the stratigraphic and sedimentological significance of late devensian ice sheet surging in holderness yorkshire u k amino acid analyses of marine mollusc valves show that the basement till of holderness yorkshire is of late devensian age ca 20 000 bp and not pre ipswichian 125 000 bp as is traditionally supposed together with the overlying skipsea and withernsea tills the basement till is argued to be a deformation till resulting from the repeated onshore surging of late devensian ice over a muddy sea floor and the subglacial transport attenuation and mixing of marine sediment silts on the surface of the basement till yielding 14c dates of 18 500 and 18 240 bp have been regarded as minimum ages for late devensian glaciation in eastern england e g the dimlington stadial of rose 1985 1989 the amino acid data presented here indicates that the maximum of the late devensian glaciation in eastern england occurred earlier sedimentological interpretation of the late devensian tills of holderness as glacially reworked marine sediments supports previous glaciological models of the british ice sheet involving surging of ice lobes along the coast of eastern england the basement skipsea and withernsea tills are separated by shallow marine sediments and may be representative of deformable bed till facies deposited below pleistocene ice sheets moving over soft fine grained sediments

styles of subglacial glaciotectonic deformation within the context of the anglian ice sheet a substantial proportion of the glacial sediments of east anglia england are chaotic and highly deformed this has been attributed to a direct glaciotectonic deformation or b slumping of glaciomarine sediments we show that interpretation a leads to a classification of subglacial glaciotectonic deposits into sites of constructional deformation where little or no erosion of the preglacial deposits has occurred and styles of deformation are preserved moving up in the sequence and excavational deformation where substantial erosion of preglacial sediments has occurred and deformational styles are superimposed on each other by using glaciological and geotechnical theory we show that the expression of these deformational styles in east anglia is a natural response to the stress fields imposed by the ice sheet and that in consequence their spatial relationships are explicable the depth of deformation is related to the geotechnical properties of till and substrate the stress and effective stress fields and found to be consistent with observations

subglacial deformation at sub freezing temperatures evidence from hagafellsjokull eystri iceland abstract we report evidence of deformation at sub freezing temperatures beneath hagafellsjokull eystri an icelandic surge type glacier the bed of a piedmont lobe that advanced during the 1999 surge comprises deformed blocks of glacier ice set within frozen sediment this material has also been injected through overlying ice to form a network of crevasse squeeze ridges this layer contains evidence for two phases of deformation under contrasting rheological conditions 1 deformation under relatively warm conditions when the blocks of glacier ice acted as competent clasts within an unfrozen deforming matrix and 2 subsequent deformation at sub freezing temperatures when the ice blocks were attenuated into the surrounding frozen matrix along fractures and planar shears enriched with excess ice this suggests that the basal thermal regime of the advancing ice margin changed from warm based to cold based during the surge event the persistence and potential prevalence of subglacial sediment deformation at sub freezing temperatures has fundamental implications for our understanding of the dynamic behaviour sediment flux and geomorphic ability of cold based glaciers

discrete element modeling of subglacial sediment deformation 1 the discrete element method dem is used in this study to explore the highly nonlinear dynamics of a granular bed when exposed to stress conditions comparable to those at the bed of warm based glaciers complementary to analog experiments the numerical approach allows a detailed analysis of the material dynamics and the shear zone development during progressive shear strain the geometry of the heterogeneous stress network is visible in the form of force carrying grain bridges and adjacent volumetrically dominant inactive zones we demonstrate how the shear zone thickness and dilation depend on the level of normal overburden stress and we show how high normal stress can mobilize material to great depths the particle rotational axes tend to align with progressive shear strain with rotations both along and reverse to the shear direction the results from successive laboratory ring shear experiments on simple granular materials are compared to results from similar numerical experiments the simulated dem material and all tested laboratory materials deform by an elastoplastic rheology under the applied effective normal stress these results demonstrate that the dem is a viable alternative to continuum models for small scale analysis of sediment deformation it can be used to simulate the macromechanical behavior of simple granular sediments and it provides an opportunity to study how microstructures in subglacial sediments are formed during progressive shear strain

