subglacial bedrock welding associated with glacial earthquakes a c 635 ma glacial striations on a neoproterozoic sandstone pavement is documented only instantaneous solidification could have preserved the breccia in situ glacier speeds of c 0 3 m s 1 would have induced peak flash temperatures sufficient 1130 c to cause partial frictional welding of the deformed quartzitic bedrock beneath an ice loaded clast these results support suggestions that glacial seismicity is due to episodic very rapid shifts of large ice masses over the substrate stick slip movement they also indicate that significant effective normal stresses occur at the base of glaciers

holocene storage of siliciclastic sediment around islands on the middle shelf of the great barrier reef platform north east australia thick sequences of sediment surround the whitsunday islands on the middle shelf of the great barrier reef gbr platform much of this sediment is siliciclastic material deposited since the sea level highstand at around 6 5 ka this raises a mass balance dilemma because modern terrigenous discharge to the gbr platform is restricted to the inner shelf shallow seismic profiles and sediment samples were collected over 450 km2 around the whitsunday islands to quantify the mass of siliciclastic sediment for a dynamic model of the shelf the sea floor and pre holocene surfaces were mapped using 4584 stations along the seismic profiles and a graphical computer program the total volume of sediment between these two surfaces is 3 67 0 45 109 m3 this volume is composed of buried reefs 0 13 0 01 109 m3 medium 0 70 0 30 109 m3 and fine grained shoals 2 84 0 35 109 m3 the volume estimates combined with measurements of carbonate concentration density and porosity indicate that 1850 380 mt of holocene siliciclastic sediment surround the whitsunday islands in medium 510 225 mt and fine grained shoals 1340 155 mt the total mass of siliciclastic material is 1 7 2 6 times that stored in cleveland bay a similar sized repository on the inner shelf a simple numerical model has been constructed to explain this large quantity of holocene siliciclastic sediment the model results in the appropriate siliciclastic mass next to the whitsunday islands by integrating regional shelf processes over time unlike the present day rivers discharged sediment to the middle shelf during the early holocene this material was subsequently focused by northward transport into the vicinity of the islands a geomorphologically complex region that serves as a sediment trap although direct riverine inputs to the middle shelf have stopped during the present sea level highstand previously deposited siliciclastic sediment is continually being winnowed from the middle shelf and redeposited next to the whitsunday islands the transport and distribution of siliciclastic sediment on the gbr platform is thus influenced significantly by storage around islands on the middle shelf

volcanic ash retrieved from the grip ice core is not from thera 1 tephra found in an ice core from greenland grip has been claimed to be from the minoan eruption of thera santorini greece if true this would date the eruption thereby resolving a decades long debate in chronology herein it is shown that the methods used to match the greenlandic tephra with thera are flawed and that the geochemical data imply the tephra is not from thera

late precambrian glacially related grooved and striated surfaces in the tillite group of central east greenland abstract the tillite group of central east greenland comprises five formations of which two are dominantly glacigenic the ulveso formation formerly lower tillite and the storeelv formation formerly upper tillite both of which embrace a wide range of facies including glaciomarine nearshore and terrestrial glacial sediments evidence of periodic terrestrial conditions is indicated especially by a variety of grooved and striated surfaces on the island ella o in the ulveso formation grooves of complex geometry 0 4 m wide and 0 3 m deep were cut into a cemented conglomerate horizon they are interpreted as having formed as a result of subglacial meltwater cavitation at the base of the storeelv formation boulders thought to have been dropped into partially cemented sandstone by floating glacier ice were largely planed off by a subsequent advance of grounded ice in the middle of the storeelv formation a striated boulder pavement occurs within diamictite on the basis of stone fabric striation orientation and cross bedding data this pavement is inferred to have formed by the packing of stones along a shear surface within an accumulating lodgement till such features have rarely been observed in pre pleistocene glacigenic sequences they imply that each glacigenic formation accumulated during a complex series of advances and retreats rather than just one

carbonates within a pleistocene glaciomarine succession yakataga formation middleton island alaska uplifted during the 1964 alaskan earthquake extensive intertidal flats around middleton island expose 1300 m of late cenozoic early pleistocene yakataga formation glaciomarine sediments these outcrops provide a unique window intooutershelfandupperslopestratathatareotherwiseburiedwithinthesoutheast alaska continental shelf prism the rocks consist of five principal facies in descending order of thickness i extensive pebbly mudstone diamictite containing sparse marine fossils ii proglacial submarine channel conglomerates iii burrowedmudstoneswithdiscretedropstonelayers iv boulderpavements whose upper surfaces are truncated faceted and striated by ice and v carbonates rich in molluscs bryozoans and brachiopods the carbonates are decimetre scale in thickness typically channellized conglomeratic event beds interpreted as resedimented deposits on the palaeoshelf edge and upper slope biogenic components originated in a moderately shallow ca 80 m relatively sediment free mesotrophic sub photicsetting thesecomponentsareamixture of parautochthonous large pectenids or smaller brachiopods with locally important serpulid worm tubes and robust gastropods augmented by sand size bryozoan and echinoderm fragments ice rafted debris is present throughout these cold water carbonates that are thought to have formed during glacial periods of lowered sea level that allowed coastal ice margins to advance near to the shelf edge such carbonates were then stranded during subsequent sea level rise productivity was enabled by attenuation of terrigenous mud deposition during these cold periods via reduced sedimentation together with active wave and tidal current winnowing near the ice front redeposition was the result of intense storms and possibly tsunamis these sub arctic mixed siliciclasticcarbonate sediments are an end member of the phanerozoic global carbonate depositional realm whose skeletal attributes first appeared during late palaeozoic southern hemisphere deglaciation

grain size trends associated with net sediment transport patterns an example from the belgian continental shelf grain size trends in relation to net sediment transport pathways are examined using some commonly used grain size parameters the results of the trend analysis for 15 types of the trends are compared with a known net sediment transport pathways established on the basis of the sandbank hydro and sediment dynamics i e general water and sediment movement patterns and bedform asymmetry it is shown that 1 the results for grain size trends associated with a worsening in sorting along the transport pathways have little similarity to transport pathways 2 the results for grain size trends associated with an improvement in sorting along the transport pathways have relatively high degree of similarity to the identified pathways and 3 the residual pattern derived on the basis of a combined grain size trend used elsewhere gao and collins 1992 appears to be most similar and suitable for defining transport pathways

age and impacts of the caldera forming aniakchak ii eruption in western alaska the mid holocene eruption of aniakchak volcano aniakchak ii in southwest alaska was among the largest eruptions globally in the last 10 000 years vei 6 despite evidence for possible impacts on global climate the precise age of the eruption is not well constrained and little is known about regional environmental impacts a closely spaced sequence of radiocarbon dates at a peatland site over 1000 km from the volcano show that peat accumulation was greatly reduced with a hiatus of approximately 90 120 yr following tephra deposition during this inferred hiatus no paleoenvironmental data are available but once vegetation returned the flora changed from a cyperaceae dominated assemblage to a poaceae dominated vegetation cover suggesting a drier and or more nutrient rich ecosystem oribatid mites are extremely abundant in the peat at the depth of the ash and show a longer term increasingly wet peat surface across the tephra layer the radiocarbon sample immediately below the tephra gave a date of 1636 1446 cal yr bc suggesting that the eruption might be younger than previously thought our findings suggest that the eruption may have led to a widespread reduction in peatland carbon sequestration and that the impacts on ecosystem functioning were profound and long lasting

