ice flow patterns and glacial transport in the eastern hudson bay region implications for the late quaternary dynamics of the laurentide ice sheet recent field surveys in the eastern hudson bay region have led to the discovery of regional ice flow sequences that require a significant reassessment of the late quaternary dynamics of the laurentide ice sheet two regional ice flow phases can be recognized from till compositional data and from crosscutting relationships observed on striated bedrock surfaces the oldest is directed toward the northwest and north northwest while the youngest is directed toward the west and includes a late glacial deflection toward the southwest the wide regional distribution of striae formed during the early northwestward glacial movement together with the recognition of palimpsest glacial dispersal trains associated with this phase suggest that it was a long lived time transgressive regional event the ensuing glacial movement is a regionally dominant westward ice flow phase during which several large glacial dispersal trains were formed downglacier from distinctive bedrock sources the largest of these trains extends

lunar global shape and polar topography derived from kaguya lalt laser altimetry a global lunar topographic map with a spatial resolution of finer than 0 5 degree has been derived using data from the laser altimeter lalt on board the japanese lunar explorer selenological and engineering explorer selene or kaguya in comparison with the previous unified lunar control network ulcn 2005 model the new map reveals unbiased lunar topography for scales finer than a few hundred kilometers spherical harmonic analysis of global topographic data for the moon earth mars and venus suggests that isostatic compensation is the prevailing lithospheric support mechanism at large scales however simple rigid support is suggested to dominate for the moon venus and mars for smaller scales which may indicate a drier lithosphere than on earth especially for the moon and venus

estimates of primary ejecta and local material for the orientale basin implications for the formation and ballistic sedimentation of multi ring basins abstract a clear understanding of thickness distributions of primary ejecta and local material is critical to interpreting the process of ballistic sedimentation provenances of lunar samples the evolution of the lunar surface and the origin of multi ring basins the youngest lunar multi ring basin orientale provides the best preserved structure for determining the thicknesses of primary ejecta and local material in general the primary ejecta thickness was often estimated using crater morphometry however previous methods ignored either crater erosion the crater interior geometry or both in addition ejecta deposits were taken as mostly primary ejecta and as far as we know the local material thickness had not been determined for the orientale in this work we proposed a model based on matching measurements of partially filled pre orientale craters pfpocs with the simulations of crater erosion to determine their thicknesses we provided estimates of primary ejecta thickness distribution with the thickness of 0 85 km at cordillera ring and a decay power law exponent of b 2 8 the transient crater radius of 200 km excavation volume of 2 3 10 6 km 3 primary ejecta volume of 2 8 10 6 km 3 these results suggest that previous works e g fassett et al 2011 moore et al 1974 might overestimate the primary ejecta thicknesses of orientale and the primary ejecta thickness model of pike 1974a for multi ring basins may give better estimates than the widely cited model of mcgetchin et al 1973 and the scaling law for impacts into ottawa sand housen et al 1983 structural uplift decays slower than previously thought and rim relief is mostly rim uplift for orientale the main reason for rim uplift may be the fracturing and squeezing upward of the surrounding rocks the proportion of local material to ejecta deposits increases with increasing radial distance from basin center and the thickness of local material is larger than that of primary ejecta at distance larger than certain distance 1 5 basin radius for orientale these results suggest ballistic sedimentation is important for multi ring basins and ejecta deposits can t be considered as mostly primary ejecta everywhere

the succession of ice flow patterns in north central quebec labrador canada abstract quebec labrador canada is one of the postulated dispersal centres of the late wisconsinan laurentide ice sheet and also contains an unusually complex and rich glacial geological record a reliable reconstruction of glaciation history represented by this record would improve our comprehension of the dynamics of the eastern sector of the laurentide ice sheet such as the evolution of the basal thermal regime and the timing of enhanced ice flow into ungava bay we mapped glacial landforms over a 200 000 km2 area using aerial photographs and conducted a detailed field based survey of the central part of the area to establish the relative age of crosscutting glacial geological features the position of the last ice remnant was south of ungava bay well north of what has commonly been perceived as the location for this dispersal centre southward radial flow from a northward migrating and predominantly cold based dispersal centre was the last glacial event in north central quebec labrador a widespread occurrence of older ice flow traces towards the southwest was detected in some areas large drumlins formed during this ice flow event occur in alignment with landforms formed during the last deglaciation this older southwestward flow is however overprinted by and therefore predates traces of northward convergent flow into ungava bay ungava bay flow traces of a northeastward flow event and in some areas traces of the deglacial flow event flutes and glacial striae indicating ice flow towards northeast are superimposed on the ungava bay flow the northeastward ice flow system has been recognised across the mapped area and because of its direction and regional significance we infer formation approximately during the last glacial maximum

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr

40ar 39ar and pb pb study of individual hornblende and feldspar grains from southeastern baffin island glacial sediments implications for the provenance of the heinrich layers lead isotope analysis of individual feldspar grains and 40ar 39ar analysis of individual hornblende grains from three samples taken from baffin island ice proximal till deposits are consistent with a provenance recording a two stage geologic history including formation of continental crust in the late archean and partial isotopic resetting and or reequilibration during major paleoproterozoic regional metamorphism and magmatism significantly although the populations are small and the two stage archean paleoproterozoic history of their sources are similar the pb isotope compositions of samples from baffin island do not match the results from heinrich layer h2 in core hu87 033 009 from northeast of the hudson strait in the labrador sea specifically in h2 samples from hu87 033 009 the 207pb 204pb is high for a given value of 206pb 204pb additionally the range in 206pb 204pb is greater in hu87 033 009 samples and there are two distinct trajectories of 208pb 204pb the steeper of the 208pb 204pb versus 206pb 204pb trends is not seen in the studied baffin island glacial sediment samples samples of feldspar grains from heinrich layers h1 h2 h4 and h5 from core v28 82 in the eastern north atlantic have a 207pb 204pb versus 206pb 204pb relation similar to that of baffin island till samples but there is also a high 208pb 204pb that we have not captured in this suite of glacial sediment samples our results highlight the strength of a combined radiogenic isotope approach to characterizing the geological history of sediment sources and provide a first step in assessing the variability in isotope character in this region of dynamic glacial events during the late quaternary period

the transition from complex craters to multi ring basins on the moon quantitative geometric properties from lunar reconnaissance orbiter lunar orbiter laser altimeter lola data united states national aeronautics and space administration nasa lunar reconnaissance orbiter lunar orbiter laser altimeter lola experiment nnx09am54g

