming bed mechanism for the rapid motion of antarctic ice streams the rheology of water saturated till is probably highly nonlinear according to information from soil mechanics and preliminary experiments on till from the base of ice stream b the equivalent flow law exponent n is probably as high as 100 and the nonlinearities of the shear stress and effective pressure dependences are closely linked the high nonlinearity has important consequences for the deforming bed mechanism a flow system operating by this mechanism can be unstable as a result of feedback from the generation of basal water by shear heating of basal till the short term feedback effect is analyzed for a perturbation in a model ice stream in which the basal meltwater is transported through a distributed system of narrow gap conduits at the ice till interface although the analysis is approximate and some of the system parameters are poorly known the results suggest that the deforming bed mechanism is unstable for n 20 the apparent lack of such an instability in the currently active ice streams implies that their motion is controlled not by the deforming bed mechanism but by some other as yet unidentified mechanism

high frequency miocene sequence stratigraphy offshore louisiana cycle framework and influence on production distribution in a mature shelf province the regressive miocene succession of offshore louisiana comprises 10 third order sequences and no fewer than 58 fourth order sequences which average approximately 1 1 and 0 19 m y in duration respectively comparable to durations measured in the gulf coast basin and basins worldwide upper lower to middle miocene distal third order sequences comprise mostly lowstand prograding wedge slope fan and basin floor fan deposits in contrast middle to upper miocene medial sequences record progressively more landward systems tracts 1 the lateral transition between on shelf incised valley fills and the proximal parts of basinward thickening lowstand prograding wedges and 2 cyclic on shelf highstand and transgressive systems tracts upper miocene inner shelf and marginal marine systems tracts and more abundant incised valleys dominate the thinner proximal third order sequences this genetic framework has a major influence on hydrocarbon distribution although a strong structural trapping component is present in the fields more than 90 of cumulative production originates where fourth order systems tracts stack to form third order lowstand systems tracts in all 10 third order sequences the development of a high frequency sequence framework for the prolific miocene succession and the discovery that hydrocarbons are pooled within the miocene third order lowstand systems tracts yield a focused model for the development of abundant undiscovered miocene reserves in the mature northern gulf of mexico shelf province

geomorphology under ice streams moving from form to process ice streams are integral components of an ice sheet s mass balance and directly impact on sea level their flow is governed by processes at the ice bed interface which create landforms that in turn modulate ice stream dynamics through their influence on bed topography and basal shear stresses thus ice stream geomorphology is critical to understanding and modelling ice streams and ice sheet dynamics this paper reviews developments in our understanding of ice stream geomorphology from an historical perspective with a focus on the extent to which studies of modern and palaeo ice streams have converged to take us from a position of near complete ignorance to a detailed understanding of their bed morphology during the 1970s and 1980s our knowledge was limited and largely gleaned from geophysical investigations of modern ice stream beds in antarctica very few palaeo ice streams had been identified with any confidence during the 1990s however glacial geomorphologists began to recognise their distinctive geomorphology which included distinct patterns of highly elongated mega scale glacial lineations ice stream shear margin moraines and major sedimentary depocentres however studying relict features could say little about the time scales over which this geomorphology evolved and under what glaciological conditions this began to be addressed in the early 2000s through continued efforts to scrutinise modern ice stream beds at higher resolution but our current understanding of how landforms relate to processes remains subject to large uncertainties particularly in relation to the mechanisms and time scales of sediment erosion transport and deposition and how these lead to the growth and decay of subglacial bedforms this represents the next key challenge and will require even closer cooperation between glaciology glacial geomorphology sedimentology and numerical modelling together with more sophisticated methods to quantify and analyse the anticipated growth of geomorphological data from beneath active ice streams