multiple pre wisconsinan glaciations along the northwestern edge of the allegheny plateau in ohio and pennsylvania stratigraphic studies of the glacial geology of the northwestern margin of the allegheny plateau of northeastern ohio and northwestern pennsylvania include evidence for multiple pre wisconsinan gla

controls on early permian shelf deglaciation in the karoo basin of south africa abstract the late carboniferous to early permian glacigene dwyka group has at its top three types of transitions from diamictite to postglacial mudrock in the southern karoo the type 1 transition sequence consists of clast poor carbonate rich diamictite having a sharp contact with the overlying mudrocks this diamicton accumulated slowly by rain out and sediment gravity flow processes during sedimentation the polar ice front was stabilised by a possible ice shelf collapse of the ice shelf caused rapid ice margin retreat the type 2 transition sequence consists of clast poor diamictite with sandstone bodies overlain by mudrock with ice rafted debris or thin interbedded debris flow diamictites deposition of the diamictite facies occurred during oscillations of a subpolar ice front with short stable periods of surface melting of the ice resulting in the formation of small subaqueous outwash fans the type 3 transition sequence consists of bouldery diamictite interbedded with massive or sandstone bearing diamictite overlain by mudrock with ice rafted debris or thin interbedded debris flow diamictites the sequence represents highly unstable ice front conditions and fluctuating sea levels with periods of rapid deposition probably by collapsing ice streams climatic conditions may have alternated between polar and subpolar the type 1 transition sequence is predominant in the western part and the type 3 sequence in the eastern part of the karoo basin long term controls on deglaciation was climatic warming as gondwana drifted over the pole as well as changes in land sea configuration superimposed on these were local controls such as relative sea level changes presence of deformable substrate and meltwater production

the permo carboniferous dwyka formation of southern africa deposition by a predominantly subpolar marine ice sheet abstract the 800 m thick glacigene dwyka formation was deposited along the northern margin of the permo carboniferous dwyka basin which covered an area of approximately 2 10 6 km 2 in southwestern gondwana the palaeogeographic setting geochemical data of the mudrocks and diamictites and the palaeontology indicate marine conditions during sedimentation ice lobes from spreading centres in the north east and south coalesced in the basin to form an extensive ice cover from the westphalian to the late sakmarian lodgement rain out and subaqueous debris flow diamictons subaqueous and subglacial melt water sands suspended mud and turbidity current sands and silts accumulated in the dwyka basin sedimentation started on the continental shelf during a grounded ice sheet stage predominantly lodgement processes then a floating ice stage predominantly debris rain out and finally an ice sheet disintegration stage debris rain out sand fall out and suspension settling of mud the palaeogeographic setting presence of marine conditions in the basin and the scale of glaciation indicate deposition from a predominantly mid latitudinal marine ice sheet the overall characteristics of the glacial sequence are neither typical of a polar nor a temperate setting and for such ancient glaciations a subpolar setting with the presence of unstable ice shelves is suggested

the polonez cove formation of king george island antarctica stratigraphy facies and implications for mid cenozoic cryosphere development the middle to late oligocene polonez cove formation exposed on south eastern king george island south shetland islands provides rare evidence of mid cenozoic west antarctic cryosphere evolution a revised lithostratigraphy and facies analysis and a review of the palaeoenvironmental significance of the formation are presented here the diamictite dominated basal member of the formation krakowiak glacier member records the presence and retreat of marine based ice on a shallow continental shelf five overlying members are recognized these consist of basaltic sourced sedimentary rocks and lavas and represent a variety of shoreface and shallow continental shelf environments in an active volcanic setting these units contain diverse reworked and ice rafted exotic clasts that become sparse towards the top of the formation suggesting a continuing but waning glacial influence new 40ar 39ar dates from interbedded lava flows indicate a late oligocene age 25 6 27 2 ma for the polonez cove formation but are slightly younger than skeletal carbonate sr isotope ages obtained previously 28 5 29 8 ma there is evidence for wet based subice conditions at the base of the polonez cove formation but no sedimentary facies to suggest substantial meltwater this may reflect a subpolar setting or may result from lack of preservation or a high energy depositional environment a northern antarctic peninsula south shetland islands provenance is probable for most non basaltic clasts but certain lithologies with possible origins in the transantarctic and ellsworth mountains also occur sparsely throughout the formation there is evidence to suggest that the presence of such far travelled clasts within subglacially deposited facies at the base of the formation reflects the advance of a local ice cap across marine sediments containing the clasts as ice rafted material the presence of these clasts suggests that extensive marine based ice drained into the southern weddell sea region and that a strong weddell sea surface current operated both before and during deposition of the polonez cove formation

subglacial sedimentary features in late palaeozoic sedimentary rocks central victoria australia abstract a section of late palaeozoic diamictite in the bacchus marsh formation in central victoria southeastern australia contains 1 1 striated boulders with long axes tilted up glacier 2 2 layers of pebbles lacking evidence of water sorting 3 3 thin sheets and lenses of sandstone 4 4 slab like and contorted sandstone bodies cut by closely spaced step fractures 5 5 thin slabs of rhythmically bedded siltstone and claystone the boulders indicate that the diamictite was deposited by lodgement beneath moving wet based glacial ice the sandstone sheets and lenses indicate periods of rapidly increasing subglacial water pressure and the deformed sandstone bodies indicate flow of some of the newly deposited diamicton in response to shear imparted by the overlying ice the pebble layers probably formed as a moving carpet of clasts close to the controlling obstacle size of the glacier during deposition from a layer of mobile subsole debris which then halted these features indicate changes in subglacial conditions that may have led to surging behaviour by the glacier the model of pebble layer formation allows an estimate of ice sliding velocity during layer formation and thus provides a method of quantifying one aspect of ice behaviour for a pre pleistocene glacier

glacial and rocky shore dynamics of the karlebotn monadnocks late neoproterozoic of northern norway located near the village of karlebotn in east finnmark norway a cluster of six gneissic monadnocks is unconformably surrounded by weakly metamorphosed sandstone strata of late neoproterozoic age in the smalfjord formation differentiated by faults the monadnocks were further sculpted by a glacier that flowed through a coastal valley concordant with the present day varangerfjorden the largest of the monadnocks is 337 m long and 167 m wide there remains ample evidence of glacial activity associated with this feature relict lodgement tillites are preserved on the west side of the monadnock flowtillites on the steep surfaces of the north and southwest sides and a small esker directly on top the valley was later inundated due to glacial eustacy and the karlebotn monadnocks became an archipelago in a shallow estuary of a broad fjord interpretation of exposed versus sheltered rocky shores on opposite sides of the largest monadnock island is supported by mineralogical variations in basal strata along t

