seasonal changes in beach morphology along the sheltered coastline of perth western australia abstract seasonal change in beach morphology is traditionally ascribed to a variation in the incident wave energy level with calm conditions in summer resulting in wide beaches with pronounced subaerial berms and energetic conditions in winter causing narrow beaches with nearshore bar morphology the coastline of perth western australia is characterised by a large seasonal variation in the incident wave height and local beaches exhibit a distinct seasonal change in morphology however these morphological changes are better explained by a seasonal reversal in the littoral drift direction than by variations in the incident wave energy conditions in summer when northward sediment transport prevails due to sea breeze activity beaches located south of coastal structures headlands or rocky outcrops become wider due to the accumulation of sediment against the obstacle these beaches will subsequently erode in winter during storms when the longshore sediment transport is toward the south in contrast beaches located north of obstacles will become narrower during summer and wider during winter the usefulness of the dimensionless fall velocity h b w s t where hb is the breaker height ws is the sediment fall velocity and t is the wave period as a predictor of presence absence of bar morphology and beach type was investigated it was found that fluctuates around the threshold of bar formation 1 5 2 over a variety of time scales daily weekly and seasonally these temporal variations in in conjunction with the relatively low wave energy level that characterises the coast negates the development of beach and nearshore morphology that is in equilibrium with the hydrodynamic conditions as a result bar occurrence and beach type can not be readily predicted using along the perth coast

multistage zircon and titanite growth and inheritance in an archean gneiss complex winnipeg river subprovince ontario abstract the paper presents results from a u pb isotopic investigation of zircon and titanite from three phases of the archean sioux lookout gneiss complex at the margin of the gneissic plutonic winnipeg river subprovince wrs of the superior province the data reveal an intricate data pattern caused by inheritance and multistage crystallization and record the timing of two major orogenic events followed by a protracted post metamorphic evolution a zircon age of 2889 3 ma for a foliated pink granite correlates with that of similar units elsewhere in the superior province and reflects one of the first major episodes of crustal assembly in the pre kenoran evolution of the region tonalite and diorite from the same site yield zircon and titanite ages of 2704 2 and 2705 5 ma which correspond to the timing of major magmatism related to kenoran collision of the wrs with the adjacent wabigoon volcanic domains tonalite and diorite both contain abundant xenocrystic zircon moreover the late kinematic diorite also includes xenocrystic titanite which has a minimum age of 2730 ma and coexists with younger ca 2705 ma titanite and zircon two generations of titanite are also present in both the tonalite and the granite the earliest one reflects the 2704 ma plutonic metamorphic event whereas the second one indicates post metamorphic crystallization at 2650 and 2627 ma caused by superimposed hydrothermal activity and retrogression the data support the concept that the titanite can remain a closed system to pb diffusion at elevated temperatures as long as the crystals escape magmatic or metamorphic recrystallization conversely they also show that titanite can easily be produced by low temperature processes hence generalized interpretation of titanite ages in terms of regional cooling can be inappropriate

the role of granitic intrusions in the evolution of the abitibi belt canada abstract the abitibi greenstone belt canada constitutes a segment of a broad volcanic arc that experienced three distinct periods of volcanic activity peaking at 2730 2725 2718 2714 and 2705 2700 ma during all three volcanic episodes which were generated by subduction related processes mantle plume derived komatiitic volcanic rocks were also added to the composite abitibi arc plume activity resulted in local regional extension and restricted later volcanism within the belt north south shortening produced thrusts and folds followed by se trending dextral strike slip faults between 2700 and 2692 ma the principal belt wide deformation subsequent e w dextral strike slip faulting limited to the major faults continued to 2690 2680 ma destor porcupine fault and even to 2680 2670 ma cadillac larder fault extensional structures resulting from the thickened crust developed around 2660 ma granitic intrusions are associated with all the volcanic episodes and phases of deformation and have been used to constrain the age of many of these events their role in stabilizing and adding to the volcanic crust is perhaps less well appreciated large synvolcanic tonalite dominated intrusions of the tonalite trondhjemite granodiorite ttg suite are associated with the three major periods of volcanic activity of the abitibi greenstone belt and are genetically related to the volcanism these synvolcanic intrusions occur both as high level flat cauldron subsidence complexes and as batholithic masses at the base of the oceanic crust tonalite gneiss dated from 2714 to 2703 ma occurring in structural culminations is considered to be arc underplated material the gneiss crystallized at depth and in contrast to the other tonalites does not appear to have intruded volcanic rocks the gneisses also record the earliest compressional deformation numerous intrusions of island arc affinity both the ttg and monzodiorite mzd suites form stocks and small plutons emplaced during the late phases of major deformation these are more numerous than voluminous and most of the tonalite end members of the suites probably intruded deeper within the sequence retarded by the now thickened crust large tonalite intrusions dominate the uplifted deeper sections adjacent to the abitibi belt in the kapuskasing uplift and grenville parautochthon and presumably make up much of the deep crust of the abitibi belt imaged in lithoprobe seismic profiles small stocks and dyke complexes characterize the late localized strike slip deformational phase these intrusions are porphyritic syenites and related rocks including some alkaline compositions garnetiferous granites of crustal derivation were emplaced in the final period of extensional activity around 2660 ma thus plutonic rocks are a volumetrically major overall component of the abitibi belt and were implicated in every phase of its development from early volcanic arc magmatic activity through the various stages of deformation to final stabilization as continental crust

chronostratigraphic constraints on the genesis of archean greenstone belts northwestern superior province ontario canada abstract the paper reports new u pb zircon ages for supracrustal assemblages of the red lake mcinnes lake hornby lake north spirit lake and favourable lake greenstone belts of northwestern superior province and evaluates their significance within the regional stratigraphic and tectonic context the new data confirm and refine the existing picture of a multistage evolution lasting ca 300 m y initiated at ca 3000 ma by periods of coeval calc alkalic tholeiitic and komatiitic magmatic activity followed by the eventual accretion of a microcontinental nucleus by ca 2900 2800 ma and concluded by an intense period of extensive tholeiitic and calc alkalic volcanism and by widespread plutonism and tectonism between 2750 and 2680 ma volcanic sequences in the mcinnes lake and hornby lake greenstone belts of the berens river subprovince yield ages of 2974 2 2969 3 2928 2 and 2901 2 ma showing a correlation with the early stages of greenstone belt development in the region similarly a sedimentary unit in the northern red lake greenstone belt was probably deposited at ca 2900 ma as indicated by a range of detrital zircon ages between 2989 and 2916 ma and the absence of younger components by contrast a volcanic unit in the center of the red lake greenstone belt previously thought to be 2830 ma yields an age of 2745 7 4 ma which correlates with some of the latest volcanic episodes in the uchi subprovince a detrital zircon population in a sedimentary unit of the western favourable lake greenstone belt of the sachigo subprovince displays single grain ages ranging from 2843 to 2727 ma the youngest age corresponding to that of an unconformably underlying tonalite gneiss the geological and geochronological relationships suggest that the north spirit lake and favourable lake greenstone belts represent back arc basins that evolved between ca 2750 and 2730 ma in a transtensional setting within the older north caribou terrane sedimentation was initiated during opening and deepening of the basins and accompanied most of the concluding compression and imbrication stages the period between 2750 and 2680 ma was also characterized by widespread granitoid plutonism that enveloped the mcinnes lake and hornby lake greenstone belts and older tonalites and formed the bulk of the berens river subprovince

prediction of cross shore sediment transport at different spatial and temporal scales abstract coastal sediment transport and associated morphological change can be examined at many spatial and temporal scales these scales range from instantaneous movement of single grains on a time scale of the local turbulence to seasonal and long term movement of large sand bodies such as longshore bars and tidal deltas researchers have begun to recognize that a first principles microscale approach to calculating long term and large scale megascale coastal change may not be possible or even adequate therefore new approaches are being sought to advance quantitative understanding and prediction of large scale coastal behavior the present paper opens with a discussion of compatible spatial and temporal scales for calculating sediment transport and morphology change motivation is given for the need to calculate morphologic change at scales greater than the typical one of microscale example calculations are made at different scales showing the assumptions and approximations of each the main conclusion is that calculations at different scales can be related and reconciled if limitations in prediction of initial and boundary conditions and in the fluid forcing are recognized

architecture and sedimentary processes on the mid norwegian continental slope a 2 7 myr record from extensive seismic evidence abstract quaternary architectural evolution and sedimentary processes on the mid norwegian continental slope are investigated using margin wide three and two dimensional seismic datasets of 100 000 km3 sediments delivered to the mid norwegian shelf and slope over the quaternary 75 000 km3 comprise the slope succession the structural high of the voring plateau characterised by initially low 1 2 slope gradients and reduced accommodation space exerted a strong control over the long term architectural evolution of the margin slope sediment fluxes were higher on the voring plateau area increasing up to 32 km3 ka 1 during the middle pleistocene when fast flowing ice streams advanced to the palaeo shelf edge resulted in a more rapid slope progradation on the voring plateau these rates of sediment delivery are high compared to the maximum of 7 km3 ka 1 in the adjacent sectors of the slope characterised by steeper slope 3 5 more available accommodation space and smaller or no palaeo ice streams on the adjacent shelves in addition to the broad scale architectural evolution identification of more than 300 buried slope landforms provides an unprecedented level of detailed process based palaeoenvironmental reconstruction channels dominate the early pleistocene record 2 7 0 8 ma during which glacimarine sedimentation on the slope was influenced by dense bottom water flow and turbidity currents morphologic signature of glacigenic debris flows appear within the middle late pleistocene 0 8 0 ma succession their abundance increases towards late pleistocene marking a decreasing role for channelized turbidity currents and dense water flows this broad scale palaeo environmental shift coincides with the intensification of northern hemispheric glaciations highlighting first order climate control on the sedimentary processes in high latitude continental slopes

archaean cratonization and deformation in the northern superior province canada an evaluation of plate tectonic versus vertical tectonic models abstract the archaean minto block northeastern superior province is dominated by tonalite trondhjemite enderbite pyroxene tonalite granodiorite and granite with subordinate mafic rocks and supracrustal belts the plutons have been interpreted as the batholithic roots of andean type plate margins and intra oceanic arcs existing horizontal tectonic models propose that penetrative recrystallization and transposition of older fabrics during terrane assembly at 2 77 and 2 69 ga produced a n nw tectonic grain in the douglas harbour domain northeastern minto block tonalite and trondhjemite dominate the faribault thury complex 2 87 2 73 ga and enderbite constitutes 50 100 km scale ovoid massifs troie and qimussinguat complexes 2 74 2 73 ga magmatic muscovite and epidote in tonalite trondhjemite have corroded edges against quartz plagioclase suggesting resorption during ascent of crystal charged magma foliation maps and air photo interpretation show the common development of 2 10 km scale ovoid structures throughout the douglas harbour domain outcrop and thin section scale structures imply that many plutons experienced a phase of syn magmatic deformation typically followed by high temperature sub magmatic overprints thermobarometric data for plutons indicate near solidus recrystallization at 4 6 kbar pressures the common preservation of syn magmatic fabrics in plutons of different ages seems incompatible with the origin of these fabrics through superimposed regional orogenesis the broad uniformity of intrusion ages and lithologies throughout the minto block and the rarity of shallowly dipping planar fabrics also seem inconsistent with accretion of disparate older terranes each of which should preserve distinct histories a possible alternative explanation for these features is provided by vertical tectonic models whereby buoyant felsic magmas ascended as crystal slurries while dense supracrustal rocks and solidified felsic intrusions emplaced into them subsided as cold fingers 10 20 km scale instabilities shear between upwelling and downwelling limbs would have concentrated in the weak intrusions generating steeply plunging syn magmatic fabrics and producing ductile overprints in solidified rocks