regional and local ice flow history in the vicinity of the izok lake zn cu pb ag deposit nunavut ice directional indicators were compiled from detailed field mapping at the izok lake volcanogenic massive sulphide deposit in the point lake region of western nunavut and eastern northwest territories cross cutting erosional relationships and depositional landforms indicate that the izok lake area was affected by four ice flow phases it is this new glacial history interpretation that is used to explain the gahnite indicator mineral dispersal train down ice of the izok lake deposit as being the net effect of all ice flow phases its fan shaped morphology is a function of the duration and intensity of two dominant glacial trajectories field based mapping of all glacial flow indicators are essential and must be properly interpreted to detect palimpsest dispersal trains the resultant dispersal fan serves as a model for future exploration in the glaciated terrain of the north central part of the slave province

marine episodes in hudson strait and hudson bay canada during the wisconsin glaciation abstract twelve reworked shell fragments were recovered from five till samples collected on nottingham island in western hudson strait canada total hydrolysate amino acid ratios d alloisoleucine l isoleucine for each fragment range from 0 043 to 0 154 and are significantly higher than ratios for radiocarbon dated holocene shells which average 0 022 one shell fragment with an aile ile ratio of 0 065 yielded an ams radiocarbon age of 44 200 2300 yr b p aa 3254 absolute age estimates based on the amino acid ratios are constrained by reasonable limits on the estimated effective diagenetic temperature of the fossils on this basis the shell fragments are correlated with oxygen isotope substages 5e and 5a and stage 3 in the preferred hypothesis theses results independently support hypotheses calling for at least partial deglaciation and marine incursion in hudson strait and hudson bay at these times

volumetric analysis of complex lunar craters implications for basin ring formation the transition in hypervelocity impact structures from complex craters to basins is defined by a transition in uplifted central features from peaks to peak rings volumetric and morphologic analyses of the central peaks in complex lunar craters are combined with subsurface data from terrestrial complex impact structures in an effort to characterize the crater to basin transition central peak volumes in lunar craters with rim diameters drc 80 km central peak volume is relatively reduced and a second power law relationship is defined the relative reduction in peak volume in craters with drc 80 km is associated with a relative decrease in peak height and is accompanied by the development of a concentric zone of enhanced floor roughening studies of terrestrial complex impact structures suggest that the amount of uplifted material as judged from its depth of origin continues to increase with increasing rim diameter this suggests that a redistribution of uplifted material away from a centralized peak may occur in the larger craters the volumetric and morphologic changes described occur over a rim diameter range of 51 80 km far below the previously proposed range for the crater to basin transition drc 140 175 km evidence is presented to support a crater to basin transition beginning at drc 51 80 km characterized by a relative reduction in central peak volume and the development of rings of floor roughening which are suggested to be precursors of peak ring development

stratigraphy paleoecology and glacial history of the gillam area manitoba sections along the nelson river in northern manitoba outcropping upstream and downstream from limestone dam record a long succession of late quaternary events the oldest sediment exposed consists of sandy nonfossiliferous sundance till of northwestern provenance and related to a kansan or illinoian glaciation the paleosol developed in the sundance till is assigned to the yarmouthian or sangamon interglacial on its stratigraphic position and depth of weathering fossiliferous clayey amery till of eastern provenance overlies the sundance till and underlies the nonglacial nelson river sediments aspartic acid d l ratios of wood fragments from the nelson river sediments correlate with an aspartic acid d l ratio of similar wood from the missinaibi formation in ontario beetle analysis indicates the nelson river sediments were deposited north of the tree line under conditions more severe than those found in the area today the deposits are believed to be of latest sangamon or possibly mid wisconsinan age

investigating the transition from central peak to peak ring basins using central feature volume measurements from the global lunar dtm 100 m 1 several theories have been suggested to explain the transition from peak to peak ring crater morphology in order to explore the transition and assess the currently advocated peak ring formation theories we have collected measurements of central feature volumes and heights for relatively fresh lunar impact craters we employed the global lunar dtm 100 m which has the vertical precision and spatial coverage necessary to accurately measure peak and peak ring volumes in more craters than previously possible the similarity in both trend and magnitude of peak and peak ring volumes suggests that peak ring formation is closely related to the development of central peaks as crater size increases our data thus lends support to those peak ring formation theories involving peak collapse

refining the ice flow chronology and subglacial dynamics across the migrating labrador divide of the laurentide ice sheet with age constraints on deglaciation geological survey of canada s geo mapping for energy and minerals ii gem2 program 2013 2020

a review and analysis of varve thickness records from glacial lake ojibway ontario and quebec canada abstract varve thickness records from glacial lake ojibway provide a detailed picture of ice margin dynamics and paleohydrology for the laurentide ice sheet during the early holocene the demise of lake ojibway occurred when an ice dam of the shrinking laurentide ice sheet failed perhaps releasing enough freshwater to impede atlantic meridional overturning circulation and precipitate the 8200 cal bp climatic cooling event previously published records and new information from lacustrine sediment cores are combined to reconstruct a varve record over 2100 years long between varves numbered 1 to 1500 average rates of ice margin retreat increased from 140 m y to as high as 400 m y beginning prior to varve year 1800 and lasting to around year 2000 ice readvanced along a front at least 300 km wide during an episode called the cochrane the total distance of the advance was at least 75 km and averaged around 200 m y during the advance a series of 50 anomalously thick varves were deposited the cause of which is uncertain a widespread unconformity occurred during the end of the cochrane advance which was followed at around varve year 2060 by the return of varves which thicken abruptly unlike previous varves these varves include ice rafted pellets of lake ojibway sediment these varves may have resulted from rapid melting and calving rather than a second advance and may relate to basin re filling following a lowstand there are no direct dates on the lake ojibway varves but a sudden increase in varve thickness at year 1528 is a regional stratigraphic marker that may have resulted from glacial sediment and meltwater including lake agassiz overflow diverted from routes to the lake superior basin into the lake ojibway basin dates from the lake superior basin suggest that the diversion of this water occurred at around 9040 cal bp applying this date to varve 1528 constrains the age of the lake ojibway varves numbered 1 to 2100 to between 10 570 and 8470 200 cal bp