surge of bering glacier and bagley ice field alaska an update to august 1995 and an interpretation of brittle deformation patterns in the summers of 1993 1994 and 1995 video and global positioning system location data and 35 mm photographs were collected in a series of systematic survey flights undertaken over the bering glacier and bagley ice field system alaska in an effort to characterize surge crevasse patterns and surge propagation during survey flights in late august 1995 we observed that the 1993 94 bering glacier surge was continuing and still expanding affecting new areas farther up in bagley ice field new crevasse fields similar in pattern to the first surge crevasses we had observed in june 1993 below khitrov hills and in other isolated areas of central bering glacier and in july 1994 near the head of bering glacier near the junction of bering glacier and bagley ice field in both upper bering glacier and bagley ice field were opening in eastern bagley ice field and in the steller side of bagley ice field the type of crevasses seen in the new fields suggested that the surge was propagating into these areas by analysis and interpretation of the brittle deformation patterns apparent in the crevasse patterns some aspects of the past kinematic framework of the surge can be deduced this approach may lead to a more general classification of ice surface structures and to their linkage to ongoing processes

40ar 39ar dating and phytolith analysis of the early pleistocene sequence of kvemo orozmani republic of georgia chronological and palaeoecological implications for the hominin site of dmanisi abstract the kvemo orozmani locality contains a 15 m thick early pleistocene fluvio lacustrine sequence bracketed by two lava flows located 13 km up the masavera river valley west from the dmanisi site kvemo orozmani is therefore of prime interest to improve the paleoecological framework as well as dating dmanisi 40 ar 39 ar ages obtained on the two lava flows indicate that kvemo orozmani sequence brackets the dmanisi sediments between 1 825 0 021 ma and 1 765 0 021 ma phytolith assemblages found at the bottom of the sequence indicate a warm and humid climate while the assemblages found at the top suggest dryer colder climatic conditions the shift in phytolith assemblages observed at kvemo orozmani is similar to the one already described in dmanisi and occurs near the end of the oluduvai subchron the new palaeoecological and chronological data confirm the hypothesis made in dmanisi about the palaeoenvironmental context during the hominin occupation that suggests a temperate climate becoming drier the age we obtained on the overlying lava flow establishes a minimum age for the climatic transition at both kvemo orozmani and the dmanisi site of 1 765 0 021 ma

how glaciers entrain and transport basal sediment physical constraints abstract simple insights from the physics of ice water and sediment place constraints on the possible sediment transport behavior of glaciers and ice sheets because glaciers concentrate runoff streams generated by glaciers transport much sediment and may erode bedrock rapidly deforming glacier beds also can transport much sediment particularly in marginal regions rapid sediment entrainment producing thick debris rich basal zones may occur by regelation into subglacial materials and by freeze on from rising supercooled waters numerous other mechanisms may be important but primarily near ice margins especially those of advancing or fluctuating glaciers several sediment entrainment mechanisms may be active beneath a single glacier but one process is likely to be dominant at any place and time

flow thermal structure and subglacial conditions of a surge type glacier temperature measurements in a subpolar surge type glacier reveal a distinctive thermal structure associated with the boundary between the ice reservoir and receiving areas in the receiving area the glacier is cold based but bottom temperature has increased as much as 0 5 c between 1981 and 1982 and the basal heat flux is roughly 10 times the expected geothermal flux water percolation through permeable subglacial material is the probable energy source deformation of the substrate could destroy this drainage system and trigger a surge

the structural glaciology of kongsvegen svalbard and its role in landform genesis mapping of the structural glaciology of kongsvegen svalbard reveals evidence for four main deformational structures these are stratification longitudinal foliation thrusts and crevasse traces these structures are considered in terms of their contribution to debris entrainment transport and subsequent landform development stratification is associated with small amounts of supraglacial debris that has been folded with flow parallel axes longitudinal foliation in places incorporates basal glacial sediments along folds with flow parallel axes and thrusts transport basal debris to the glacier surface crevasse traces are not significant in terms of debris entrainment the entrainment of basal debris along longitudinal foliation is not a universally recognised process at kongsvegen this process is attributed to the development of a transposition foliation in combination with incorporation of debris rich basal ice or soft basal sediment in the fold complex mapping of the landforms in the proglacial area shows that debris incorporated along longitudinal foliation is released as foliation parallel ridges and that transverse ridges mark debris bearing thrusts the role of longitudinal foliation in landform development has never been documented in this manner although the preservation potential of such ridges may be limited recognition of foliation parallel ridges in the pleistocene landform record has important implications for the interpretation of the dynamics of former ire masses

