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

a low order model of the heinrich event cycle if heinrich events result from free oscillations in the size and basal melting conditions of the laurentide ice sheet a rigorous quantitative analysis of ice sheet physics should describe their dynamics to explore this possibility i exploit two characteristic timescales which arise from ice sheet physics to construct a relaxation oscillator model of the north atlantic s heinrich events the numerical implementation of this model confirms the notion that the periodicity of heinrich events approximately 7 000 years is determined by the gross properties of a steady glacial climate e g an annual average sea level temperature of 10 c and an adiabatic lapse rate

stratigraphy and origin of an area of hummocky glacial topography northern wisconsin u s a abstract the winegar phase of the wisconsin glaciation produced a band of hummocky glacial topography in northern wisconsin some hummocks are composed of flow till and somewhat sorted till like debris flow sediment these hummocks formed when ice melted beneath a cover of debris that flowed to its present position other hummocks contain interlayered lodgement and meltout till flow till debris flow and slopewash sediment lake and meltwater stream sediment these hummocks formed where sediment was stacked near the ice margin in other hummocks till and supraglacial debris flow and slopewash sediment deposited during the winegar phase form a thin veneer over older meltwater stream sediment these hummocks formed when ice buried in pre winegar phase stream sediment melted and the overlying sediment collapsed

late ordovician to early devonian adakites and nb enriched basalts in the liuyuan area beishan nw china implications for early paleozoic slab melting and crustal growth in the southern altaids abstract we report newly defined nb enriched basalts adakites and dacites from the beishan nw china of the southern altaids based on field geochemical isotopic and geochronology studies two phases of adakites adakite i and adakite ii have been defined which are calc alkaline and characterized by high na 2 o k 2 o ratios 1 49 1 71 and 2 32 3 64 and sr contents 494 1213 ppm and 325 494 ppm negligible to positive eu anomalies strong depletion of hree e g yb 0 48 0 93 ppm and 0 50 0 99 ppm and y 6 87 9 80 ppm and 6 02 10 30 ppm and enriched in rb sr ba k and depleted nb and ti they are characterized by relatively low e nd t values 0 8 to 0 9 and 0 6 to 3 8 and relatively constant high 87 sr 86 sr i ratios 0 70635 0 70636 and 0 70583 0 70651 the zircons of adakite i have relatively low e hf t 0 8 to 2 7 the nb enriched basalts are sodium rich n 2 o k 2 o 1 31 4 44 with higher tio 2 p 2 o 5 zr and nb contents and nb th pm nb la pm and nb u ratios than typical arc basalts they are relatively enriched in rb ba u pb and k depleted in nb and minor negative to positive ba zr sr and ti they have low positive e nd t 0 9 to 2 3 and relatively high 87 sr 86 sr i 0 70556 0 70691 ratios the dacites are typical arc magmas with moderately enriched lile distinctly negative eu nb sr and ti anomalies they have positive e nd t 2 2 and relatively high 87 sr 86 sr i 0 70786 we argue that the liuyuan adakites were most probably related to the melting of young hot subducted crust of the paleo asian ocean which included tectonically subducted radiogenic crustal material and or inheritance from highly radiogenic oceanic crust e g oib the nb enriched basalts likely resulted from mantle wedge peridotites metasomatized by adakites and or further changed by components other than adakites e g minor slab derived fluids based on own zircon sims u pb dating of these key rock types we further propose that from the late ordovician to early devonian large volumes of magma consisting of late ordovician nb enriched basalts 451 ma and dacites 442 ma late silurian adakite i 424 ma early devonian adakite ii 374 ma and i s a type granites 436 ma 380 ma developed in the southern altaids together with other geochronological data from the literature we conclude that subducted oceanic slab melting was frequent from 470 ma to 370 ma our results suggest that frequent hot and or young oceanic crustal subduction and slab melting were important mechanisms in the accretionary growth of the southern altaids

southern laurentide ice lobes were created by ice streams des moines lobe in minnesota usa abstract regional mapping in southern minnesota has illuminated a suite of landforms developed by the des moines lobe that delimit the position of the lobe at its maximum and at lesser readvances the ice lobe repeatedly advanced discharged its subglacial water and subsequently stagnated recent glaciological research on antarctic ice streams has led some glacial geologists to postulate that ice streams drained parts of the marine based areas of the laurentide ice sheet i postulate that such ice streams may develop in land based areas of an ice sheet as well and that the des moines lobe 200 km wide and 900 km long was an outlet glacier of an ice stream it appears to have been able to advance beyond the laurentide ice sheet as long as adequate water pressure was maintained however the outer part of the lobe stagnated because subglacial water that facilitated the flow was able to drain away through tunnel valleys stagnation of the lobe is not equivalent to stoppage of the ice stream because ice repeatedly advanced into and onto the stagnant margins stacking ice and debris similar landforms are also seen in other lobes of the upper midwestern united states

constraints on the preservation of diamict facies melt out tills at the margins of stagnant glaciers abstract this paper reviews the formation and preservation of diamict facies melt out till generated in situ by the downwasting of stagnant ice at the margins of continental glaciers melt out tills are commonly envisaged as being the product of the top supraglacial or bottom subglacial melt of stagnant debris rich glacier ice but such deposits are relatively uncommon in modern glacier environments where they are thin laterally discontinuous and have a low preservation potential theoretical considerations of the distribution of englacial debris in continental ice sheets show that only the glacier margin is likely to contain the necessary volumes of englacial debris for melt out to be a significant diamict forming process however the application of thaw consolidation theory to the formation of such deposits shows that substantial porewater pressures and self sediment deformations can be induced during melt out especially in matrix rich debris the classic melt out mechanism widely invoked in the literature since 1875 and involving passive in situ aggregation from thawing englacial debris can be seen to operate under restricted conditions diamict facies produced by melt out are likely to form spatially disjunct and laterally discontinuous elements within complex and ice marginal stratigraphics the melt out process is not a mechanism whereby regionally extensive diamiet ite units can be deposited across glaciated basins

geomorphic evidence for active and inactive phases of late devensian ice in north central ireland abstract in the clogher valley north central ireland subglacial and proglacial landforms provide evidence for phases of active and inactive subglacial and ice marginal processes during the late devensian weichselian glaciation 22 13 kyr bp patterns of subglacial rogen moraines and drumlins record east to west ice flow along the valley axis during maximal ice stages eskers superimposed upon the rogen moraines and drumlins record later west to east ice flow and mark the first stages of ice retreat areas of large ice marginal non aligned ridges and mounds are separated by more extensive areas in which no deglacial landforms or sediments are present the non aligned ridges and mounds do not show a dominant directional signature have intricate basal outlines steep slopes and high relief

great jurassic thrust sheets in beishan north mountains gobi areas of china and southern mongolia abstract jurassic thrust sheets with minimum displacements of 120 180 km have been discovered within the hercynian indosinian orogenic belt of the beishan of china and south gobi area the thrusts strike e w extend over 1200 km in length and carried meso proterozoic massive dolomitic limestones over strata ranging from neoproterozoic cryogenian and terminal proterozoic to lower middle jurassic slip linear plots based on kinematic indicators such as slickenlines and groove lineations fiber lineations and drag folds adjacent to the fault surface vergence of folds and imbricated thrusts in the upper plate indicate northward movement in the beishan area to the west and southward movement in the south gobi area to the east the two major thrust faults the beishan thrust and south gobi thrust are presumably separated by a major tear fault the ruo shui fault the major thrust faults were later deformed into a series of e w antiforms and synforms and the sheets are separated due to erosion into a number of klippen mainly located on synforms of the major faults the yagan metamorphic core complex which is a result of an extensional event that postdates the thrust event yields an 40 ar 39 ar plateau age of 155 1 10 ma and rb sr isochron age of 153 6 2 ma the thrust sheets formed during the late middle jurassic long after the closure of any oceans in the study area previously reported for this region and are ascribed to a phase of intracontinental deformation the closing of the jurassic tethys or retroarc deformation behind an active continental margin at the southern edge of asia prior to the tethyan collision or and the closing of mongolo okhotsk oceans might be responsible for this event

pleistocene glaciations of central asia results from 10be surface exposure ages of erratic boulders from the pamir tajikistan and the alay turkestan range kyrgyzstan abstract we have determined the timing of glaciations in the pamir tajikistan and the alay turkestan range kyrgyzstan using 10be surface exposure dating glacial advances in the area have occurred 93 136 60 80 40 55 27 25 22 20 19 17 16 15 15 13 and 11 9 cal ka bp all late pleistocene glaciers in the pamir and the alay turkestan ranges have been valley glaciers except for the most extended glaciers on the pamir plateau which have formed local piedmont glaciations in the eastern pamir these are characterized by ela depressions of 370 380 m thar 0 5 in the turkestan range and alay range ela depressions at the same time were 750 and 600 m respectively late pleistocene glacier advances all over western high asia were contemporaneous with climatic cold phases rather than monsoonal maxima their maximum extent and that of the western hemisphere ice sheets were asynchronous due to increasing aridity in the region over the course of the last glacial late pleistocene climate in central asia seems to have been influenced by the interplay of the westerly circulation and the siberian anticyclone some indirect monsoonal influence in the eastern pamir may be responsible for the existence of some of the lateglacial moraine stages in this area high altitude glaciers seem to have reached their maximum extent earlier mis 5 4 than low altitude glaciers first half of mis 3 possibly due to prolonged glacial aridity imparting with moisture advection onto high altitude sites inducing glacial retreat but prolonged cold during the same time imparting with glacier ablation at lower altitude sites inducing glacial advance