dynamics of the laurentide ice sheet in hudson bay canada abstract interpretation of sidescan sonar and high resolution sub bottom seismic data from hudson bay has led to the development of a model for final disintegration of the laurentide ice sheet the well preserved geomorphic features observed from the bay floor are similar to subaerially exposed glaciogenic features observed around the perimeter of hudson bay and can be related to the following three paleoenvironments subglacial erosion deposition ice margin deposition and reworking and post glacial reworking and burial subglacial features observed include fluted terrains with superimposed rogen moraines relief attributed to ice surging eskers subglacial channels and dead ice topography de geer moraines and recessional moraine complexes indicate the mechanism and orientation of ice breakup retreat the orientations of these features are locally consistent with regional trends indicating the overall direction of ice sheet flow and retreat during the last phase of the laurentide ice sheet subglacial channel complexes incised into the till and bedrock and high energy bedforms deposited on the till surface occur in present day water depths down to 160 m due to isostatic depression these could have been as much as 315 m deeper at the time of formation they indicate a wet based glacier with high energy subglacial drainage during the late phases of ice retreat the sidescan sonar data also reveal numerous zones of distinctive morphology attributed to ice breakup and post glacial processes cross cutting relationships allow the determination of the relative ages of these zones from oldest to youngest these zones are interpreted to represent scouring of the bedrock and overburden by the remnants of the ice sheet seafloor scouring by tabular icebergs which moved in a tidally induced arcuate pattern or under the influence of massive glacial lake outflows development of dead ice topography under residual ice masses and random iceberg scouring by smaller icebergs restricted to present day water depths of less than 180 m recently published reconstructions of the collapse of the laurentide ice sheet based on dated quaternary sections on land suggest that the bay was rapidly deglaciated in approximately 400 years the well preserved and distinctive glaciogenic and stratigraphic indicators observed from the bay floor suggest that they were formed by a succession of mechanisms which were too short lived to obliterate the underlying evidence of deglaciation this supports the interpretation of extremely rapid deglaciation of hudson bay and points out that subsequent marine processes have not been sufficient to obliterate the glacial features

early holocene deglaciation of the eastern coast of hudson bay abstract the final stage of deglaciation of hudson bay was a major holocene catastrophic event marked by the drainage of lake agassiz ojibway at 8 47 ka cal bp and the rapid collapse of the laurentide ice sheet previous work undertaken in the nastapoka river area eastern hudson bay demonstrated that during the relative sea level highstand that shortly followed the drainage of the lake the western margin of the quebec labrador ice sector rapidly retreated eastward to reach a stillstand position in a coastal hill range in this study an analysis of landsat 7tm images has allowed a mapping of large scale glacial landforms outwash deposits eskers flutings and de geer and rogen moraines between kuujjuaraapik se hudson bay and puvirnituq ne hudson bay the key results from this mapping are i ice contact outwash deposits mapped along the entire arc shaped coastline of the eastern hudson bay outline a major ice stillstand phase in the coastal hills that extended at least from kuujjuaraapik to inukjuak the presence of these hills allowed a stabilisation of the ice margin that led to the accumulation of thick and extensive ice contact submarine fans ii the position of these deposits on the down ice side west of large sets of flutings indicates an important phase of sediment delivery by a rapid ice flow phase toward a marine based ice margin iii a second system of outwash deposits observed farther inland indicates a subsequent phase of stabilisation of the ice margin during its retreat toward central quebec labrador

the subglacial mosaic of the laurentide ice sheet a study of the interior region of southwestern hudson bay abstract reconstructions of past ice flow provide useful insights into the long term behaviour of past ice sheets and help to understand how glaciated landscapes are shaped here we present reconstruction of a 10 phase ice flow history from southwestern hudson bay in northeastern manitoba canada a dynamic region situated between two major ice dispersal centres of the laurentide ice sheet we utilize a diverse geologic dataset including 1900 field based erosional indicators 12 streamlined landform flowsets esker and meltwater corridor orientations 103 till fabrics analyses and 1344 till clast lithology counts our reconstruction suggests that both pre mis 2 and mis 2 glaciations followed similar growth patterns where ice advanced into study area from ice centered to the east probably in northern quebec followed by a switch in ice flow direction indicating flow from the keewatin ice centre to the northwest and north the cause for this switch in ice flow orientation is uncertain but the youngest switch may relate to retreat of ice during mis 3 that then left space for keewatin sourced ice to advance over the study area while modelling experiments indicate widespread cold based conditions in the study area during the last glacial cycle uniformly relict landscapes are not common instead the glaciated landscape is palimpsest and commonly fragmented forming a subglacial bed mosaic of erosional and depositional assemblages that record both shifting ice flow direction through time and shifting subglacial conditions each assemblage formed or modified during times of dynamic warm based ice and later preserved under conditions below or close to the pressure melting point slow and sluggish or cold based

sedimentary record of ice divide migration and ice streams in the keewatin core region of the laurentide ice sheet abstract the aberdeen lake region of central mainland nunavut is a former core region of the laurentide ice sheet that is characterized by streamlined glacial landforms classified into multiple crosscutting flow sets and near continuous till blanket the presence of widespread till near the centre of the keewatin ice dome raises questions about its origin detailed drillcore logging revealed a complex stratigraphy consisting of at least 6 till units variably preserved across the study area till provenance analysis indicates deposition by near opposite trending ice flow phases interpreted as evidence of reconfiguration of the keewatin ice divide at the surface large north northwesterly aligned landforms are present across the study area the till stratigraphy within these landforms indicates the same nnw ice flow phase is responsible for considerable till production this ice flow phase is also correlated to a long regional dispersal train of erratics toward the gulf of boothia the production of an extensive thick 12 m till sheet during the nnw trending ice flow phase occurred far from the ice margin at a time of extensive ice cover of mainland nunavut likely from an east west oriented ice divide a deglacial westerly trending ice flow phase formed small drumlins atop the larger nnw streamlined till ridges and deposited a surficial till unit that is too thin to mask the nnw flow set across the study area it is proposed that the boothia paleo ice stream catchment area propagated deep into the laurentide ice sheet and contributed to significant till production in this core region of the keewatin sector prior to the westerly ice flow shift the apparent relationship between till thickness and the size of the associated or correlated drumlins flow sets and dispersal trains indicates complex erosion deposition interplay is involved in the formation of streamlined subglacial landforms

glacial and nonglacial events in the eastern james bay lowlands canada marine and continental records indicate that the laurentide ice sheet lis was highly dynamic throughout the last glacial cycle with significant reorganizations of its ice divide system the extensive depositional record of the hudson bay lowlands hbl is particularly important for the study of ice sheet dynamics because this region was once located near two important ice domes of the lis however important stratigraphic issues remain in the hbl notably regarding the chronology of nonglacial units and the direction and extent of the ice flow events recognized here we focus on the northeastern sector of the lis by documenting the provenance of tills and dating nonglacial deposits in sedimentary sequences east of james bay our investigations indicate that the regional stratigraphy comprises at least five distinct tills and an important unit of glaciolacustrine rhythmites this glacial sequence lies on massive lacustrine clay and fluvial sand containing abundant organic debris and wood fragments the