concertina eskers bruarjokull iceland an indicator of surge type glacier behaviour abstract bruarjokull is a surging outlet lobe of vatnajokull in se iceland geological evidence indicates that during quiescent phases water drainage takes place in ice walled and ice roofed channels as the glacier retreats the courses of these channels are represented by eskers eskers which were englacial prior to surges become deformed during a surge the deformation of eskers reflects strong longitudinal compression of the ice in the terminal zone the wavelength of one of the compressed eskers indicates that ice in a 4 km wide margin was compressed by 50 of its original length compressed or concertina eskers date from the 1963 1964 and the 1890 surges of bruarjokull they have not been found from the 1810 surge indicating that they may not survive if overrun by subsequent surges

geomorphology and sedimentology of surging glaciers a land systems approach the identification of surging glaciers and ice streams in glaciated landscapes is of major importance to the understanding of ice sheet dynamics and for reconstructing ice sheets and climate no single landform or diagnostic criterion has yet been found with which to identify surging glaciers a surging glacier land system model is constructed using observations and measurements from contemporary surging glacier snouts in iceland svalbard u s a and canada for differentiating ancient surging margins from other non surging palaeoglaciers this integrates the suite of landforms sediments and stratigraphy produced at surging glacier margins landforms produced during surging include thrust moraines concertina eskers and subglacial crevasse squeeze ridges sedimentary sequences are usually characterized by multiple stacked diamictons and stratified interbeds which display severe glaciotectonic contortion and faulting hummocky moraine comprising interbedded stratified sediments and mass flow diamictons has also been associated with surge margins where large quantities of supraglacial and englacial debris entrained during the surge event have melted out in situ an example of the application of the land system model is presented for east central alberta canada a surging palaeo ice stream is identified within this part of the southwestern laurentide ice sheet where thrust block moraines crevasse squeeze ridges flutings hummocky moraine and glaciotectonized sediments are juxtaposed

the interpretation and classification of subglacially deformed materials abstract a general classification of subglacially deformed materials is proposed based on sediment properties and their relationship to styles of subglacial strain deformation till is defined as homogenized usually diamictic material formed by glacially induced shear of subsole materials three types are recognized type a formed by pervasive ductile deformation type b formed by brittle shear and comminution till produced by the reduction of void space by in situ crushing and abrasion the term glacitectonite is adopted for materials that have undergone subglacial shear but retain some of the structural characteristics of the parent material original structures may be truncated by glacitectonic fabric elements type a or distorted but not truncated type b the principles of strain and material response particle orientation mechanisms fabric development and drainage conditions are reviewed examples of deformation tills and glacitectonites from breidamerkurjokull iceland slettmarkbreen norway loch lomond scotland and east yorkshire england are used to illustrate the nature of subglacially deformed materials and their relationship to former subglacial strain and drainage conditions the geologic evidence suggests a continuum of bed strengths can be recognized in deforming substrata ranging from high strength low strain type b deformation till and comminution till to low strength high strain type a deformation till

how do glaciers surge a review the quasi periodic oscillations between normal and fast motion exhibited by surge type glaciers provide the best observational opportunity to determine limiting conditions that allow fast motion the measurements from variegated glacier prove that its surge motion is caused by rapid sliding induced by high water pressure this arises from a major restructuring of the basal hydraulic system which impedes water discharge prior to and during surge although the evolving glacier geometry and stress distribution play a principal enabling role the seasonal timing of two distinct surge pulses each initiated in winter and terminated in summer indicates a major influence from variable external water inputs this influence is not considered in existing surge models and should promote caution in the use of data from temperate and subpolar surge type glaciers to deduce surge potential in polar ice masses the spatial spreading of surge motion from a zone of local initiation occurs by stress redistribution which may spread the surging zone rapidly upglacier or downglacier inside a region of active ice and by mass redistribution with compressional thickening at the surge front which enables down glacier propagation into less active ice the data from other surge type glaciers including the extensive data from medvezhiy glacier are not inconsistent with the above processes but are inadequate to establish whether completely different mechanisms operate in some surges the way by which water accumulates and produces fast sliding is not established in detail for any surge type glacier and may be different on different glaciers depending for example on the presence or absence of unconsolidated debris between the ice and rock

the formation of a geometrical ridge network by the surge type glacier kongsvegen svalbard traditionally geometrical ridge networks are interpreted as the product of the flow of subglacial sediment into open basal crevasses at the cessation of a glacier surge crevasse fill ridges they are widely regarded as a characteristic landform of glacier surges understanding the range of processes by which these ridge networks form is therefore of importance in the recognition of palaeosurges within the landform record the geometrical ridge network at the surge type glacier kongsvegen in svalbard does not form by crevasse filling the networks consist of transverse and longitudinal ridges that can be seen forming at the current ice margin the transverse ridges form as a result of the incorporation of basal debris along thrust planes within the ice the thrusts were apparently formed during a glacier surge in 1948 longitudinal ridges form through the meltout of elongated pods of debris which on the glacier surface are subparallel to the ice foliation and pre date the surge this work adds to the range of landforms associated with glacier surges