beach cusp morphodynamics detailed measurements of three dimensional beach cusp morphology were made on a steep gradient low energy microtidal beach in perth western australia during the field campaign a variety of wave conditions and tidal ranges were experienced and these differing hydrodynamic conditions were reflected in a consistent pattern of morphological changes to the beach cusp system a useful parameter to delineate between trends of cusp destruction and re formation appeared to be the surf similarity parameter tan i0 l0 where h0 is offshore wave height l0 is deep water wave length and tan is beach gradient for 1 2 a small storm was experienced at the start of the field campaign period and resulted in considerable erosion of the beach face the cusp morphology across the lower beachface was destroyed but a subtle remnant of the pre storm cusp morphology was preserved on the upper beachface when cusps reformed after the storm under the influence of declining wave conditions they appeared at the same location and with the same dimensions as the pre storm cusp morphology hence it is considered that the cusp re formation was controlled more by the antecedent morphology than the hydrodynamic conditions this indicates that positive feedback between swash hydrodynamics and beachface morphology necessary to form beach cusps does not require a large variation in relief 1997 john wiley sons ltd

geological control of beach morphodynamic state the concept of beach morphodynamic states has achieved widespread acceptance in the coastal geological literature since its inception in the mid 1980s and expansion in the 1990s much of the pioneering work was undertaken in australia under a range of environmental conditions in microtidal environments and a close empirical relationship between beach 3 dimensional morphology and the dean s parameter h b wst was established subsequently the relative tidal range parameter h b tr was extended to beaches of all tidal ranges in this paper observations are presented from 25 beaches around the north coast of ireland these beaches exist on an environmental gradient that encompasses marked tidal and wave energy variability micro to macrotidal and low to high wave energy each beach was visually categorised into one of several established beach states described in the literature on the basis of field observations for each beach the rtr and dean s parameter were calculated for the immediately antecedent period and used to predict the beach state using published relationships observed and predicted beach states were then compared comparison of observed and predicted beach states showed that while beaches with observed dissipative morphology typically matched the expected criteria most other beach states did not lack of agreement between predicted and observed beach states has been reported elsewhere and attributed to failings in the rtr and dean s parameter in addition this study identifies geological factors as important constraints on actual beach state in the majority of beaches studied inherited geological factors appear to be more important determinants of beach morphology than contemporary dynamics c 2005 elsevier b v all rights reserved

the norwegian greenland sea continental margins morphology and late quaternary sedimentary processes and environment the continental margins surrounding the norwegian greenland sea are to a large degree shaped by processes during the late quaternary the paper gives an overview of the morphology and the processes responsible for the formation of three main groups of morphological features slides trough mouth fans and channels several large late quaternary slides have been identified on the eastern norwegian greenland sea continental margin the origin of the slides may be due to high sedimentation rates leading to a build up of excess pore water pressure perhaps with additional pressure caused by gas bubbles triggering might have been prompted by earthquakes or by decomposition of gas hydrates trough mouth fans tmf are fans at the mouths of transverse troughs on presently or formerly glaciated continental shelves in the norwegian greenland sea seven tmfs have been identified varying in area from 2700 km2 to 215 000 km2 the trough mouth fans are depocentres of sediments which have accumulated in front of ice streams draining the large northwest european ice sheets the sediments deposited at the shelf break upper slope by the ice stream were remobilized and transported downslope mostly as debris flows the trough mouth fans hold the potential for giving information about the various ice streams feeding them with regard to velocity and ice discharge two large deep sea channel systems have been observed along the norwegian continental margin the lofoten basin channel and the inbis channel along the east greenland margin several channel systems have been identified the deep sea channels may have been formed by dense water originating from cooling sea ice formation and brine rejection close to the glacier margin or they may originate from small slides on the upper slope transforming into debris flows and turbidity currents

the physiography of high arctic cross shelf troughs abstract a comprehensive inventory of high arctic cross shelf troughs is compiled from international bathymetric chart of the arctic ocean ibcao bathymetric data v 3 0 the location of 75 cross shelf troughs is presented alongside a synthesis of their key physiographic characteristics and available glacial geological evidence the troughs are interpreted to have been occupied intermittently and eroded by marine terminating ice streams that traversed the shelf during at least one and often many quaternary full glacial periods considerable variation in cross shelf trough physiography exists in the high arctic trough lengths range between 35 and 1400 km widths from 12 to 260 km and maximum depths from 200 to 1000 m the longest cross shelf troughs extend through inter island channels on the beaufort sea queen elizabeth islands and barents kara sea margins the gradient of the upper slope beyond high arctic glacial troughs which ranges between 0 3 and 13 is shown to have a negative relationship with palaeo ice stream drainage basin area and trough length glacial sedimentary depocentres or trough mouth fans tmfs are inferred on the basis of bathymetric and where available seismic evidence to exist beyond the majority of high arctic cross shelf troughs evidence of shelf progradation or tmf development is absent from the slope beyond several high arctic troughs probably as a result of limited sediment supply to the margin and or sediment by passing of the upper slope on the south greenland continental margin it is likely that steep slopes prevented the development of significant glacial sedimentary depocentres beyond cross shelf troughs or that glacigenic debris has been removed from the upper slope by submarine slope failure

archean continental assembly in the southeastern superior province of canada between 1988 and 1993 seismic reflection and refraction surveys were acquired across the medium to high grade opatica plutonic gneiss belt the low grade abitibi greenstone belt and the pontiac metasedimentary belt all of which form part of the late archean superior province shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 ma of various exotic terranes against a backstop provided by the opatica belt this rapid southward growth of the archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65 km depth in the upper mantle below the opatica belt in contrast to the mainly orthogneissic opatica and pontiac belts the midcrust of the abitibi belt comprises metasedimentary and igneous rocks plus imbricated units of unknown affinity relict midcrustal accretionary complexes of substantial size whieh are indicative of primary suture zones are interpreted near the northern and southern limits of the abitibi belt an interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous evidence of large scale extension appears to be confined to the southern volcanic zone of the abitibi which developed into a half graben as the original suture zone was reactivated in extension unusually high seismic p wave velocities 7 5 8 2 kms 1 are present in the lower 8 km of the abitibi crust and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust crustal xenolith studies suggest that this process may be linked to early proterozoic magmatism

intracrustal extension of an archean orogen revealed using single grain u pb zircon geochronology the metaplutonic wawa gneiss domain in the southern superior province is part of an uplifted block that was exhumed during the proterozoic exposing the lower crustal 30 km levels of the late archean 2720 to 2670 ma abitibi wawa orogen previous structural and geophysical studies of the midcrustal wawa gneiss domain reveal subhorizontal shear zones that formed during orogen parallel extension and correspond to gently dipping seismic reflectors present at depth elsewhere in the orogen in order to bracket the age of this extensional deformation the ages of metre scale igneous bodies that crystallized before and after extension have been determined in four outcrops using single grain u pb zircon dating techniques the results constrain the time of extension to the interval between 2660 and 2645 ma and show that deformation was coeval with pegmatite emplacement partial melting of tonalite gneisses as old as 2925 35 ma and regional circa 2660 ma granulite facies metamorphism of adjacent more deeply eroded crust this extension and associated high grade metamorphism in the wawa gneiss domain occurred 10 to 25 m y after the cessation of regional compression major transcurrent fault movement and sediment deposition in the upper structural levels of the orogen the middle and lower crust was therefore mechanically decoupled from the upper crust during this period of orogen parallel extension this intracrustal style of deformation is interpreted to be the consequence of gravitational forces working on a thermally softened middle crust the detachment of lithosphere from the orogen base is the favored mechanism for generating these forces and for introducing the heat that caused synextension metamorphism cyclic construction and delamination of orogen lithosphere could account for even younger metamorphic strain events evident in nearby exposures of lower crust

morpho sedimentary dynamics of a micro tidal mixed sand and gravel beach playa granada southern spain abstract this paper addresses the changes in the morphology and sedimentology of a micro tidal mixed sand and gravel beach playa granada southern spain forced by wave and water level variations and human intervention through nourishment monthly and storm event driven beach surveys consisting of topographical measurements and sediment sampling in two selected areas were carried out over a one year period three prevailing sediment fractions sand fine gravel and coarse gravel and two end member morphological states of the upper beach profile convex with multiple berms and concave with a single storm berm were identified between them several transitional profiles were formed characterized by developing berms that progressively overlapped generating sediment variability both across the beach profile and with depth the results indicate that the total run up including water level reached during an event represents a more accurate threshold for differentiating between erosional and depositional conditions than wave height they also suggest that mixed sand and gravel beaches recover faster from storm erosion than sandy beaches the long term benefit of the artificial nourishment that took place at the end of the survey period was very limited and this is attributed to the too fine sediment used for the nourishment and its placement too high on the beach clearly nourishment interventions must take into account the natural sediment distribution and the profile shape to avoid rapid losses of the nourished sediment

morphological changes in a cuspate sandy beach under persistent high energy swells renaca beach chile abstract the present work proposes a morphological evolution model for sandy cuspate beaches which depends on two parameters swash excursion s and offshore wave steepness h0 l0 the model was derived from observations of morphology and wave climate at renaca beach in central chile comprising in situ instrumentation topographic surveys wave modeling and optical imagery it was found that wave steepness alone was unable to explain the morphological changes observed on the beach rather results indicate that swash excursion is better to distinguish between erosive and accretionary events with swash excursion exceeding 29 5 m for more than 12 h being indicative of erosive events while morphological changes are led by s wave steepness correlates with the erosive potential of waves especially when h0 l0 is greater than 0 017 although derived for a specific beach under a limited range of conditions this work adds to the understanding of the dynamic behavior of beaches exposed to persistent high energy swells