tectonics of complex crater formation as revealed by the haughton impact structure devon island canadian high arctic the results of a systematic field mapping campaign at the haughton impact structure have revealed new information about the tectonic evolution of mid size complex impact structures these studies reveal that several structures are generated during the initial compressive outward directed growth of the transient cavity during the excavation stage of crater formation 1 sub vertical radial faults and fractures 2 sub horizontal bedding parallel detachment faults and 3 minor concentric faults and fractures uplift of the transient cavity floor toward the end of the excavation stage produces a central uplift compressional inward directed deformation results in the duplication of strata along thrust faults and folds it is notable that haughton lacks a central topographic peak or peak ring the gravitational collapse of transient cavity walls involves the complex interaction of a series of interconnected radial and concentric faults while the outermost concentric faults dip in toward the crater center the majority of the innermost faults at haughton dip away from the center complex interactions between an outward directed collapsing central uplift and inward collapsing crater walls during the final stages of crater modification resulted in a structural ring of uplifted intensely faulted sub vertical and or overturned strata at a radial distance from the crater center of 5 0 6 5 km converging flow during the collapse of transient cavity walls was accommodated by the formation of several structures 1 sub vertical radial faults and folds 2 positive flower structures and chaotically brecciated ridges 3 rollover anticlines in the hanging walls of major listric faults and 4 antithetic faults and crestal collapse grabens oblique strike slip i e centripetal movement along concentric faults also accommodated strain during the final stages of readjustment during the crater modification stage it is clear that deformation during collapse of the transient cavity walls at haughton was brittle and localized along discrete fault planes separating kilometer size blocks

multiple deglaciations of the hudson bay lowlands canada since deposition of the missinaibi last integlacial formation abstract the stratigraphic record in the james and hudson bay lowlands indicates that the sequence of glacial events at the geographical center of the 12 6 10 6 km 2 laurentide ice sheet may have been more complex than hitherto imagined isoleucine epimerization ratios of in situ and transported shells recovered from till and associated marine and fluvial sediments cluster into at least 4 discrete groups two alternative explanations of the data are offered of which we strongly favor the first hypothesis 1 setting the age of the last interglacial marine incursion the bell sea at 130 000 yr b p results in a long term average diagenetic temperature for the lowlands of 0 6 c using this temperature enables us to predict the age of shells intermediate in age between the last interglaciation and the incursion of the tyrrell sea 8000 yr ago between these two interglacial marine inundations hudson bay is predicted to have been free of ice along its southern shore about 35 000 75 000 and 105 000 yr ago based on amino acid ratios from shells occurring as erratics in several superimposed tills and fluvial sediments these results suggest 1 that traditional concepts of ice sheet build up and decay must be reexamined 2 that high sea levels may have occurred during the wisconsin glaciation and 3 that a critical reappraisal is required of the open ocean 18 o record as a simple indicator of global ice volume an alternative hypothesis 2 is also examined it is based on the assumption that the 35 000 yr old deposits calculated on the basis of hypothesis 1 date from the last interglaciation this in effect indicates that the missinaibi formation commonly accepted as sediments of the last interglaciation are about 500 000 yr old and that the effective diagenetic temperature in the lowlands during approximately the last 130 000 yr has been close to 6 c we argue for rejection of this alternative hypothesis

southeastward ice flow in central quebec and its paleogeographic significance directional glacial erosional marks located systematically on the southwest side of rock outcrops in the chibougamau and lac mistassini areas in west central quebec indicate unequivocally a sustained regional event of southeast ice flow prior to the last or regional southwest flow from new quebec striations from the former ice flow are consistently oriented at around 120 azimuth and are observed albeit at scattered occurrences within an area covering thousands of square kilometres although the age of the southeast ice flow event cannot be determined it is considered more likely to be of wisconsinan age on the basis of the well preserved state of its features and the lack of deep weathering on surfaces where these are observed it is suggested that the early ice flow event might have been fed by a local outflow centre on the east side of james bay perhaps extending as a ridge east of hudson bay

sakami moraine quebec a 500 km long moraine without climatic control the laurentide ice sheet in eastern canada disintegrated step by step as evidenced by several morainic complexes although commonly interpreted as reflecting climatic events it seems probable that the disintegration simply related to changes in the dynamics of the ice margin without climatic control from the example of the late glacial sakami moraine in quebec formed during the drainage of lake ojibway into the tyrrell sea the concept of a re equilibration moraine already briefly proposed is developed the construction of such a moraine implies a stabilization of the ice front when the glacier margin previously floating or calving in a lacustrine or marine basin is suddenly grounded because of topographic blocking or abrupt drop of water level due to drainage the glacial retreat then pauses until the equilibrium profile of the ice is restored and a morainic accumulation can form during the halt

ice streams in the laurentide ice sheet identification characteristics and comparison to modern ice sheets this paper presents a comprehensive review and synthesis of ice streams in the laurentide ice sheet lis based on a new mapping inventory that includes previously hypothesised ice streams and includes a concerted effort to search for others from across the entire ice sheet bed the inventory includes 117 ice streams which have been identified based on a variety of evidence including their bedform imprint large scale geomorphology topography till properties and ice rafted debris in ocean sediment records despite uncertainty in identifying ice streams in hard bedrock areas it is unlikely that any major ice streams have been missed during the last glacial maximum laurentide ice streams formed a drainage pattern that bears close resemblance to the present day velocity patterns in modern ice sheets large ice streams had extensive onset zones and were fed by multiple tributaries and where ice drained through regions of high relief the spacing of ice streams shows a degree of spatial self organisation which has hitherto not been recognised topography exerted a primary control on the location of ice streams but there were large areas along the western and southern margin of the ice sheet where the bed was composed of weaker sedimentary bedrock and where networks of ice streams switched direction repeatedly and probably over short time scales as the ice sheet retreated onto its low relief interior several ice streams show no correspondence with topography or underlying geology perhaps facilitated by localised build up of pressurised subglacial meltwater they differed from most other ice stream tracks in having much lower length to width ratios and have no modern analogues there have been very few attempts to date the initiation and cessation of ice streams but it is clear that ice streams switched on and off during deglaciation rather than maintaining the same trajectory as the ice margin retreated we provide a first order estimate of changes in ice stream activity during deglaciation and show that around 30 of the margin was drained by ice streams at the lgm similar to that for present day antarctic ice sheets but this decreases to 15 and 12 at 12 cal ka bp and 10 cal ka bp respectively the extent to which these changes in the ice stream drainage network represent a simple and predictable readjustment to a changing mass balance driven by climate or internal ice dynamical feedbacks unrelated to climate or both is largely unknown and represents a key area for future work to address

