Processes of annual moraine formation at a temperate alpine valley glacier: insights into glacier dynamics and climatic controls

This paper presents the first detailed sedimentological study of annual moraines formed by an alpine valley glacier. The moraines have been forming since at least AD 1980 by a subsidiary lobe of Gornergletscher, Switzerland that advances up a reverse bedrock slope. They reach heights of 0.5–1.5 m, widths of up to 6 m and lengths of up to several hundreds of metres. Sediments in these moraines are composed of proglacial outwash and debris flow units; subglacial traction till is absent entirely. Based on four representative sections, three genetic process combinations have been identified: (i) inefficient bulldozing of a gently sloping ice margin transfers proglacial sediments onto the ice, causing differential ablation and dead‐ice incorporation upon retreat; (ii) terrestrial ice‐contact fans are formed by the dumping of englacial and supraglacial material from point sources such as englacial conduit fills; debris flows and associated fluvial sediments are stacked against a temporarily stationary margin at the start, and deformed during glacier advance in the remainder, of the accumulation season; (iii) a steep ice margin without supraglacial input leads to efficient bulldozing and deformation of pre‐existing foreland sediments by wholesale folding. Ice‐surface slope appears to be a key control on the type of process responsible for moraine formation in any given place and year. The second and third modes result in stable and higher moraines that have a higher preservation potential than those containing dead ice. Analysis of the spacing and climatic records at Gornergletscher reveals that winter temperature controls marginal retreat and hence moraine formation. However, any climatic signal is complicated by other factors, most notably the presence of a reverse bedrock slope, so that the extraction of a clear climatic signal is not straightforward. This study highlights the complexity of annual moraine formation in high‐mountain environments and suggests avenues for further research.     

Ice marginal dynamics during surge activity,Kuannersuit Glacier,Disko Island,West Greenland

The Kuannersuit Glacier surged 11 km between 1995 and 1998. The surge resulted in the formation of an ice cored thrust moraine complex constructed by subglacial and proglacial glaciotectonic processes. Four main thrust zones are evident in the glacier snout area with phases of compressional folding and thrusting followed by hydrofracture in response to the build-up of compressional stresses and the aquicludal nature of submarginal permafrost and naled. Various types of stratified debris-rich ice facies occur within the marginal zone: The first (Facies I) comprises laterally continuous strata of ice with sorted sediment accumulations, and is reworked and thrust naled ice. The second is laterally discontinuous stratified debris-rich ice with distinct tectonic structures, and is derived through subglacial extensional deformation and localised regelation (Facies II), whilst the third type is characterised by reworked and brecciated ice associated with the reworking and entrainment of meteoric ice (Facies III). Hydrofracture dykes and sills (Facies IV) cross-cut the marginal ice cored thrust moraines, with their sub-vertically frozen internal contact boundaries and sedimentary structures, suggesting supercooling operated as high-pressure evacuation of water occurred during thrusting, but this is not related to the formation of basal stratified debris-rich ice. Linear distributions of sorted fines transverse to ice flow, and small stratified sediment ridges that vertically cross-cut the ice surface up-ice of the thrust zone relate to sediment migration along crevasse traces and fluvial infilling of crevasses. From a palaeoglaciological viewpoint, marginal glacier tectonics, ice sediment content and sediment delivery mechanisms combine to control the development of this polythermal surge valley landsystem. The bulldozing of proglacial sediments and the folding and thrusting of naled leads to the initial development of the outer zone of the moraine complex. This becomes buried in bulldozed outwash sediment and well-sorted fines through surface ablation of naled. Up-ice of this, the heavily thrust margin becomes buried in sediment melted out from basal debris-rich ice and subglacial diamicts routed along thrusts. These mechanisms combine to deliver sediment to supraglacial localities, and promote the initial preservation of structurally controlled moraines through insulation, and the later development of kettled dead ice terrain.     