sedimentation in a river dominated estuary the mgeni estuary on the wave dominated east coast of south africa occupies a narrow bedrock confined alluvial valley and is partially blocked at the coast by an elongate sandy barrier fluvial sediment extends to the barrier and marine deposition is restricted to a small flood tidal delta sequential aerial photography sediment sampling and topographical surveys reveal a cyclical pattern of sedimentation that is mediated by severe fluvial floods which exceed normal energy thresholds during severe floods up to 10x 103 m3 s 1 lateral channel confinement promotes vertical erosion ofbed material eroded material is deposited as an ephemeral delta in the sea after floods the river gradient is restored within a few months through rapid fluvial deposition and formation of a shallow braided channel over an extended period approximately 70 years the estuary banks and bars are stabilised by vegetation and mud deposition subsequent downcutting in marginal areas transforms the channel to an anastomosing pattern which represents a stable morphology which adjusts to the normal range of hydrodynamic conditions this cyclical pattern of deposition produces multiple fill sequences in such estuaries under conditions of stable sea level the barrier and adjacent coastline prograde temporarily after major floods as the eroded barrier is reformed by wave action but excess sediment is ultimately eroded as waves adjust the barrier to an equilibrium plan form morphology deltaic progradation is prevented by a steep nearshore slope and rapid sediment dispersal by wave action and shelf currents during transgression estuarine sedimentation patterns are controlled by the balance between sedimentation rates and receiving basin volume if fluvial sedimentation keeps pace with the volume increase of a basin an estuary may remain shallow and river dominated throughout its evolution and excess fluvial sediments pass through the estuary into the sea only if the rate of volume increase of the drowned river valley exceeds the volume of sediment supply are deep water environments formed under such conditions an estuary becomes a sediment sink and infills by deltaic progradation and lateral accretion as predicted by evolutionary models for microtidal estuaries bedrock valley geometry may exert an important control on this rate of volume increase independently of variations in the rate of relative sea level change if estuarine morphology is viewed as a function of the balance of wave tidal and fluvial processes the mgeni estuary may be defined as a river dominated estuary in which deltaic progradation at the coast is limited by high wave energy it is broadly representative of other river dominated estuaries along the natal coast and a conceptual regional depositional model is proposed refinement of a globally applicable model will require further comparative studies of river dominated estuaries in this and other settings but it is proposed that river dominated estuaries represent a distinct type of estuarine morphology

the paleoclimatic significance of deformation structures in neoproterozoic successions abstract this paper reviews the different types of soft sediment deformation structures that can form in glacial and non glacial settings and explores the potential use of these structures in resolving long standing debates in paleoenvironmental reconstructions of neoproterozoic glacigenic successions soft sediment deformation structures are created when compressional gravitational or shear stress is applied to unlithified sediments during or shortly after deposition in subglacial or ice marginal glacial settings shear and compressional stress imparted by ice moving on top of a deformable substrate or advancing ice buldozing unlithified ice marginal sediments can result in a wide range of folding faulting and shear structures in glaciofluvial or stagnant ice marginal setting gravitational collapse and remobilization of sediments associated with the melting of buried ice can result in normal faulting and broad folding in glaciolacustrine or glaciomarine settings compressional shear and gravitational types of deformation structures can occur as a result of grounding ice or icebergs rapid sedimentation and reworking downslope associated with high sedimentation rates in non glacial settings similar deformation structures can form as a result of slope instability and reworking of sediments downslope rapid sedimentation seismic shaking wave induced shearing or loading in this context two case studies are presented to demonstrate the type of paleoenvironmental information that an analysis of deformation structures can provide in the first case study analysis of deformation in the port askaig formation scotland reveals a distinctive stratigraphic distribution of deformation structures the types of deformation observed together with their recurrence over several 100s of meters and their basinal context are used to infer a seismic origin for the deformation which in turn suggests a significant tectonic control on sedimentation atop a record of ice margin fluctuations in a glaciomarine setting in the second example analysis of deformation in the smalfjord formation northern norway provides strong evidence for deformation by active ice overriding glaciofluvial deposits the types of deformation in this example together with its complexity scale and associated facies provide the strongest case for ice marginal deformation in sum analysis of deformation structures together with analysis of structural geology stratigraphy facies and facies associations can provide additional constraints on paleoenvironmental conditions at the time of deposition which can help us refine or test paleoenvironmental models proposed for this critical time period in earth history

the sedimentary record of drifting ice early wisconsin sunnybrook deposit in an ancestral ice dammed lake ontario canada abstract outcrops of pebbly mud diamict at scarborough in southern ontario canada the so called sunnybrook till are associated with the earliest incursion of the laurentide ice sheet lis into mid continent north america some 45 000 years ago the sunnybrook is a blanket like deposit containing deepwater ostracodes and occurs conformably within a thick 100 m succession of deltaic and glaciolacustrine facies that record water depth changes in a large proglacial lake contextual evidence associated facies sedimentary structures deposit geometry and landforms indicates a low energy depositional setting in an ice dammed ancestral lake ontario in which scouring by floating ice masses was an important process u shaped iceberg cut scours with lateral berms up to 7 m deep occur on the upper surface of the sunnybrook and are underlain by sub scour structures that extend several meters below the scour base ice rafted concentrations of clasts clast layers grooved surfaces formed by floating ice glissading over a muddy lake floor soft sediment striations and melanges of sand and mud mixed by grounding ice keels ice keel turbates are present and are all well known from modern cold environments the wider significance of this depositional model is that the lis margin lay east of scarborough and did not overrun southern ontario this finding is in agreement with recent data from the erie basin of canada ohio and indiana where deposits formerly correlated with the sunnybrook and thus implying an extensive early wisconsin ice sheet are now regarded as illinoian a speculative hypothesis is proposed that relates deposition of the sunnybrook and two younger deposits of similar sedimentology to surge like instabilities of the southern lis margin

loophole for snowball earth the snowball earth hypothesis posits an ice covered planet new climate simulations of snowball conditions allow ice free equatorial oceans that may be crucial for a theory about early animal evolution

glacially derived shallow marine gold placers of the cape yakataga district gulf of alaska abstract economic gold bearing coastal sediment of holocene age 10 ka occurs along the gulf of alaska coastal margin from cape yakataga in the west to icy bay in the east a distance of 50 km deposits have been worked since 1898 and total production since then is about 15 000 ounces gold is recovered from discontinuous sheets of ruby garnet rich beach sand exposed at low tide along the high energy storm dominated coastline similar auriferous sand facies are also present within raised marine terraces cut across outwash fans along the narrow mountainous coastal margin the area has been uplifted by glacio isostatic rebound following the last glacial maximum and by high magnitude earthquakes the area lies along the collision zone between the north american and pacific plates study of uplifted and well exposed late cenozoic strata the glaciomarine yakataga formation that outcrop along the coastal margin confirms this succession as the source of placer gold the yakataga formation records the rapid accumulation of over 6 km of gold bearing glaciomarine sediments in a rapidly subsiding forearc basin these strata have in turn been uplifted as coastal mountains and eroded by glaciers the white river glacier and yakataga glacier drainage basins can be identified as the principal source of detrital gold along the yakataga coastline auriferous beach sediment is the product of westward reworking of glacial outwash from the white and yakataga rivers and is accompanied by a well defined downcurrent reduction in the size and number of gold particles

evidence of glacial abrasion in the calingasta uspallata and western paganzo basins mid carboniferous of western argentina abstract evidence of glacial abrasion is present in the basal part of the carboniferous section of the calingasta uspallata and paganzo western argentina where subglacial and proximal glacial marine diamictites with pebbly dropstone shales constitute the dominant facies association intraformational striated pavements are exposed in the hoyada verde and leoncito formations of the calingasta uspallata basin marine invertebrate fossils levispustula zone in associated shales indicate a mid carboniferous age for both units a single layer striated boulder pavement is superbly exposed in the hoyada verde formation local a axis clast imbrication in the underlying bouldery diamictite and strong parallel striae on the flattened upper surfaces of boulders indicate a subglacial origin for the pavement paleo ice flow directions have an nne ssw strike the leoncito intraformational pavement is shaped on bioturbated fine grained sandstones with plant remains glacial striations are straight and parallel with subcircular cross sections dome like features are present in the surface which is covered in turn by a sandy bouldery diamictite four striated surfaces are present in the western paganzo basin three of them pavements at the san juan river valley loma de los piojos and agua hedionda are developed on basement rocks they contain parallel to subparallel striations nailhead striations crescentic gouges impact marks and furrows the pavements are overlain by thin up to 1 5 m laterally discontinuous diamictite beds with striated clasts some of them bullet shaped paleo ice flow directions are n ne directed an intraformational pavement is also present at the san juan river valley section it is carved on the top of the basal diamictite and exhibits furrows probably caused by subglacial soft sediment grooving the dispersion of paleo ice flow directions in the calingasta uspallata and paganzo basins indicates i regional slopes toward the north into a brackish water embayment and northwest into the open marine conditions ii glacial erosion by ice grounded below sea level and confined to the margins of the basins