sedimentological record of subglacial conditions and ice sheet dynamics of the vistula ice stream north central poland during the last glaciation abstract deposits of the vistula ice stream draining the scandinavian ice sheet during the last glaciation were investigated at four field sites in north central poland using micro and macroscale features the study reveals several till units with specific structural textural and lithological characteristics the individual till units are either macroscopically massive or bedded and the contacts between the units are either sharp or transitional the nature of the contacts with the underlying sediments ductile deformation structures largely undeformed clayey clasts tectonic lamination thin horizontal stringers of sorted sediments ploughing marks boulder pavements and striated upper surfaces of pebbles in the till indicate both bed deformation and enhanced basal sliding under high subglacial water pressure conditions it is suggested that the till is a hybrid deposit generated by some combination of lodgement deformation and ploughing punctuated by periods of basal decoupling the depth of deformation at any point in time was thinner up to several decimetres than the maximum till thickness c 2 5 m the ice sliding velocity estimations indicate velocities of less than 100 to over 2000 m yr 1 which suggests an unstable and highly dynamic ice lobe consistent with spatial variability of till characteristics sand wedges in the deposits beneath the till and the nature of the till bed interface indicate permafrost under the advancing ice sheet we suggest that under the increasing ice thickness a layer of thawed water saturated sediment formed on top of the still frozen ground due to inefficient drainage and contributed to ice streaming by promoting pervasive deformation and basal sliding

identifying areas of low profile ice sheet and outcrop damming in the antarctic ice sheet by ers 1 satellite altimetry a digital elevation model dem of the surface of the antarctic ice sheet is compared with a simple two dimensional ice flow model to illuminate gross distortions 500 m of the ice surface elevation we use a dem derived from ers l satellite altimetry airborne data and twerle balloon data this is compared with an ice sheet elevation model generated by applying theoretical surface elevations calculated for twodimensional ice flow to isolines of distance from the grounding line continentality the model is scaled using only one parameter to match the measured surface elevation at dome argus the model is far from rigorous violating continuity conditions ignoring variations in surface mass balance and temperature and assuming uniform basal conditions however the comparison of model and observed surface elevations is illuminating in terms of the behaviour of the ice sheet at a continental scale across the ice sheet the rms difference between modelled elevation and the dem is around 300 m but much of this results from isolated areas of much greater disagreement we ascribe these gross differences to the effects of basal conditions in four areas the observed surface is more than 500 m higher than the modelled surface most of these are immediately upstream of substantial areas of rock outcrop and are caused by the damming effect of these mountain ranges in nine areas the measured surface is more than 500 m lower than predicted eight of these areas in west antarctica and the l ambert glacier basin are associated with suspected areas of basal sliding the ninth is an area of 250 000 km2 in east antarctica not previously noted as having unusual flow characteristics but for which very few data exist we speculate that this area results from unusual basal conditions resulting in a low profile ice sheet a low profile ice sheet of this size within the east antarctic ice sheet indicates that basal conditions are perhaps more variable than previously thought

development of the topography controlled upper odra ice lobe scandinavian ice sheet in the fore mountain area of southern poland during the saalian glaciation abstract during the saalian drenthe glaciation the upper odra ice lobe constituted the most southern part of the scandinavian ice sheet in central europe the lobe was situated in an area with varied topography in the foreland of a mountain range the ice lobe had a complex form because it was subdivided into several smaller sublobes the primary factors that controlled the lobe were investigated it was found that the ice lobe development depended largely on the topography but the lithology of the ice sheet substratum was also highly important the topography controlled the main movement direction of the ice and caused a concentration of the ice flow in the axial part of the silesian lowland between subglacial morphological barriers which thereby formed a corridor of relatively fast flowing ice which was probably streaming and which supplied the lobe the topography also determined the ice flow within the lobe which strongly matched the valley system the presence of sediments with relatively low permeability at the base of the lobe the quaternary deposits are underlain by neogene clays caused high pore water pressure in the subglacial zone and triggered the basal movement of the ice sheet which promoted a rapid flow of the ice and favoured the development of the lobe the basal tills deposited from the active and passive ice and extant till patches without a fluvioglacial cover atop some parts of the studied area indicate spatially varying ice flow dynamics especially in the later stage of glaciation certain parts of the lobe underwent stagnation while in other parts the ice was probably still active the varied behaviour of the ice within the lobe was determined primarily by the topography and lithology of the substratum which produced spatially varied marginal compression and basal friction a model of the development of the lobe is proposed