Depositional evidence for marginal oscillations of the Irish Sea ice stream in southeast Ireland during the last glaciation

Late Devensian/Midlandian glacial deposits on the southeast Irish coast contain a record of sedimentation at the margins of the Irish Sea ice stream (ISIS). Exposures through the Screen Hills reveal a stratigraphy that documents the initial onshore flow of the ISIS ('Irish Sea Till') followed by ice stream recession and readvances that constructed glacitectonic ridges. Ice-contact fans (Screen Member) were deposited in association with subglacial deformation tills and supraglacial/subaqueous mass flow diamicts. In SE Ireland, the ISIS moved onshore over proglacial lake sediments which were intensely folded, thrust and cannibalized producing a glacitectonite over which laminated and massive diamictons were deposited as glacitectonic slices. Ice marginal recession and oscillations are documented by: (a) ice-proximal, subaqueous diamict-rich facies; (b) isolated ice-contact glacilacustrine deltas; (c) syn-depositional glacitectonic disturbance of glacilacustrine sediments and overthrusting of ice-contact outwash; (d) offshore moraine ridges; and (e) changing ice flow directions and facies transitions. Diagnostic criteria for the identification of dynamic, possibly surging, ice-stream margins onshore include thrust-block moraines, tectonized pitted outwash and stacked sequences of glacitectonites, deformation tills and intervening stratified deposits. In addition, the widespread occurrence of hydrofracture fills in sediments overridden and locally reworked by the ISIS indicate that groundwater pressures were considerably elevated during glacier advance. The glacigenic sediments and landforms located around the terrestrial margins of the ISIS are explained as the products of onshore glacier flow that cannibalized and tectonically stacked pre-existing marine and glacilacustrine sediments. Localized tectonic thickening of subglacially deformed materials at the former margins of glaciers results in zones of net erosion immediately up-ice of submarginal zones of net accretion of subglacial till. The more stable the ice-stream margin the thicker and more complex the submarginal sedimentary stack.     

Common origin for De Geer moraines of variable composition in Raudvassdalen,northern Norway

The study of De Geer moraines in Raudvassdalen shows that most De Geer moraines are likely to have a common origin at the grounding line of glaciers despite variability in composition of the ridges. Pebble fabric, grain‐size analysis and structures within exposures of De Geer moraines in the Raudvassdalen area, with compositions ranging from mostly till to mostly sorted sediment, indicate that the ridges all formed at the grounding line of a tidewater glacier by common processes: deposition of sorted sediments beyond the grounding line followed by deformation of pre‐existing sediments and deposition of till as the glacier overrode the ridges. The compositional variation of the ridges is probably related to the position of the section studied relative to the location of the outlet of subglacial streams. Ridges composed entirely of till form at locations remote from the outlet of subglacial streams, and ridges with a component of sorted sediments form in closer proximity to these streams. This unifying theory of De Geer moraine formation, along with theoretical and geological evidence showing that there are limited physical conditions where basal crevasses can form, suggests that the number of De Geer moraines interpreted to have formed in basal crevasses is probably unrealistic. Copyright © 2000 John Wiley & Sons, Ltd.     

Holocene glacial evolution of Mohnbukta in eastern Spitsbergen

Submarine geomorphology is one of the main tools for understanding past fluctuations of tidewater glaciers. In this study we investigate the glacial history of Mohnbukta, on the east coast of Spitsbergen, Svalbard, by combining multibeam‐bathymetric data, marine sediment cores and remote sensing data. Presently, three tidewater glaciers, Heuglinbreen, Königsbergbreen and Hayesbreen calve into Mohnbukta. Hayesbreen surged at the end of the Little Ice Age, between 1901 and 1910. The submarine landform assemblage in Mohnbukta contains two large transverse ridges, interpreted as terminal moraines, with debrisflow lobes on their distal slopes and sets of well‐preserved geometric networks of ridges, interpreted as crevasse‐squeeze ridges inshore of the moraines. The arrangement of crevasse‐squeeze ridges suggests that both landform sets were produced during surge‐type advances. The terminus position of the 1901–1910 Hayesbreen surge correlates with the inner (R.2) terminal moraine ridge suggesting that the R.1 ridge formed prior to 1901. Marine sediment cores display ¹⁴C ages between 5700–7700 cal. a BP derived from benthic foraminifera, from a clast‐rich mud unit. This unit represents pre‐surge unconsolidated Holocene sediments pushed in front of the glacier terminus and mixed up during the 1901 surge. An absence of retreat moraines in the deeper part of the inner basin and the observation of tabular icebergs calving off the glacier front during retreat suggest that the front of Hayesbreen was close to flotation, at least in the deeper parts of the basin. As the MOH15‐01 core does not penetrate into a subglacial till and the foraminifera in the samples were well preserved, the R.1 ridge is suggested to have formed prior to the deposition of the foraminifera. Based on these data we propose that a surge‐type advance occurred in Mohnbukta in the early Holocene, prior to 7700 cal. a BP, which in turn indicates that glaciers can switch to and from surge mode.     