propagation of a glacier surge into stagnant ice the propagation of the surge of variegated glacier into its terminal lobe was observed by daily surveying of a longitudinal line of closely spaced markers extending up glacier from a stagnant zone of thin 40 m nearly motionless ice near the terminus across a front zone to a surge zone of thicker 100 m rapidly moving 20 m d 1 ice above within the front zone there were large angles of upward motion 30 compressive longitudinal strain rates 0 1 d 1 and upward vertical velocities 5 m d 1 within the surge zone the ice motion was nearly parallel to the ice surface and longitudinal gradients were small the deformation pattern produced microcracking exfoliation and buckling of surface ice longitudinal cracking longitudinal chasms and an increase in glacier volume the front zone propagated at an average speed of about 40 m d 1 or roughly twice the surge speed of the ice above the front this factor is determined by the thickness jump across the front finite element modeling constrained by measured geometry and surface velocity was used to deduce the velocity and stress at depth the velocity in the surging zone occurs almost entirely by sliding the sliding rate drops abruptly beneath the center of the front zone to zero in the stagnant zone a comparison of basal velocity and shear stress distributions indicates a highly lubricated area beneath the upper part of the front zone and a locked area beneath the lower part of the front zone normal stress at the bed is not significantly different from overburden and there is no high compressive normal stress on the bed to act as a pressure dam to obstruct water flow through the front zone

theory of glacial erosion transport and deposition as a consequence of subglacial sediment deformation a theory of erosion transport and deposition of unlithified sediments by glaciers is presented it predicts the large scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle together with the resultant intensity of large scale lincations drumlins which will be incised in the landscape the theory also predicts the dispersal patterns of subglacial lithologies together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till it predicts major erosional discontinuities within tills and the formation of boulder pavements it suggests that the dominant proportion of the lowland tills produced by pleistocene mid latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine grained the theory is based on an analysis of glacier dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition

stratigraphy and sedimentology of devensian dimlington stadial glacial deposits east yorkshire england the stratigraphy and sedimentology of the glacial deposits exposed along the coast of east yorkshire are reviewed critical sections at filey brigg barmston and skipsea are examined to reassess the stratigraphy of devensian dimlington stadial glacial deposits in the light of recent developments in glacial sedimentology sedimentary and glaciotectonic structures studied in the field and by using scanning electron microscopy are emphasised two hypotheses are considered for the genesis of the interbedded diamictons and stratified sediments the first involves the deposition of lodgement till and or deformation till followed by meltout till which was overridden to produce more deformation till reflecting periods of ice stagnation punctuated by glacier thickening the second hypothesis which is favoured on the basis of field evidence and micromorphology involves the vertical accretion of a deforming till layer associated with cavity channel or tunnel valley fills beneath active ice at barmston the upper part of the diamicton contains elongate pendant structures containing gravels indicating that the diamicton was saturated and able to flow the diamictons therefore represent a complex sequence of tills deposited and deformed by active ice during the dimlington stadial previously published stratigraphical schemes involving classifications of multiple tills in east yorkshire should be simplified and it is more appropriate to assign these to a single formation the skipsea till formation rhythmic glaciolacustrine and proglacial glaciofluvial sediments overlie the tills at barmston and skipsea these were deposited in sag basins during deglaciation as the tills settled and deformed under thickening sediment and as buried ice melted out extensive sands and gravels cap the succession and were deposited on a sandur during the later stages of deglaciation

unstable behavior of the laurentide ice sheet over deforming sediment and its implications for climate change geologic records of fluctuations of the laurentide ice sheet margin following the most recent glacial maximum ca 20 000 14c yr b p identify fundamental differences in ice sheet behavior depending on subglacial bed conditions rapid and irregular icemargin fluctuations occurred only over areas of deforming sediment indicating nonclimatic forcing controlled by the inherent instability of coupled ice sheet deforming sediment dynamics in contrast largely uninterrupted ice margin retreat with no evidence of significant readvance occurred over rigid bed areas indicating stable behavior unstable ice sheet behavior was most pronounced from 15 000 until 10 000 14c yr b p by which time most of the ice margin had retreated onto a rigid bed unstable ice sheet behavior would have been an integral component in controlling variable fluxes of icebergs and meltwater as well as meltwater routing to the north atlantic thus affecting thermohaline circulation the abrupt climate oscillations in the north atlantic region that ended at 10 000 14c yr b p may thus have their origin in the inherently unstable behavior of the laurentide ice sheet overriding deforming sediment