subsurface heterogeneity in the geological and hydraulic properties of the hummocky paris moraine guelph ontario the advance and retreat of ice lobe margins of the laurentide ice sheet formed moraines that are a prevalent feature throughout southwestern ontario in contrast to the well studied stratified moraine complexes recessional and end moraines have largely been ignored in the context of hydrogeological studies recent urban growth has led to development pressures on these moraines and a need to better understand their hydrogeology this study presents data sets from the paris moraine near guelph ontario to examine its geomorphology internal composition and the corresponding hydraulic properties of these ice marginal features the moraine s geomorphic elements were mapped using high resolution global positioning system transects aerial photograph analysis and ground truthing nine continuous sediment cores were recovered to determine the nature and distribution of subsurface sedimentary units and their relation to the regional stratigraphic framework cores were described in detail using standard sedime

sedimentation in isolated glaciomarine embayments during glacio isostatically induced relative sea level fall northern champlain sea basin the nature of glaciomarine sediments deposited during ice margin retreat can vary according to physiographic setting and relative sea level fluctuations to understand the effects of these two parameters on sedimentation we analyzed the sediment records of four lakes located within former isolated glaciomarine embayments of the northern champlain sea basin these lakes were initially inundated by marine water of the champlain sea following deglaciation and have subsequently experienced basin isolation owing to glacio isostatic rebound three of these lakes reveal a common litho and acoustic stratigraphic succession characterized by an ird free glaciomarine to marine facies consisting of homogeneous to faintly laminated clayey silts grading into well laminated silts with rapidly deposited layers these two units recorded the transitional environment from glaciomarine sedimentation below multiyear shorefast ice to increased terrestrial runoff and rapid glacio isostatic rebound once the ice margin retre

late wisconsin ice flow history in the buffalo head hills kimberlite field north central alberta ice flow of the last glaciation in the buffalo head hills kimberlite field of northern alberta is reconstructed from landform interpretations and clast orientations for the purpose of aiding kimberlite exploration in the region the paucity of bedrock outcrop and the absence of preserved striae and other erosional ice flow indicators on the soft cretaceous marine sediments inhibit detailed interpretations on glacial flow chronology poorly developed bedrock drumlins on the buffalo head hills and erosional ice flow indicators preserved on the kimberlite outcrops indicate southwestward ice flow during the maximum extent of ice during the last glaciation during the deglaciation of northern alberta later phases of ice flow were controlled by lobes of surging ice which surged into proglacial lakes west of the buffalo head hills the maximum phase of southwest flow was followed by southeastward ice movement of the peace river ice lobe similarly east of the buffalo head hills the maximum phase of ice flow was superceded by a south southwest ice advance of the wasbasca ice lobe

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

s type granite from the gongpoquan arc in the beishan orogenic collage southern altaids implications for the tectonic transition abstract voluminous paleozoic intrusions occur in the beishan orogenic collage boc and their genesis and tectonic background are important to reconstruct the accretion collision processes in the southernmost altaids paleozoic is an important period for arc development in the boc where the gongpoquan and huaniushan arcs are located there are two pulses of magmatism in the huaniushan and gongpoquan arcs i e the ca 470 423 ma i type and ca 424 395 ma s and a type granitoids in this study we focus on two peraluminous granitic plutons in the gongpoquan arc i e the baitoushan muscovite granite and haergen two mica granite aiming at unraveling their petrogenesis and tectonic background zircon la icp ms u pb dating yields emplacement ages of ca 409 395 ma and ca 409 ma for the baitoushan and haergen plutons respectively both the granitic plutons are strongly peraluminous with a cnk ratios of 1 10 1 20 indicative of s type affinities the rocks are characterized by high sio 2 and k 2 o contents with high cao na 2 o ratios moreover the rocks possess low mgo contents rb sr and rb ba ratios together with their relatively high initial 87 sr 86 sr ratios 0 7139 0 7152 and less radiogenic end t values 3 15 to 5 17 implying a clay poor and plagioclase rich crustal source compared with earlier pulse of arc related magmatism ca 470 423 ma the latter pulse of magmatism ca 424 395 ma consists mainly of normal granite characterized by higher sio 2 66 and k 2 o contents weaker fractionated ree patterns and lower eu values and gabbroic to dioritic intrusions are only sporadic moreover the granitoids of the latter pulse show variable but more crust like sr nd isotopic compositions 87 sr 86 sr 0 0 7038 0 7327 end t 6 70 to 0 33 than the earlier ones 87 sr 86 sr 0 0 7024 0 7080 end t 2 56 to 8 86 indicating that the early devonian ca 424 395 ma experienced extensive crustal melting with minor involvement of mantle materials considering early devonian geological evidence the transition from i to s and a type magmatism probably reflects the early devonian amalgamation between the gongpoquan and huaniushan arcs which caused not only regional unconformity and strong deformation metamorphism but also extensive melting of the accreted crustal materials in the boc

a hybrid origin for lachlan s type granites the murrumbidgee batholith example the murrumbidgee batholith is typical of other composite s type granite batholiths in the lachlan fold belt lfb of southeastern australia consisting of discrete peraluminous cogenetic granite suites however it contains a unique suite murrumbucka at its southern extremity that has chemical and petrological features transitional with metaluminous i type granites previous detailed structural and metamorphic studies have shown that the batholith is tilted northward exposing subvolcanic plutons in the north and root zones in the south originally located at depths of not vert similar10 km a transition from mafic foliated sheet like granites in the south to felsic generally less foliated homogeneous granites in the north is consistent with magma ascent via subvertical structurally controlled sheets to emplacement in an overlying magma chamber in the inferred root zones the murrumbucka suite hosts migmatitic metasedimentary and gabbroic rocks both of which have transitional contacts and show evidence for mafic felsic interaction with the murrumbucka suite the migmatites extend southward to become part of the high t low p cooma metamorphic complex which contains a core of heterogeneous remobilised diatexitic granite cooma suite lenses of which also occur throughout the southern deeper parts of the murrumbidgee batholith the composition of the most mafic rocks from the batholith lies on a chemical tie line between cooma suite granites and the gabbros for almost all elements this chemical coincidence is interpreted to reflect derivation of parental murrumbidgee s type granite magmas by bulk mixing between a felsic crustal and mafic mantle derived component consistent with sr and nd isotopic results and field observations based on the tie lines and simple numerical modelling the murrumbucka suite is estimated to be a 50 50 mix of mantle and crustal derived magmas whereas the more peraluminous and widespread clear range suite which is very typical of lachlan s type granites is modelled as a 40 60 mix chemical variation trends diverge from the mixing lines and suggest that much of the fractionation process operated after mixing mixing occurred within sheets during the ascent of the magmas whereas fractionation was the dominant process generating chemical diversity during emplacement in the magma chamber generation of the s type granites probably occurred in an extensional tectonic environment

timing and climatic drivers for glaciation across semi arid western himalayan tibetan orogen abstract mapping and forty seven new 10 be ages help define the timing of glaciation in the ladakh and pangong ranges in northwest india five new local glacial stages are defined for the ladakh range from oldest to youngest these include the ladakh 4 glacial stage at 81 20 ka the ladakh 3 glacial stage not dated the ladakh 2 glacial stage at 22 3 ka the ladakh 1 glacial stage not dated and the ladakh cirque glacial stage at 1 8 0 4 ka three local glacial stages are defined for the pangong range which include the pangong 2 glacial stage at 85 15 ka the pangong 1 glacial stage at 40 3 ka and the pangong cirque glacial stage at 0 4 0 3 ka the new 10 be ages are combined with 645 recalculated 10 be ages from previous studies to develop the first regional framework of glaciation across the dryland regions of the greater himalaya transhimalaya pamir and tian shan at the western end of the himalayan tibetan orogen nineteen regional glacial stages are recognized that are termed semi arid western himalayan tibetan stages swhts these include swhts 9 at 311 32 ka swhts 7 at 234 44 ka tentative swhts 6 at 146 18 ka swhts 5e at 121 11 ka swhts 5a at 80 5 ka swhts 5a at 72 8 ka swhts 4 at 61 5 ka swhts 3 at 46 4 ka swhts 2f at 30 3 ka swhts 2e at 20 2 ka swhts 2d at 16 9 0 7 ka swhts 2c at 14 9 0 8 ka swhts 2b at 13 9 0 5 ka swhts 2a at 12 2 0 8 ka swhts 1e at 8 8 0 3 ka tentative swhts 1d at 6 9 0 2 ka tentative swhts 1c at 3 8 0 6 ka swhts 1b at 1 7 0 2 ka and swhts 1a at 0 4 0 1 ka regional glacial stages older than 21 ka are broadly correlated with strong monsoons swhts that are 21 ka or younger have smaller uncertainties and broadly correlate with global ice volume given by marine oxygen isotope stages and northern hemisphere climatic events oldest dryas older dryas younger dryas roman humid period and little ice age

petrogenesis and implications for tectonic setting of cambrian suprasubduction zone ophiolitic rocks in the central beishan orogenic collage northwest china abstract the hongliuhe ophiolite is a fragment of the early paleozoic oceanic crust situated in a suture zone central to the beishan orogenic collage which is a subset to the central asian orogenic belt in northwest china it has lithologic geochemical and chronological similarities to the yueyashan xichangjing ophiolite 400 km east along strike allowing for correlative interpretations of marginal seafloor spreading processes the hongliuhe ophiolite is explored in this study with complete petrologic geochemical and structural descriptions the ultramafic body comprises cumulate layered lherzolite and gabbro plutonic lithologies tectonized with syn magmatic extension a u pb age of 520 3 5 8 ma was obtained by shrimp analysis of zircons from a pegmatitic gabbro the ultramafic rocks are variably depleted and some show refertilization of lile s consistent with slab dehydration in a supra subduction mantle wedge mineral chemistry shows high cr 36 82 populations of spinel and wide ranging lower mg 8 50 consistent with known forearc peridotite chemistry partially re equilibrated by tectonism or through staged magmatism olivine and pyroxene chemistries are consistent with this interpretation a thin volcanic cover is interlayered with red cherts and has a bimodal set of geochemical signatures both island arc tholeiite and calc alkaline they bear distinct nb ta anomalies consistent with slab derived fluid initiated melting in a supra subduction environment coarse volcaniclastic turbidites infilled paleo topography along the ophiolite and they are interbedded with quartzofeldspathic sandstones indicating an initial proximity to an arc the sandstones contain detrital spinel with analyses showing relation to both the ophiolite and possibly a secondary alkalic volcanic source the supracrustal sequence continues with a thick contiguous covering of margin related rocks including interlayered limestone pyroclastic and siliciclastic rocks and basalts as a constraint on emplacement we provide an u pb zircon shrimp date 413 6 3 5 ma from an undeformed granitoid body intruded within the overlying stratigraphy the entire analysis paints a picture of subduction rollback related extension affecting the margin of an emerging or mature arc just prior to a known magmatic cycle that ended with a collision in the devonian at the latest this suture is one of many in the beishan and our interpretation provides evidence in a long running debate supporting accretionary orogenesis in an island arc archipelago as the main system for growing sialic crust in the paleozoic central asia

paleozoic multiple accretionary and collisional processes of the beishan orogenic collage in the paper by xiao and others american journal of science v 310 p 1553 1594 1 please substitute figure 16 reproduced here for figure 16 that appears between pages 1573 and 1574 the original figure 16 2 was printed at an improper size figure 3 1 lookup volpage 310