希夏邦马峰东南富曲河谷的冰川沉积和冰川构造

在希夏邦马峰（海拔８０１２ｍ）东南富曲河谷，中更新世以来有三次冰期；即聂拉木、富曲和普罗冰期。它们均可再分为两个亚阶段。聂拉木南的高冰碛平台长３.５ｍ，宽１.５ｋｍ，厚２００ｍ。属于中更新世聂拉木冰期（聂聂雄拉冰期）的巨大山谷冰川沉积，中尼公路从高冰碛平台尾端通过，形成数公里长的冰碛剖面，呈现出美丽多姿的冰川成因类型沉积和冰川构造现象，包括冰下，冰上融出碛，冰内.冰下河道沉积，冰湖沉积，坠碛，流磺等。冰川运动时造成的冰川构造，如断层、滑动面-…等也很清楚，代表了海洋型（暖冰川）冰川沉积和冰川构造特征，是中国目前研究冰川构造最理想的场所。     

Fluid-pressure controlled soft-bed deformation sequence beneath the surging Breiðamerkurjökull (Iceland, Little Ice Age)

The general subject of this paper is subglacial deformation beneath Breiðamerkurjökull, a surging Icelandic glacier. More specifically it discusses the evolution and the role of fluid pressure on the behaviour of subglacial sediments during deformation. During Little Ice Age maximum, the two outcrops studied, North Jökulsarlon (N-Jk) and Brennhola-Alda (BA), were located at 2550 m and 550 m respectively from the front of the Breiðamerkurjökull. Sedimentological analysis at the forefield of the glacier shows thick, coarse glaciofluvial deposits interbedded with thin, fine-grained shallow lacustrine/swamp deposits, overlain by a deformed till unit at N-Jk. BA outcrop shows fine-grained shallow lacustrine/swamp deposits overlain by a deformed till unit. The sequence of deformation events from one outcrop to the other is similar. First, major thrust planes, which were rooted in shallow lacustrine/swamp deposits developed by glacially induced simple shear. Next, the thrusts were folded, indicating the deformation of hydroplastic sediment assisted by moderate fluid pressure. Then clastic dyke swarms crosscut the sedimentary succession, proving that fluid overpressure caused hydrofracturing associated with fluidisation. Finally, as water escaped from the glacier bed, fluid pressure dropped, and normal faulting occurred in brittle-state subglacial sediments. Fluid-pressure variations are related to glacier dynamics. They control the deformation sequence by modifying subglacial rheological behaviour and the nature of the subglacial tectonism.     

Ribbed moraine formed by subglacial folding, thrust stacking and lee-side cavity infill

Transverse-to-iceflow ribbed moraine occurs in abundance in the coastal zone of northern Sweden, particularly in areas below the highest shoreline (200–230 m a.s.l.), but occasionally also slightly above. Based on detailed sedimentological and structural investigations of machine-dug sections across five ribbed moraine ridges, it is concluded that these vertically and distally prograding moraine ridges were formed as a result of subglacial folding/thrust stacking and lee-side cavity deposition. The proximal part of the moraines (Proximal Element) was formed by subglacial folding and thrust stacking of sequences of pre-existing sediments, whereas the distal part (Distal Element) was formed by glaciofluvial and gravity-flow deposition in lee-side cavities. The initial thrusting and folding is suggested to be a result of differences in bed rheology at the ice-marginal zone during the early or late melt season, and that generated a compressive zone transverse to ice flow as a result of a more mobile bed up-glacier compared to a less mobile bed down-glacier. It is considered that the lee-side cavities were formed as a result of ice-bed separation on the distal slope of the thrust/fold-created obstruction. The lee-side cavities formed an integral part of a subglacial linked-cavity drainage network regulated in their degree of interconnection, size and shape by fluctuations in basal meltwater pressure/discharge and basal iceflow velocity. The proximal and distal elements of the ribbed moraine ridges are erosively cut and/or draped with a consistently more homogeneous deforming bed till (Draping Element) marking the final phase of ribbed moraine formation considered to be contemporaneous with De Geer moraine formation further down-flow at the receding ice-sheet margin.     