variable history of quaternary ice sheet advance across the beaufort sea margin arctic ocean the seismic stratigraphy and architecture of the beaufort sea shelf and slope are investigated using a comprehensive grid of high resolution two dimensional seismic reflection data three cross shelf troughs representing locations of former ice streams draining a 1000 km long section of the laurentide ice sheet lis are examined the mackenzie amundsen gulf and m clure strait systems dynamics of these paleo ice streams influenced ice sheet configuration and may have forced abrupt climatic change through delivery of ice and freshwater to the arctic ocean a comprehensive understanding of their geometry and dynamics is crucial for constraining numerical models of the lis evidence for two quaternary ice advances to the shelf break is interpreted from the mackenzie trough by contrast seismic stratigraphy of the amundsen gulf trough 400 km east of the mackenzie trough records at least nine quaternary ice advances here the outer shelf consists of stacked till sheets extending to the shelf break and forming a trough mouth fan the contrasting glacial histories of these neighboring ice streams are explained by their positions within the lis the mackenzie trough ice stream was situated at the extreme northwest ice sheet margin whereas the amundsen gulf ice stream had a more central location and larger drainage basin supplying significant quantities of ice and sediment to the arctic ocean through much of the quaternary the m clure strait trough probably possesses a similar architecture to the amundsen gulf trough and an even larger trough mouth fan

late quaternary glacial dynamics and sedimentation variability in the bering trough gulf of alaska ice dynamics tectonic setting and sediment supply are the key parameters controlling the architecture of high latitude margins and the formation of trough mouth fans tmfs current understanding of these archives of paleo ice streams is based on studies of ice sheets adjacent to stable passive margins while the behavior of active convergent glacier influenced margins remains relatively unconstrained we integrate high resolution seismic data and chronology from integrated ocean drilling program expedition 341 cores in southeast alaska across the actively converging yakutat terrane margin to examine the late quaternary evolution of the bering glacier the largest outlet glacier of the poorly understood cordilleran ice sheet cis we interpret at least eight glacial advances to the shelf break since the end of the mid pleistocene transition showing a more dynamic cis than hitherto realized during the past 130 k y the temperate meltwater charged bering glacier delivered 925 km 3 of sediment to the shelf and slope providing one of the highest rates of sustained sediment accumulation 5 10 m k y ever reported globally rapid formation of a tmf reaching 600 m thick in 130 k y emphasizes the extreme sediment flux that can be produced by wet based glacial systems and its critical role in the development of high latitude margin stratigraphy tmf formation despite initially steep tectonically controlled slopes in this active setting reflects an autogenic shift in the evolution of the bering trough suggesting that major transitions between sedimentary regimes need not reflect some externally driven change in climate variability

morphological evolution of beach cusps and associated swash circulation patterns abstract a numerical model capable of simulating the motion of water particles on beach cusp morphology under the influence of an initial velocity and gravity is presented the model indicates that the typical swash pattern on beach cusps is three dimensional with wave uprush diverging at the cusp horns resulting in concentrated backwash streams in the embayment the degree of horn divergence is an increasing function of the parameter e s 2 where e quantifies the prominence of the beach cusps s is the horizontal swash excursion length and is the cusp spacing the numerical experiments are supplemented with detailed field measurements of beach cusp morphological change the field data include three types of morphological response each characterised by a particular pattern of swash circulation that can be delineated using e s 2 when e s 2 beach cusp morphology is large and or subdued in relation to the swash length swash circulation is essentially two dimensional oscillatory flow and results in steepening of the beachface and infilling of the cusp embayments for e s 2 0 015 to 0 15 wave uprush is deflected from the cusp horns and flows into the embayments where it exits in a concentrated backwash stream horn divergent flow the ensuing swash backwash inequality reinforces cusp development and maintains existing cusp morphology when e s 2 0 15 beach cusps are small and or pronounced in relation to the length of the swash overtopping and ponding of the cusp horn takes place and swash circulation is from the embayment to the horn horn convergent flow as a result cusp horns are eroded and accretion occurs in the embayments

an arctic ocean ice shelf during mis 6 constrained by new geophysical and geological data the hypothesis of floating ice shelves covering the arctic ocean during glacial periods was developed in the 1970s in its most extreme form this theory involved a 1000 m thick continuous ice shelf covering the arctic ocean during quaternary glacial maxima including the last glacial maximum lgm while recent observations clearly demonstrate deep ice grounding events in the central arctic ocean the ice shelf hypothesis has been difficult to evaluate due to a lack of information from key areas with severe sea ice conditions here we present new data from previously inaccessible unmapped areas that constrain the spatial extent and timing of marine ice sheets during past glacials these data include multibeam swath bathymetry and subbottom profiles portraying glaciogenic features on the chukchi borderland southern lomonosov ridge north of greenland morris jesup rise and yermak plateau sediment cores from the mapped areas provide age constraints on the glaciogenic features combining these new geophysical and geological data with earlier results suggests that an especially extensive marine ice sheet complex including an ice shelf existed in the amerasian arctic ocean during marine isotope stage mis 6 from a conceptual oceanographic model we speculate that the cold halocline of the polar surface water may have extended to deeper water depths during mis 6 inhibiting the warm atlantic water from reaching the amerasian arctic ocean and thus creating favorable conditions for ice shelf development the hypothesis of a continuous 1000 m thick ice shelf is rejected because our mapping results show that several areas in the central arctic ocean substantially shallower than 1000 m water depth are free from glacial influence on the seafloor

nd isotopic evidence for early to late archean 3 4 2 7 ga crustal growth in the western superior province ontario canada abstract new sm nd analyses of archean metavolcanic plutonic gneissic and metasedimentary rocks from sachigo and berens river subprovinces north caribou terrane and winnipeg river and wabigoon subprovinces in conjunction with previously published data are used to trace the crustal evolution of the western superior province wsp from ca 3 4 to 2 7 ga the extensive wsp geochronological database which contains 3 17 2 55 ga plutonic rocks allows the calculation of precise e t nd values and the identification of the main periods of new crustal addition at 3 4 0 1 3 02 2 92 and 2 76 2 69 ga mass balance calculations indicate that the western superior province represents a collage of archean crusts comprising 7 44 and 48 wt of crust created at 3 4 3 0 and 2 7 ga respectively although these calculations depend on the isotopic signatures of the mantle and those of the average crust at different times several important observations can be made 1 the importance of pre 2 7 ga terranes recycled in the 2 7 ga orogen 2 the existence of time intervals of 150 300 ma dominated by intra crustal processes resulting in a large range of crystallization ages u pb zircon ages that do not reflect new additions from the mantle 3 much of the new crustal additions at 2 7 ga were contaminated by crustal material subducted into the source region ttg suites or were contaminated during intrusion into the crust sanukitoids

revision of the nw laurentide ice sheet implications for paleoclimate the northeast extremity of beringia and arctic ocean sedimentation abstract for the past half century reconstructions of north american ice cover during the last glacial maximum have shown ice free land distal to the laurentide ice sheet primarily on melville and banks islands in the western canadian arctic archipelago both islands reputedly preserve at the surface multiple laurentide till sheets together with associated marine and lacustrine deposits recording as many as three pre late wisconsinan glaciations the northwest corner of banks island was purportedly never glaciated and is trimmed by the oldest and most extensive glaciation banks glaciation considered to be of matuyama age 780 ka bp inside the limit of banks glaciation younger till sheets are ascribed to the thomsen glaciation pre sangamonian and the amundsen glaciation early wisconsinan stade the view that the western canadian arctic archipelago remained largely ice free during the late wisconsinan is reinforced by a recent report of two woolly mammoth fragments collected on banks and melville islands both dated to 22 ka bp these dates imply that these islands constitute the northeast extremity of beringia a fundamental revision of this model is now warranted based on widespread fieldwork across the adjacent coastlines of banks and melville islands including new dating of glacial and marine landforms and sediments on dundas peninsula southern melville island ams 14 c dates on ice transported marine molluscs within the most extensive laurentide till yield ages of 25 49 ka bp these dates require that late wisconsinan ice advanced northwestward from visount melville sound excavating fauna spanning marine isotope stage 3 laurentide ice that crossed dundas peninsula 300 m asl coalesced with melville island ice occupying liddon gulf coalescent laurentide and melville ice continued to advance westward through m clure strait depositing granite erratics at 235 m asl that require grounded ice in m clure strait as do streamlined bedforms on the channel floor deglaciation is recorded by widespread meltwater channels that show both the initial separation of laurentide and melvile ice and the successive retreat of laurentide ice southward across dundas peninsula into viscount melville sound sedimentation from these channels deposited deltas marking deglacial marine limit forty dates on shells collected from associated glaciomarine rhythmites record near synchronous ice retreat from m clure strait and dundas peninsula to north central victoria island 11 5 ka bp along the adjacent coast of banks island deglacial shorelines also record the retreat of laurentide ice both eastward through m clure strait and southward into the island s interior the elevation and age 11 5 ka bp of deglacial marine limit there is fully compatible with the record of ice retreat on melville island the last retreat of ice from mercy bay northern banks island previously assigned to northward retreat into m clure strait during the early wisconsinan is contradicted by geomorphic evidence for southward retreat into the island s interior during the late wisconsinan this revision of the pattern and age of ice retreat across northern banks island results in a significant simplification of the previous quaternary model our observations support the amalgamation of multiple till sheets previously assigned to at least three pre late wisconsinan glaciations into the late wisconsinan this revision also removes their formally named marine transgressions and proglacial lakes for which evidence is lacking erratics were also widely observed armouring meltwater channels originating on the previously proposed never glaciated landscape an extensive late wisconsinan laurentide ice sheet across the western canadian arctic is compatible with similar evidence for extensive laurentide ice entering the richardson mountains yukon farther south and with the innuitian ice sheet to the north widespread late wisconsinan ice in a region previously thought to be too arid to sustain it has important implications for paleoclimate ice sheet modelling arctic ocean ice and sediment delivery and clarifying the northeast limit of beringia

crustal magnetization and long wavelength aeromagnetic anomalies of the minto block quebec measured susceptibilities of 3500 samples from a 500 500 km area of the minto block superior province quebec have an average value of 0 03 si equivalent to induced magnetizations of 1 5 a m charnockitic lithologies igneous orthopyroxene bearing granodiorite granite diorite that constitute large parts of the minto region yield the highest susceptibilities the variations in susceptibility show a good correlation with the variations in the observed magnetic field including those at long wavelengths 100 km suggesting that charnockitic and related granitoid rocks could be responsible for the long wavelength anomalies modeling of a magnetic profile perpendicular to the strike of mapped lithotectonic subprovinces in the minto block supports this contention charnockitic lithologies are not usually considered a possible sources of long wavelength anomalies mafic rock types are generally favored the dominant minto lithologies are more felsic but are more efficient in the production of magnetite from the available iron

the role of swash infiltration in determining the beachface gradient a numerical study on sand and gravel beaches the increase of the gradient of the beachface with increasing sediment size has been attributed to swash infiltration the present study uses a process based numerical model to examine in detail the role of swash infiltration in determining the beachface gradient it is found that swash infiltration increases the onshore asymmetry in the swash flow thereby enhancing onshore sediment transport and resulting in relatively steep beachface gradients however the accretionary effects of swash infiltration are only evident when the rate of infiltration is sufficiently large i e when the total infiltration volume over a wave cycle exceeds c 2 of the uprush volume this threshold is attained when the hydraulic conductivity exceeds 1 cm s which is approximately equivalent to a grain size coarser than 1 5 mm this finding suggests that the correlation between beachface gradient and sediment size observed on sandy beaches is not due to enhanced swash asymmetry caused by swash infiltration for gravel beaches however swash infiltration is likely to be the dominant factor in controlling the beachface gradient with increased hydraulic conductivity i e sediment size resulting in steeper beachface gradients c 2001 elsevier science b v all rights reserved