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

erosional origin of hummocky terrain in south central alberta canada we propose that hummocky terrain in south central alberta is the product of subglacial erosion rather than supraglacial letdown during ice disintegration a common view of hummock formation extensive exposures in hummocks contain sediments marking the history of laurentide ice advance and deglaciation in the region regardless of the genesis of those sediments units are abruptly truncated at the surface that represents the hummock and trough morphology indicating that the hummocks are the product of differential erosion subglacial sediments predating the erosion and subglacial eskers overlying the erosion surface suggest that the erosion was also subglacial hummock morphology lithostratigraphy correlated from hummock to hummock truncation at the land surface and widespread coarse boulder lags support a glaciofluvial origin for hummocky terrain in this region

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

hummock fields in the norway basin and eastern iceland plateau rayleigh taylor instabilities side scan imagery and 3 5 khz profiles in the norway basin reveal 3 4 10 4 km 2 of sea floor covered by regularly spaced 500 2000 m low relief hummocks previously attributed to bottom currents extrapolating from nearby borehole data i attribute these mound fields to gravitational rayleigh taylor instabilities resulting from superposition of 50 100 m of thick pliocene pleistocene glacigenic glacier derived sandy clays density 1800 kg m 3 on a several hundred metre thickness of miocene biosiliceous oozes having a density of 1300 kg m 3 this hypothesis predicts or explains similar hummock fields in many oceanic regions where such density inversions exist

late pleistocene stratigraphy and implications for deglaciation and subglacial processes of the flathead lobe of the cordilleran ice sheet flathead valley montana usa abstract the flathead lobe of the cordilleran ice sheet advanced and retreated across the flathead valley a 90 km long by 15 25 km wide basin the glacier terminated at the southern end of the flathead valley during its last glacial maximum i used geologic mapping sedimentologic study and mapping from 3300 descriptive water well logs to define the stratigraphy of glacial and post glacial deposits in the flathead valley and to infer subglacial processes and retreat rates units in the 30 200 m thick stratigraphic sequence include deep sand and gravel proglacial alluvium diamicton till locally gravelly silt and clay coarse grained subaqueous deposits at recessional moraines and glaciolacustrine deposits and surficial sand and gravel outwash and holocene stream deposits a nearly continuous till overlies alluvium and is in turn directly overlain by glaciolacustrine deposits glaciolacustrine shoreline indicators at altitudes of 940 958 m show that the flathead lobe receded as a proglacial lake expanded in that range of altitudes this lake the ancestral flathead lake was impounded behind bedrock that was downcut 60 78 m since the late pleistocene glaciolacustrine deposits are as much as 180 m thick in discontinuous troughs that locally cut through till to alluvium and bedrock the troughs are mostly 1 5 km in width 6 25 km in length sinuous in plan view and undulatory in longitudinal profile the troughs extend below the elevation of flathead lake and have been below local base level since glaciation these characteristics point to incision by meltwater hydrostatically pressurized beneath the flathead lobe in subglacial tunnel channels sand and gravel at the trough bases and part of the glaciolacustrine fill in the troughs are subglacial deposits tunnel channels that terminate at recessional moraines suggest excavation occurred sporadically during deglaciation the distribution of glaciolacustrine silts indicates that the flathead lobe retreated and the ancestral flathead lake expanded by 87 km before the lake outlet stream could downcut its dam by 500 m year 1

what controls dead ice melting under different climate conditions a discussion abstract in the geological record hummocky dead ice moraines represent the final product of the melt out of dead ice processes and rates of dead ice melting in ice cored moraines and at debris covered glaciers are commonly believed to be governed by climate and debris cover properties here backwasting rates from 14 dead ice areas are assessed in relation to mean annual air temperature mean summer air temperature mean annual precipitation mean summer precipitation and annual sum of positive degree days the highest correlation was found between backwasting rate and mean annual air temperature however the correlation between melt rates and climate parameters is low stressing that processes and topography play a major role in governing the rates of backwasting the rates of backwasting from modern glacial environments should serve as input to de icing models for ancient dead ice areas in order to assess the mode and duration of deposition a challenge for future explorations of dead ice environments is to obtain long term records of field based monitoring of melt progression furthermore many modern satellite borne sensors have high potentials for recordings of multi temporal digital elevation models dems for detection and quantification of changes in dead ice environments in recent years high accuracy dems from airborne laser scanning altimetry lidar are emerging as an additional data source however time series of high resolution aerial photographs remain essential for both visual inspection and high resolution stereographic dem production

younger dryas glacial landsystems in north west scotland an assessment of modern analogues and palaeoclimatic implications abstract by explicitly linking process to form landsystem models developed in modern environments provide powerful tools for the reconstruction of past environments from the sedimentological and geomorphological record some landform characteristics however may originate by more than one process equifinality so although a particular depositional model may fit the data it is not necessarily the correct one here we critically examine the use of modern analogues to interpret glacial landforms focusing on hummocky moraine a distinctive landform association consisting of numerous sharp crested moraine mounds and ridge fragments which occurs on many valley floors in upland britain detailed studies of examples of younger dryas age in the north west highlands of scotland show that they represent ice contact fans which have been affected to varying degrees by ice push and subglacial deformation and that they formed at active oscillating ice margins the sedimentological data are inconsistent with an origin by englacial thrusting as has been proposed for similar moraines elsewhere in upland britain on the basis of morphological criteria comparison with modern glacial landsystems shows that although aspects of hummocky moraines in the study area have close modern counterparts none provides an exact analogue in all respects this may reflect the distinctive climate of western scotland during the younger dryas estimates of mean july temperatures in the younger dryas in scotland based on chironomid assemblages are c 8 c at sea level when combined with reconstructed equilibrium line altitudes for contemporaneous glaciers these estimates indicate mean annual precipitation totals in the range of 2358 337 mm yr 1 at sea level in nw scotland c 26 higher than present day values the implied heavy snowfall suggests that glaciers in western scotland during the younger dryas were of high activity type unlike modern glaciers in svalbard for example high glacier velocities and high winter precipitation would suppress permafrost development despite low mean annual temperatures the climatic environment also favoured rapid oscillations of glacier margins during both glacier build up and retreat phases climatic factors combined with the availability of large quantities of subglacial debris led to the development of distinctive glacial landsystems which may have no direct modern analogue

landform assemblage in isidis planitia mars evidence for a 3 ga old polythermal ice sheet abstract the floor of isidis planitia a giant impact basin located close to the martian equator exhibits a landform assemblage nicknamed thumbprint terrain made of arcuate ridges aligned cones isolated cones cone fields associated with a peripheral network of sinuous ridges linear depressions and mounds from a new comprehensive mapping initiative of these landforms and from comparisons with terrestrial analogues ribbed moraines dirt cones kettle holes eskers tunnel valleys and moraine plateaux we demonstrate that this distinctive assemblage is a glacial landsystem inherited from the presence of a massive polythermal ice sheet over the basin during the hesperian the flow of the ice sheet was controlled by its basal thermal regime wet based conditions led to the formation of arcuate ridges and aligned cones in most parts of the basin while a negative geothermal anomaly due to impact related crustal thinning was responsible for cold based conditions in its central part where only isolated cones and cones fields are present sinuous ridges linear depressions and mounds at the basin margins are interpreted as relicts of a radial network of subglacial channels which drained the glacial meltwater produced within the interior of the ice sheet across its cold based periphery

till stratigraphy and sedimentology at the margins of terrestrially terminating ice streams case study of the western canadian prairies and high plains complex glacigenic sediment sequences arranged in large arcuate assemblages and demarcating the lobate termini of fast ice flow corridors in western canada represent the terrestrial equivalents of the trough mouth fans of the marine palaeo ice stream record intensive sedimentological and stratigraphic investigations at a regional scale are employed in the interpretation of thick units or sequences of diamicton and two depositional scenarios are proposed first glacier marginal till thickening and the stacking of till wedges takes place in areas where an ice stream margin was stationary for short periods of time stacks of subglacial traction tills potentially record annual sub marginal incremental thickening at ice stream margins especially in areas where densely spaced recessional push moraines appear to record seasonal climatic forcing second proglacial lake and valley infilling with glacilacustrine rhythmites and mass flow diamictons is associated with the advection of sub marginal till into preglacial bedrock depressions and records ice stream marginal oscillations and debris influx into discontinuous proglacial lakes which act as subaqueous sediment sinks for materials that would otherwise have been used to construct sub marginal till wedges and moraines glacial overriding modifies the deposits produced in both scenarios through deformation and the dislocation and entrainment of bedrock and sediment rafts at the margins of bedrock valley walls resulting in the widespread development of glacitectonites and megablocks the occurrence of the thickest deposits as preglacial valley fills may be significant in groundwater siphoning away from subglacial deforming layers and the promotion of sticky spots