glacier surge mechanism 1982 1983 surge of variegated glacier alaska the hundredfold speedup in glacier motion in a surge of the kind the kind that took place in variegated glacier in 1982 1983 is caused by the buildup of high water pressure in the basal passageway system which is made possible by a fundamental and pervasive change in the geometry and water transport characteristics of this system the behavior of the glacier in surge has many remarkable features which can provide clues to a detailed theory of the surging process the surge mechanism is akin to a proposed mechanism of overthrust faulting

sedimentation and stratigraphy at eyjabakkajokull an icelandic surging glacier a model for sedimentation by surging glaciers is developed from analysis of the debris load sedimentary processes and proglacial stratigraphy observed at the icelandic surging glacier eyjabakkajokull three aspects of the behavior of surging glaciers explain the distinctive landformsediment associations which they may produce a sudden loading of proglacial sediments during rapid glacier advances results in the buildup of excess pore pressures failure and glacitectonic deformation of the overridden sediments b reactivation of stagnant marginal ice by the downglacier propagation of surges is associated with large longitudinal compressive stresses these induce intense folding and thrusting during which basal debris rich ice is elevated into an englacial position in a narrow marginal zone as the terminal area of the glacier stagnates between surges debris from this ice is released supraglacially and deposited by meltout and sediment flows c local variations in overburden pressure beneath stagnant crevassed ice cause subglacial lodgement tills which are sheared during surges to flow into open crevasses and form crevasse fill ridges

subglacial till a physical framework for its properties and processes the continuum theory of mixtures provides a framework around which present knowledge of subglacial till can be organized the proposed framework is provisional in two senses the constitutive description of till will require improvement and not all potentially significant processes have been included till is treated as a three component mixture of water clasts coarse grained solids and fine grained matrix solids these components obey mass balance equations and their behavior is subject to constitutive relations that describe such processes as darcian water transport fine grained sediment transport consolidation shear deformation dilatancy and comminution five state variables uniquely determine the compressibility permeability shear strength and other physical properties numerical solutions of the state evolutions equations illustrate that the properties of till are highly responsive to changes in the subglacial environment

setting characteristics and age of the andean porphyry copper belt in colombia three distinct porphyry copper subbelts are defined in the colombian andes 1 a western subbelt emplaced during the early to middle eocene in an oceanic domain underlain by ophiolitic basement along the western side of the western cordillera 2 a composite eastern subbelt generated during the middle jurassic to earliest cretaceous in a continental domain underlain by elements of the guyana shield along the eastern side of the central cordillera and the western side of the eastern cordillera and between the two 3 a central subbelt formed in the early to late miocene at the contact of the oceanic and continental domains along the cauca patia depression and its flanks k ar geochronometry undertaken as part of this study provided preferred minimum ages of 54 7 or 1 3 m y for murindo 48 1 or 1 0 m y for acandi and 42 7 or 0 9 m y for pantanos pegadorcito in the western subbelt 166 or 4 m y for mocoa 166 or 4 m y for dolores 144 or 3 m y for california and 131 or 2 m y for infierno chili in the eastern subbelt and 17 4 or 0 4 m y for piedrasentada in the central subbelt the porphyry copper prospects are associated with dacite to tonalire porphyry stocks which in the western and eastern subbelts intruded batholiths emplaced perhaps 15 to 30 m y earlier but which in the central subbelt are isolated within metamorphic or sedimentary host rocks the porphyry copper systems exhibit a variety of alteration patterns with potassium silicate and sericitic alteration important in the highest grade prospects and propylitization predominant in low grade occurrences none of the prospects displays significant supergene enrichment although porphyry coppers in the continental domain tend to be more molybdenum rich than those in the oceanic domain gold rich prospects occur in both settings as do major lode gold deposits the jurassic and miocene subbelts are interpreted to have been emplaced during eastward subduction at andean type continental margins whereas the western eocene subbelt could have been generated in either a modified andean type setting or in an intraoceanic island arc prior to its accretion to the continental edge

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