the laurentide and innuitian ice sheets during the last glacial maximum the late wisconsinan advance of the laurentide ice sheet started from a middle wisconsinan interstadial minimum 27 30 14 c ka bp when the ice margin approximately followed the boundary of the canadian shield ice extent in the cordillera and in the high arctic at that time was probably similar to present ice advanced to its late wisconsinan stage 2 limit in the northwest south and northeast about 23 24 14 c ka bp and in the southwest and far north about 20 21 14 c ka bp in comparison to some previous reconstructions of ice extent our current reconstruction has substantially more late wisconsinan ice in the high arctic where an innuitian ice sheet is generally acknowledged to have existed in the atlantic provinces where ice is now thought to have extended to the continental shelf edge in most places and on eastern baffin island where ice probably extended to the fiord mouths rather than to the fiord heads around most of the ice margin the late wisconsinan maximum ice extent either exceeded the extent of earlier wisconsinan advances or it was similar to the early wisconsinan advance ice marginal recession prior to 14 14 ck a bp occurred mainly in deep water and along the southern terrestrial fringe however heinrich event 1 probably drew down the entire central ice surface at 14 5 14 c ka bp sufficiently to displace the labrador sector outflow centre 900 km eastward from the coast of hudson bay the onset of substantial ice marginal recession occurred about 14 14 c ka bp in the northwest southwest and south but not until about 10 11 14 c ka bp in the northeast and in the high arctic thus the period of maximum ice extent in north america generally encompasses the interval from b24 21 to 14 14 c ka bp or considerably longer than the duration of the lgm defined as occurring during a period of low global sea level as well as during a time of relative climate stability b18 14 c ka bp the interval of advance of much of the laurentide ice sheet to its maximum extent between b27 14 c ka bp and b24 14 c ka bp coincides with a suggested interval of rapid fall in global sea level to near lgmlevels r 2001 elsevier science ltd all rights reserved

beach erosion and recovery during consecutive storms at a steep sloping meso tidal beach this study analyses beach morphological change during six consecutive storms acting on the meso tidal faro beach south portugal between 15 december 2009 and 7 january 2010 morphological change of the sub aerial beach profile was monitored through frequent topographic surveys across 11 transects measurements of the surf swash zone dimensions nearshore bar dynamics and wave run up were extracted from time averaged and timestack coastal images and wave and tidal data were obtained from offshore stations all the information combined suggests that during consecutive storm events the antecedent morphological state can initially be the dominant controlling factor of beach response while the hydrodynamic forcing and especially the tide and surge levels become more important during the later stages of a storm period the dataset also reveals the dynamic nature of steep sloping beaches since sub aerial beach volume reductions up to 30 m3 m were followed by intertidal area recovery 2 z 3 m with rates reaching 10 m3 m however the observed cumulative dune erosion and profile pivoting imply that storms even of regular intensity can have a dramatic impact when they occur in groups nearshore bars seemed to respond to temporal scales more related to storm sequences than to individual events the formation of a prominent crescentic offshore bar at 200 m from the shoreline appeared to reverse the previous offshore migration trend of the inner bar which was gradually shifted close to the seaward swash zone boundary the partially understood nearshore bar processes appeared to be critical for storm wave attenuation in the surf zone and were considered mainly responsible for the poor interpretation of the observed beach behaviour on the grounds of standard non dimensional morphological parameters copyright 2011 john wiley sons ltd

persistence of storm induced morphology on a modal low energy beach a case study from nw iberian peninsula abstract the morphology of low energy beaches is inherited from high energy events showing a non equilibrium state with the prevailing conditions an example of a sheltered low energy beach rodas beach nw iberian peninsula is examined in this paper the evolution of six beach profiles was analysed during three years in order to investigate the response of this beach under fair weather and storm conditions the beach presented significant morphological differences alongshore due to its geographical location with respect to the wave approach these differences determined the sediment transfer pattern longshore or cross shore sediment transport from the emerged beach towards the adjacent inner shoreface zone significant morphological changes at each profile seem to be related to the incidence of sea waves along the beach superimposed on persistent swell during winter morphological changes were recorded under high energy storms during winter whereas the recovery to prestorm conditions occurred under prevailing fair weather conditions during summer the traditional beach classification parameter relative tidal range rtr which takes into account the effect of the tide range was found unsuitable to properly classify such a mesotidal low energy beach therefore a new parameter the relative tidal range maximum rtrmax was introduced which is a modification of the commonly used rtr rtrmax uses the highest wave breaking height observed during high energy storm conditions instead of the modal wave breaking height by using this parameter the studied beach could be properly classified into the traditional beach classification models taking into account the tide range since low energy beaches generally show a morphology that is inherited from higher energy conditions than the prevailing conditions it is expected that the rtrmax will be applicable for the classification of other low energy sheltered beaches

video observations of beach cusp morphodynamics beach cusps are a common feature of steep reflective and intermediate beaches although many observations of beach cusp spacing exist there are few observations quantifying the incipient formation evolution and eventual destruction of these features beach cusp morphodynamics were analyzed using a 3 year dataset of video images collected at tairua beach new zealand twenty four beach cusp episodes were selected to monitor the cycle of beach face changes from planar to the appearance of the cusp patterns and back to planar again observations show that beach cusp disappearance can be ascribed not only to the erosive influence of storms but also to the persistence of accretionary conditions leading to the infilling of beach cusp bays and the development of an alongshore continuous berm we also report observations of changes in beach cusp spacing over time which can be attributed to the merging of adjacent cusps within the cusp field with the overall cusp spacing re adjusting to accommodate the disappearance of a horn although the self organization theory provides a better fit to the data and theory we were unable to conclusively refute any of the mechanisms causing beach cusp formation since both existing theories subharmonic standing edge wave and self organization can predict the trend in the observed beach cusp spacing these observations show that initial beach cusp formation primarily occurs around 70 of the episodes analyzed under mildly accretionary conditions and when accretion persists the pattern disappearance is likely to occur as a result of bay infilling to form a featureless berm

analysis of the scale of errors in nearshore bathymetric data most studies of nearshore hydrodynamics sediment transport and morphology focus on bathymetric variability within a narrow band of spatial and temporal scales typically these studies rely on bathymetry estimates derived from field observations consisting of discrete samples in space and time with varying degrees of measurement error sampling limitations which result in aliasing and measurement errors can significantly contaminate variability at resolved scales and may lead to large errors in the representation of the scales of interest using a spectral analysis interpolation errors were analyzed for three different nearshore bathymetric data sets each of which targeted a different range of spatial scales bathymetric features that were unresolved or poorly resolved e g beach cusps introduced the potential for contamination in two of the data sets this contamination was significantly reduced using an appropriate scale controlled interpolation method leading to more accurate representations of the actual bathymetry an additional benefit of using scale controlled interpolation is that interpolation errors may be estimated independently of actual observations which allows one to design bathymetric sampling strategies that ensure that dominant scales are either resolved or largely removed finally interpolation errors corresponding to a particular sample design can be used to determine which interpolated values contribute usefully to a band limited analysis of bathymetric variability

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

minto block superior province missing link in deciphering assembly of the craton at 2 7 ga plate tectonic models of the superior province are rooted in the granite greenstone and metasedimentary belts of the southern part of the craton north striking domains of the minto block in the northeastern part of the province evolved at similar times in 3 1 2 8 2 725 and 2 69 ga events requiring expansion of models of late archean assembly to accommodate minto geology western and eastern protocratons 3 1 2 8 ga rifted at 2 79 ga to produce an ocean basin that was mostly consumed by subduction at 2 725 ga the leaf river plutonic suite of cale alkalic hornblende biotite orthopyroxene clinopyroxene granodiorite represents magmatic arcs built on the protocratons whereas the intervening goudalie domain containing fault bounded fragments of rifted continental crust rift volcanics primitive oceanic crust 2724 ma island arc rocks and a 2718 ma back arc assemblage marks the suture terminal collision at 2 7 ga led to thickening and crustally derived granitoid magmatism the southern superior province also experienced vigorous activity between 2 725 and 2 69 ga as island arcs oceanic plateaus continental fragments and accretionary prisms were amalgamated progressively from north to south in a regime of dextral transpression then stitched by granites a northern proto superior craton had continental magmatic arcs built on its eastern and southern flanks in response to west northwest directed subduction orthogonal convergence in the east produced wide plutonic arcs in contrast to terrane accretion tectonics in the more oblique regime along the southern margin

swash zone sediment transport step dynamics and morphological response on a gravel beach abstract state of the art morphodynamic field data were collected from slapton sands uk a fine gravel beach d50 2 10 mm over a 10 day period under a range of wave tide conditions comprising a mixture of southerly swell and easterly wind waves hs 0 5 1 m ts 4 8 s tide range 3 4 m over the field period the upper beach experienced accretion and berm formation during tides with low steepness swell dominated wave conditions h l 0 01 surprisingly the intertidal beach showed an opposite trend with sediment losses under swell conditions and an increase in sediment volume due to wind waves and suggests that upper beach morphological change can be decoupled from the overall sediment budget a submerged accretionary feature the beach step grew and migrated onshore on the rising tide typically reaching a maximum relief of around 0 5 m then gradually decreased in prominence while migrating offshore as the tide fell resulting in little net morphological change the step was better developed during the falling tide than the rising tide and through its control on the swash hydro and sediment dynamics the step had a significant effect on the development of the berm swash flow velocities under the conditions measured were often in excess of 2 m s 1 for both the uprush and the backwash onshore mean flows occurred in the upper swash region 40 inundation was characterised by offshore mean flows swash infiltration losses in the swash zone increased from 40 in the lower swash region to 100 in the upper swash region the most energetic swash flows and the strongest offshore mean flows occurred at the transition between swash zone and surf zone 90 100 inundation

the role of tides in beach cusp development 1 field measurements of morphology and swash flow during three episodes of beach cusp development indicate that tides modulate the height and cross shore position of beach cusps during rising tide beach cusp height decreases as embayments accrete more than horns and the cross shore extent of beach cusps decreases during falling tide beach cusp height increases as embayments erode more than horns and cross shore extent increases a numerical model for beach cusp formation based on self organization extended to include the effects of morphological smoothing seaward of the swash front and infiltration into the beach reproduces the observed spacing position and tidal modulation during rising tide water particles simulating swash infiltrate preferentially in embayments causing enhanced deposition during falling tide exfiltration of water particles combined with diversion of swash from horns causes enhanced erosion in embayments smoothing of beach morphology in the swash zone seaward of the swash front and in the shallow surf zone accounts for most of the observed tidal modulation even in the absence of infiltration and exfiltration despite the qualitative and in some cases quantitative agreement of the model and measurements the model fails to reproduce observed large deviations of horn orientation from shore normal some aspects of beach cusp shape and deviations from the basic tidal modulation possibly because of the simplified parameterization of cross shore sediment transport and the neglect of the effects of sea surface gradients on flow index terms 4546 oceanography physical nearshore processes 4255 oceanography general numerical modeling 3220 mathematical geophysics nonlinear dynamics keywords beach cusp tides infiltration exfiltration erosion

repeated pleistocene glaciation of the east siberian continental margin during glacial periods ice sheets covered continental margins through much of arctic north america greenland and western eurasia marine structures suggest that an ice sheet up to a kilometre in depth periodically covered the east siberian continental shelf as well