quaternary geology and geomorphology of the dinosaur provincial park area and surrounding plains alberta canada the identification of former glacial lobes drainage diversions and meltwater flood tracks abstract the quaternary geology and geomorphology of the lower red deer river drainage basin centred on the dinosaur provincial park badlands provides information on pre late wisconsinan drainage patterns and the dynamics of former lobes of the laurentide ice sheet in western canada patterns of deglaciation proglacial lake evolution and spillway incision are also reconstructed based upon the distribution of surface materials and glacial glaciofluvial landforms the empress group fluvial and glaciolacustrine sediments which could be as young as 27 ka bp infill the precursor red deer river and its tributaries and document the initial advance of glacier ice into southern alberta glaciotectonic disturbance of older sediments and bedrock the production of deformation tills and the construction of a megafluting complex and cupola hills record the advances of a glacier lobe centred over the study area stratified inter and intra till beds record pulses of subglacial meltwater between phases of subsole deformation the thickening of tills towards the margin of the lobe represents a till wedge an expected product of sediment advection by glaciers moving over deformable beds the eastern margin of the glacier lobe is demarcated by the interlobate suffield hummocky moraine belt which contains overprinted thrust ridges which record diachronous oscillations of neighbouring lobes within the ice sheet proglacial and glaciofluvial sediments were deposited in the area in association with proglacial lake bassano patricia which drained eastwards when the suffield moraine was dissected by spillways changes in the size of glacial lake bassano patricia are clearly documented by a sequence of spillway incisions which culminated in the erosion of scabland topography and the initiation of a new course for the red deer river a 15 km southward diversion of the main channel in distinct contrast to the documented incision histories of other small rivers in alberta one tree creek and little sandhill creek did not start major incisions of the quaternary sediments over buried valley positions until the late holocene when environmental conditions were characterized by higher precipitation

multiple origins of large hummock deposits in alai valley northern pamir implications for palaeoclimate reconstructions abstract this paper interprets the origin of several of the largest hummocky landform assemblages in the alai valley northern pamir a formerly glaciated intermontane depression the vast hummocky deposits in the koman suu and achik tash river catchments are found to be of two contrasting modes of formation glacial hummocks deposited during koman and lenin glaciers withdraw and avalanche hummocks produced during catastrophic koman and lenin rock avalanches the origins of the deposits we assessed through remote and field based geomorphological mapping as well as sedimentological investigations which included clast analysis and the identification of micro scale agglomerates indicative of rock avalanche emplacement both the koman and lenin rock avalanches were large catastrophic events with run outs of 34 and 24 km respectively and a volume over 1 109 m3 each that occurred subsequent to glacier withdrawal from the area the complex conditions on the moment of the rock avalanche emplacement promoted unusual deposits geomorphology and extensive run outs the landslide landforms formed over the pre existing glacial hummocks and fluvial deposits and are geomorphologically and sedimentologically distinct from the larger glacial hummocks the reconstruction of this sequence of events has implications for how hummock dating should be interpreted this research illustrates large scale catastrophic landsliding in the glacial environment and adds to the ongoing debate about the misidentification of rock avalanche deposits as of glacial origin and their relevance to palaeoclimatological and palaeoseismological reconstructions

geochronology petrogenesis and tectonic implications of triassic granitoids from beishan nw china abstract five triassic granitic plutons have been identified from the beishan orogen by zircon u pb dating that defines two age groups 240 238 ma and 225 217 ma they belong to the high k calc alkaline and shoshonitic series and have alkalic and alkali calcic characteristics and are weakly peraluminous metaluminous these granitoids are mainly highly fractionated i type granite and a type granite they have high i sr values 0 7049 0 7096 and negative e nd t values of 0 9 to 2 8 with nd model ages of 0 89 1 13 ga suggesting a crustal contribution they also exhibit relatively high zircon e hf t values 1 3 to 6 1 and two stage hf model ages of 1 34 0 87 ga suggesting a mixture of mantle and various amounts of crustal components the mantle component probably originated from underplated magmas produced by partial melting of lithospheric mantle metasomatized by fluids or melts possibly derived from pre existing subducted materials the mid triassic 240 238 ma pluton has relatively high sr y 15 9 41 8 ratios and mgo 1 22 3 77 contents with negligibly to weakly negative eu anomalies eu 0 57 0 80 indicating adakite like features this was likely the result of partial melting of crustal components in response to voluminous underplated mantle derived magmas probably linked to lithospheric delamination and asthenospheric upwelling the late triassic 217 225 ma granitic plutons show strong negative anomalies of eu ba nb ta sr p and ti these were likely the result of fractional crystallization and crustal contamination for the granitic magma emplaced in a post orogenic setting

melt out till in the edmonton area alberta canada till at two localities in the edmonton area alberta was formed mainly by passive melt out from glacier ice and not by lodgment by active ice or as a result of supraglacially derived flows this interpretation is based on the implications of in situ stratified glacial sediment below and within till and of large unlithified clasts of sediment derived from subtill deposits the interpretation is also supported by more general regional stratigraphic and structural observations such associated properties as subtill deformation a sharply planar till preglacial sediment contact sole marks and fabric support formation by melt out or illustrate local exceptions to this process regionally consistent preferred orientation of clasts is interpreted to represent preferred englacial orientation collective analysis of the principal eigenvectors of individual englacial fabrics and flow or resedimented till fabrics from a modem glacial environment and similar analysis for till at villeneuve near edmonton show the edmonton till to occupy the intermediate position in terms of orientation strength predicted by the melt out hypothesis in places preferred orientation is considered to have been disturbed by flow and diapirism induced by the process of thaw consolidation thus preferred clast orientation in till may show considerable deviation from a regional trend stratigraphy and sedimentology are used to classify the till and to relate individual exposures to regional environments of deposition

flow pattern evolution of the laurentide and cordilleran ice sheets across west central alberta canada implications for ice sheet growth retreat and dynamics during the last glacial cycle this paper presents a reconstruction of the geometry dynamics and flow pattern of the laurentide and cordilleran ice sheets along the albertan portion of the eastern canadian rocky mountains during the last glaciation sediment landform associations relating to the evolution of these ice sheets document their initial convergence and mutual deflection across west central alberta the continued advance of laurentide ice locally displaced cordilleran ice westward into the front ranges and deflected trunk ice emerging from the athabasca river valley south eastwards along the foothills these convergent flow patterns remained major features in the regional geometry of both ice sheets throughout the last glacial maximum the onset of deglaciation was characterized by the northward retreat of the laurentide ice sheet which progressively unblocked valleys along the mountain front the configuration of buttress removal initiated the lateglacial expansion and sequential drawdown of the eastern cordilleran ice sheet along fast flowing outlet glaciers subsequent regional retreat of laurentide and cordilleran ice was associated with the development of large proglacial lakes which amplified deglaciation by destabilizing the margins of both ice sheets triggering extensive readvances due to periodic changes in water level associated with the opening and closing of outlets by ice margin fluctuations

compression characteristics and index properties of tills and intertill clays in southern saskatchewan canada compression characteristics and index properties of unweathered tills and clays from repeated glaciations follow a predictable pattern at 10 sites along a 1300 km transect across southern saskatche

outburst flood origin of the central kalamazoo river valley michigan usa abstract geomorphic evidence and stratigraphic information from boreholes suggest that the oversized central kalamazoo river valley ckrv in southwest michigan resulted from a catastrophic outburst flood emanating from subglacial channels under the saginaw lobe of the laurentide ice sheet the ckrv occurs as a deeply incised trench over 2 km wide and in excess of 50 m deep situated in a reentrant formed by the lake michigan saginaw and huron erie lobes the course of the ckrv follows an irregular flow path that bisects the kalamazoo moraine of the lake michigan lobe erosional terraces near the mouth of the channel indicate that lake michigan lobe meltwater drained eastward prior to the westward saginaw outburst prior to valley formation the lake michigan lobe had retreated westward to at least the lake border moraine with the lake michigan lobe absent to impede flow drainage from the ckrv proceeded southwesterly until draining into glacial lake chicago near st joseph michigan the outburst originated from a system of saginaw tunnel channels that display convex up flow profiles and contain eskers meltwater drainage transitioned from subglacial to ice marginal and proglacial environments during the interval represented by the outburst the saginaw lobe appears to have been in a relatively stationary position