late holocene sediment in nara inlet central great barrier reef platform australia sediment accumulation on the middle shelf of a tropical mixed clastic carbonate system nara inlet in the whitsunday islands is a natural depocenter for clastic and carbonate sediment on the middle shelf of the central great barrier reef gbr platform three sediment cores were collected from the inlet to understand sediment accumulation in this tropical mixed clastic carbonate system over time surface and subsurface sediment is comprised of two main components terrigenous clay and shallow marine carbonate similar to other regions on the middle shelf of the gbr platform the carbonate component 25 80 by weight and dominated by sr poor foraminifera tests and mollusc shells generally increases towards the surface radiocarbon ages derived from tests of benthic foraminifera indicate that the top 3 m of sediment accumulated within the last 3000 years but that the rate has slowed toward present day the change in clastic and carbonate abundance therefore indicates a system where both clastic and carbonate accumulation has decreased over time but where clastic accumulation has decreased faster reduced clastic accumulation may reflect progressive impedance of terrigenous input by a laterally growing fringing reef at the mouth of nara inlet likewise reduced carbonate accumulation may reflect preferential reef growth and carbonate deposition outside of the inlet even though clastic accumulation has decreased significantly over time the late holocene mass composition and distribution of clastic material in nara inlet implies a terrigenous source previously unrecognized in models for the evolution of the gbr platform large amounts of terrigenous sediment probably were stored around topographic highs on the middle shelf during lowstand and early transgression this clastic material was then removed over time mixed with autochthonous carbonate and deposited in embayments on the middle shelf the accumulation of sediment on the middle shelf of a tropical mixed clastic carbonate margin is not straightforward

a model for striated boulder pavement formation on glaciated shallow marine shelves an example from the yakataga formation alaska abstract laterally extensive striated boulder pavements crop out within glaciomarine diamicts of the early pleistocene yakataga formation on middleton island alaska boulder pavements consist of planes of one clast thickness with faceted upper surfaces showing consistently oriented striation directions cropping out over several kilometers along strike clasts do not have the preferred long axis alignment nor the characteristic bullet shape of boulders transported at a glacier base and deposited by lodgement processes in most cases pavements identify the planar bounding surfaces of thick up to 100 m tabular units of fossiliferous muddy diamict deposited by the settling of fine grained sands and muds from suspension together with ice rafted debris in water depths between 20 and 250 m diamict facies both underlying and overlying the pavements contain abundant unabraded macro and microfossils and show no evidence of deformation by subglacial shear processes pavements appear to have originated as boulder lag surfaces produced by wave and tidal current winnowing of diamict on relatively shallow bank areas during episodes of relative sea level lowering lags were subsequently overridden and abraded by a grounding ice shelf the glacially abraded pavements are thought to record the repeated expansion of a partially floating ice shelf to the edge of the gulf of alaska continental shelf during the early pleistocene recognition of boulder pavements formed in this manner may help establish the former presence of ice shelves whose depositional record remains enigmatic

the athabasca fluting field alberta canada implications for the formation of large scale fluting erosional lineations abstract current drumlin and fluting hypotheses centred on processes of formation erosion and deposition by subglacial deformation accretion by lodgement glacial erosion and meltwater erosion and deposition rarely receive comprehensive field testing this paper documents the morphology internal structure and landscape associations of the athabasca fluting field alberta and discusses how well these characteristics are explained by postulated fluting forming processes fluting hypotheses are comprehensively tested by this approach and their rejection or acceptance are firmly based on field observation and knowledge of processes using multiple criteria fluting is shown to be erosional and not to be a product of pervasive subglacial deformation or accretion by lodgement morphology landscape associations bed truncation plus glaciofluvial lags and gravel on fluting ridges are inferred to support erosion by meltwater intact primary bedding synsedimentary faults and injection structures very close to the landsurface showing no sign of shear deformation are used to reject erosion by subglacial deformation fluting is shown to be closely associated with tunnel channels and indicates that these channels were overtopped by broad floods that formed the fluting field and eroded anabranching tracts over the length of the alberta plains prior to fluting formation sequences with melt out and lodgement till conduit sediment and boulder lags record net deposition over an extended time period diapirism prior to fluting formation marks a period of extreme pressure gradients which are related to bed separation following fluting formation there was only minimal landscape modification dominated by glaciofluvial and glaciolacustrine deposition not glacial processes this progression is presented as evidence for two very different glacial regimes separated by a short period with diapirism and extensive landscape modification by meltwater erosion we note that the study area was glaciated only once during the late wisconsinian and it follows that glacial sedimentation and landscape modification were confirmed to this single glacial episode

volcanism and the greenland ice cores the tephra record abstract in recent years increased awareness of the considerable potential offered by tephrochronology in palaeoclimatic studies has fuelled a renewed interest in the tephra record preserved within the greenland ice cores this renewal has occurred in tandem with the development of continuous flow analysis techniques which provide high resolution volcanic aerosol records these chemical records are of considerable value for identifying tephra horizons composed of glass shards and also provide the basis for a new high precision ice core timescale thus allowing ages to be assigned to volcanic events tephra horizons of glass shards in the ice cores play a critical role for 1 testing and building chronological frameworks for the ice cores and other sequences 2 synchronising disparate palaeoclimatic sequences and testing phase relationships 3 atmospheric pathway reconstruction and 4 verifying the source of volcanic aerosol peaks here we review the results of over 25 years of research into tephra horizons in the grip gisp2 and ngrip cores we provide high precision ages for horizons based on recently developed timescales and draw together geochemical characterisations of the products of 45 volcanic events deposited on the greenland ice sheet and identified in the cores there is a clear disparity between the number of volcanic aerosol peaks and the number of tephra horizons of glass shards identified thus far however this synthesis highlights the critical role of the greenland ice cores firstly in the reconstruction of the history of icelandic volcanic eruptions and secondly the considerable value of some tephra horizons in facilitating the correlation of these key climatic archives to north atlantic marine sequences other noteworthy issues include the potential of these cores to preserve tephra horizons from more distal volcanic sources such as north america to resolve the debate surrounding the thera eruption and the observation that some tephra horizons are preserved within the ice without coeval volcanic aerosol signals