a framework for attenuation method selection evaluated with ice penetrating radar data at south pole lake all radar power interpretations require a correction for attenuative losses moreover radar attenuation is a proxy for ice column properties such as temperature and chemistry prior studies use either paired thermodynamic and conductivity models or the radar data themselves to calculate attenuation but there is no standard method to do so and before now there has been no robust methodological comparison here we develop a framework meant to guide the implementation of empirical attenuation methods based on survey design and regional glaciological conditions we divide the methods into the three main groups 1 those that infer attenuation from a single reflector across many traces 2 those that infer attenuation from multiple reflectors within one trace and 3 those that infer attenuation by contrasting the measured power from primary and secondary reflections to assess our framework we introduce a new ground based radar survey from south pole lake comparing selected empirical methods to the expected attenuation from a temperature and chemistry dependent arrhenius model based on the small surveyed area lack of a sufficient calibration surface and low reflector relief the attenuation methods that use multiple reflectors are most suitable at south pole lake

controls on the evolution of passive margin salt basins structure and evolution of the salina del bravo region northeastern mexico we would like to thank cgg and tgs for permission to use seismic data nancy cottington drafted the diagrams cari breton constructed the geographic information system database eva silverfine ott edited the text the paper was greatly improved by reviews from chris jackson and an anonymous reviewer the project was funded by the applied geodynamics laboratory consortium consisting of the following companies anadarko bhp billiton bp cgg chevron condor ecopetrol emgs eni equinor exxonmobil fieldwood hess ion geophysical midland valley murphy nexen noble petrobras petronas pgs repsol rockfield saudi aramco shell spectrum geo stone talos tgs total west erngeco and woodside additional funding came from the jackson school of geosciences juan i soto also acknowledges the financial support provided by the ministry of education in spain with a salvador de madariaga fellowship publication was authorized by the director of the bureau of economic geology jackson school of geosciences the university of texas at austin

glacial and glaciofluvial deposits in the interlobate areas of the scandinavian ice sheet abstract rapid flow within ice streams took place in a broad marginal zone of the scandinavian ice sheet during deglaciation ice streams and interlobate zones have been characterised by analysis of glacial flow patterns and examination of sedimentary features of interlobate glacial and glaciofluvial deposits reconstructed glaciodynamic systems are compared with ice streams of modem ice sheets wide interstream areas of inactive ice were left between some adjacent ice streams where they separated during the retreat of the margin these areas are characterised by morainic hummocks composed of both till and glaciofluvial sediment and sections show indications of subglacial deformation observations from fennoscandia suggest that these interstream areas were sites of net accumulation of glacial and glaciofluvial sediment during deglaciation glaciofluvial sediment was deposited in subglacial tunnels or cavities simultaneously with till deformation and deposition resulting from subglacial melting substantial glaciofluvial complexes were formed in narrow interlobate joints where adjacent ice lobes coalesced a case study made in southern finland shows converging flow patterns of two ice lobes and intervening glaciofluvial deposits concentration of meltwater in interlobate zones was a consequence of ice sheet configuration thinner ice in interlobate zones caused convergence of supraglacial and subglacial drainage the crevassed and strain softened ice may also have allowed surface meltwater to penetrate down to the ice bed interface close to the ice margin steep ice velocity gradients in interlobate zones would have increased subglacial melting rates and this environment was favourable for establishment of a subglacial drainage network and deposition of glaciofluvial sediment

ice dynamics at the mouth of ice stream b antarctica field data collected at the mouth of ice stream b show that the flow dynamics of this region are distinctly different than either the major portion of the ice stream upstream or the ice shelf downstream surface slopes in this region are as low as ice shelf surface slopes yet with the exception of patches of ice which may be floating the ice is grounded basal shear stress is negligible the resistance to flow being partitioned between shear at the sides and longitudinal gradients of longitudinal and transverse stress the surface is generally crevasse free features similar to ice rises are observed upstream of the grounding line their origin is uncertain but they move at velocities comparable to the surrounding ice the flow is laterally extensive and longitudinally compressive but there are large local variations of the strain rate from the regional trends the boundary between the two major tributaries to ice stream b followed with the radar is characterized by a band of strain rates much smaller than average detailed measurements at the downstream b network highlight this local variability of strain rates but confirm that there is a strong correlation between surface topography and strain rates the strain rates indicate that the undulating topography is locally generated the lower elevation ice is thicker and moves faster a velocity profile across the crevassed northern margin shows that the decrease of velocity toward the edge is nearly linear a calculation of ice stream discharge at this location agrees closely with two rather rough estimates of balance flux and is considerably larger than a third estimate the discharge of ice stream b does not appear to be significantly out of balance with published estimates of total ice accumulation within the present catchment basin