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

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

structure age and tectonic development of the huoshishan niujuanzi ophiolitic melange beishan southernmost altaids abstract the huoshishan niujuanzi ophiolitic melange hno is located near the central part of the beishan orogenic belt in the southernmost altaids the hno consists of ultramafic rocks cumulate gabbros gabbros plagiogranites diorites diabases basalts andesites rhyolitic volcaniclastic rocks and siliceous sedimentary rocks many of which are in a schist matrix gongpoquan group geochemical data of the mafic rocks indicate a calc alkaline or a mixture of calc alkaline and tholeiitic rocks with negative nb ta and positive pb ba and la anomalies suggesting formation in an island arc or supra subduction zone setting a gabbro from a block in the melange in the niujuanzi area has a zircon age of 435 0 1 9 ma and a plagiogranite with an age of 444 3 1 9 ma and another gabbro from the huoshishan area has an age of 410 5 3 7 ma the schist matrix has a zircon age of 512 5 3 ma and contains silurian devonian and carboniferous fossils thus the melange formed in the late carboniferous or later our structural analysis of fault planes in the hno the crenulation cleavages s 2 of the schist and fold axial planes of early permian sandy limestone quartz veins and late permian sandstones indicates that the melange underwent a north to south compression and the orientation of stretching lineations slickensides and fold hinge lines implies that the hno experienced top to the north or northwest movement the entire planar and linear structural data set suggests that the subduction polarity was probably to the south in the late paleozoic the emplacement age of the hno was probably near the end permian based on the age of the youngest rocks in the ophiolitic melange and by the presence of a late permian unconformity from our work integrated with published regional data we outline a comprehensive geodynamic model for the central boc

quantitative assessment of the hydraulic role of subglaciofluvial interbeds in promoting deposition of deformation till northern till ontario the northern till is a thick 65 m deformation till underlying some 7500 km2 of southern ontario canada including the peterborough drumlin field it was deposited below the lake ontario ice stream of the laurentide ice sheet the till rests on glaciotectonized aquifer sediments and consists of multiple beds of till up to 6 m thick these are separated by boulder lags sometimes in the form of striated pavements with thin 30 cm interbeds of poorly sorted waterlaid sand the composite till stratigraphy indicates punctuated aggradation where the subglacial bed was built up incrementally by the repeated immobilization of deforming overpressured till layers boulders and sands indicate pauses in subglacial aggradation marked by sluggish sheet flows of water that reworked the top of the underlying till interbeds are laterally extensive and correlated using downhole electrical conductivity core recovery and natural gamma data a 3 d finite element model feflow using data from 200 cored and geophysically logged boreholes and a large digital water well dataset of 3400 individual records shows that the till functions as a leaky aquitard as a consequence of water flow through interbeds it is proposed that interbeds played a similar role in the subglacial hydraulic system below the laurentide ice sheet by allowing drainage of excess porewater pressures in deforming sediment and promoting deposition of till this is in agreement with theoretical studies of deforming bed dynamics and observations at modern glaciers where porewater in the deforming layer is discharged into underlying aquifers in this way the presence of interbeds may be fundamental in retarding downglacier transport of deforming bed material thereby promoting the build up of thick subglacial till successions

hummocky moraine sedimentary record of stagnant laurentide ice sheet lobes resting on soft beds abstract over large areas of the western interior plains of north america hummocky moraine hm formed at the margins of laurentide ice sheet lis lobes that flowed upslope against topographic highs current depositional models argue that hm was deposited supraglacially from stagnant debris rich ice disintegration moraine across southern alberta canada map and outcrop data show that hm is composed of fine grained till as much as 25 m thick containing rafts of soft glaciotectonized bedrock and sediment chaotic non oriented hm commonly passes downslope into weakly oriented hummocks washboard moraine that are transitional to drumlins in topographic lows the same subsurface stratigraphy and till facies is present throughout these landforms and others such as doughnut like rim ridges flat topped moraine plateaux and linear disintegration ridges are identified as belonging to subglacially deposited soft bed terrain this terrain is the record of ice lobes moving over deformation till derived from weakly lithified bentonite rich shale drumlins record continued active ice flow in topographic lows during deglaciation whereas hm was produced below the outer stagnant margins of ice lobes by gravitational loading pressing of remnant dead ice blocks into wet plastic till intervening zones of washboard moraine mark the former boundary of active and stagnant ice and show hybrid drumlins whose streamlined form has been altered by subglacial pressing humdrums below dead ice the presence of hummocky moraine over a very large area of interior north america provides additional support for glaciological models of a soft bedded laurentide ice sheet

paleozoic accretionary orogenesis in the eastern beishan orogen constraints from zircon u pb and 40ar 39ar geochronology abstract the continental growth mechanism of the altaids in central asia is still in controversy between models of continuous subduction accretion versus punctuated accretion by closure of multiple oceanic basins the beishan orogenic belt located in the southern altaids is a natural laboratory to address this controversy key questions that are heavily debated are the closure time and subduction polarity of former oceans the emplacement time of ophiolites and the styles of accretion and collision this paper reports new structural data u pb and ar ar ages from the eastern beishan orogen that provide information on the accretion process and tectonic affiliation of various terranes our geochronological and structural results show that the younging direction of accretion was northwards and the subduction zone dipped southwards under the northern margin of the shuangyingshan micro continent this long lived and continuous accretion process formed the hanshan accretionary prism our field investigations show that the emplacement of the xiaohuangshan ophiolite was controlled by oceanic crust subduction beneath the forearc accretionary prism of the shuangyingshan mazongshan composite arc to the south moreover we address the age and terrane affiliation of lithologies in the eastern beishan orogen through detrital zircon geochronology of meta sedimentary rocks we provide new information on the ages subduction polarities and affiliation of constituent structural units as well as a new model of tectonic evolution of the eastern beishan orogen the accretionary processes and crustal growth of central asia were the result of multiple sequences of accretion and collision of manifold terranes

geomorphology and till architecture of terrestrial palaeo ice streams of the southwest laurentide ice sheet a borehole stratigraphic approach abstract a multidimensional study utilising geomorphological mapping and the analysis of regional borehole stratigraphy is employed to elucidate the regional till architecture of terrestrial palaeo ice streams relating to the late wisconsinan southwest laurentide ice sheet detailed mapping over a 57 400 km2 area of southwestern saskatchewan confirms previous reconstructions of a former southerly flowing ice stream demarcated by a 800 km long corridor of megaflutes and mega scale glacial lineations ice stream 1 and cross cut by three formerly southeast flowing ice streams ice streams 2a b and c analysis of the lithologic and geophysical characteristics of 197 borehole samples within these corridors reveals 17 stratigraphic units comprising multiple tills and associated stratified sediments overlying preglacial deposits the till thicknesses varying with both topography and distance down corridor reconciling this regional till architecture with the surficial geomorphology reveals that surficial units are spatially consistent with a dynamic switch in flow direction recorded by the cross cutting corridors of ice streams 1 2a b and c the general thickening of tills towards lobate ice stream margins is consistent with subglacial deformation theory and variations in this pattern on a more localised scale are attributed to influences of subglacial topography including thickening at buried valley margins thinning over uplands and thickening in overridden ice marginal landforms

subglacial bedforms of the zemgale ice lobe south eastern baltic abstract presented in this paper are the results of the mapping of 6600 subglacial bedforms of the zemgale ice lobe zil their composition and internal structure in the south eastern baltic region topographic maps at scale 1 10 000 were mainly used to map bedforms in latvia while a digital elevation model with cell size of 5 m was used for north central lithuania the zil operated during the deglaciation of the late weichselian scandinavian ice sheet sis at least in the middle lithuanian and north lithuanian glacial phases and created a subglacial landform assemblage consisting of glacial lineations such as drumlins mega flutings mega scale glacial lineations msgls and transversal morphologies such as ribbed moraines it is observed that lineations of different scales co exist suggesting subglacial bedform transition and possibly a continuum of bedforms highly elongated lineations with a length of up to 24 km and an elongation ratio of up to 1 50 interpreted as msgls are observed in the ne part of the middle lithuanian plain in the so called zemgale drumlin field 20 of lineations have elongation ratios 10 1 indicating the fastest ice flow in the central part of the main body of the lobe based on test surveys and investigated outcrops the drumlin cores consist of sorted sediments with different levels and depths of glaciotectonic deformation overlain by subglacial till the main drumlin forming ice stress was mostly parallel to the crests of drumlins while some ice stress from inter drumlin depressions acted during the final episodes of their formation the observed sediment structures indicate multiple episodes of the basal ice bed coupling and decoupling which can best be explained by the mosaic ice bed deformation model ribbed moraines are characterized by a complicated structure of multiple till units interbedded with sorted sediments that have been formed by repeated subglacial overthrusting

reconstructing the confluence zone between laurentide and cordilleran ice sheets along the rocky mountain foothills south west alberta landform mapping near the athabasca valley was fundamental to determining whether the late wisconsinan laurentide and cordilleran ice sheets lis cis coalesced in this paper we examine the detailed landform record using new lidar digital elevation and hillshade models of the area of confluence and eventual desuturing of these ice sheets this work reveals an earlier more extensive cordilleran advance before coalescence with the lis when the ice sheets coalesced the flow pattern was dominated by ice flow along the mountain front with cordilleran ice flowing out of major trunk valleys but laurentide ice flowing into the foothills up smaller valleys this flow pattern implies that when the ice sheets coalesced the cis was already waning or at least was not at its maximum deglaciation was interrupted by several re advances probably related to the destabilizing effect of proglacial lakes along the ice margin

ice rich terrain in gusev crater mars abstract the morphology of materials on the floor of gusev crater 14 s 175 w mars imply a history of volcanism and subsequent removal of an ice rich deposit fluid lava flows observed in the western portion of gusev crater paradoxically terminate in a steep thick 60 m flow front adjacent to hummocky terrain the hummocky terrain is morphologically similar to deglaciated terrain on earth generated when glacial debris are left behind after the glacier has retreated we propose the following scenario for the floor of gusev crater first ice rich material was deposited adjacent to thira crater second fluid lavas were emplaced and ponded against the ice rich deposits at some later time the ice within the deposit sublimated leaving hummocky terrain current age estimates for the gusev flows are hesperian suggesting that the ice removal occurred in the upper hesperian or more recently if this hypothesis is correct quench features glassy rinds columnar jointing should be observed at the lava flow margin the hummocky deposit should be poorly sorted angular debris

numerical reconstruction of a soft bedded laurentide ice sheet during the last glacial maximum we used a numerical ice sheet model to reconstruct the north american laurentide ice sheet during the last glacial maximum our model simulates ice sheet conditions that can be specified experimentally as either a rigid substrate hard bed or a wet deformable till soft bed basal sliding is excluded we use geologic records of former basal ice sheet processes to prescribe the distribution of hard and soft beds our reconstruction of the laurentide ice sheet is significantly lower in ice surface height and contains less ice volume than the climap maximum reconstruction in contrast our reconstruction agrees well with the ice 4g reconstruction both in height and volume because the ice 4g reconstruction is based on the inversion of relative sea level data whereas our reconstruction is based on glacial geology and ice mechanics this agreement suggests that soft beds provide a glaciological mechanism to explain the shape and volume of the laurentide ice sheet that is most consistent with observations of relative sea level change and other geodynamic considerations