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

u pb geochronology of archean metasedimentary rocks in the pontiac and abitibi subprovinces quebec constraints on timing provenance and regional tectonics abstract u pb isotopic ages of single detrital zircons have been obtained from metasandstones in the northern part of the pontiac subprovince and from the cadillac kewagama and lac caste groups within the southern abitibi subprovince youngest detrital zircon ages from these sequences are 2685 2686 2687 and 2693 ma respectively the lac fourniere pluton which intrudes pontiac group metasedimentary rocks is dated at 2682 1 ma a similar age was previously measured from a pluton that cuts the kewagama group these data bracket deposition of greywacke in the pontiac and abitibi subprovinces to 2685 3 ma in contrast the youngest zircon found in a volcaniclastic unit interbedded with timiskaming group metawacke is 2673 2 ma while the age of a pluton that intrudes the timiskaming is 2672 1 ma thus the timiskaming group is distinctly younger than the other metasedimentary rocks the normalized probability distributions of detrital zircon ages from the pontiac and the abitibi metasedimentary rocks as well as of dated igneous rocks from the southern abitibi subprovince all show that their major zircon populations formed in the age range 2685 2710 ma the distributions for the abitibi metaturbidites and dated igneous rocks are very similar suggesting that metasedimentary rocks in the abitibi belt were mostly derived from the surrounding granite greenstone terrain the largest peak of the pontiac distribution is a few million years younger than that from the abitibi metasedimentary rocks but there is a correlation between major peaks and both groups show similar age ranges for older pre abitibi components 2750 2850 and 3020 3030 ma deposition of the ca 2685 ma sediments was virtually syn deformational based on previous field geochemical and isotopic studies as well as the detrital zircon age data the pontiac group may represent a foreland basin that may have developed from an accretionary prism the source of much of the sediment may have been an uplifted extinct continental arc to the north the arc was mostly eroded into the sediment basin and perhaps tectonically overridden by a southward advancing nappe complex comprised of its back arc basin now partly preserved in the southern volcanic zone of the abitibi subprovince metaturbidites exposed between metavolcanic assemblages within the greenstone belt may represent overturned unconformities at the structural base of the greenstones these were brought up by high angle faults which also localized deposition of non marine timiskaming type sediments it is tentatively suggested that pontiac type metasedimentary rocks extend most of the way beneath the abitibi subprovince the oldest part of the basin emerging as the quetico group in the western superior province

quaternary geology of the duck hawk bluffs southwest banks island arctic canada a re investigation of a critical terrestrial type locality for glacial and interglacial events bordering the arctic ocean duck hawk bluffs southwest banks island is a primary section 8 km long and 60 m high in the western canadian arctic archipelago exposing a long record of quaternary sedimentation adjacent to the arctic ocean a reinvestigation of duck hawk bluffs demonstrates that it is a previously unrecognized thrust block moraine emplaced from the northeast by laurentide ice previous stratigraphic models of duck hawk bluffs reported a basal unit of preglacial fluvial sand and gravel beaufort fm forested arctic overlain by a succession of three glaciations and at least two interglacials our observations dismiss the occurrence of preglacial sediments and amalgamate the entire record into three glacial intervals and one prominent interglacial the first glacigenic sedimentation is recorded by an ice contact sandur containing redeposited allochthonous organics previously assigned to the beaufort fm this is overlain by fine grained sediments with ice wedge pseudomorphs and well preserved bryophyte assemblages corresponding to an interglacial environment similar to modern the second glacial interval is recorded by ice proximal mass flows and marine rhythmites that were glacitectonized when laurentide ice overrode the site from amundsen gulf to the south sediments of this interval have been reported to be magnetically reversed 780 ka the third interval of glacigenic sedimentation includes glacifluvial sand and gravel recording the arrival of laurentide ice that overrode the site from the northeast island interior depositing a glacitectonite and constructing the thrust block moraine that comprises duck hawk bluffs sediments of this interval have been reported to be magnetically normal 780 ka the glacitectonite contains a highly deformed melange of pre existing sediments that were previously assigned to several formally named marine and interglacial deposits resting in an undeformed sequence in contrast the tectonism associated with the thrust block moraine imparted pervasive deformation throughout all underlying units highlighted by a previously unrecognized raft of cretaceous bedrock during this advance laurentide ice from the interior of banks island coalesced with an ice stream in amundsen gulf depositing the interlobate sachs moraine that contains shells as young as 24 cal ka bp late wisconsinan during deglaciation meltwater emanating from these separating ice lobes deposited outwash that extended to deglacial marine limit 11 m asl along the west coast of banks island our new stratigraphic synthesis fundamentally revises and simplifies the record of past quaternary environments preserved on southwest banks island which serves as a key terrestrial archive for palaeoenvironmental change

erosion accretion patterns and multiple beach cusp systems on a meso tidal steeply sloping beach abstract this study examines daily variations in intertidal topography and beach cusps on a meso tidal steeply sloping beach during a five month period the observations are based on coastal imagery and frequent topographic surveys and are extracted from patterns of contours positioned 1 and 3 m above msl lower higher beach face respectively the intertidal topography was very sensitive to variations in the wave regime triggering rapid transitions from erosive to accretive states and vice versa as well as transformations of the cusp systems often involving severe horn erosion and beach face scarping distinct beach cusp systems were observed at different levels of the beach with their evolution being controlled by i wave forcing ii tidal modulation of the wave power at different levels of the beach and iii interactions between the existing beach cusp systems the lower beach face rhythmical patterns changed constantly and merged with the upper beach face cuspate features typically when the significant wave height exceeded h o 1 5 m the upper beach face pattern was persistent during the monitoring period re emerging with similar spacing and alignment despite being eroded during energetic events this persistence is also maintained by the interactions between swash and irregularities e g embayments in the dune backshore morphology the spacing of the observed beach cusps ranged from c min 8 m to c max 67 m these values were underestimated by the edge wave and self organization theories and were better predicted by the sunamura 2004 formula beach cusp growth was linked to breaker angles of near normal incidence and narrow banded frequency wave conditions bay infilling was found to be the most dominant cusp reduction mechanism for larger features c 30 m while smaller features vanished during erosive conditions the most intense cusp growth episodes were linked to accretive conditions while larger features were shown to emerge during more energetic erosion events these findings highlight the need for more longer term observations of diverse beach morphologies to formulate a global theory regarding beach cusp formation and evolution

the seismic architecture and geometry of grounding zone wedges formed at the marine margins of past ice sheets grounding zone wedges form where halts of decades to centuries during ice sheet retreat across polar shelves allow sediment buildup at the grounding zone through delivery of deforming basal debris from fast flowing ice streams thirty grounding zone wedges were identified on two dimensional seismic reflection profiles across the continental shelves of ne and nw greenland grounding zone wedges close to the seafloor are probably of late weichselian age others are on paleoshelves buried beneath thick prograding glacial sediments several greenland shelf grounding zone wedges occur on topographic highs that provide stabilizing pinning points in relatively shallow water grounding zone wedges are asymmetric in the ice flow direction with steeper ice distal sides typical grounding zone wedges are approximately 5 20 km long and 50 100 m thick with a lateral width of several tens of kilometers forty grounding zone wedges from the greenland norwegian and antarctica margins show considerable variability about these values grounding zone wedge dimensions are controlled by sediment flux the duration of halts in ice retreat subice cavity shape and ice stream width low angle ice shelf cavity roofs immediately beyond the grounding zone probably restrict vertical accommodation space preventing formation of high amplitude ridges grounding zone wedges are mainly transparent or chaotic on seismic profiles probably resulting from delivery of diamictic debris offlapping reflectors represent progradation into an ice roofed cavity reflector truncations of grounding zone wedge bases indicate erosion during initiation channels are present within some grounding zone wedges meltwater flow is under high pressure and v shaped incisions suggest high energy flow channels are not ubiquitous within greenland grounding zone wedges and may represent nonsteady meltwater release perhaps through lake drainage where ice sheet mass loss is dominated by meltwater runoff grounding zone wedges probably contain more sorted sediment than those from greenland simple models of grounding zone wedge architecture are presented grounding zone wedges indicate episodic rather than catastrophic ice sheet retreat and are a mechanism for ice shelf stabilization because wedge growth counteracts collapse induced by ice sheet thinning and sea level rise

test of edge wave forcing during formation of rhythmic beach morphology 1 this paper reports on concurrent measurements of evolving rhythmic beach morphology and the associated water motion on a macrotidal coarse sandy beach beach cusps with a spacing of 19 m developed over a 3 day period of low energy swell after which they were destroyed by energetic wind waves the beach cusps were clearly accretionary features and developed mainly during the rising tide detailed hydrodynamic measurements were conducted at three locations in the swash and inner surf zone in the vicinity of a developing cusp horn and revealed that energetic standing mode zero subharmonic edge waves were not present during the initiation of the beach cusps specifically analysis of the hydrodynamic data indicated that 1 the amount of subharmonic energy was limited 10 of the total energy 2 none of the computed energy spectra exhibited a significant peak at the subharmonic frequency 3 the cross shore velocity amplitude at the subharmonic frequency was 2 3 times larger than the longshore velocity amplitude 4 the longshore current motion at the subharmonic frequency was barely coherent between adjacent locations in the surf zone and 5 the time series of pressure and cross shore current were in phase at either side of the developing cusp horn it is concluded that at least during this event of beach cusp formation energetic standing edge waves are not a prerequisite to initiate the development of rhythmic beach morphology

a major trough mouth fan on the continental margin of the bellingshausen sea west antarctica the belgica fan a 330 km length of the little known continental shelf edge and slope of the bellingshausen sea west antarctica is investigated using multibeam swath bathymetric and sub bottom profiler evidence the shelf break is at 650 700 m across the 150 km wide belgica trough and to either side is about 500 m when full glacial ice advanced across the shelf to reach the shelf break it was partitioned into fast and slow flowing elements with an ice stream filling the trough this had important consequences for the nature and rate of sediment delivery to the adjacent continental slope off belgica trough the upper continental slope has convex outward contours indicating a major sedimentary depocentre of gradient 1 2 degrees acoustic profiles and cores from the depocentre show a series of diamictic glacigenic debris flows the depocentre is interpreted as a trough mouth fan built largely by debris delivered from the ice stream the slope is steeper beyond the trough margins at up to 6 degrees the main morphological features on the bellingshausen sea slope are gully systems and channels major canyons and late quaternary slides are absent most gullies and channels are found on the fan gullies are about 15 25 m deep a few hundred metres wide and some are 25 km long the largest channel is over 60 km long about a kilometre wide and 10 to 15 m deep the channels provide pathways for sediment by passing of the upper slope and transfer to the continental rise and beyond by turbidity currents gullies on the bellingshausen sea margin cut through debris flows on the slope assuming the debris flows are linked mainly to downslope transport of diamictic debris when ice was at the shelf edge under full glacial conditions then those gullies cut into them formed during deglaciation belgica fan is 22 000 km 2 in area and about 60 000 km 3 in volume it is the largest depocentre identified to date on the continental margin of the west antarctic ice sheet fed by an interior ice sheet basin of approximately 200 000 km 2 c 2008 elsevier b v all rights reserved