landform and sediment imprints of fast glacier flow in the southwest laurentide ice sheet evidence for former fast glacier flow ice streaming in the southwest laurentide ice sheet is identified on the basis of regional glacial geomorphology and sedimentology highlighting the depositional processes associated with the margin of a terrestrial terminating ice stream preliminary mapping from a digital elevation model of alberta identifies corridors of smoothed topography and corridor parallel streamlined landforms megaflutes to mega lineations that display high levels of spatial coherency ridges that lie transverse to the dominant streamlining patterns are interpreted as a series of minor recessional push moraines b thrust block moraines or composite ridges hill hole pairs constructed during readvances surges and c overridden moraines cupola hills apparently of thrust origin together these landforms demarcate the beds and margins of former fast ice flow trunks or ice streams that terminated as lobate forms localised cross cutting and or misalignment of flow sets indicates temporal separation and the overprinting of ice streams lobes the fast flow tracks are separated by areas of interlobate or inter stream terrain in which moraines have been constructed at the margins of neighbouring competing ice streams outlet glaciers this inter stream terrain was covered by more sluggish non streaming ice during full glacial conditions thin tills at the centres of the fast flow corridors in many places unconformably overlying stratified sediments suggest that widespread till deformation may have been subordinate to basal sliding in driving fast ice flow but the general thickening of tills towards the lobate terminal margins of ice streams outlet glaciers is consistent with subglacial deformation theory in this area of relatively low relief we speculate that fast glacier flow or streaming was highly dynamic and transitory sometimes with fast flowing trunks topographically fixed in their onset zones and with the terminus migrating laterally the occurrence of minor push moraines and flutings and associated landforms because of their similarity to modern active temperate glacial landsystems are interpreted as indicative of ice lobe marginal oscillations possibly in response to seasonal climatic forcing in locations where meltwater was more effectively drained from the glacier bed further north the occurrence of surging glacier landsystems suggests that persistent fast glacier flow gave way to more transitory surging possibly in response to the decreasing size of ice reservoir areas in dispersal centres and also locally facilitated by ice bed decoupling and drawdown initiated by the development of ice dammed lakes copyright 2008 john wiley sons ltd

neogene tectonic stratigraphic and play framework of the southern laguna madre tuxpan continental shelf gulf of mexico neogene shelf slope canyon and slope to basin floor transition plays in the southern laguna madre tuxpan lm t continental shelf reflect a variety of structural and stratigraphic controls including gravity sliding and extension compression salt evacuation and lowstand canyon and fan systems the neogene in the lm t area was deposited along narrow shelves associated with a tectonically active coast affected by significant uplift and erosion of carbonate and volcanic terrains this study characterizes 4 structurally defined trends and 32 neogene plays in a more than 50 000 km2 19 300 mi2 area linking the veracruz and burgos basins the caonero trend in the southern part of the lm t area contains deep seated basement faults caused by laramide compression many of these faults are directly linked to the interpreted mesozoic source rocks providing potential pathways for vertically migrating hydrocarbons in contrast the lankahuasa trend north of the caonero trend contains listric faults which detach into a shallow horizon this trend is associated with thick pliocene shelf depocenters the dominant plays in the faja de oro nyade trend in the central part of the lm t area contain thick lower and middle miocene successions of steeply dipping slope deposits reflecting significant uplift and erosion of the carbonate tuxpan platform these slope plays consist of narrow channel fill and levee sandstones encased in siltstones and mudstones plays in the north end of the lm t area in the southern part of the burgos basin contain intensely deformed strata linked to salt and shale diapirism outer shelf slope and proximal basin floor plays in the lamprea trend are internally complex and contain muddy debris flow and slump deposits risk factors and the relative importance of play elements vary greatly among lm t plays reservoir quality is a critical limiting play element in many plays especially those in the caonero trend directly downdip from the trans mexican volcanic belt as well as carbonate rich slope plays adjacent to the tuxpan platform in contrast trap and source are low risk play elements in the lm t area because of the abundance of large three way and four way closures and the widespread distribution of organic rich upper jurassic tithonian age source rock the potential for hydrocarbon migration in lm t plays is a function of the distribution of deep seated faults inferred to intersect the primary mesozoic source their distribution is problematic for the lankahuasa trend where listric faults sole out into the paleocene seal is poorly documented for lm t plays although the presence of overpressured zones and thick bathyal shales is favorable for seal development in middle and lower miocene basin and slope plays