Formation of a stratified subglacial ‘till’ assemblage by ice‐marginal thrusting and glacier overriding

Ó Cofaigh, C., Evans, D. J. A. & Hiemstra, J. F. 2010: Formation of a stratified subglacial ‘till’ assemblage by ice‐marginal thrusting and glacier overriding. Boreas, 10.1111/j.1502‐3885.2010.00177.x. ISSN 0300‐9483. A thick sequence of glaciotectonically stacked till and outwash is preserved in a coastal embayment at Feohanagh, southwest Ireland. The sequence contains a variety of diamicton lithofacies, including laminated, stratified and massive components, but stratified diamictons dominate. Stratification/lamination is imparted by the presence of numerous closely spaced subhorizontal and anastomosing partings, which give a fissile appearance to the diamictons. Many partings are the result of sandy or thin gravelly layers within the diamictons. Some diamictons contain interbeds and lenses of sand, mud and gravel, which still preserve the original stratification. The sequence at Feohanagh is the product of a two‐stage depositional process in which initial glaciolacustrine sedimentation in an ice‐dammed lake was followed by glaciotectonic thrusting and overriding, during which the lake sediments were reworked and variably deformed. Similar late Quaternary sequences of glaciotectonically stacked stratified sediments and till have been described from around the coastal margins of Ireland and Britain, where they constitute glaciotectonite–subglacial traction till continuums rather than true lodgement tills as traditionally implied. Thick stratified diamicton assemblages are likely to occur in areas where steep topography provides pinning points for the glacier margin to stabilize and deliver large volumes of sediment into a glaciolacustrine or glaciomarine setting before proglacial and subglacial reworking of the sediment pile. The resulting geological–climatic unit, often defined as ‘till’, will contain a large amount of stratified and variably deformed material (laminated and stratified diamictons will be common), including intact sediment rafts, reflecting low strain rates and short sediment transport distances.     

Late Glacial De Geer moraines with glaciofluvial sediment in the Chapais area, Québec (Canada)

Glaciofluvial De Geer moraines have rarely been described in detail in the literature. This study presents a model for the genesis of moraines of this type in the Chapais area, Québec. The model is based mainly on facies and deformatin structures analysis, and geomorphological data. Well-stratified glaciofluvial material is commonly found in the core of the moraines, whereas till or glacial diamicton may be present as surficial cover on their proximal side or as injected lenses in the sorted sediments. The paleocurrents are systematically directd downglacier. The moraines were built up in subglacial crevasses in areas where meltwater was channelized. Water flowed under pressure from small upglacier cavities, carrying a load of coars-grained material When flowing water entered crevasses already occupied by water, flow sparation occurred, reducing the capacity of the flow to carry the particles, and avalanching glaciofluvial material on the leeside of the piled sediments. The occurrence, in these sediments, of glaciotectonic deformation structures such as overturned to recumbent folds and thrust faults is evidence that the glacier was still active to some degree during and after the sedimentation phase.     

Styles of ice-marginal deformation at Hagafellsjökull-Eystri, Iceland during the 1998/99 winter-spring surge

This paper describes the internal architecture of a push moraine formed by a winter-spring surge of Hagafellsjökull-Eystri (Iceland) in 1998/99. The sedimentary architecture of this push moraine consists of a multilayered slab of glaciofluvial sediments with a monoclinal structure that has been displaced laterally by the advancing ice margin. The crest and ice-distal face of the moraine consist of subhorizontal sediment sheets, while the ice-proximal face dips steeply (45° to 90°) towards the ice margin. The core of the moraine consists of frozen sediment and thin slabs of glacier ice are embedded in its proximal face. The sediment slabs are characterized by both brittle and ductile styles of deformation. We argue that the observed variation in deformation style is dependent on whether the glacial foreland was frozen or unfrozen at the time of displacement. Frozen foreland would behave in a brittle fashion, while unfrozen foreland is likely to have deformed in a more ductile manner. The associated spatial variations in the degree of foreland freezing could be explained by variation in ice-marginal snow cover. We conclude that the thermal regime of the foreland, and the timing of the ice advance, is of importance to the style of internal deformation found within ice-marginal push moraines.     