evidence for multiple quaternary ice advances and fan development from the amundsen gulf cross shelf trough and slope canadian beaufort sea margin abstract the seismic stratigraphy and sedimentary architecture of the amundsen gulf trough and adjacent slope canadian beaufort sea margin are investigated using a grid of 2 d seismic reflection data the inner shelf of the amundsen gulf trough is interpreted to be composed predominantly of exposed or near surface bedrock overlain by a spatially discontinuous veneer of glacimarine to open marine sediment there is a seaward transition from exposed bedrock on the inner shelf to a thick up to 500 m outer shelf prograding wedge of acoustically semi transparent sediment eight seismic sequences divided into four megasequences are described from the outer shelf stratigraphy eight till sheets are identified from megasequences a to c providing evidence for at least eight quaternary ice stream advances through the amundsen gulf trough to the shelf break a trough mouth fan with a minimum volume of about 10 000 km 3 is present on the adjacent slope the amundsen gulf ice stream probably represented the most northwesterly marine terminating ice stream of the laurentide ice sheet through much of the quaternary providing a major route for ice and sediment transfer to the arctic ocean the youngest till sheet within the amundsen gulf trough megasequence d was probably deposited by a subsidiary ice stream the anderson ice stream subsequent to retreat of the last late wisconsinan amundsen gulf ice stream this provides evidence of dynamic ice sheet behaviour and the reorganisation of the northwest laurentide ice sheet margin during the last deglaciation a number of buried glacigenic landforms including palaeo shelf break gullies and a grounding zone wedge with a volume of 90 km 3 are described from the amundsen gulf trough stratigraphy lateral grounding zone wedges are identified at the northern and southern lateral margins of the amundsen gulf and m clure strait troughs respectively and are interpreted to have been formed roughly contemporaneously by ice streams in amundsen gulf and m clure strait

comparison of storm cluster vs isolated event impacts on beach dune morphodynamics two dimensional numerical simulations were used to investigate the impacts of storm clustering on the beach dune evolution of the sefton coast liverpool bay uk a storm cluster consisting of a series of closely spaced seven events was identified using observed wave and surge data during the 2013 2014 winter period first event in this cluster is regarded as exceptionally intense and the occurrence of seven storms within a very short time period is unique the xbeach coastal area model was used to simulate beach change from 1 the storm sequence clustered events and 2 the same storms considering them as isolated events offshore metadata was transformed to the nearshore area using the delft3d and swan models resulting evolution was first compared with the available post storm profiles measured at a number of locations along the sefton coast analysis of the clustered and isolated simulations showed the effect of clustering on the sefton beach dune system when compared to the impact of isolated events occurring on a fully recovered beach system morphological change occurred during each storm in the cluster was influenced by the preceding storm s such that the evolution is not proportional to the storm power of the event as it would be for isolated events both storm cases resulted in heavy erosion at formby point i e central of the sefton coast and accretion in the north and south the cluster prevented system recovery with the area of erosion continually extending south along the coast compared with that in isolated events the initial storm within the cluster caused large bed level changes in the nearshore ridge runnel system enabling the subsequent storms to penetrate further south the local convex geometry of the sefton coast is found to have more influence on the beach dune morphodynamics than the clustering effect this study enhances the understanding beach dune response to storm clusters to interpret observed morphological changes and to develop tools for sustainable coastal management particularly in the sefton coast and generally in similar systems worldwide

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

chemical sedimentary protoliths in the 3 75 ga nuvvuagittuq supracrustal belt quebec canada abstract analyses of chemical sedimentary precipitates such as banded iron formation bif provide a direct means to explore the nature and composition of the early hydrosphere the recently discovered 3750 myr old nuvvuagittuq supracrustal belt nsb in the northeast superior province quebec canada hosts a suite of iron oxide rich pyroxene and amphibole units that are interpreted to be the metamorphosed equivalents of fe oxide facies bif and a collection of bif like ca fe mg silicate rocks the nsb rocks provide a rare glimpse of trace metal availability in eoarchean ca 3800 ma seawater as they may be contemporaneous with the relatively well studied isua supracrustal belt of southern west greenland their comparison provides an opportunity to enhance our basic understanding of the eoarchean oceans at a global scale work since the initial discovery of the nsb in 2001 has established the basic lithological geochemical and petrographic characteristics of these bif and bif like rocks here we review the current state of knowledge of nsb rocks of probable chemical sedimentary origin including aspects of their geology likely origin and age we conclude by examining the implications of results thus far for our understanding of early seawater compositions and for the emergence of life in the context of early metallo enzyme evolution

late wisconsinan erosion and eolian deposition summer island area pleistocene mackenzie delta northwest territories optical dating and implications for glacial chronology the summer island area of the pleistocene mackenzie delta western canadian arctic is thought to have been last glaciated during the early wisconsinan 35 ka with a suggested late wisconsinan 13 ka glacial limit located 80 100 km to the south cryostratigraphic observations in the summer island area identify eolian and fluvial sediments overlying an erosion surface on summer island eolian sand occurs within sand wedges the tops of which are contiguous with an eolian sand sheet deposited on a formerly bare ice surface this paleo ice surface is attributed to fluvial erosion and that in turn is attributed to proglacial meltwater activity the age of erosion is inferred to be shortly before 14 ka determined by optical dating using infrared stimulation of luminescence from feldspar of the eolian sand wedges this 14 ka meltwater erosion is significantly north of the generally accepted limit of late wisconsinan glaciation in this coastal area suggesting that ice last covered the whole of rich

late archean structural and metamorphic history of the wind river range evidence for a long lived active margin on the archean wyoming craton the archean rocks of the wind river range in western wyoming record a late archean history of plutonism that extends for more than 250 m y the range is dominated by granitic plutons including the 2 8 ga native lake gneiss the 2 67 ga bridger batholith the 2 63 ga louis lake batholith and late 2 54 ga granites these plutons provide a means of distinguishing the complex metamorphism and deformation that affected the range in the late archean five deformation events are recorded d1 is a penetrative deformation that occurred during the earliest granulite facies metamorphism d2 is a folding event probably in amphibolite facies that deforms porphyritic dikes that cut the d1 fabrics both d1 and d2 predate the intrusion of the ca 2 8 ga native lake gneiss d3 is a folding event accompanied by upper amphibolite to granulite metamorphism that deformed the medina mountain sequence a sequence of rocks that was either deposited or thrust upon the native lake gneiss d4 is a fabric forming event associated with the mount helen structural belt mhsb it is represented by mylonites in the mhsb a penetrative fabric in the bridger batholith and folding of the d3 structures in the medina mountain sequence we consider d3 and d4 to be coeval with the emplacement of the bridger batholith and hence to date at ca 2 67 ga the latest structures d5 are fabrics associated with the folding and thrusting of the 2 65 ga south pass sequence we recognize at least four metamorphic events m1 is associated with the d1 fabrics and occurred at high t 750 c and high p 7 8 kilobars m2 650 750 c and 4 5 5 kilobars is associated with the intrusion of the bridger batholith and formation of the d3 and d4 structures the d5 structures of the south pass sequence record m3 which is low p 2 3 kilobars and low t 500 c the final metamorphism m4 is a contact metamorphism around the louis lake batholith in the south against the south pass sequence the metamorphism occurred at 3 kilobars and at temperatures 700 c in contrast in the north where the louis lake batholith is charnockitic the metamorphism occurred at 6 kilobars and 800 c this pressure gradient is probably a reflection of tilting of the wind river block during the laramide orogeny the composition of the plutons and the structural and metamorphic history of the wind river range indicate that during the late archean this area occupied the active margin of the wyoming province this tectonic environment is similar to the long lived phanerozoic margins of north america the wind river range represents the best documented active margin of archean age

imaging evidence for hubbard glacier advances and retreats since the last glacial maximum in yakutat and disenchantment bays alaska high resolution 2 d multichannel seismic data collected during the 2012 utig usgs national earthquake hazards reduction program survey of disenchantment and yakutat bays in southeast alaska provide insight into their glacial history these data show evidence of two unconformities appearing in the form of channels and are interpreted to be advance pathways for hubbard glacier the youngest observable channel thought to have culminated near the main phase of the little ice age lia is imaged in disenchantment bay and ends at a terminal moraine near blizhni point an older channel thought to be from an advance that culminated in the early phase of the lia extends from disenchantment bay into the northeastern edge of yakutat bay turning southward at knight island and terminating on the southeastern edge of yakutat bay our interpretation is that hubbard glacier has repeatedly advanced around the east side of yakutat bay in knight island channel possibly due to the presence of malaspina glacier cutting off access to central yakutat bay during times of mutual advance we observe two distinct erosional surfaces and retreat sequences of hubbard glacier in yakutat bay supporting the hypothesis that minor glacial advances in fjords do not erode all prior sediment accumulations interpretation of chaotic seismic facies between these two unconformities suggests that hubbard glacier exhibits rapid retreats and that disenchantment bay is subject to numerous episodes of outburst flooding and morainal bank collapse these findings also suggest that tidewater glaciers preferentially reoccupy the same channels in bay and marine settings during advances

subglacial bedform evidence for a major palaeo ice stream and its retreat phases in amundsen gulf canadian arctic archipelago ascertaining the location of palaeo ice streams is crucial in order to produce accurate reconstructions of palaeo ice sheets and examine interactions with the ocean climate system this paper reports evidence for a major ice stream in amundsen gulf canadian arctic archipelago mapping from satellite imagery landsat etm and digital elevation models including bathymetric data is used to reconstruct flow patterns on southwestern victoria island and the adjacent mainland nunavut and northwest territories several flow sets indicative of ice streaming are found feeding into the marine trough and cross cutting relationships between these flow sets and utilising previously published radiocarbon dates reveal several phases of ice stream activity centred in amundsen gulf and dolphin and union strait a large erosional footprint on the continental shelf indicates that the ice stream ca 1000 km long and ca 150 km wide filled amundsen gulf probably at the last glacial maximum subsequent to this the ice stream reorganised as the margin retreated back along the marine trough eventually splitting into two separate low gradient lobes in prince albert sound and dolphin and union strait the location of this major ice stream holds important implications for ice sheet ocean interactions and specifically the development of arctic ocean ice shelves and the delivery of icebergs into the western arctic ocean during the late pleistocene copyright c 2006 john wiley sons ltd