the distribution of basal water between antarctic subglacial lakes from radar sounding antarctica s subglacial lakes have two end member geophysical expressions as hydraulically flat radar reflective regions highlighted in ice surface topography and radar sounding profiles definit

palaeoglaciology of an ice sheet through a glacial cycle the european ice sheet through the weichselian abstract satellite images provide unique means of identifying large scale flow generated lineations produced by former ice sheets they can be interpreted to reconstruct the major elements which make up the integrated large scale structure of ice sheets ice divides ice streams interstream ridges ice shelves calving bays the evolving palaeoglaciological structure of the european ice sheet during its decay from the last glacial maximum lgm is reconstructed by reference to these components and in the context of a new map showing isochrons of retreat during the retreat phase in particular the time dependent dynamic evolution of the ice sheet and the pattern of ice stream development are reconstructed crossing lineations are widespread the older ones are suggested to have formed during molten bed phases of ice sheet growth and preserved by frozen bed conditions during the glacial maximum particularly in areas which lay during deglaciation beneath ice divides and inter ice stream ridges both areas of slow flow and possibly frozen bed conditions four phases of growth a1 to a4 and five phases of decay r1 to r5 are used to describe the major climatically and dynamically determined stages in the evolution of the ice sheet through the last glacial cycle the growth and decay patterns are quite different and associated with major shifts in the ice divide reflecting growth from the fennoscandian mountains and decay away from marine influenced margins these patterns were determined by the locations of nucleation areas spatial patterns of climate and calving at marine margins the prevalence of streaming within the retreating ice sheet suggests that the mean elevation of the ice sheet was lower than predicted from glaciological models which do not include streaming and that this might reconcile glaciological models and earth rheology models which infer paleao ice sheet thickness by inverting sea level data

structural styles and depositional history of north and south padre island ocs areas offshore south texas abstract north and south padre island ocs areas were studied using 2 d seismic reflection data check shot data well logs and paleontological reports six salt structures occur beneath the present shelf edge shale occurs in over pressured ridges beneath fault footwalls we found that growth faults in the study area are similar in style to that of growth faults in the brazos ocs area however they differ in maximum expansion periods the clemente tomas fault system late oligocene early miocene is located above an over pressured shale ridge the corsair fault system early middle miocene formed due to a large sediment influx that forced salt to withdraw basinward the wanda fault system formed during the late miocene due to further salt withdrawal we defined five depositional sequences from the miocene 18 0 16 3 13 4 10 5 and 6 2 ma nine mini basins are in the study area and they are connected by three main sediments pathways mechanism of the basin formation is controlled by the nature of depocenter shift depocenters formed by salt withdrawal and sediment influx do not shift on the other hand depocenters shift is controlled by fault expansion and sediment influx we generated five stratigraphic cross sections using detailed well log correlation to show the sand andmore shale sequences discontinuities in these sequences are common due to subsidence and the large number of faults in the study area presence of faults and shale bodies may cause formation of structural traps less