kinematic evolution of thrust related structures in the umbro romagnan parautochthon northern apennines italy in the internal part of the umbro marchean romagnan apennines the foredeep clastic wedge constituting the neogene part of the sedimentary cover is completely detached from the underlying mesozoic palaeogene succession the resulting umbro romagnan parautochthon consists of tectonostratigraphic units with a general geometry of broad synclinal blocks separated by narrow faulted anticlines thrust related structures observed in the field require thrust ramp propagation to have occurred within already folded rocks therefore they cannot be restored using simple fault bend fold or fault propagation folding models evidence for a passive fold origin in the studied rocks suggests that an early detachment folding episode preceded ramp propagation the latter was facilitated by the enhanced thickness of incompetent material in the cores of detachment anticlines which became the preferential sites where thrust ramps cut up section depending on the trajectory of such thrust ramps different types of fault related structures could develop hanging wall anticlines which give way to monoclinal structures higher up in the section are associated with listric thrust ramps whereas hanging wall monoclines approximately parallel to the underlying fault surface are associated with straight trajectory ramps this kinematic evolution which occurred partly during syn depositional compression also accounts for the observed lithofacies distribution the latter reflects an early differentiation of the foredeep trough into sub basins that are progressively younger towards the foreland the detachment anticlines that originally bounded such sub basins were the site of later thrust propagation leading to a tectonic juxtaposition of different tectonostratigraphic units consisting of broad nw se elongate synclinal blocks

topography of the moon from the clementine lidar range measurements from the lidar instrument carried aboard the clementine spacecraft have been used to produce an accurate global topographic model of the moon this paper discusses the function of the lidar the acquisition processing and filtering of observations to produce a global topographic model and the determination of parameters that define the fundamental shape of the moon our topographic model a 72nd degree and order spherical harmonic expansion of lunar radii is designated goddard lunar topography model 2 gltm 2 this topographic field has an absolute vertical accuracy of approximately 100 m and a spatial resolution of 2 5 the field shows that the moon can be described as a sphere with maximum positive and negative deviations of 8 km both occurring on the farside in the areas of the korolev and south pole aitken s p aitken basins the amplitude spectrum of the topography shows more power at longer wavelengths as compared to previous models owing to more complete sampling of the surface particularly the farside a comparison of elevations derived from the clementine lidar to control point elevations from the apollo laser altimeters indicates that measured relative topographic heights generally agree to within 200 m over the maria while the major axis of the lunar gravity field is aligned in the earth moon direction the major axis of topography is displaced from this line by approximately 10 to the east and intersects the farside 24 north of the equator the magnitude of impact basin topography is greater than the lunar flattening 2 km and equatorial ellipticity 800 m which imposes a significant challenge to interpreting the lunar figure the floors of mare basins are shown to lie close to an equipotential surface while the floors of unflooded large basins except for s p aitken lie above this equipotential the radii of basin floors are thus consistent with a hydrostatic mechanism for the absence of significant farside maria except for s p aitken whose depth and lack of mare require significant internal compositional and or thermal heterogeneity a macroscale surface roughness map shows that roughness at length scales of 10 1 10 2 km correlates with elevation and surface age

provenance of heinrich layers in core v28 82 northeastern atlantic 40ar 39ar ages of ice rafted hornblende pb isotopes in feldspar grains and nd sr pb isotopes in the fine sediment fraction abstract several correlatable layers of sediment rich in ice rafted grains have been documented in the north atlantic the most notable within the last glacial cycle are the heinrich layers layers extremely rich in ice rafted detritus and generally barren of foraminifera within the north atlantic ice rafted detritus ird belt the view of these layers is that they represent events where great armadas of icebergs were launched into the north atlantic the importance of the heinrich layers lies in their connection with abrupt climate change in the north atlantic and perhaps globally there is a growing number of published provenance studies of the heinrich layers in the north atlantic based on a variety of methods however there is little overlap of methods applied to the same samples in this contribution we present a multi component provenance study of heinrich layers h1 h2 h4 and h5 from core v28 82 in the eastern north atlantic our results indicate that virtually the entire inventory of terrigenous clastic detritus in heinrich layers h2 h4 and h5 came from ancient continental sources surrounding the labrador sea although heinrich layer h1 is similar in many respects it appears to have some significant differences relative to the other three

topographic characterization of lunar complex craters 1 we use lunar orbiter laser altimeter topography data to revisit the depth d diameter d and central peak height hcp diameter relationships for fresh complex lunar craters we assembled a data set of young craters with d 15 km and ensured the craters were unmodified and fresh using lunar reconnaissance orbiter wide angle camera images we used lunar orbiter laser altimeter gridded data to determine the rim to floor crater depths as well as the height of the central peak above the crater floor we established power law d d and hcp d relationships for complex craters on mare and highlands terrain our results indicate that craters on highland terrain are on average deeper and have higher central peaks than craters on mare terrain furthermore we find that the crater depths for both mare and highlands craters are significantly deeper than previously reported this likely reflects the inclusion of transitional craters and or older and or modified craters in previous work as well as the limitations of the stereophotogrammetric and shadow length data sets used in those studies there is substantial variability in the depths and the central peak heights for craters in a given diameter range we suggest that the differences in mean d and hcp as a function of crater diameter for highlands and mare craters result from differences in bulk physical properties of the terrain types whereas the variability in d and hcp at a given diameter may reflect variations in impactor properties and impact parameters