transverse supraglacially derived crevasse infillings in a pleistocene ice sheet margin zone eastern poland genesis and sedimentary record abstract the so called crevasse infillings in the marginal zone of the saalian ice sheet in eastern poland are atypical relief forms for lowlands glaciated in the pleistocene they are located on a high of the cretaceous palaeogene substratum and form isolated ridges arranged in trains parallel to the former ice sheet margin i e transverse to the movement of the ice sheet the sedimentary succession of the crevasse infillings consists mainly of undeformed glaciodeltaic deposits we propose a model of the crevasse infilling development in three phases against the background of ice mass dynamics 1 ice sheet advance over a high of the substratum compressive ice flow that bumped against the high s slope and enrichment of the ice with debris 2 an overriding of the substratum high by ice masses a tensional ice flow regime resulted in significant crevassing and 3 ice mass stagnation low energy supraglacial deltaic sedimentation in isolated ponds between disintegrated ice blocks under frozen bed conditions considering this genesis we suggest classifying these forms as kames instead of crevasse infillings

glacial geomorphology of terrestrial terminating fast flow lobes ice stream margins in the southwest laurentide ice sheet glacial geomorphological mapping of southern alberta canada reveals landform assemblages that are diagnostic of terrestrial terminating ice streams fast flowing outlet glaciers with lobate snouts spatial variability in features that comprise the landform assemblages reflects changes in a palaeo ice stream activity switch on off and b snout basal thermal regimes associated with climate sensitive steady state flow palaeo ice stream tracks reveal distinct inset sequences of fan shaped flowsets indicative of receding lobate ice stream margins former ice lobe margins are demarcated by a major often glacially overridden transverse moraine ridges commonly comprising glacitectonically thrust bedrock and b minor closely spaced recessional push moraines and hummocky moraine arcs details of these landform types are well exhibited around the former southern margins of the central alberta ice stream where larger scale more intensive mapping identifies a complex glacial geomorphology comprising minor transverse ridges mtr types 1 3 hummocky terrain ht types 1 3 flutings and meltwater channels spillways the mtr type 1 constitute the summit corrugation patterns of glacitectonic thrust moraines or major transverse ridges and have been glacially overrun and moderately streamlined the mtr type 2 sequences are recessional push moraines similar to those developing at modern active temperate glacier snouts the mtr type 3 document moraine construction by incremental stagnation because they occur in association with hummocky terrain the close association of hummocky terrain with push moraine assemblages indicates that they are the products of supraglacial controlled deposition on a polythermal ice sheet margin where the ht type 3 hummocks represent former ice walled lake plains the ice sheet marginal thermal regime switches indicated by the spatially variable landform assemblages in southern alberta are consistent with palaeoglaciological reconstructions proposed for other ice stream fast flow lobes of the southern laurentide ice sheet where alternate cold polythermal and temperate marginal conditions associated with climate sensitive steady state flow sequentially gave way to more dynamic streaming and surging activity

the blackspring ridge flute field south central alberta canada evidence for subglacial sheetflow erosion abstract the blackspring ridge bsr located in south central alberta canada is dominated by a prominent flute field flutes elongated streamlined depressions and ridges elongate streamlined hills are up to 15 km long and are composed of two material types in situ bedrock and in situ pre laurentide glaciation fluvial sand and gravel beds the preglacial beds are tertiary or early quaternary in age the beds are undisturbed maintain primary bedding structures and even maintain clast imbrication no till overlies the gravel beds although in places large granite boulder erratics lie on the surface indicating that ice was present in the region in the past because the ridges are composed of preglacial materials they are remnant erosional landforms rather than constructional landforms geomorphic and sedimentary evidence favor subglacial meltwater as the erosional agent rather than ice we suggest that the elevation of the bsr relative to basal ice would have resulted in confined subglacial meltwater flow with associated flow acceleration and increased scouring resulting in flute formation this meltwater stripped away any till cover leaving behind only a few boulders observations at the bsr flute field preclude the possibility that flutes and remnant ridges are the result of deformation of soft clayey beds

ice thrust terrains and glaciotectonic settings in central alberta airphoto analyses geologic field mapping and study of borehole logs and surficial topographic maps in ice thrust terrains in central alberta suggest that all these techniques are needed to identify ice thrust terrains that may or may not have topographic expressions three geomorphological settings susceptible to glaciotectonic deformation are described as escarpment valley and plains settings the bedrock slopes in the escarpment setting ranged from 19 to 51 m km the bedrock slopes in the valley setting ranged from 23 to 32 m km and the valley was 8 5 km wide between crests and 4 km at the bottom the bedrock slopes of the plains setting ranged from 2 to 5 m km ice thrust features are found in topographic troughs in front of an ice sheet where water bodies were impounded these proglacial water bodies thawed the permafrost in front of the glacier the disintegration of proglacial permafrost decreased the resistance of subglacial strata to ice thrusting ice thrust features can be expected in areas wh

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

irish ice sheet dynamics during deglaciation of the central irish midlands evidence of ice streaming and surging from airborne lidar abstract high resolution digital terrain models dtms generated from airborne lidar data and supplemented by field evidence are used to map glacial landform assemblages dating from the last glaciation midlandian glaciation oi stages 2 3 in the central irish midlands the dtms reveal previously unrecognised low amplitude landforms including crevasse squeeze ridges and mega scale glacial lineations overprinted by conduit fills leading to ice marginal subaqueous deposits we interpret this landform assemblage as evidence for surging behaviour during ice recession the data indicate that two separate phases of accelerated ice flow were followed by ice sheet stagnation during overall deglaciation the second surge event was followed by a subglacial outburst flood forming an intricate esker and crevasse fill network the data provide the first clear evidence that ice flow direction was eastward along the eastern watershed of the shannon river basin at odds with previous models and raise the possibility that an ice stream existed in this area our work demonstrates the potential for airborne lidar surveys to produce detailed paleoglaciological reconstructions and to enhance our understanding of complex palaeo ice sheet dynamics

high resolution mapping of glacial landforms in the north alpine foreland austria abstract in this study results from traditional field mapping were merged with precise elevation information from airborne lidar light detection and ranging surveys morphological and sedimentological data provide new results from the austrian eastern part of the salzach piedmont glacier during times of and shortly after the last glacial maximum lgm the variations in meltwater discharge had a major impact on the development of glacial landforms in areas with high meltwater supply erosional or debris reworking processes play a major role represented by drainage channels drumlins and kettled low relief hummocky moraine with low slope angles low discharge areas are associated with distinct depositional forms such as high relief end moraines up to 30 m and hummocky moraine averaging 20 m with high slope angles isolated conical kames may reach heights up to 45 m fluvial activity is supposed to rise towards the end of the glacial cycle causing high melting rates and comprehensive debris reworking the formation of terminal lakes and associated widespread inorganic lake clays are the last deposits within the study area before the salzach glacier completely receded to its main valley the survey of glacial landforms through the combination of field mapping and high resolution dem derived from airborne lidar missions gives precise information on transport and deposition during the last glacial cycle of the eastern salzach glacier piedmont lobe

morphometric analysis of ice walled lake plains in northern illinois implications of lake elongation by wind induced dual cycle currents abstract ice walled lake plains iwlps are rounded flat topped mounds that formed in stagnant ice environments along the margins of the laurentide ice sheet we conducted detailed morphometric and statistical analyses of the shape size and orientation of more than 400 iwlps identified from aerial photos aided with lidar data in dekalb county illinois usa lake elongation theories include extraterrestrial impact e g the carolina bays ice flow dynamics and crevasses and wind induced currents that preferentially erode the shorelines perpendicular to the dominant wind direction the results indicate that elliptical iwlps with a perimeter greater than 3050 m have preferred orientations roughly normal to the paleo wind direction as indicated by contemporaneous parabolic dunes located 50 km to the west the orientations of the iwlps with a perimeter less than 1220 m are scattered and show no apparent trend the iwlp orientation is not related to ice flow dynamics or glacial crevasses because no statistically significant relationship exists with regard to the ice flow as proxied by the moraine direction the orientation of large iwlps in dekalb county are consistent with wind induced lake elongation observed in modern permafrost thaw lakes suggesting that the prevailing wind also played an important role in controlling the orientation of iwlps during the last glacial period and led to the preferred orientation we see today

incipient tunnel channels abstract glaciated terrains in east central alberta and south central michigan contain channels that have hummocks and transverse ridges separating depressions along their floors this association imparts a linked pothole appearance similar channels are often interpreted as tunnel channels or subaerial channels partly filled with sediment from a subsequent glacial advance a stagnating ice roof or slumped sediment from the channel margins however the truncation of sedimentary packages in the channel walls and intrachannel hummocks indicates that they are erosional landforms cut into glacial sediments till bedrock or gravel eskers overlie and are found within a few channels indicating that these channels formed before the final stagnation that produced the eskers these two characteristics combined with the observation that many channels have convex up long profiles indicate that the channels were eroded by pressurized subglacial water because the formative mechanisms for this type of channel are not clear and modern environments that could produce this type of landform are inaccessible we draw on several morphologic analogues to propose mechanisms for channel erosion we conclude that the erosion of these linked pothole channels incipient tunnel channels was the product of the complex interaction between complex turbulent flow structures and various scales of roughness elements