Marginal formation of De Geer moraines and their implications to the dynamics of grounding‐line recession

De Geer moraine ridges occur in abundance in the coastal zone of northern Sweden, preferentially in areas with proglacial water depths in excess of 150 m at deglaciation. From detailed sedimentological and structural investigations in machine‐dug trenches across De Geer ridges it is concluded that the moraines formed due to subglacial sediment advection to the ice margin during temporary halts in grounding‐line retreat, forming gradually thickening sediment wedges. The proximal part of the moraines were built up in submarginal position as stacked sequences of deforming bed diamictons, intercalated with glaciofluvial canal‐infill sediments, whereas the distal parts were built up from the grounding line by prograding sediment gravity‐flow deposits, distally interfingering with glaciolacustrine sediments. The rapid grounding‐line retreat (ca. 400 m yr^?1) was driven by rapid calving, in turn enhanced by fast iceflow and marginal thinning of ice due to deforming bed conditions. The spatial distribution of the moraine ridges indicates stepwise retreat of the grounding line. It is suggested that this is due to slab and flake calving of the ice cliff above the waterline, forming a gradually widening subaqueous ice ledge which eventually breaks off to a new grounding line, followed by regained sediment delivery and ridge build‐up. Copyright © 2005 John Wiley & Sons, Ltd.     

Neoglacial (<3000 years) till and flutes at Saskatchewan Glacier,Canadian Rocky Mountains,formed by subglacial deformation of a soft bed

Understanding the processes that deposit till below modern glaciers provides fundamental information for interpreting ancient subglacial deposits. A process‐deposit‐landform model is developed for the till bed of Saskatchewan Glacier in the Canadian Rocky Mountains. The glacier is predominantly hard bedded in its upper reaches and flows through a deep valley carved into resistant Palaeozoic carbonates but the ice margin rests on a thick (<6 m) soft bed of silt‐rich deformation till that has been exposed as the glacier retreats from its Little Ice Age limit reached in 1854. In situ tree stumps rooted in a palaeosol under the till are dated between ca 2900 and 2700 yr bp and record initial glacier expansion during the Neoglacial. Sedimentological and stratigraphic observations underscore the importance of subglacial deformation of glaciofluvial outwash deposited in front of the advancing glacier and mixing with glaciolacustrine carbonate‐rich silt to form a soft bed. The exposed till plain has a rolling drumlinoid topography inherited from overridden end moraines and is corrugated by more than 400 longitudinal flute ridges which record deformation of the soft bed and fall into three genetically related types: those developed in propagating incipient cavities in the lee of large subglacial boulders embedded in deformation till, and those lacking any originating boulder and formed by pressing of wet till up into radial crevasses under stagnant ice. A third type consists of U‐shaped flutes akin to barchan dunes; these wrap around large boulders at the downglacier ends of longitudinal scours formed by the bulldozing of boulders by the ice front during brief winter readvances across soft till. Pervasive subglacial deformation during glacier expansion was probably facilitated by large boulders rotating within the soft bed (‘glacioturbation’).     

Within-valley asymmetry and related problems of Neoglacial lateral moraine development at certain Jotunheimen glaciers, southern Norway

A difference in the size of Neoglacial lateral moraines on either side of a valley axis (within-valley asymmetry of lateral moraine development) is described. Analysis of clast roundness has revealed subangular material in latero-terminal and terminal moraines; lateral moraines, however, exhibit a compositional gradient of increasing angularity with distance from the former glacier snout. Comparisons with clasts of known origin suggest that this 'roundness gradient' may be explained with reference to either or both of two hypotheses: (1) a variable proportion of supraglacial (or englacial) to subglacial transported material; and (2) the variable composition of regolith incorporated by a push mechanism from the valley sides. Within-valley asymmetry is inferred to result where the supply of debris to lateral moraines from these sources is unequal either side of a valley axis. Both interpretations are also consistent with the relatively large size of latero-terminal sections of end moraines. In order to account for the discrepancy between moraine size and apparent debris supply rates, it is suggested that the largest lateral moraines may have been formed over a longer time scale than the 'Little Ice Age', and that reworking of deposits may have occurred. The supply of debris to the north-facing lateral moraine at Nordre Illåbreen has been so great that it has developed into a rock glacier; this suggests the possibility that subglacial material and valley-side regolith, as well as supraglacial material, contributes to the formation of ice-cored rock glaciers.     