contemporary sedimentation in the cocos keeling islands indian ocean interpretation using settling velocity analysis abstract textural parameters of grain size distributions derived from sieve analysis and component analysis have been used to the exclusion of other methods to examine sedimentary processes in modern reef environments however because these methods do not reflect the hydraulic behaviour of bioclastic sediment this study uses settling velocity analysis accounting for grain size density and shape variability and component analysis of separated settling velocity fractions to describe and differentiate bioclastic deposits and infer transport pathways in the cocos keeling islands an indian ocean atoll principal component and cluster analysis of settling 251 samples and component 90 samples characteristics discriminated eight settling velocity and five component classes a derived settling velocity classification reflecting the hydraulic properties of the sediments describes a lagoonward gradient from those with fast settling characteristics on the outer reef flat to those with moderate settling character on sand aprons the active sediment transport system is constrained between these zones component analysis of bulk samples identifies two broad sediment types 1 those with reef derived components found throughout the active transport zone reef flat to sand aprons and 2 those characterised by greater proportions of gastropod and halimeda fragments which are produced and deposited in situ in the lagoon component analysis of settling fractions further resolves details of sediment movement in the active transport zone sediment is transported from the productive reef flat to passage entrances which form initial depositional environments slower settling material is selectively transported from the entrances through the passage conduits to passage exit environments these form the main depositional areas under mean energy conditions during moderate to high energy events sediment is flushed from the passage to sand apron zones with slower settling grains deposited in the deep lagoon sinks results show that settling velocity analysis and component analysis of settling fractions can resolve the nature of sediment transport patterns in reef environments at a much finer scale than studies in which analysis is based on grain size distributions

maximum extent and dynamic behaviour of the last british irish ice sheet west of ireland a complete reconstruction of the last british irish ice sheet biis is hindered by uncertainty surrounding its offshore extent and dynamic behaviour this study addresses this problem by reconstructing the depositional history of four sediment cores taken from a series of sinuous glacigenic sediment ridges on the continental shelf west of ireland we present new geomorphic sedimentary and micropaleontological data that record a maximum westward biis extent that was at least 80 km farther offshore from any previous estimates the data suggests that a large ice shelf formed over parts of the shelf prior to retreat this new data increases the areal extent of grounded biis ice by 6700 km2 from previous estimates which represents a 3 increase in the irish sector of the ice sheet three new ams radiocarbon dates demonstrate for the first time that the biis advanced to the shelf edge during last glaciation late midlandian late devensian with ice advance onto the porcupine bank occurring after 24 720 260 yr cal bp deglaciation was complete by 19 182 155 yr cal bp thus constraining biis occupation over the porcupine bank to less than 5500 years estimated retreat rates of marine terminating ice across the shelf range from 70 to 180 myr 1

intertidal boulder pavements in the northeastern gulf of alaska and their geological significance abstract striated boulder pavements consisting of planar concentrations of clasts having striated upper surfaces are a common feature of glacigenic deposits but their origin is not well understood laterally extensive pavements are currently forming in the intertidal zone west of icy bay in the gulf of alaska pavements comprise armoured layers of interlocking boulders one clast thick that have been eroded from underlying outcrops of late cenozoic glaciomarine diamictites they originate essentially as lag surfaces along a high energy storm dominated mesotidal shoreline boulder pavements are either flat or show a nucleated plan form where successively smaller boulders have been accreted around a large core boulder nucleation imparts a hummocky surface topography to the pavements and suggests that some form of size sorting of clasts has occurred packing is promoted by repeated tamping of the clast lag by floating masses of glacier ice which become grounded across the intertidal zone at low tide repeated abrasion of the pavement surface by debris contained within ice blocks produces smooth flattened clast upper surfaces and short randomly oriented striations data from icy bay can be used to constrain the origin of laterally extensive boulder pavements exposed in late cenozoic glaciomarine sediments on middleton island the significance of such pavements in the geologic record is that they form along erosional unconformities and may identify sequence boundaries

evidence for a glacial origin of neoproterozoic iii striations at oaibaccannjar ga finnmark northern norway the origin of the famous late precambrian striated pavement at oaibaccannjar ga bigganjargga northern norway remains controversial most investigators have accepted a glacial formation but some prefer a soft sediment mechanism however a newly discovered c 2 5 mm thick zone of brecciation under rare polished striations indicates a hard substrate during formation and thus a glacial origin for the striations other points indicating that the striations formed in a hard substrate are 1 the striated platform in the veidnesbotn formation is c 150 ma older than the overlying diamictite smalfjord formation having been buried to c 2 5 km depth cementation should have started before smalfjord times 2 the marked irregularity of the sub smalfjord formation palaeotopography on skjaholmen 3 the presence of rounded veidnesbotn formation boulders in the diamictite above the striations imprints of clasts appearing to lie across the striations are re interpreted as relicts of mud flakes within the veidnesbotn formation which were cut across and quarried out during pavement formation the origin of the overlying diamictite tillite vs debris flow is not constrained by the presence of glacial striations and most probably was deposited some time after striation formation

identifying the controls over downstream fining of river gravels abstract a wide range of rates of downstream fining in gravel bed rivers has been reported as a consequence explanations of the phenomenon range from those that consider only abrasion to others that consider only selective sediment transport this apparent confusion results in part from inconsistent definitions of downstream fining analysis of results from a small gravel bed stream is used to show how the method of sediment sampling and the percentile used to characterize fining rates can affect the results when fining is adequately defined a theoretical relationship between fining rate and drainage basin area is derived from established alluvial fan relations this relationship is remarkably consistent with a range of studies by numerous authors using often different methods such results suggest that fining is achieved through the mechanism of selective transport but that the overall control on fining patterns is the volume of sediment supplied from within the drainage basin as such the rate of size decline in sedimentary deposits such as river terraces has the potential to be used to infer past changes in climate tectonics and or relative base level models of this are presented and these are shown to be broadly consistent with observations made in an ancient sandstone body full use of changing rates of fining as paleo environmental indicators requires further work on the rate of adjustment of fining profiles with respect to the rates of change in the forcing environmental conditions

late quaternary sedimentation in st george s bay southwest newfoundland acoustic stratigraphy and seabed deposits shallow seismic reflection data collected in st george s bay southwest newfoundland reveal a complex pattern of subsurface topography and acoustic facies two basins in the inner bay are underlain by glacially overdeepened valleys that extend to depths in excess of 180 m within the thick quaternary sequence in the inner bay we recognize eight acoustic units units 1 ice contact 2 subaqueous outwash and 3 draped glaciomarine record the presence and retreat of a major late wisconsinan ice margin unit 4 postglacial mud has resulted from reworking of glaciogenic sediments in response to changes in relative sea level unit 5 postglacial sand is a shoreface deposit on the seaward front of the former moraine unit 6 postglacial delta was formed by fluvial reworking of glaciogenic sediments during the postglacial lowstand of relative sea level unit 7 postglacial barrier platform comprises seaward fining clinoform prisms that have prograded into the basins and underlie gravel beach ridge pla

lagoon shelf sediment exchange by storms evidence from foraminiferal assemblages east coast of grand cayman british west indies abstract the abundance and size distribution of ecologically diagnostic foraminiferal species are ideal tracers of sediment transportation and storm processes on the windward shelf of grand cayman during storm peak sediment in the lagoon and fore reef is suspended by large storm waves sediments of sand size and finer including forereef foraminifera species are moved shoreward by wind driven currents and deposited in a blanket behind the reef the fore reef foraminiferal assemblage is winnowed and the mean and median sizes of amphistegina gibbosa progressively decrease from the shelf edge into the inner lagoon as the storm wanes water piled in the lagoon drains via channels and sediment is transported offshore consequently lagoonal sediment that includes lagoonal foraminiferal t sts is carried out of the lagoon and deposited on the fore reef terrace and down the shelf escarpment this process produces winnowed lagoonal sediments because suspended fine sediments are preferentially transported during the long inter storm periods tranquil conditions are once again established and sediment exchange between the lagoon and on the fore reef terrace ceases during this time lagoonal organisms once again colonize the lagoon and bioturbation causes mixing of surface and subsurface sediments