reconciling records of ice streaming and ice margin retreat to produce a palaeogeographic reconstruction of the deglaciation of the laurentide ice sheet abstract this paper reconstructs the deglaciation of the laurentide ice sheet lis including the innuitian ice sheet from the last glacial maximum lgm with a particular focus on the spatial and temporal variations in ice streaming and the associated changes in flow patterns and ice divides we build on a recent inventory of laurentide ice streams and use an existing ice margin chronology to produce the first detailed transient reconstruction of the ice stream drainage network in the lis which we depict in a series of palaeogeographic maps results show that the drainage network at the lgm was similar to modern day antarctica the majority of the ice streams were marine terminating and topographically controlled and many of these continued to function late into the deglaciation until the ice sheet lost its marine margin ice streams with a terrestrial ice margin in the west and south were more transient and ice flow directions changed with the build up peak phase and collapse of the cordilleran laurentide ice saddle the south eastern marine margin in atlantic canada started to retreat relatively early and some of the ice streams in this region switched off at or shortly after the lgm in contrast the ice streams draining towards the north western and north eastern marine margins in the beaufort sea and in baffin bay appear to have remained stable throughout most of the late glacial and some of them continued to function until after the younger dryas yd the yd influenced the dynamics of the deglaciation but there remains uncertainty about the response of the ice sheet in several sectors we tentatively ascribe the switching on of some major ice streams during this period e g m clintock channel ice stream at the north west margin but for other large ice streams whose timing partially overlaps with the yd the drivers are less clear and ice dynamical processes rather than effects of climate and surface mass balance are viewed as more likely drivers retreat rates markedly increased after the yd and the ice sheet became limited to the canadian shield this hard bed substrate brought a change in the character of ice streaming which became less frequent but generated much broader terrestrial ice streams the final collapse of the ice sheet saw a series of small ephemeral ice streams that resulted from the rapidly changing ice sheet geometry in and around hudson bay our reconstruction indicates that the lis underwent a transition from a topographically controlled ice drainage network at the lgm to an ice drainage network characterised by less frequent broad ice streams during the later stages of deglaciation these deglacial ice streams are mostly interpreted as a reaction to localised ice dynamical forcing flotation and calving of the ice front in glacial lakes and transgressing sea basal de coupling due to large amount of meltwater reaching the bed debuttressing due to rapid changes in ice sheet geometry rather than as conveyors of excess mass from the accumulation area of the ice sheet at an ice sheet scale the ice stream drainage network became less widespread and less efficient with the decreasing size of the deglaciating ice sheet the final elimination of which was mostly driven by surface melt

measurement and analysis of lunar basin depths from clementine altimetry altimetric profiles from the clementine lidar are used to calculate the depths of 29 large craters and basins on the moon plotting the depths of the best preserved structures together with values for simple and complex craters measured in pre clementine studies reveals an inflection in the depth diameter d d curve in addition to the one revealed by pre clementine data this inflection occurs in the diameter range that corresponds to the morphologic transition from complex crater to basin the best empirical power law fit for basin depths is log10 d 0 41 log10 d 0 57 this relationship is characterized by a lower slope than that for complex craters demonstrating that this morphologic transition corresponds to a further decrease in the depth of an impact structure relative to its diameter with increasing size qualitative consideration of possible causes for the second inflection leads to the conclusion that it is most likely a consequence of a short term modification mechanism that influences fundamental crater morphology such as the increasing influence of gravity with diameter thicknesses of maria in the major basins are calculated by assuming that their unfilled depths would follow thed drelation results are compared with previous estimates and yield thicknesses that are generally greater than those determined by studies of flooded craters and less than those obtained from analysis of gravity

sediment sources of northern quebec and labrador glacial deposits and the northeastern sector of the laurentide ice sheet during ice rafting events of the last glacial cycle provenance studies of anomalously high flux layers of ice rafted detritus ird in north atlantic sedi ments of the last glacial cycle show evidence for massive iceberg discharges coming from the hudson strait region of the laurentide ice sheet lis although these so called heinrich events h events are commonly thought to be associated with abrupt drawdown of the lis interior uncertainties remain regarding the sector s of this multi domed ice sheet that conveyed ice through hudson strait in northern quebec and labrador nql large scale patterns of glacial lineations indicate massive ice flows towards ungava bay and hudson strait that could reflect the participation of the labrador quebec ice dome in h events here we evaluate this hypothesis by constraining the source of nql glacial deposits which provide an estimate of the provenance characteristics of ird originating from this sector specifically we use 40 ar 39 ar ages of detrital hornblende grains in 25 till samples distributed along a latitudinal transect lat 58 extending east and west of ungava bay the data show that tills located west and southwest of the ungava bay region are largely dominated by hornblende grains with archean ages 2 6 ga while tills located east of ungava bay are characterized by grains with early paleo proterozoic ages 2 0 1 8 ga although most samples contain a few archean age grains ird derived from the nql region should thus be characterized by a large proportion of archean age detrital grains which contrasts significantly with the predominant paleoproterozoic 40 ar 39 ar ages 1 8 1 6 ga typically reported for the dominant age population of hornblende grains in h layers comparisons with ird through the last glacial cycle from a western north atlantic core off newfoundland do not show evidence for any prominent ice rafted event with the provenance characteristics of nql glacial deposits thereby suggesting that significant ice calving event s from the labrador quebec sector may have been limited throughout that interval although these results tend to point towards a relative stability of this ice dome during h events our study also indicates that further provenance work is required on ird proximal to the hudson strait mouth in order to constrain with a greater confidence the sector s of the lis that fed ice into hudson strait during h events alternatively these results and other paleogeographic considerations tend to support models suggesting that part of the ungava bay glacial lineations could be associated with a late glacial ice flow across hudson strait

north american ice sheet build up during the last glacial cycle 115 21 kyr the last glacial maximum lgm outline and subsequent retreat pattern 21e7 kyr of north americanice sheets are reasonably well established however the evolution of the ice sheets during their build upphase towards the lgm between 115 and 21 kyr has remained elusive making it difficult to verifynumerical ice sheet models for this important time interval in this paper we outline the pre lgm icesheet evolution of the laurentide and cordilleran ice sheets by using glacial geological and geomorphologicalrecords to make a first order reconstruction of ice sheet extent and flow pattern we mappedthe entire area covered by the laurentide and cordilleran ice sheets in landsat mss images andapproximately 40 of this area in higher resolution landsat etm images mapping in aerial photographsadded further detail primarily in quebec labrador the cordilleran region and on baffin island our analysis includes the recognition of approximately 500 relative age relationships from crosscuttinglineations together with previously published striae and till fabric data these are used as the basis forrelative age assignments of regional flow patterns for the reconstruction of the most probable ice sheetevolution sequence we employ a stepwise inversion scheme with a clearly defined strategy for delineatingcoherent landforms swarms reflecting flow direction and configuration and linking these topreviously published constraints on relative and absolute chronology our results reveal that icedispersalcentres in keewatin and quebec were dynamically independent for most of pre lgm time andthat a massive quebec dispersal centre rivalling the lgm in extent existed at times when the sw sectorof the ice sheet had not yet developed the oldest flow system in eastern quebec labrador atlanticswarm had an ice divide closer to the labrador coast than later configurations a northern keewatin central arctic ice sheet existed prior to the lgm but is poorly chronologically constrained there is alsoevidence for older and more easterly cordilleran ice sheet divide locations than those that prevailedduring the late wisconsinan in terms of ice sheet build up dynamics it appears that residual ice capsafter warming phases may have played an important role in particular the location and size of remnantice masses at the end of major interstadials i e ois 5c and 5a must have been critical for subsequentbuild up patterns because such remnant uplands may have fostered much more rapid ice sheetgrowth than what would have occurred on a fully deglaciated terrain the ice sheet configuration duringstadials would also be governed largely by the additional topography that such residual ice constitutesbecause of inherent mass balance topography feedbacks