terrestrial ice streams a view from the lobe abstract the glacial landforms of minnesota are interpreted as the products of the lobate extensions of ice streams that issued from various ice sheds within the laurentide ice sheet low relief till plains trough shaped lowlands boulder pavements and streamlined forms make up the subglacial landsystem in minnesota that is interpreted as having been formed by streaming ice extremely uniform tills are created subglacially in a way that remains somewhat mysterious at the ice margins thrust moraines and hummocky stagnation topography are more common than single crested simple moraines if the ice lobes had repeated advances subglacial drainage features are obscure up ice but are present down ice in the form of tunnel valleys eskers spooner hills and associated ice marginal fans ice streaming may occur when basal shear stress is lowered as a result of high subglacial water pressure subglacial conditions that allow the retention of water will allow an ice lobe to extend far beyond the ice sheet as long as the ice shed also supports the advance by supplying adequate ice even with adequate ice flux however the advance of an ice lobe may be terminated at least temporarily if the subglacial water is drained through tunnel valleys or perhaps a permeable substrate thrust moraines and ice stagnation topography will result from sudden drainage although climate change is ultimately responsible for the accumulation of ice in the laurentide ice sheet the asynchronous advances and retreats of the ice lobes in the mid continent are strongly overprinted by the internal dynamics of individual ice streams as well as the interaction of ice sheds which obscure the climate signal

bed radar reflectivity across the north margin of whillans ice stream west antarctica and implications for margin processes surface based ice penetrating radar profiles were made across the active north margin the snake of the upper part of whillans ice stream formerly ice stream b branch b2 west antarctica at three locations low frequency about 2 mhz and the ground deployment of the radar allowed penetration through the near surface zone of fracturing to detect internal layering and bed reflection characteristics on continuous profiles spanning from the slow moving ice of engelhardt ridge well into the chaotic zone of the shear margin internal layers were tracked beneath the chaotic zone where they are warped but remain continuous the energy returned from internal layers showed no systematic changes associated with the transition from the undisturbed surface of the slow moving ice into the fractured surface of the shear margin thus indicating little effect from the surface crevasses on the penetration of the radar signal based on this calibration of the near surface effects and corrections for path length spreading and attenuation we examine the spatial variation of bed reflectivity low bed reflectivity found under engelhardt ridge extends under the chaotic zone of the margin into fast moving ice we argue that the fast motion in a band along the margin is mediated by processes other than deformation of thick dilated till that is the source of lubrication allowing fast motion in the interior of the ice stream

water flow through temperate glaciers understanding water movement through a glacier is fundamental to several critical issues in glaci ology including glacier dynamics glacier induced floods and the prediction of runoff from glacierized drainage basins to this end we have synthesized a conceptual model of water movement through a temper ate glacier from the surface to the outlet stream pro cesses that regulate the rate and distribution of water input at the glacier surface and that regulate water movement from the surface to the bed play important but commonly neglected roles in glacier hydrology where a glacier is covered by a layer of porous perme able firn the accumulation zone the flux of water to the glacier interior varies slowly because the firn tempo rarily stores water and thereby smooths out variations in the supply rate in the firn free ablation zone in con trast the flux of water into the glacier depends directly on the rate of surface melt or rainfall and therefore varies greatly in time water moves from the surface to the bed through an upward branching arborescent net work consisting of both steeply inclined conduits formed by the enlargement of intergranular veins and gently inclined conduits spawned by water flow along the bottoms of near surface fractures crevasses engla cial drainage conduits deliver water to the glacier bed at a limited number of points probably a long distance downglacier of where water enters the glacier englacial conduits supplied from the accumulation zone are quasi steady state features that convey the slowly varying water flux delivered via the firn their size adjusts so that they are usually full of water and flow is pressurized in contrast water flow in englacial conduits supplied from the ablation area is pressurized only near times of peak daily flow or during rainstorms flow is otherwise in an open channel configuration the subglacial drainage system typically consists of several elements that are distinct both morphologically and hydrologically an up glacier branching arborescent network of channels in cised into the basal ice conveys water rapidly much of the water flux to the bed probably enters directly into the arborescent channel network which covers only a small fraction of the glacier bed more extensive spatially is a nonarborescent network which commonly includes cav ities gaps between the glacier sole and bed channels incised into the bed and a layer of permeable sediment the nonarborescent network conveys water slowly and is usually poorly connected to the arborescent system the arborescent channel network largely collapses during winter but reforms in the spring as the first flush of meltwater to the bed destabilizes the cavities within the nonarborescent network the volume of water stored by a glacier varies diurnally and seasonally small temper ate alpine glaciers seem to attain a maximum seasonal water storage of 200 mm of water averaged over the area of the glacier bed with daily fluctuations of as much as 20 30 mm the likely storage capacity of subglacial cavities is insufficient to account for estimated stored water volumes so most water storage may actually occur englacially stored water may also be released abruptly and catastrophically in the form of outburst floods