volcano meteorological tsunamis thec ad 200 taupo eruption new zealand and the possibility of a global tsunami meteorological tsunamis are long period waves that result from meteorologically driven disturbances they are also generated by phase coupling with atmospheric gravity waves arising through powerful volcanic activity the ad 1883 krakatau eruption generated volcano meteorological tsunamis that were recorded globally because of its extreme violence and energy release 150 50 megatons explosive yield and by analogy with the krakatau event it is highly possible that the ignimbrite emplacement phase of the c ad 200 taupo eruption of north island new zealand generated a similar volcano meteorological tsunami that may have reached coastal areas worldwide tsunami deposits of identical age to the taupo eruption occur in central coastal new zealand and probably relate to that event definitive evidence elsewhere has not yet been found in theory volcano meteorological tsunamis are likely to be produced during comparable eruptive events at other explosive volcanoes and thus represent an additional volcan

glacial or non glacial origin for the bigganjargga tillite finnmark northern norway bigganjargga is the classic locality for the varangian ice age in northern scandinavia the presence of two sets of striations on the underlying quartzite basement has been taken as evidence for a glacial origin of the bigganjargga diamictite however soft sediment surface structures on the underlying sandstone indicate that it was not consolidated when the diamictite was emplaced together with structures in the diamictite this shows that the deposit formed as a debris flow moving northwestwards over an unconsolidated substrate there is no evidence for glacial origin or contribution for the diamictite material

facies and architecture of a carboniferous grounding line system from the guandacol formation paganzo basin northwestern argentina abstract new outcrops of middle carboniferous glacigenic deposits found in the guandacol formation western paganzo basin are described in this paper the study locality of los pozuelos creek northwestern argentina includes coarse grained diamictites rhythmites laminated pebbly mudstones and shales that represent an expanded column of the gondwanic glaciation in this region thirteen lithofacies recorded at the measured section have been grouped into three facies associations facies association i is composed of coarse grained massive and stratified diamictites lithofacies dmm dms dmg dcs laminated siltstones with dropstones fld and interstratified sandstones and mudstones fl sr these rocks represent both tillites and resedimented diamictites closely associated to small water bodies where laminated siltstones with dropstones and stratified sandstones and mudstones were deposited facies association ii comprises couplets of matrix supported thinly bedded diamictites dmld and laminated mudstones with dropstones fld this facies association results from the combination of three different processes subaqueous cohesionless debris flows coeval rainout of ice rafted debris and settling of fine grained particles from supension finally facies association iii is made up of laminated mudstones without dropstones thin marl levels and scarce fine to very fine grained sandstones this assemblage clearly suggests sedimentation in a deep marine environment below the wave base the architecture of the glacigenic deposits has been investigated using photomosaic panels the geometry of the depositional bodies and facies suggest that los pozuelos creek outcrops exhibit a well preserved three dimensional example of a grounding line system in particular three different subenvironments of a morainal bank were interpreted a bank front a bank core and a bank back the bank front assemblage is characterized by coarse grained mainly resedimented diamictites grading laterally to prograding clinoforms composed of interbedded matrix supported thinly bedded diamictite and mudstones the bank core assemblage is formed by a stacking of coarse grained diamictites where at least five major erosional surfaces bounding four multistory diamictite bodies can be recognized finally the bank back assemblage corresponds to discontinuous intervals of striated lodgement till and coarse grained resedimented diamictites showing important post depositional deformation the retrogradational stacking of the morainal banks indicate an overall glacial retreat and a glacioeustatic sea level rise erosional surfaces at the base of each morainal bank suggest intervening short term episodes of ice advance the new data presented here confirm the existence of true tillites in western paganzo basin and suggest several at least four pulses of glacial advance and retreat during the namurian glaciation in the region and permit a more refined interpretation of the glacial deposits in the huaco area

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

barriers barrier platforms and spillover deposits in st george s bay newfoundland paraglacial sedimentation on the flanks of a deep coastal basin abstract st george s bay and adjacent coastal areas of southwest newfoundland contain thick sequences of late quaternary glaciogenic sediments and have been subject to relative sea level fluctuations over a rang of 70 m during the past 13 5 ka glaciogenic sediments have been mobilized by littoral processes to form two types of deposits these are 1 the prograded barrier beach and spit at stephenville and flat island respectively which are associated with large subaqueous sandy platforms and 2 a wedge of well stratified sand and gravel on the landward flank of the shallow sill which extends across st george s bay at an average depth of about 25 m the subaerial barriers and spits are composed of gravel beach ridges overlain locally by thin aeolian deposits with freshwater and salt marsh peats in swales these barriers developed during the last several thousand years as relative sea level approached its present position from an early postglacial minimum of approximately 25 m the prerequisite for beach ridge formation was the deposition of large sandy subaqueous platforms which appear on shallow seismic reflection records as clinoform wedges prograded into the deep basin of inner st george s bay the barrier platform at stephenville is constructed across a deep submarine valley the flat island barrier platform sits on the flank of a glacially overdeepened trough partly filled by glacial and postglacial sediments and envelops a delta formed during the early postglacial low stand of relative sea level the platform and barrier together constitute a thick coastal sequence coarsening up from sandy mud at the base to gravel at the top the wedge of well stratified coarse sediment up to 50 m thick situated on the landward flank of the shallow sill overlies older glaciogenic sediments and passes laterally into postglacial basin muds steep foreset reflections dipping 10 20 are evident on shallow seismic reflection records suggesting that the wedge was formed by spillover of sediment entrained by wave and current action into the relatively deep basins of the inner bay active gravel ripples testify to the continued mobility of coarse sediment on the sill the formation of the spillover deposits barriers and platforms demonstrates the potential for glacially overdeepened basins to host thick heterogeneous coarse grained coastal and inner shelf deposits

ams c 14 dating of deglacial events in the irish sea basin and other sectors of the british irish ice sheet sedimentary sequences deposited by the decaying marine margin of the british irish ice sheet biis record isostatic depression and successive ice sheet retreat towards centres of ice dispersion radiocarbon dating by accelerator mass spectrometry ams of in situ marine microfaunas that are commonly associated with these sequences constrain the timing of glacial and sea level fluctuations during the last deglaciation enabling us to evaluate the dynamics of the bus and its response to north atlantic climate change here we use our radiocarbon dated stratigraphy to define six major glacial and sea level events since the last glacial maximum 1 initial deglaciation may have occurred 18 3 kyr c 14 bp along the northwestern irish coast in agreement with a deglacial age of similar to 22 cl 36 kyr bp for southwestern ireland ice retreated to inland centres and areas of transverse moraine began to form across the north irish lowlands 2 channels cut into glaciomarine deglacial sediments along the western irish sea coast are graded to below present sea level identifying a fall of relative sea level rsl in response to isostatic emergence of the coast 3 marine mud that rapidly infilled these channels records an abrupt rise in global sea level of 10 15 m similar to 16 7 c 14 kyr bp that flooded the irish sea coast and may have triggered deglaciation of a marine based margin in donegal bay 4 intertidal boulder pavements in dundalk bay indicate that rsl similar to 15 0 c 14 kyr bp was similar to present 5 a major readvance of all sectors of the biis occurred between 14 and 15 kyr 14 c bp which overprinted subglacial transverse moraines and delivered a substantial sediment flux to tidewater ice sheet margins this event the killard point stadial indicates that the bus participated in heinrich event 1 6 subsequent deposition of marine muds on drumlins 12 7 c 14 kyr bp indicates isostatic depression and attendant high rsl resulting from the killard point readvance these events identify a dynamic biis during the last deglaciation as well as significant changes in rsl that reflect a combination of isostatic loading and eustatic changes in global sea level c 2005 elsevier ltd all rights reserved