time perspectives on glacial landscape formation glacial flow chronology at lac aux goelands northeastern quebec canada we used mapping from aerial photographs and field investigations around lac aux goelands whitegull lake northeastern quebec to establish a glacial flow chronology that involves three different glacial landform systems we interpret the youngest of these systems as related to the late wisconsinan deglaciation this system has an up glacier discontinuity that marks a temporal shift from warm based subglacial conditions to non erosive cold based subglacial conditions whereby older glacial landscapes are preserved in the interior of quebec labrador furthermore our results support theories suggesting a northward migrating ice divide to a final position south of ungava bay during the final deglaciation the recognition that a glacial landscape is fragmented and consists of smaller temporally unrelated units is of considerable significance in understanding ice sheet dynamics and patterns of subglacial preservation and destruction this is because a fragmented landscape implies a fragmented formation history where formation of subglacial landforms occurred in restricted subglacial zones during restricted time periods periods of formation must have been separated by periods of preservation and inhibited subglacial reshaping based on estimates of the ice margin retreat velocity and the velocity of the warm based inward migrating zone we suggest that periods of preservation were orders of magnitude longer than periods of subglacial landform formation copyright 2003 john wiley sons ltd

late wisconsinan glacial history in the bonaparte lake map area south central british columbia implications for glacial transport and mineral exploration1this article is one of a series of papers published in this special issue on the theme of new insights in cordilleran intermontane geoscience reducing exploration risk in the mountain pine beetle affected area british columbia 2earth sciences sector contribution number 20100093 this study is centred on the bonaparte lake map area located in the southern interior plateau of south central british columbia the reconstruction of the late wisconsinan glacial history of this part of the southern sector of the cordilleran ice sheet incorporates i the analysis and interpretation of landforms of various scales ii the sedimentology and stratigraphy of glacial sediments and iii the geochemical and mineralogical composition of till and analysis of regional glacial dispersal of these components the onset of the last glacial event was initiated by ice advancing westerly and southwesterly into the study area from an alpine source region located in the cariboo mountains as glaciation intensified ice from the coast mountains coalesced with the cariboo mountain ice over the interior plateau and developed into an ice divide around 52 north latitude which resulted in ice flow to be diverted to a southerly direction over the study area the two dominant ice flow directions produced pal

evidence for a relict glacial landscape in quebec labrador abstract two major glacial landform systems occur in labrador these are the radial lineation and esker swarm reflecting late wisconsinan decay and the ungava bay lineation and esker swarm reflecting convergent northward flow in some sectors the two landform systems give conflicting evidence regarding deglaciation pattern we have interpreted the ice sheet dynamics in labrador from morphological data using a new inversion model that treats spatial patterns of deglacial meltwater landforms separately from lineation patterns in the george river area we have found that the ungava bay swarm of deglacial landforms has been overprinted at a right angle by a younger regional meltwater pattern from the last deglaciation a similar overprint also exists along the intersection line in west central labrador these relations show that the previously accepted relative ages of the two landform systems hughes 1964 boulton and clark 1990a b klassen and thompson 1993 have to be reversed we interpret the ungava bay lineation and esker swarm to represent a 0 25 106km2 pre late wisconsinan relict landscape formed during the deglaciation of an older ice sheet and later preserved in a dry based central zone of the labrador dome

glacial geology of western hudson strait canada with reference to laurentide ice sheet dynamics hudson strait is a large marine trough that connected the marine based interior of the laurentide ice sheet with the ocean field investigations of the glacial geology of western hudson strait have helped to define the role of the strait in laurentide ice sheet dynamics in general ice converged on western hudson strait from the north west and south upon reaching the strait ice flowing from foxe and ungava peninsulas turned eastward within less than 10 km of the coast to parallel eastwardly flowing ice in the strait glacial erosion of western hudson strait is identified as slight the texture carbonate content ph and the presence of several types of erratics distinguish till on nottingham salisbury and mill islands in western hudson strait from till on southern foxe peninsula and northern ungava peninsula till on the islands is characterized by higher ph and higher proportions of silt clay and carbonate than till on either adjacent peninsula in addition the presence of red sandstones porphyritic intrusives stromatolites omarolluks and iron formation are restricted to the islands with few exceptions that can be explained by ice rafting carbonate is present in the pebble and boulder fractions of till on foxe peninsula and in all fractions of till on the islands but is absent from till on ungava peninsula these data suggest that ice from distant and widely distributed source regions affected western hudson strait and along with evidence for a rapid change in ice flow direction along the margins of hudson strait suggest that the strait was occupied by a relatively fast moving ice stream that had a substantial impact on the interior of the ice sheet through downdraw

ice 3g a new global model of late pleistocene deglaciation based upon geophysical predictions of post glacial relative sea level change a new high resolution global model of late pleistocene deglaciation is inferred on the basis of geophysical predictions of postglacial relative sea level variations in which the ice ocean solid earth interaction is treated in a gravitationally self consistent fashion for the purpose of these analyses the radial viscoelastic structure of the planet is assumed known on the basis of previously published sensitivity tests on solutions of the forward problem only radiocarbon controlled relative sea level histories from sites that were actually ice covered with one or two additions are employed to constrain the model leaving relative sea level rsl data from sites that were not ice covered to be employed to confirm its consistency results for these confirmatory analyses are reported elsewhere here the new deglaciation model referred to as ice 3g is compared to previous models derived by several independent means and tested against a number of additional observations other than sea level histories including geologically controlled retreat isochrones oxygen isotope data from deep sea sedimentary cores and coral terrace elevations the latter two observations strongly constrain the net sea level rise that has occurred since the onset of deglaciation and therefore the mass of ice that melted during the last glacial interglacial transition

a highly mobile laurentide ice sheet revealed by satellite images of glacial lineations satellite images of the north american continent reveal previously unrecognized patterns of crossing drift lineations reflecting unexpected shifts of centres of mass of the laurentide ice sheet throughout the last glacial cycle such shifts must must have been associated with major global atmospheric circulation changes