u pb age and nd isotopic evidence for archean terrane development and crustal recycling in the south central wabigoon subprovince canada u pb geochronology is presented for 19 volcanic and associated plutonic rocks from nine greenstone belts within the southern part of the central wabigoon subprovince cws western superior province data indicate that early felsic volcanism 3 014 2 999 ma was coeval with emplacement of the marmion batholith at 3 002 ma this was followed by periods of mafic volcanism with minor felsic and komatiitic compositions between 2 963 and 2 898 ma across the south cws plume driven mafic and komatiitic volcanism occurred in the lumby lake and lac des mille lacs belts at ca 2 828 ma eruption of komatiitic tuffs in the steep rock belt at 2 780 ma was proceeded by more widespread mafic and minor intermediate to felsic volcanism at 2 729 2 726 ma zircon inheritance and nd isotope data indicate that the cws south of the garden lake belt comprises a juvenile 3 0 ga terrane the marmion terrane subsequent magmatism was ensialic occurring in a continental although submerged setting recycling of the 3 0 ga marmion basement during successive volcanic events although somewhat cryptic was widespread and represents a significant archean process 2 726 ma rhyolite in the garden lake belt has a 3 22 ga nd model age and lies at the southern boundary of the distinct north cws which has a 3 3 3 2 ga crustal prehistory the north cws may represent a heavily reworked extension of the adjacent 3 3 ga winnipeg river subprovince and was probably separate from the marmion terrane at 3 0 ga these two recycled terranes differ significantly from the juvenile neoarchean volcanic dominated western wabigoon although the boundaries between these terranes are in most places cryptic they may indeed represent suture zones which were fundamental to the development of the wabigoon subprovince

ice sheet grounding zone wedges gzws on high latitude continental margins abstract grounding zone wedges gzws are asymmetric sedimentary depocentres which form through the rapid accumulation of glacigenic debris along a line source at the grounding zone of marine terminating ice sheets during still stands in ice sheet retreat gzws form largely through the delivery of deforming subglacial sediments the presence of gzws in the geological record indicates an episodic style of ice retreat punctuated by still stands in grounding zone position moraine ridges and ice proximal fans may also build up at the grounding zone during still stands of the ice margin but these require either considerable vertical accommodation space or sediment derived from point sourced subglacial meltwater streams by contrast gzws form mainly where floating ice shelves constrain vertical accommodation space immediately beyond the grounding zone an inventory of gzws is compiled from available studies of bathymetric and acoustic data from high latitude continental margins the locations and dimensions of gzws from the arctic and antarctic alongside a synthesis of their key architectural and geomorphic characteristics are presented gzws are only observed within cross shelf troughs and major fjord systems which are the former locations of ice streams and fast flowing outlet glaciers typical high latitude gzws are less than 15 km in along flow direction and 15 to 100 m thick gzws possess a transparent to chaotic acoustic character which reflects the delivery of diamictic subglacial debris many gzws contain seaward dipping reflections which indicate sediment progradation and wedge growth through continued delivery of basal sediments gzw formation is inferred to require high rates of sediment delivery to a fast flowing ice margin that is relatively stable for probably decades to centuries although the long term stability of the grounding zone is controlled by ice sheet mass balance the precise location of any still stands is influenced strongly by the geometry of the continental shelf the majority of high latitude gzws occur at vertical or lateral pinning points which encourage grounding zone stabilisation through increasing basal and lateral drag and reducing mass flow across the grounding zone

new marine evidence for a late wisconsinan ice stream in amundsen gulf arctic canada abstract amundsen gulf and adjoining dolphin and union strait and coronation gulf form the southwestern end of the northwest passage adjacent to the beaufort sea extensive high resolution multibeam sonar imagery and sub bottom profiles of the seabed have been acquired primarily in amundsen gulf by arcticnet and the ocean mapping group at the university of new brunswick these data reveal a variety of seabed landforms including mega scale glacial ridge and groove lineations drumlins moraines iceberg scours bedrock outcrops and discontinuous sediment deposits of variable thickness the lineations are widespread especially in southeastern amundsen gulf they resemble modern and paleo bedforms reported from antarctica svalbard greenland and other canadian arctic channels where they have been ascribed to ice streams the glacial sole marks on the seabed in amundsen gulf and regional data from the adjacent mainland and islands outline the configuration of a glacial ice stream from the laurentide ice sheet that occupied amundsen gulf and adjoining waterways during the late wisconsinan part of the northwestward flowing ice stream was deflected around the colville mountains on victoria island and rejoined the main ice stream in amundsen gulf by way of prince albert sound the grounded amundsen gulf ice stream extended northwestward to the outer slope in the beaufort sea where it was buttressed by arctic shelf ice maximum ice stream extent is inferred to have been coincident with the late glacial maximum multi sequence ice contact sediments and stratigraphic relations with glaciomarine sediments indicate that several ice advances and retreats occurred in the northwestern part of the gulf final retreat from the maximum position began prior to 13 000 cal yr bp and terrestrial dates indicate that the retreating ice front had reached dolphin and union strait by about 12 5 cal ka bp

beach cusps as self organized patterns computer simulations of flow and sediment transport in the swash zone on a beach demonstrate that a model that couples local flow acceleration and alongshore surface gradient is sufficient to produce uniformly spaced beach cusps the characteristics of the simulated cusps and the conditions under which they form are in reasonable agreement with observations of natural cusps the self organization mechanism in the model is incompatible with an accepted model in which standing alongshore waves drive the regular pattern of erosion and deposition that gives rise to beach cusps because the models make similar predictions it is concluded that currently available observational data are insufficient for discrimination between them

multiple meltwater discharge and ice rafting events recorded in the deglacial sediments along the beaufort margin arctic ocean abstract the first regional model for the deglacial history of the beaufort margin slope in the western arctic is presented the conceptual model was developed using new high resolution chirp seismic reflection multibeam bathymetry and sediment core data acquired along the margin this synthesis provides important constraints on sediment source and dispersal patterns for the last deglaciation the continental slope from barrow canyon to the mackenzie trough is characterized by thick holocene sediments mostly sourced from barrow canyon and continental discharge this acoustically transparent fine grained unit overlies acoustically laminated sediments sourced from the mackenzie river and ice rafting the deglacial history of the margin from mackenzie trough to the amundsen gulf is characterized by many ice rafting and meltwater discharge events ice rafted debris layers were deposited around 14 6 ka and 14 1 ka and likely document enhanced ice shedding events from the amundsen and m clure ice streams as they retreated there are three inferred meltwater discharge events all sourced from the mackenzie region the oldest discharge event had two phases one sometime between 14 5 ka and 14 2 ka and another between 14 0 ka and 13 0 ka this discharge deposited finely laminated sediments more than 7 m thick sourced from proglacial lakes in the area following this was a flood event starting at 12 94 ka which generated high amplitude reflectors deposited coarse debris and caused a lowering in the 18o record this is possibly a major outburst flood from glacial lake agassiz with an additional component of agassiz meltwater that was diverted from its southern drainage pathway down the mississippi to the gulf of mexico finally a third discharge event was initiated by 11 3 ka which deposited coarse laminated sediments focused in the mackenzie trough timing of these second and third events correlates with the onset of the younger dryas cold period and the preboreal oscillation event respectively suggesting that fresh water discharge may have reached a tipping point leading to those climate events

high frequency in situ field measurements of morphological response on a fine gravel beach during energetic wave conditions this paper presents novel data collected over four weeks at loe bar a fine gravel barrier d50 3 5 mm exposed to energetic wave conditions hs10 2 4 m combined remote and in situ measurements were used to identify 3d morphological response profile change and runup behaviour during successive energetic periods hs 2 5 m small intertidal beach volume losses occurred under high steepness waves h0 l0 0 01 but in general the overall beach volume was remarkably stable in fact the greatest morphological response was evident not in cross shore sediment exchanges but due to the alongshore redistribution related to the three dimensional beach cusp system specifically during energetic wave conditions the cusp morphology underwent horn growth and embayment deepening leading to increased cusp definition an additional explanation for the stability of the cross shore beach profile is related to the development of the beach step during energetic conditions and long period waves significant deposition c 1 m occurred at the top of the beach step above the still water line the enhanced step provided a focal point for wave breaking considerably curtailing the swash motion and reducing the runup limit thereby providing protection to the upper beach this study provides much needed insight into complex morphological response and runup characteristics on gravel beaches fundamental for improved model development and leading towards better predictive tools

data for enhanced deposition of coal fragments at the chukchi margin western arctic ocean implications for deglacial drainage history from the laurentide ice sheet abstract distribution and composition of coarse particles 250 m were investigated in 13 sediment cores from the chukchi margin western arctic ocean petrographic clast identification is supplemented by xrf core scanning eds analysis of coal fragments and ams 14c dating of planktic foraminifera for age control coal debris distribution is also investigated in surface sediments of the study region for provenance interpretation the sediment core study focuses on the interval between the last glacial maximum and the holocene for constraining provenance and timing of the deglacial discharge we find that this interval is characterized by a distinct enrichment in sedimentary rock fragments including coal comparison with longer stratigraphic records indicates that this composition is unique for the time interval since at least maine isotope stage 5 based on the timing interpreted provenance and geographic distribution of the coal enrichment we conclude that the most likely primary source was the deglacial discharge from the mackenzie drainage basin of the laurentide ice sheet lis the identified coal bearing layer also expressed in the xrf sulfur record can be thus used as a stratigraphic marker for the lis discharge to the arctic ocean during the last deglaciation

arctic ocean glacial history while there are numerous hypotheses concerning glacial interglacial environmental and climatic regime shifts in the arctic ocean a holistic view on the northern hemisphere s late quaternary ice sh

mafic to felsic magmatism and crustal recycling in the obonga lake greenstone belt western superior province evidence from geochemistry nd isotopes and u pb geochronology abstract the central wabigoon subprovince of the western superior province contains mesoarchean granitoid and supracrustal rocks 3 01 2 83 ga and 2 78 2 69 ga granitoid plutons and supracrustal sequences it is a key area for understanding the relationship between greenstone belts and surrounding granitoid rocks that may have acted as basement the obonga lake greenstone belt contains two distinct assemblages 1 a ta nb th they have end values of 0 7 to 2 4 a dacitic unit in the northern assemblage has high la yb high sr y low nb y and heavy ree and is interpreted to represent a mantle modified slab melt similar to adakites in cenozoic arcs the associated enriched basalts may represent melts from the mantle wedge modified by slab melt adakitic metasomatism or they may represent an enriched oib like asthenospheric source rocks of the southern assemblage mostly show th and light ree enrichment and negative nb and ta anomalies low end values down to 0 9 can be modelled through contamination of a mafic liquid by 3 3 3 2 ga sialic crust although rocks of that age have not been observed suggested mesoarchean basement to the southern assemblage may therefore have had a complex history spanning several hundred million years the southern assemblage volcanism may have occurred in a continental arc to continental back arc system where crustal recycling played an important role