the nature and controls on downstream fining within sedimentary links abstract at scales of 100 to 102 km grain size change in gravelly alluvial systems is characterized by punctuated downstream fining significant lateral sediment inputs define a sequence of sedimentary links within which fluvial processes produce fining trends using a new set of surface grain size data eighteen such links along two canadian gravel bed rivers are examined negative exponential functions are found to be most appropriate for describing within link grain size changes development of a pragmatic means of predicting grain size is then pursued by searching for a means to relate model diminution coefficients to link attributes consideration of lithologically differentiated fining trends indicates that lithology plays an insignificant role in controlling fining rate examination of within link longitudinal profiles indicates a reasonable relation between fining rate and the rate of change of channel slope the predictive value of this relation is limited by the regional nature of the data upon which it is based and the relative difficulty of obtaining accurate gradient information however as previously noted for alluvial fans although for different reasons channel gradient is correlated with link length and a reasonable predictive tool is apparent in a relation between link length and diminution rate the validity of the derived relation cannot be tested for want of a suitable data set several lines of evidence indicate that abrasion is unimportant as a fining mechanism on these rivers and it is argued that this may be typical of sedimentary links fining rate must then be a function of the efficiency with which sorting processes operate and it is suggested that this efficiency is dependent on aggradation rates associated with the lateral sediment influxes at the head and terminus of individual links

net sediment transport patterns inferred from grain size trends based upon definition of transport vectors reply abstract a new approach to the analysis of the grain size trends associated with net sediment transport directions in shallow marine environments is developed here two specific trends modified from mclaren and bowles 1985 on the basis of a literature review are used to define a grid of trend vectors such vectors are filtered and transformed into transport vectors it is suggested that an ordered pattern of the transport vectors represents net transport paths the orderliness of the derived pattern is examined on the basis of a significance test using the average length of the transport vectors as a criterion both the filtering operation and the significance test procedure are based upon the assumption that the trend vectors which are not associated with net transport have the same probability of being in any direction the technique is applied to data from yangpu harbour southern china the technique can be applied to inner continental shelf and coastal environments for such an analysis the area under investigation should be sampled at an interval large enough to reflect the real grain size trends resulting from net sediment transport but small compared with the dimensions of the sedimentary environment the time scale of the results depends upon the sampling depth applicability of the technique is likely to be somewhat limited where the environment is highly variable over short distances furthermore due to its statistical character the analysis may fail to identify net transport patterns

sediment accumulation in low sedimentation wave dominated glaciated inlets abstract many fjords situated in lowland areas of resistant bedrock are characterized by a comparatively low fluvial sediment flux and are often dominated by wave processes several such fjords in eastern canada are used as examples to develop a generalized sediment distribution model physiographically the fjords can be subdivided into nearshore platforms basin slopes and basin floors where there has been marine transgression the principal sediment source is shoreline erosion of glacial till in areas of crustal rebound late glacial and early holocene muds deposited on highs are eroded as they come within the zone of wave erosion with the falling sea level storm waves resuspend fine grained sediment in water depths of many tens of metres and produce near bottom turbid water zones within basins fine sand and silt settle out rapidly leading to the deposition of graded beds on the basin floors residual sand and gravel accumulate on the near shore platforms while silt and mud prograde over or are advected beyond basin slopes burying older basin fill sediment

sedimentology and chronology of the advance and retreat of the last british irish ice sheet on the continental shelf west of ireland the last british irish ice sheet biis had extensive marine terminating margins and was drained by multiple large ice streams and is thus a useful analogue for marine based areas of modern ice sheets however despite recent advances from investigating the offshore record of the biis the dynamic history of its marine margins which would have been sensitive to external forcing s remain inadequately understood this study is the first reconstruction of the retreat dynamics and chronology of the western marine terminating margin of the last late midlandian biis analyses of shelf geomorphology and core sedimentology and chronology enable a reconstruction of the late midlandian history of the biis west of ireland from initial advance to final retreat onshore five ams radiocarbon dates from marine cores constrain the timing of retreat and associated readvances during deglaciation the biis advanced without streaming or surging depositing a bed of highly consolidated subglacial traction till and reached to within 20 km of the shelf break by 24 000 cal bp ice margin retreat was likely preceded by thinning grounding zone retreat and ice shelf formation on the outer shelf by 22 000 cal bp this ice shelf persisted for 2500 years while retreating at a minimum rate of 24 m yr and buttressing a 150 km long 20 km wide bathymetrically controlled grounding zone a large 150 km long arcuate flat topped grounding zone wedge termed here the galway lobe grounding zone wedge glgzw was deposited below this ice shelf and records a significant stillstand in biis retreat geomorphic relationships indicate that the biis experienced continued thinning during its retreat across the shelf which led to increased topographic influence on its flow dynamics following ice shelf break up and grounding zone retreat past the glgzw at this stage of retreat the western biis was comprised of several discrete asynchronous lobes that underwent several readvances sedimentary evidence of dilatant till deposition suggests that the readvances may have been rapid and possibly associated with ice streaming or surging the largest lobe extended offshore from galway bay and deposited the galway lobe readvance moraine by 18 500 cal bp further to the north an ice lobe readvanced at least 50 km offshore from killary harbour possibly by 15 100 cal bp the existing chronology currently does not allow us to determine conclusively whether these readvances were a glaciodynamic internally driven response of the ice sheet during deglaciation or were climatically driven following the 18 500 cal bp readvance the galway lobe experienced accelerated eastward retreat at an estimated rate of 113 m yr

global and continental scale glaciations on the precambrian earth abstract earth s climate during the proterozoic eon was marked by major glacial events with evidence for large continental ice sheets on many cratons and with sedimentological data indicating that glaciers had extended to sea level this paper emphasizes the sedimentological and sequence stratigraphic responses to glaciations and evaluates the major driving forces of glaciations during the precambrian first and second order sequences are recognized related to continental scale fragmentation and formation of marine rift basins wherein sedimentary rocks indicate glacial influences and pronounced tectonic climatic linkages coarse syn rift deposits are typically characterized by mass flow diamictites and conglomerates it is important to undertake sedimentological facies and sequence stratigraphic analysis of these syn rift and capping passive margin sequences as well as of slope turbidite deposits which formed if enhanced subsidence of the basins was occurring more generally latitude and syn rift tectonic uplift can cause formation of glacial ice and enhance distinctive glacial influences on extensional basin sedimentation thus supporting a causal relationship between thermal subsidence and the formation of glacier ice on inland areas during the precambrian however it is suggested that long lived marine terminated glaciers also situated at low paleolatitudes were related to an extensional tectonic setting in such settings glacial deposits associated with sedimentary sequences of distinctively different origin e g carbonate and chemically mature siliciclastic sequences can well be used to detect the prominent sequence boundaries to verify depositional systems tracts internal sediment stacking patterns in sequences are indicative of dynamic processes along glaciated continental margins and without always having the need for global synchroneity in glacially influenced rift basins and continental margins it is important to recognize the sequence boundaries of significant subaerial unconformities and their correlative conformities a sequence boundary is a chronostratigraphically significant surface always produced as a consequence of a change in relative sea level these can then be well related to initiation and decay of glaciations however on land glacial deposits in a very few cases are prevented from later erosion attenuation of continental crust during rifting and breakup of the continent raises relative sea level and also many of the shallow intra cratonic basins subsided below sea level in favourable conditions being affected by major continental glaciations paleoproterozoic and neoproterozoic glacial deposits are known in north and south america south africa india western australia and fennoscandia against this background continental scale to global glaciations in the precambrian appear to be possible however views on the causes and timing of these glaciations as well as on planetary extent of ice cover are still contradictory there is a need to continue detailed sedimentological studies of pre glacial and post glacial deposits as well as to interpret syn glacial lithofacies for their inferred transportation and depositional processes pre glacial deposits especially should provide a new target to help us understand the processes that initiated these precambrian glaciations the sequence stratigraphic approach with understanding of the stacking pattern of depositional systems could prevent oversimplification and use of just single events to explain the complexity of evolution of glacially influenced precambrian continental margin sediments