application of till pebble lithology to the differentiation of glacial lobes in southern alberta variations in the pebble composition of surface till in southern alberta indicate the presence of three contemporaneous lobes in the last continental glacier the ice lobe domains have similar pebble associations and differ only in the frequency of occurrence of rock constituents a large part of the percentage variability within each domain is caused by irregular entrainment of locally derived stones which dilutes pebble contents and results in a lithology substantially different from the provenance composition the study has shown that the regional trends are preserved and can be recognized in the locally modified lithologies but the latter have to be isolated and evaluated differently from the regular provenance lithologies the common diluting agent in the area is fluvial gravel that is widespread on erosional bedrock surfaces and in preglacial valleys because the gravel is generally coarse pebble fractions of larger size are more affected by local dilution than are successively finer fractions q

geotechnical model for great plains hummocky moraine formed by till deformation below stagnant ice abstract hummocky or disintegration moraine is a widespread late wisconsin glacial terrain type across the glaciated great plains of mid continent north america this terrain consists of chaotic hummocks and enclosed kettle hollows that are widely underlain by clay rich till the prevailing interpretation of this terrain is one of supraglacial deposition from debris covered stagnant ice and is based largely on geomorphologic comparisons with modern valley glacier margins recently in contrast it has been argued that hummocky moraine can form subglacially by the pressing of fine grained deformable till under the overburden pressure of stagnant ice in this paper we test the postulated subglacial origin of hummocky moraine using a geotechnical model of a deforming soil till the model confirms that hummocky topography can be the result of failure and deformation of fine grained till by differential loading below stagnant ice differential pressures at the ice bed contact arise from the complete or partial perforation of thin stagnant ice sheet lobes by numerous water filled doline like depressions the growth and decay of ponds as ice down wastes result in repeated cycles of subglacial till failure and landform development resulting in the observed random linear circular elongate and other patterns of hummocks ridges and hollows formerly streamlined preferentially oriented subglacial landforms such as drumlins and rogen moraines can also be overprinted forming washboard moraine

glacial geomorphology of the neutral hills uplands southeast alberta canada the process form imprints of dynamic ice streams and surging ice lobes abstract the neutral hills uplands of southern alberta canada is an area of complex and varied glacial landforms dominated by glacitectonic compressional structures but also containing expansive areas of hummocky terrain and kame and kettle topography it lies between the strongly streamlined trunks of the former central alberta cais and maskwa palaeo ice streams of the sw laurentide ice sheet lis and hence comprises an inter ice stream regional moraine zone constructed at around 15 5 cal ka bp this study aimed to compile a regional map of the glacial geomorphology of central southeast alberta in order to decipher the landform sediment signatures of overprinted ice stream margins in terrestrial continental environments and to refine the palaeoglaciological reconstructions for the southwest lis detailed mapping from lidar and aerial imagery identifies distinctive glacial landsystems diagnostic of the partial overprinting of cross cutting ice stream trunks and fast flow lobes widespread evidence of surge diagnostic features indicates that the ice streams experienced repeated flow instabilities consistent with the broader scenario of a highly dynamic and unstable sw lis characterised by markedly transitory and cross cutting palaeo ice streams the inter ice stream moraine zone is characterised by spectacular glacitectonic compression of bedrock cupola hill construction and mega raft displacement but also displays evidence of multi phase stagnant ice melt out where partially overprinted surge lobes advanced into large areas of buried glacier ice contemporaneous ice melting led to the widespread development of glacier karst and the production of eskers at a range of scales the largest of which record deranged drainage patterns indicative of ice walled channel sedimentation controlled by the regional bedrock slope towards the northeast these process form regimes have created a significant local relief that is a product of not only glacitectonic compression of bedrock but also the creation and melting of a melange of ice and bedrock sediment blocks of variable ice volume which are representative of former buried snout ice with a glacier karst system that was repeatedly proglacially thrust due to surging widespread evidence for subglacial channel cutting is likely strongly linked to the transitory surging and cross cutting nature of the palaeo ice streams in the region whereby ice streams switched on and surged in response to the build up migration and marginal outbursts of subglacial water reservoirs in addition to the reduced basal friction caused by the low permeability of the cretaceous bedrock pressurized groundwater and potentially also shallow biogenic gas deposits were likely important to the process form regimes of surging lobes of soft bedded ice streams in a region where ice flow was against an adverse bed slope a scenario that gave rise to a variety of enigmatic landforms such as doughnuts doughnut chains apparent blow out features and possible till eskers as well as glacitectonic mega rafts

the glacial origins of relict pingos wales uk ramparted depressions doughnut shaped debris cored ridges with peat and or sediment filled central basins are commonly perceived to represent the relict collapsed forms of permafrost ground ice mounds i e pingos or lithalsas in wales uk ramparted depressions of late pleistocene age have been widely attributed to permafrost related processes however a variety of alternative glacial origins for these enigmatic landforms are also consistent with the available geological and geomorphological evidence although previous studies have barely considered such alternative processes of formation from detailed geophysical sedimentological and remote sensing studies at two field sites we demonstrate that i the wastage of stagnating glacier ice is a viable alternative explanation for the formation of ramparted depressions in wales ii the glacial geomorphology and geology of these landforms is analogous to supraglacial and subglacial landforms from the last laurentide and fennoscandian ice sheets iii these landforms have significant potential for characterising the nature of deglaciation around the margins of the irish sea during the last glacial cycle and may record evidence for the overextension and stagnation of the south eastern margin of the irish sea ice stream and iv investigations of ramparted depressions within formerly glaciated terrains must consider both glacial and periglacial mechanisms of formation

plate tectonic characteristics during the early paleozoic in beishan near the sino mongolian border region china abstract the tectonic units during the early paleozoic in the beishan area comprise the northern margin of the eastern end of the tarim plate and the northern flank of the mingshui hanshan microplate these two paleoplates were separated by an ophiolitic melange belt all of them underwent the tectonic evolution typical of the continental margins of atlantic andean and west pacific ocean types the continental margin was an open continental shelf and a continental slope from the early cambrian to the early ordovician at the southern flank of the continental shelf an isolated shallow sea basin developed separated from the shelf by an upwarping zone its northern flank shibanjing xiaohuangshan is an ocean basin north of this ocean basin is the mingshui hanshan microcontinent or microplate this area is situated in a tensional tectonic environment it shows the characteristics of a passive atlantic type margin during the middle and late parts of the early ordovician the continental crust on the southern fringe of the shallow sea basin was pulled open and a narrow ocean rift formed in the mid ordovician this area underwent a period of considerable development in which extension and subduction existed simultaneously the southern fringe of the continental margin came to the end of the passive margin on its northern side a marginal volcanic zone with calc alkali volcanism formed in the late ordovician silurian the oceanic crust was folded and subducted southward and the hengluanshan xichanjing marginal basin developed into an interarc basin where the hongliuhe niujuazi backarc basin and the baiyunshan dongqiyishan volcanic arc had formed with associated granitic magma at the beginning of the devonian the tarim plate finally came into collision with the mingshui hanshan microplate and the ocean basin closed forming the mingshui shibanjing xiaohuangshan suture belt there were volcanic molasse basins on both sides of this orogenic zone from the late paleozoic the beishan area was incorporated into an intraplate mobile zone

a change in the geodynamics of continental growth 3 billion years ago models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout earth s history the oxygen isotopic composition of zircons for which uranium lead and hafnium isotopic data provide age constraints is a key archive of crustal reworking we identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time growth of continental crust appears to have been a continuous process albeit at variable rates a marked decrease in the rate of crustal growth at 3 billion years ago may be linked to the onset of subduction driven plate tectonics

present crustal movement and strain distribution in central europe inferred from gps measurements in 1992 an international collaboration was initiated to study the present day geodynamics of central europe the primary tool of this research is the global positioning system gps therefore the central european gps geodynamic reference network was established in 1994 the network extends from the south in the eurasian african plate boundary zone to the east european pre cambrian platform in the northeast during the first phase of the project four gps campaigns were organized between 1994 and 1997 analysis of gps data provided rms repeatabilities around 2 mm in the horizontal and 5 6 mm in the vertical component for each campaign the velocities of gps sites situated on the eurasian plate were compared with those predicted by the nnr nuvel 1a plate velocity model the intraplate velocity field shows that the northern part of central europe seems generally stable with velocities under 2 mm yr however the southern sites close to the plate boundary zone show higher values and abrupt changes of velocities indicating the existence of narrow deformation zones the primary feature revealed by the strain calculation in the alpine pannonian dinaric system is the dominant north south contraction resulting from the collision of the african and eurasian plates transmitted by the adriatic microplate however the direction of contraction in the alpine pannonian dinaric junction zone changes from nw se at the northeastern boundary of the adriatic microplate to ne sw toward the bohemian massif the magnitudes of these strain rates are similar 8 0 5 3 ppb yr and 8 6 2 5 ppb yr respectively over 200 350 km subnetworks we observe e w oriented 93 13 right lateral shear 12 5 ppb yr between the alpine north pannonian unit and the southernmost sites in the dinarides and southern alps along with left lateral shear 97 7 10 2 ppb yr relative to the stable bohemian massif in the north both the strain field and the intraplate velocities presented show that the alpine north pannonian unit is presently crushed out to the east with 2 mm yr velocity on average along two main fault zones as a consequence of the compression caused by the adria alpine collision