transient slip events from near field seismic and geodetic data on a glacier fault whillans ice plain west antarctica 1 bidaily tidally modulated stick slip speed ups of the whillans ice plain wip provide insight into glacier dynamics and failure at a naturally repeating fault asperity we installed a network of continuously operating gps receivers in 2007 and deployed on ice broadband seismometers during the austral summer of 2008 on whillans ice stream wis west antarctica and recorded 26 glacier speed up events previous work during the 2004 field season suggested that these speed ups initiate as failure of an asperity on or near ice raft a that triggers rupture across the entire wip our results for 2008 locate the slip initiation farther to the south of this feature closer to the grounding line and the southernmost extent of the ross ice shelf the initiation may be controlled by a discontinuity in basal boundary conditions at the suture between two ice streams a strong correlation between the amplitude of seismic waves generated at the rupture front and the total slip achieved over the duration of the slip event 30 min suggests slip predictable behavior i e the ability to forecast the eventual slip based on the first minute of seismic radiation successive slip events propagate with different rupture speeds 100 300 m s that strongly correlate r 2 0 73 with the interevent duration in addition the amount of slip achieved during each event appears to be correlated with the rupture speed we use these observations to constrain basal shear stress to be 4 kpa by calculating conditions for basal freezing our observations yield information regarding mechanics and dynamics of ice streambeds at the scale of tens to hundreds of kilometers subglacial processes are notoriously difficult to constrain on these large scales which are relevant to the understanding of regional and continental ice motion

outburst flooding and the initiation of ice stream surges in response to climatic cooling a hypothesis abstract outburst flooding from subglacial reservoirs and associated surging of ice streams are caused by processes following climatic cooling that produce ice shelf grounding on proglacial sills trapping subglacial water according to an hypothesis presented here glaciers often advance into proglacial water bodies cooling may allow ice shelf formation and advance causing grounding on a proglacial sill trapping subglacial water ice shelf freeze on to the sill and development of a local reversal in ice air surface slope over the sill then are likely allowing ice thickening and water overpressurization if basal thawing then occurs as is likely an outburst flood and a surge may be triggered these processes may have been involved in heinrich events generation of antarctic subglacial lakes and meltwater scouring of some regions

changes in the west antarctic ice sheet the portion of the west antarctic ice sheet that flows into the ross sea is thinning in some places and thickening in others these changes are not caused by any current climatic change but by the combination of a delayed response to the end of the last global glacial cycle and an internal instability the near future impact of the ice sheet on global sea level is largely due to processes internal to the movement of the ice sheet and not so much to the threat of a possible greenhouse warming thus the near term future of the ice sheet is already determined however too little of the ice sheet has been surveyed to predict its overall future behavior

basal topography and ice flow in the bailey slessor region of east antarctica an airborne radio echo sounding campaign carried out in the upper reaches of bailey ice stream and slessor glacier in coats land east antarctica has revealed that tributaries of enhanced flow lie within well defined basal troughs and are separated from each other by bed highs these new data indicate significant differences in ice thickness compared with those estimated in the bedmap database a numerical modeling study has revealed that driving stresses are high enough to account for flow by ice deformation alone in intertributary areas most flow in the enhanced flow tributaries of slessor glacier may also be explained by ice deformation alone however although ice deformation is also significant in the bailey ice stream tributary a large amount of basal motion is also required to fully explain flow velocities here it is proposed that the trough in which bailey tributary lies is sufficiently deep that marine sediments may have accumulated here in preglacial times along with water produced by geothermal heating frictional heating and through a reduction in the pressure melting point basal motion may therefore be facilitated by the presence of a deformable saturated till layer at the bed