sediment transport patterns determined from grain size parameters overview and state of the art abstract grain size trends have been applied in many diverse sedimentary environments to determine sediment transport paths generally coinciding with information from tracer studies current measurements and the orientation of sedimentary structures the different methods proposed to date are critically analysed and compared with reference to recent field studies it is concluded that the two dimensional methods produce comparable results and may in fact complement each other in spite of the advances several problems still exist which include the sampling method and density the choice of trend types the relative weight of grain size parameters and the interpretation of results these are discussed together with possible solutions

are some tillites impact related debris flow deposits abstracta number of ancient glacial deposits or tillites are currently interpreted as originating by subaqueous debris flow processes at glacial termini problems in identifying the glacial origin of such deposits can arise because some criteria commonly used for establishing glaciation especially outsize clasts interpreted as dropstones from icebergs can apparently be produced by nonglacial debris flows and debris falls could some of these debris flow diamictite deposits be related to large body impacts especially those that occurred at times other than established glacial periods debris flow ejecta of known impacts seem to be marked by mildly shocked clasts showing evidence of plastic behavior with brittle failure commonly resulting in multiple partially displaced fractures grading into crushed and brecciated clasts these deformation features entail brief periods of high confining pressures in accord with a hypervelocity impact origin i report here several debris flow diamictites currently

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

glacial influence on neoproterozoic sedimentation the smalfjord formation northern norway there is much debate regarding the intensity and geographic extent of glaciation during the neoproterozoic particularly in response to recent geochemical work suggesting that the neoproterozoic earth was at times ice covered from equator to poles the snowball earth hypothesis a detailed sedimentological analysis of the neoproterozoic smalfjord formation of northern norway was conducted in order to determine the extent and intensity of glacial influence on sedimentation in the tarmfjorden area the smalfjord formation consists of a stacked succession of diamictites interbedded with fine grained laminated mudstones containing rare outsized clasts diamictites and interbedded mudstones are interpreted as the product of subaqueous mass flows generated along the basin margin in the varangerfjorden area chaotically interbedded diamictites conglomerates and sandstones are overlain by a thick succession of stacked sandstone beds one diamictite unit at bigganjargga overlies a striated pavement the varangerfjorden outcrops appear to record deposition on a subaqueous debris apron although diamictites contain rare striated and faceted clasts suggesting a glacial sediment source their origin as subaqueous mass flows prevents the interpretation of ice mass form or distribution rare lonestones may be associated with floating ice in the basin which may be of glacial or seasonal origin glacial ice may have contributed poorly sorted glacial debris to the basin margin either directly or through fluvioglacial systems but there is no evidence of direct deposition by ice at varangerfjorden or tarmfjorden the overall fining upward trend identified in the smalfjord formation and overlying nyborg formation is consistent with depositional models of rift basin settings this fining upward trend the predominance of mass flow facies including breccias associated with scarps and the evidence for extensional tectonic activity in the region suggest that tectonic activity may have played an important role in the development of this neoproterozoic succession the smalfjord formation at tarmfjorden and varangerfjorden does not exhibit sedimentological characteristics consistent with severe glacial conditions suggested by the snowball earth hypothesis

evaluating the influence of tidal currents on the distribution of silt in nara inlet central great barrier reef australia a simple model is developed to infer flood tide dispersal of silt during fair weather conditions from spatial grain size variation of mixed carbonate siliciclastic silt on the bed of nara inlet a 3 km long shallow tropical embayment on the mid shelf of the central great barrier reef region australia the model uses settling velocities calculated for three dominant silt modes 23 28 and 41 m and measured tidal current data to predict the headward transport of silt by a large flood tide modelling was aided by the use of wind and currents 2000 wc2000 freeware a visualisation and analysis tool for vectorial data the model predicts the 23 28 and 41 m modes will be transported minimum headward distances of 1250 950 and 500 m respectively and compares well with the observed silt distribution on the bed the results indicate that under fair weather conditions flood tides control the distribution of silt consistent grain size trends of bulk and siliciclastic silt implies that siliciclastic and carbonate silt sized grains may exhibit similar hydraulic behaviour during transport by flood tides in nara inlet the model may be applied to embayments where fine sediment transport is dominated by flood tides it may also have application in identifying the controls on dispersal and accumulation of particle associated contaminants in embayments characterised by mixed carbonate and siliciclastic sediments

subglacial deformation and water pressure cycles as a key for understanding ice stream dynamics evidence from the late ordovician succession of the djado basin niger subglacial deformation is crucial to reconstructing glacier dynamics sediments associated with the late ordovician ice sheet in the djado basin niger exhibit detailed structures of the subglacial shear zone three main types of subglacial shear zones ssz are discriminated the lowermost ssz developed on sandstones displays riedel macrostructures and cataclastic microstructures these resulted from brittle deformation associated with strong glacier bed coupling and low pore water pressure where they developed on a clay rich bed the overlying ssz display s c to s c fabrics sheath folds and dewatering structures these features indicate high ductile shear strain and water overpressure on fine grained sand beds the ssz exhibit homogenized sand units with sand stringers interpreted as fluidized sliding beds the succession of subglacial deformation processes depends on fluid pressure behavior in relation to subglacial sediment permeability fluid overpressure allows subglacial sediment shear strength and ice bed coupling to be lowered leading to ice streaming

till and moraine emplacement in a deforming bed surge an example from a marine environment the glacier sefstrombreen in spitsbergen surged across an arm of the sea between qsr 1882 and 1886 and rode up onto the island coraholmen marine and terrestrial geological observations and archive records show that the glacier advanced on a deforming carpet of marine mud which was eroded from its original location transported and smeared over the sea bed and coraholmen as a deformation till the glacier emplaced about 210sm 0 2 km 3 of drift in the terminal 2 km of its advance in a maximum of 14 years leaving a thickness of up to 20 m on coraholmen which was doubled in size as a result during the surge subglacial muds were characterised by high water pressures low effective pressures and low frictional resistance to glacier movement original sedimentary inhomogenities permit fold structures to be identified but repeated refolding and progressive remoulding produce mixing and homogenisation of deformation tills the surge was probably shortlived and as the heavily crevassed glacier stagnated underlying water saturated muds were intruded into crevasses and then extruded on the glacier surface reticulate crevasse intrusion ridges on coraholmen and the sea floor reflect the orientation of surge generated crevasses water and sediment was also extruded beyond the glacier at its maximum extent to form extensive flows producing till tongues both on coraholmen and the sea floor extending over 1 3 km from the glacier it is argued that subglacial deformation of pre existing sediment will almost invariably be associated with glaciation of marine areas and that this process will not only produce deformation tills through remoulding of pre existing sediments but will also play a fundamental role in glacier dynamics criteria which perrait glacial tills produced by such events from marine and glaciomarine muds are discussed copyright 1996 elsevier science ltd