controls on jokulhlaup transported buried ice melt out at skeiðararsandur iceland implications for the evolution of ice marginal environments abstract high magnitude jokulhlaups glacier margin position and ice thickness have been identified as key controls on sandur evolution existing models however have focused primarily on observations made during short windows of time and often do not account for the subsequent modification of proglacial landsystems by repeated jokulhlaups or post depositional modification due to melt out over decadal time scales digital elevation models dems were used to reconstruct the development of large depressions on skei ararsandur an outwash plain in southeast iceland these depressions measure up to 1 km in width and up to 13 m in depth and are associated with ice bodies up to 1 km in length and up to 150 m in height emplaced during a high magnitude jokulhlaup in 1903 and subsequently buried by jokulhlaups in 1913 and 1922 the continued melting of the har askri a ice bodies over a century following their emplacement together with subsequent repeated burial by high magnitude jokulhlaups demonstrates that jokulhlaups may continue to serve as important controls on sandur evolution on a decadal to centennial timescale 101 102 years the har askri a depressions developed only following the retreat of the glacier margin after 1945 which highlights the controls of margin position on the evolution of the sandur margin position and thickness of the glacier profile was seen to affect not only the distribution and thickness of sediment emplaced during jokulhaups but also the rate and pattern of melt in the decades following the decoupling of the margin from the sandur the jokulhlaup landsystem model signatures identified at this site may provide a useful analogue for interpreting landforms and strata emplaced by glacier margin fluctuations jokulhlaups and melt out generated by retreating continental pleistocene ice sheets

ice walled lake plains implications for the origin of hummocky glacial topography in middle north america abstract ice walled lake plains are prominent in many areas of hummocky till topography left behind as the laurentide ice sheet melted from middle north america the formation of the hummocky till topography has been explained by 1 erosion by subglacial floods 2 squeezing of subglacial till up into holes in stagnant glacial ice or 3 slumping of supraglacial till the geomorphology and stratigraphy of ice walled lake plains provide evidence that neither the lake plains nor the adjacent hummocks are of subglacial origin these flat lake plains up to a few kilometers in diameter are perched as much as a few tens of meters above surrounding depressions they typically are underlain by laminated fine grained suspended load lake sediment many ice walled lake plains are surrounded by a low rim ridge of coarser grained shore sediment or by a steeper rim ridge of debris that slumped off the surrounding ice slopes the ice walled lakes persisted for hundreds to thousands of years following glacial stagnation shells of aquatic molluscs from several deposits of ice walled lake sediment in south central north dakota have been dated from about 13 500 to 10 500 b p calibrated radiocarbon ages indicating a climate only slightly cooler than present this is confirmed by recent palaeoecological studies in nearby non glacial sites to survive so long the stagnant glacial ice had to be well insulated by a thick cover of supraglacial sediment and the associated till hummocks must be composed primarily of collapsed supraglacial till

sims u pb rutile age of low temperature eclogites from southwestern chinese tianshan nw china article i nfo the geological evolution in the southwestern chinese tianshan orogen in nw china is poorly understood because of controversial geochronological results of the low temperature eclogites it is likely due to sm nd and rb sr isotopic disequilibrium in different mineral fractions excess 40 ar in the high pressure k bearing minerals and multi stage growths of zircon in u pb system rutile as a typical eclogite facies mineral with high u pb ratio is a feasible candidate to constrain the eclogite facies metamorphic event in this study we measured u pb rutile age data using sims cameca ims 1280 for low temperature eclogites from southwestern chinese tianshan orogen nw china we demonstrate that sims is a powerful technique for in situ u pb dating of rutile with u content as low as 1 ppm sims u pb rutile dating yielded an age of 318 7 ma which may record the time when the rocks cooled down to the closure temperature of pb in rutile ca 500 c but should be very close to the time of eclogite facies metamorphism in terms of common fast exhumation of coesite bearing eclogite this new proof strongly suggests a late carboniferous collision between kazakhstan yili block and central tianshan microcontinent

numerical modeling of valley glacier stagnation as a paleoclimatic indicator stagnation of the terminal region of a glacier occurs in response to sufficiently large and rapid climatic warming so the presence of stagnation deposits provides quantitative information on the climate change that forced retreat here we use a simple flow line glacier model to investigate the relationship between stagnation climate forcing and aspects of the glacier bed for climatic warming greater than the threshold to cause stagnation larger or faster warming events cause longer regions of a glacier to stagnate smaller or slower warming episodes below the threshold for stagnation cause retreat while active flow persists along the entire glacier length the threshold for stagnation depends not only on the climatic forcing but also on many other aspects of the glacier with stagnation favored by factors including a lower mean bed slope with greater roughness quantitative determination of the climatic forcing consistent with the occurrence or absence of stagnation deposits requires that these site specific characteristics be incorporated in modeling

polydiapirs multiwavelength gravity structures polydiapirs domes in domes result from the evolution of a multiwavelength rayleigh taylor gravitational instability the present work investigates the initiation and development of polydiapirs by means of two dimensional finite difference models of gravitationally unstable triple layered sequences of newtonian fluids the values of viscosities thicknesses and densities were systematically varied for selected examples in triple layered sequences only two types of density stratification are likely to result in multiwavelength instabilities that will eventually develop into polydiapiric structures the limiting factors and evolution of such structures were studied for both these types of density stratifications finite difference calculations on models with physical parameters appropriate to evaporitic sequences and covered by a viscous clastic overburden showed that a very small downward density decrease in the evaporitic source may cause an internal overturn that will compete with the main overturn between source and overburden this may lead to the development of sequential or simultaneous polydiapirs the mature polydiapirs in the models exhibit spines curtain folds pendant repetitions of the source layers inside the large diapirs and anomalous interdiapiric distances all these features are known in natural salt diapirs suggesting that polydiapirs may be far more common in nature than hitherto recognized

the last and penultimate glaciation in the north alpine foreland new stratigraphical and chronological data from the salzach glacier abstract the northern alpine foreland was repeatedly covered by massive piedmont glaciers during quaternary glacial maxima the salzach palaeoglacier lobe austria germany was the easternmost of a series of pleistocene piedmont glaciers entering the foreland through major alpine valleys it covered an area of more than 1000 km2 during at least four glacial maxima here we aim to bring more light into its history by analyzing multiple drill log data two major outcrops topographic data and absolute chronologies of sediments stratigraphic and lithofacies investigations are focused on proximal i e near the lobe axis and distal i e near terminal moraine deposits of the salzach glacier lobe the glacial carving into the miocene bedrock occurred during early glacial maxima and was rather uniform across the lobe with larger values only in the proximal parts of the glacier more than 100 m of accumulated sediments during later glacial maxima indicate a change in ice sheet dynamics and a characteristic sequence development which varies from proximal to distal lobe positions new luminescence ages suggest a depositional focus at the penultimate glacial period while the impact of the lgm was rather minor sediments of gravelly braided rivers dominate the proximal parts of the former lobe where meltwater discharge was generally high while sheetflood deposits dominate the distal near terminal lobe positions

ice shaped ring forms in western canada their airphoto expressions and manifold polygenetic origins abstract a variety of ice shaped ring forms and patterns in western canada are strikingly expressed in black and white aerial photographs the most common forms are round and oval mounds and plateaus with rim ridges around summit craters it is speculated that small clusters and large fields of them originated from dead ice supraglacial and subglacial wasting processes and often from a combination of both where shallow glacial lake waters were present other ice shaped ring features and patterns illustrated in this paper result from former and present permafrost and periglacial activity groundwater discharge and winter ice mounding and expansion that transfer fine sediment from circular orifices to their margins slush formation that creates fractal like patterns on thin ice floating on shallow snow covered lakes and wind driven icebergs and lake ice that produce circular scours on the beds of glacial lakes portions of contact airphotos have been included to show better examples of ring forms and patterns in western canada speculations on their manifold polygenetic origins are presented

the origin and glaciodynamic significance of sandstone ridge networks from the hirnantian glaciation of the djado basin niger the djado basin niger was located beneath the inner part of the late ordovician ice sheet the felar felar formation consists mainly of glaciomarine deposits associated with the major ice sheet recession within the glaciation and is bounded by two glacial unconformities structures corresponding to sandstone ridges are found within the felar felar formation sandstone ridges are several metres high about 10 m wide and hundreds of metres long these structures are organized in extensive anastomosed to subpolygonal networks the association of sandstone ridge networks with the later glacial unconformity and with other glacial evidence suggests sub glacial conditions for their origin sandstone ridge sedimentological characteristics indicate that sandstone ridges result from the scouring of the felar felar formation by sub glacial turbulent and pressurized meltwater then subglacial cavities were infilled with sand derived from glacial abrasion sandstone ridge networks are comparable with tunnel channels and document unusual drainage structures of the inner part of the palaeo ice sheet

the meltwater hypothesis for subglacial bedforms this paper presents an historical and in places informal account of the meltwater hypothesis which invokes enormous outburst floods for the formation of subglacial bedforms it begins with a brief discussion of the difficulties of determining processes of formation for landforms which are not seen in formation analogy provides a solution to these difficulties analogy between erosional marks at the bases of turbidites and drumlins which were the starting point for this hypothesis rests on the idea that inverted erosional marks at the ice bed are subsequently infilled to form drumlins field tests on the sedimentology architecture and landform associations of drumlins in the livingstone lake drumlin field are outlined before more extensive work on bedrock erosional forms and flood routes is introduced bedrock erosional forms played a central part in establishing the hypothesis since their form and ornamentation are confidently interpreted as fluvial their form and genesis are discussed mainly with reference to sites at french river and wilton creek ontario though some remarkable bedrock erosional forms in antarctica support their regional extent initially in the meltwater hypothesis drumlins were thought to be cavity fills and erosional drumlins were recognized later this development is shown to be central to the realization that drumlin composition may be inferred from drumlin form the scale of drumlin fields measured at about 10 3 km 2 and the magnitude of the inferred floods require that the flood events were regional regional scale flood tracts in ontario quebec alberta and the northwest territories extending over 1000 km in length and several hundred kilometers in width support this suggestion floods had they occurred would have caused rapid rates of sea level rise and may have changed climate through their effects on ocean stratification and sea surface temperatures the meltwater hypothesis covers a range of bedforms besides drumlins and bedrock erosional marksffluting rogen moraine hummocky terrain and transverse ridges recent work shows how these forms are best explained by the meltwater hypothesis the roles of water storage and release which underpin the theory of the meltwater hypothesis remain poorly understood r 2002 published by elsevier science ltd

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