Ribbed moraine stratigraphy and formation in southern Finnish Lapland

Characteristics of ribbed moraines, the dominating moraine type in southern Finnish Lapland, have been studied in detail. The ridges are composed of several till units, of which the bottommost units consist of mature basal tills and the surficial parts are enriched with local, short‐transport rock fragments and boulders in till and at the surface of ridges. As a result of this re‐examination a two‐step model of the formation process of ribbed moraines is presented. In the first stage, while cold‐based conditions prevailed, both the bottommost part of the ice sheet and the frozen, substrate fractured under compressive ice flow. Following glacial transport of fractured blocks and formation of the transverse ridge morphology, erosion between the ridges continued owing to freeze–thaw process under variable pressure conditions. In the areas with a low pre‐existing till sheet, the process caused quarrying of the bedrock surface and subsequent deposition of rock fragments and boulders under high pressure on the next ridge. The most suitable conditions for ribbed moraine formation existed during Late Weichselian deglaciation, after the Younger Dryas when the climate warmed very quickly, leading to an imbalance between a warm glacier surface and a cold base. Copyright © 2006 John Wiley & Sons, Ltd.     

Formation of glaciolacustrine Late Pleistocene end moraines in the Tasman Valley,New Zealand

Detailed examination of the Tekapo Formation in the Tasman Valley, New Zealand has identified 20 facies, and five facies associations. These associations are delta foresets and bottomsets, sediment density flows, ice-contact lake sediments with ice-rafted debris and resedimentation deposits, and outwash gravels. Interpretation of the sediment-landform associations informed by observations at modern glacier termini suggests that the Late Pleistocene Tekapo Formation moraines have been formed by downwasting of a more expanded Tasman Glacier. During the early stages of glacier retreat, ponds on the glacier surface develop into thermokarst lakes which enlarge and coalesce to form a large supraglacial lake. Continued downwasting causes the lake outlet river to entrench into the impounding latero-frontal ice-cored moraine, lowering the lake level. This exposes lake-bottom sediments and forms shorelines on the proximal slopes of the ice-cored moraine. As the ice-cored moraine melts, these lake sediments are deformed and deposited against the Mt. John moraine. The observations and interpretations reported here suggest the Late Pleistocene end moraine is a constructional feature not a structural (glaciotectonic) feature as suggested by previous studies.     

Seafloor glacial features reveal the extent and decay of the last British Ice Sheet,east of Scotland

Three‐dimensional (3D) seismic datasets, 2D seismic reflection profiles and shallow cores provide insights into the geometry and composition of glacial features on the continental shelf, offshore eastern Scotland (58° N, 1–2° W). The relic features are related to the activity of the last British Ice Sheet (BIS) in the Outer Moray Firth. A landsystem assemblage consisting of four types of subglacial and ice marginal morphology is mapped at the seafloor. The assemblage comprises: (i) large seabed banks (interpreted as end moraines), coeval with the Bosies Bank moraine; (ii) morainic ridges (hummocky, push and end moraine) formed beneath, and at the margins of the ice sheet; (iii) an incised valley (a subglacial meltwater channel), recording meltwater drainage beneath former ice sheets; and (iv) elongate ridges and grooves (subglacial bedforms) overprinted by transverse ridges (grounding line moraines). The bedforms suggest that fast‐flowing grounded ice advanced eastward of the previously proposed terminus of the offshore Late Weichselian BIS, increasing the size and extent of the ice sheet beyond traditional limits. Complex moraine formation at the margins of less active ice characterised subsequent retreat, with periodic stillstands and readvances. Observations are consistent with interpretations of a dynamic and oscillating ice margin during BIS deglaciation, and with an extensive ice sheet in the North Sea basin at the Last Glacial Maximum. Final ice margin retreat was rapid, manifested in stagnant ice topography, which aided preservation of the landsystem record. Copyright © 2008 John Wiley & Sons, Ltd.