penetration of antarctic subglacial lakes by vhf electromagnetic pulses information on the depth and electrical conductivity of basal water bodies owing to the high level of absorption of very high frequency radio waves in water previous investigators of airborne radio echo sounding res data from antarctica have assumed that the depth of subglacial lakes cannot be measured directly by this method however we have identified a number of res returns from beneath the ice water interface at the surface of eight subglacial lakes that we have interpreted as being reflected from the lake floor the returns allow us to measure the depth of subglacial lakes since the velocity of radio waves in water 33 4 m s 1 is relatively unaffected by electrical conductivity attenuation of radio waves within water is controlled largely by its electrical conductivity consequently by examining the decay of the radio wave amplitude with depth we can gain information about the conductivity of subglacial water bodies our results indicate that the minimum water depths of eight subglacial lakes vary between 8 and 21 m the lakes from which our depth measurements were taken are distributed widely around the ice sheet thus it may be concluded for the first time that antarctic subglacial water bodies are generally at least several meters in depth by examining the attenuation of radio waves through subglacial water the electrical conductivity of the water is estimated to be extremely low i e fresh pure water

stagnation of ice stream c west antarctica by water piracy the dynamic behavior of the west antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea level rise attention is focused on the fast moving and rapidly responding ice streams that drain the ice sheet one of these ice stream c largely stopped about a century ago and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice sheet here new data are presented indicating that the slowdown of the ice stream is restricted to its lower part and occurred because of loss of lubrication on localized sticky spots at the bed of the ice stream the increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream b together with slow drawdown of the ice sheet rather than from any general stabilizing feedbacks

longitudinal variations in glacial flow theory and test using data from the byrd station strain network antarctica application de la theorie de budd 1970 des effets du lit sur les variations d ecoulement du glacier aux observations de la calotte de glace a la station byrd on utilise une rheologie visqueuse lineaire et on resout une equation biharmonique des variations des efforts et des vitesses de deformation les conditions aux limites comportent une topographie du fond avec ondulations glissements et cisaillements

abrupt climatic change related to unstable ice sheet dynamics toward a new paradigm ice streams are fast currents of ice that drain some 90 of present day ice sheets and probably drained a similar percent of ice from former ice sheets ice streams may have life cycles akin to but much longer than surge cycles of certain mountain glaciers the floating tongues of marine ice streams coalesce to form floating ice shelves that buttress marine ice sheets and the grounded lobes of terrestrial ice streams stabilize terrestrial ice sheets ice shelves and ice lobes form during the life cycles of ice streams and disintegrate rapidly to initiate new life cycles successive life cycles continue until the ice sheet collapses a new paradigm is postulated in which abrupt climatic change is driven by iceberg outbursts associated with life cycles of marine ice streams and dust storms associated with life cycles of terrestrial ice streams for pleistocene northern hemisphere ice sheets these mechanisms regulated north atlantic surface temperatures and salinity and perhaps planetary albedo and atmospheric co2 and thereby drove climatic change which tended to be abrupt because ice stream life cycles began and ended abruptly ice streams proliferated along the margins of northern hemisphere ice sheets after they advanced beyond the crystalline shields of north america and eurasia and spread over softer sedimentary rocks overlain by thick ice cemented sediments that progressively thawed especially along river valleys and marine troughs an increasing fraction of these ice sheets was then subjected to the unstable life cycles of ice streams as seen in the pronounced spikiness of the late glacial oxygen isotope stratigraphy in the greenland ice core at dye 3 life cycles of greenland ice streams during the holocene seem to be linked to episodes of regional climatic cooling most recently the little ice age when superimposed on the present milankovitch hemicycle of reduced insolation over the north atlantic this could abruptly initiate a new worldwide glaciation cycle in the aftermath of present day co2 greenhouse warming proliferation of antarctic ice stream life cycles during the period of greenhouse warming may also trigger global climatic change by forcing a reorganization of circumpolar thermohaline circulation in the southern ocean

antarctica measuring glacier velocity from satellite images many landsat images of antarctica show distinctive flow and crevasse features in the floating part of ice streams and outlet glaciers immediately below their grounding zones some of the features which move with the glacier or ice stream remain visible over many years and thus allow time lapse measurements of ice velocities measurements taken from landsat images of features on byrd glacier agree well with detailed ground and aerial observations the satellite image technique thus offers a rapid and cost effective method of obtaining average velocities to a first order of accuracy of many ice streams and outlet glaciers near their termini