beach cusp morphology on sand and mixed sand and gravel beaches south island new zealand abstract the morphology of beach cusps on a number of beaches around the south island new zealand is described cusp dimensions such as spacing elevation amplitude depth and spacing variance are interrelated cusp dimensions co vary in regular fashion under different process conditions such that the cusp form displays a high degree of regularity beach cusp amplitude is an exception to this regularity that appears to be controlled by factors such as grain size and beach slope analysis of cusp spacing and elevation dimensions on mixed sand and gravel beaches reveal that cusps having particular spacings form at distinct elevations on the beachface with the largest spacings occurring at the highest levels on the beachface the relationship between cusp elevation and spacing provides a technique through which beach cusp spacing can be used to infer mean sea level

ice sheet dynamics through the quaternary on the mid norwegian continental margin inferred from 3d seismic data reconstructing the evolution of ice sheets is critical to our understanding of the global environmental system but most detailed palaeo glaciological reconstructions have hitherto focused on the very recent history of ice sheets here we present a three dimensional 3d reconstruction of the changing nature of ice sheet derived sedimentary architecture through the quaternary ice age of almost 3 ma an extensive geophysical record documents a marine terminating calving fennoscandian ice sheet fis margin present periodically on the mid norwegian shelf since the beginning of the quaternary spatial and temporal variability of the fis is illustrated by the gradual development of fast flowing ice streams and associated intensification of focused glacial erosion and sedimentation since that time buried subglacial landforms reveal a complex and dynamic ice sheet with converging palaeo ice streams and several flow switching events that may reflect major changes in topography and basal thermal regime lack of major subglacial meltwater channels suggests a largely distributed drainage system beneath the marine terminating part of the fis this palaeo environmental examination of the fis provides a useful framework for ice sheet modelling and shows that fragmentary preservation of buried surfaces and variability of ice sheet dynamics should be taken into account when reconstructing glacial history from spatially limited datasets

widespread multi source glacial erosion on the chukchi margin arctic ocean abstract multibeam bathymetry and sub bottom profiler data acquired in 2011 from r v marcus langseth in a broad grid over the chukchi sea margin reveal multiple glacigenic features on the top and slopes of the outer chukchi shelf rise and adjacent borderland glacial lineations record a complex pattern of erosion likely formed by both local glaciation and far traveled ice shelves streams sourced from the laurentide and possibly east siberian ice sheets multiple till units and stacked debris flows indicate recurrent glacial grounding events composite till wedges of several hundred meters thick extend the shelf edge by 10 20 km in places distribution of ice marginal features on the chukchi rise suggests stepwise deglacial retreat towards the shelf backing up the broad bathymetric trough at the eastern side of the rise glacigenic features other than extensive iceberg scouring cannot be identified above 350 m depth and no glacigenic bedforms are present on the current swept shallow shelf despite the resulting uncertainty with the southern extent of the glaciation the data suggest a widespread grounded ice presence on the northern chukchi shelf which makes it an important previously underestimated component of the arctic paleo glacial system

an alternative suggestion for the pliocene onset of major northern hemisphere glaciation based on the geochemical provenance of north atlantic ocean ice rafted debris abstract the onset of abundant ice rafted debris ird deposition in the nordic seas and subpolar north atlantic ocean 2 72 millions of years ago ma is thought to record the pliocene onset of major northern hemisphere glaciation nhg due to a synchronous advance of north american laurentide scandinavian and greenland ice sheets to their marine calving margins during marine isotope stage mis g6 numerous marine and terrestrial records from the nordic seas region indicate that extensive ice sheets on greenland and scandinavia increased ird inputs to these seas from 2 72 ma the timing of ice sheet expansion on north america as tracked by ird deposition in the subpolar north atlantic ocean however is less clear because both europe and north america are potential sources for icebergs in this region moreover cosmogenic dating of terrestrial tills on north america indicate that the laurentide ice sheet did not extend to 39 n until 2 4 0 14 ma at least 180 ka after the onset of major ird deposition at 2 72 ma to address this problem we present the first detailed analysis of the geochemical provenance of individual sand sized ird deposited in the subpolar north atlantic ocean between mis g6 and 100 2 72 2 52 ma ird provenance is assessed using laser ablation lead pb isotope analyses of single ice rafted 150 m feldspar grains to track when an ice rafting setting consistent with major nhg first occurred in the north atlantic ocean during the pliocene intensification of nhg inhg we investigate when the pb isotope composition 206pb 204pb 207pb 204pb 208pb 204pb of feldspars deposited at dsdp site 611 first resembles that determined for ird deposited at this site during mis 100 2 52 ma the oldest glacial for which there exists convincing evidence for widespread glaciation of north america whilst quaternary magnitude ird fluxes exist at site 611 during glacials from 2 72 ma we find that the provenance of this ird is not constant instead we find that the pb isotope composition of ird at our study site is not consistent with major nhg until mis g2 2 64 ma we hypothesise that ird deposition in the north atlantic ocean prior to mis g2 was dominated by iceberg calving from greenland and scandinavia we further suggest that the grounding line of continental ice on northeast america may not have extended onto the continental shelf and calved significant numbers of icebergs to the north atlantic ocean during glacials until 2 64 ma

test of self organization in beach cusp formation 1 field observations of swash flow patterns and morphology change are consistent with the hypothesis that beach cusps form by self organization wherein positive feedback between swash flow and developing morphology causes initial development of the pattern and negative feedback owing to circulation of flow within beach cusp bays causes pattern stabilization the self organization hypothesis is tested using measurements from three experiments on a barrier island beach in north carolina beach cusps developed after the beach was smoothed by a storm and after existing beach cusps were smoothed by a bulldozer swash front motions were recorded on video during daylight hours and morphology was measured by surveying at 3 4 hour intervals three signatures of self organization were observed in all experiments first time lags between swash front motions in beach cusp bays and horns increase with increasing relief representing the effect of morphology on flow second differential erosion between bays and horns initially increases with increasing time lag representing the effect of flow on morphology change because positive feedback causes growth of beach cusps third after initial growth differential erosion decreases with increasing time lag representing the onset of negative feedback that stabilizes beach cusps a numerical model based on self organization initialized with measured morphology and alongshore uniform distributions of initial velocities and positions of the swash front at the beginning of a swash cycle reproduces the measurements except for parts of one experiment where limited surveys and a significant low frequency component to swash motions might have caused errors in model initialization

seismic stratigraphy sedimentary architecture and palaeo glaciology of the mackenzie trough evidence for two quaternary ice advances and limited fan development on the western canadian beaufort sea margin abstract a comprehensive grid of high resolution 2 d seismic reflection data from the western canadian beaufort sea margin is used to reconstruct past ice stream dynamics within the mackenzie trough eight seismic facies and five sequences divided into two megasequences are identified from the mackenzie trough stratigraphy evidence for two quaternary ice advances to the shelf break is provided by two sequences of acoustically chaotic to semi transparent facies interpreted as subglacial till buried landforms interpreted as lateral moraines and a grounding zone wedge record the positions of still stands or re advances in the ice margin the continental slope beyond the trough is characterised by canyons separated by inter canyon ridges and thin glacigenic debris flows correlation with the onshore record suggests that the older of the two ice advances which excavated the mackenzie trough probably occurred during the illinoian or early wisconsinan glaciations the younger ice stream advance is interpreted to have occurred during the last late wisconsinan glaciation the onset of cross shelf glaciation on the western canadian beaufort sea margin is inferred to have been initiated significantly later than on the eastern beaufort sea and eastern canadian arctic margins which have a longer history of ice advance and were less peripheral to the ice sheet centre the architecture of the slope beyond the mackenzie trough reflects this comparatively short history of ice advance and lacks the progradational architecture and major glacial sedimentary depocentre or trough mouth fan that is characteristic of slopes seaward of cross shelf troughs on formerly glaciated margins

relict subduction zones in canada 1 relict subduction zones that are interpreted on 10 lithoprobe deep seismic reflection profiles in canada provide key observational evidence that lithosphere plate interactions have operated with essentially the same resultant geometry for at least 2 7 gyr subduction zone reflections are generally found beneath known collisional belts and their orientations are often consistent with the locations of exposed arcs sutures and subduction complexes they project from the lower crust into the upper mantle and provide key geometric relationships between the crust moho and upper mantle the interpretation of subduction zone reflections as being syntectonic establishes a minimum age for the overlying lower crust and mantle wedge and precludes inferred formation of the moho and upper mantle by postorogenic delamination in these areas the orientation of subduction zone reflections may be synthetic or antithetic to exposed sutures indicating juxtaposition by subcretion or by delamination respectively the mantle wedge above the subduction zone reflections may exhibit sparse reflectivity subduction zone reflections commonly dip toward the craton indicating a preferred dip of subduction during the last stages of convergence within an orogen or else a higher likelihood of preservation beneath older domains interpreted ages of the canadian examples range from recent to neoarchean 2 7 ga providing observational evidence that plate tectonics or similar processes have been active since at least the neoarchean the locations and geometries of these relict subduction zones provide a snapshot of accretion boundaries in the mantle and establish a framework for the neoarchean recent accretionary architecture of the north american lithosphere

characterization of transpressive deformation in shear zones of the archean north caribou greenstone belt nw superior province and the relationship with regional metamorphism abstract the 2 7 3 0 ga north caribou greenstone belt ncgb host to the musselwhite bif hosted gold deposit possesses abundant shear zones on its northern margins which appear to have formed under amphibolite facies conditions protracted deformation and regional metamorphism are coeval with widespread magmatism and accretion events during crustal amalgamation of the western superior province and are responsible for folding the ore hosting bif and channeling fluids the importance of shear zones in behaving as conduits for fluids during the tectonic evolution of the ncgb is not well known and their relationship with metamorphism is equivocal yet higher grade syn to post tectonic metamorphic minerals seem to correlate with loci of higher strain structural analyses support oblique transpressive collision that produced steeply dipping planar and shallowly plunging linear fabrics with dominant dextral kinematics that trend broadly parallel to the doubly arcuate shape of the belt electron backscatter diffraction analyses were conducted on strategic samples across one shear zone in order to characterize crustal conditions during transpressive deformation the dinnick lake shear zone cuts through mafic metavolcanics and at its core is an l tectonite granite composed of recrystallized quartz whole rock geochemistry shows little variation in ca na mg and k often used as indicators of hydrothermal alteration from surrounding less deformed units suggesting deformation in a dry environment microstructural analysis indicates subgrain rotation recrystallization and deformation by prism a and c slip in quartz as well as aligned hornblende that suggest deformation temperatures above 500 c quartz in mafic rocks along the margins of the shear zone also exhibits a basal a slip component indicating a slight decrease in strain or temperature although the ncgb exhibits some first order evidence of vertical tectonism dome and keel geometries the dominant strain record within shear zones is that of horizontal oblique transpressive displacement this is in agreement with other greenstone belts in the western superior province where vertical tectonism and horizontal tectonism were coeval