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

terra mare comparison of small young craters on the moon abstract several characteristics of very small impact craters of lunar surface have recently been improved by high resolution datasets lunar impact craters with diameter below 100 m and especially below 10 m were surveyed and compared at mare and terra surfaces in this work altogether at 0 6 km2 area with 4866 craters there the surface density values of all craters km2 at the mare regions were 1 5 2 0 times larger than at the terra regions and this difference came mainly from there is an ongoing debate on the reliability and usage of small craters for age estimation xiao and strom 2012 this work provides some further information to evaluate this aspect no correlation was found between the areal density values of craters and ages determined from size frequency distributions confirming using examples of smaller craters than analysed in most earlier works that large sized craters dominate the age estimation despite the larger number of

deglaciation landforms and c 14 chronology of the lac guillaume delisle area eastern hudson bay a report on field evidence abstract this paper presents new field data pertaining to the deglaciation of the lac guillaume delisle area aiming to establish its emplacement in time and space between the breaching of the laurentide ice sheet lis over hudson bay and the final ice disintegration over northern quebec and labrador the deglaciation of the east coast of hudson bay started at 8200 cal bp in the south western sector of the lac guillaume delisle estuary when the marine limit was at 271 m above sea level asl by 8000 cal bp frontal moraines and proximal glacio marine sediments were deposited in the transverse valleys along the coastal cuesta ridges at a mean elevation of 250 m asl they are associated with a regional topographic pinning event of the western margin of the quebec labrador ice sheet qlis on the high coastal relief at that time the coastal relief encouraged a change in deglaciation mode from fast calving in the open and deep tyrrell sea to a tidewater ice sheet controlled by the topography the ice front thereafter retreated eastward under very large and fast crustal unloading where ice contact deltas fed by short sandar and eskers were deposited between 7900 and 7400 cal bp between 240 and 195 m asl as the ice front was receding inland and lost contact with tyrrell sea a series of deposits and landforms characterize this modification in the sedimentation regime when the ice sheet margin changed from tidewater to terrestrial dating shows that the deglaciation of the western margin of qlis on land was very fast i e faster than when the front of the ice sheet was in contact with the tyrrell sea as the fall of the relative sea level continued a large volume of paraglacial sediments was deposited in the river valleys making up vast fluvial terraces and deltas between 6500 and 5800 cal bp between 125 and 100 m asl since then land emergence has continued at a rate of about 1 3 m century downcutting by rivers post glacial deltaic sedimentation and shoreline changes still continue today

early holocene glacial lakes and ice marginal retreat pattern in labrador ungava canada abstract the existence of glacial lakes naskaupi mclean minto melezes and wapussakatoo during the deglaciation of labrador ungava canada was recognized at the end of 1950s however glacial lakes have seldom been included in regional reconstructions to constrain the outline of successive ice margins during the glacial retreat in labrador ungava reconstruction of the ice flow succession in labrador ungava has often primarily focused on till lineation systems these reconstructions often depict a late glacial ice dispersal center situated over central labrador ungava more recent studies on glacial geomorphology including meltwater features however suggest that north central labrador ungava exhibited cold based conditions at least during the latest deglaciation cold based conditions inhibit basal sliding and formation of landforms except for meltwater traces such as meltwater channels glacial lake shorelines and deltas this situation implies that meltwater traces are the main source of information when reconstructing the spatial retreat pattern during a cold based deglaciation evidence presented in this study such as glacial lake shorelines fossil deltas and spillway and drainage channels in north central labrador ungava indicates the existence of numerous previously unmapped glacial lakes the mapped glacial lake features are synthesized to a reconstruction of several glacial lake stages which in turn are used to constrain the late glacial ice margin retreat pattern over the inferred cold based areas of north central labrador ungava a total of 26 glacial lakes 65 sub stages existed during the deglaciation of labrador ungava these lakes were impounded along the southern margin of the shrinking ice sheet the required damming ice margins indicate that the last ice remnant of the laurentide ice sheet in labrador ungava was situated over the southern ungava bay and the adjacent southern shore

geomorphological reconstruction of the labrador sector of the laurentide ice sheet abstract the labrador sector formed one of three major domes of the laurentide ice sheet during the last wisconsinan glaciation reconstruction of its evolution is important because it modulated atmospheric and oceanic circulation over 1 100 ka time scales this paper reports new geomorphological evidence of ice flow which is used to reconstruct glimpses of the behaviour of the labrador sector during its evolution flow evidence indicates a southern position 51 n of the labrador ice divide presumed to be the usual location when the ice sheet was close to its maximum extent a previously inferred dramatic surge of labrador ice across hudson strait and onto baffin island at 9 9 9 4 ka is verified seemingly contradictory evidence regarding deglaciation in northern quebec has been analysed and a reconstruction developed that solves this problem deglacial retreat of the labrador ice sheet was highly asymmetric with rapid retreat of the southern margin northwards migration of the centre of mass by 500 km and slow retreat along the northern margin as it shrank its thermal regime changed from cold centred to entirely cold based and it finally fragmented into many residual ice caps and glaciers this dynamic and asymmetric behaviour may be due to destabilisation of the labrador ice sheet triggered by heinrich event ice streaming through hudson strait

non intrusive measurements of crater growth abstract an experimental technique to measure crater growth is presented whereby a high speed video captures profiles of a crater forming after impact obtained using a vertical laser sheet centered on the impact point unlike previous so called quarter space experiments where projectiles were launched along a transparent plexiglas sheet so that growth of half a crater could be viewed the use of the laser sheet permits viewing changes in crater shape without any physical interference to the cratering process this technique indicates that for low velocity impacts 300 m s into 220 m glass beads that are without cohesion and where the projectile is not disrupted craters initially grow somewhat proportionally but that later their depths remain essentially constant while their diameters continue to expand in addition these experiments indicate that as the impact velocity increases the rate of growth and the transient depth to diameter ratio at the end of ejecta excavation decreases these last two observations are probably due to the large time of penetration of the projectile which becomes a significant fraction of the time of crater formation this is contrary to the expectations for the scaling rules which assumes a point source very high curtain angles 45 are also seen and could be due to the low friction angle of the target significant crater modification which is rarely seen in quarter space experiments is also observed and appears to be controlled by the dynamic angle of repose of the target these latter observations indicate that differences in target friction angles may need to be considered when determining near rim ejecta mass distributions and large scale crater modification processes on the planets

postglacial paleoceanography of hudson bay stratigraphic microfaunal and palynological evidence cores containing representative sequences of postglacial sediments in northern and southern hudson bay were analyzed for their microfaunal foraminifers and ostracods and palynological dinocysts

geological characterization of the chang e 4 landing area on the lunar farside abstract on january 3 2019 china s chang e 4 probe accomplishes the first ever soft landing on moon s farside in the von karman crater floor mare volcanism plain 177 59 e 45 46 s within the south pole aitken spa basin its surface in situ measurements would provide the first ground truth for lunar farside volcanism and spa materials we present a detailed geologic context topographical morphological quantitative geochemical and mineralogical characterization of a 20 20 km area centered at the chang e 4 landing site using the latest multi source high resolution orbital data sets the landing area is very flat at a baseline of 30 m with 90 of surface having slope