Subglacial Sediment Deformation Research Articles

Subglacial deformation of permafrost beneath Wright Lower Glacier, Antarctica as revealed by direct observations and measurements from a subglacial tunnel

Subglacial processes influence the large-scale behaviour of glaciers and determine the geological processes that occur at the ice-substrate interface. In this paper we focus on subglacial deformation beneath a cold-based glacier resting on permafrost. We ask what are the drivers of subglacial permafrost deformation, how is strain distributed beneath glaciers with a permafrost substrate, how is strain transmitted from the glacier into the substrate, and how is deformed permafrost incorporated into the basal zone of the glacier? To address these questions a tunnel excavated in the margin of Wright Lower Glacier in the McMurdo Dry Valleys, Antarctica to observe the structure of the basal zone and bed of the glacier and to monitor basal ice deformation over a period of 4 years. The tunnel revealed a stacked sequence of stratified debris-bearing and clean ice layers, and thick rafts of frozen sediment. Isotopic and gas analyses reveal that despite the low local temperature of the basal ice (−18.5 °C), liquid water has played an important role in the formation of this basal sequence. The rafts of frozen sediment contain well preserved sedimentary structures and algae layers that show the material has been entrained from the underlying permafrost. Deformation measurements made with a combination of displacement transducers, strain arrays, and plumb-lines show that strain and motion within the basal zone are heterogeneous and that strain is transmitted through the permafrozen sediment and into intra-sediment ice layers. The distribution of strain in the basal zone produces a compound velocity profile resembling the deformation patterns inferred from deformation structures in Pleistocene permafrost and also resembles profiles associated with subglacial sediment deformation driven by hydrogeological processes. We conclude that ice-rich permafrost is readily deformed by glaciers and that there is no clear boundary between basal ice and deforming permafrost.

Subglacial sediment deformation in the Ross Sea, Antarctica

The Ross Sea embayment is the outlet for one-third of all Antarctic ice flow, receiving input from both East and West Antarctica. Marine-based ice streams, whose stability is susceptible to global sea-level changes, expand and contract across the Ross Sea continental shelf, eroding and deforming the substrate and transporting sediment subglacially. Deposits from the Last Glacial Maximum in the western Ross Sea provide an opportunity to investigate paleo-ice stream processes using sediment cores associated with glacial landforms. This study combines micro-to macro-sedimentological analyses to characterize lithofacies that represent a glacial-interglacial succession. We observe subtle variations in sediment properties within the subglacial unit that suggest till heterogeneity. Physical characteristics that vary both laterally and vertically indicates different sediment response to glacial stress, resulting in non-pervasive deformation within the till. These observations are consistent with the interpretation that a dynamic ice sheet with variable flow regimes existed across the Ross Sea during the Last Glacial Maximum. Further microscopic evidence of dynamic subglacial conditions includes sediment aggregates preserved as discrete grains in the form of soft sediment clasts or till pellets. Till pellet-rich sediment is commonly associated with the transition from a subglacial to proximal glaciomarine environment, although here we demonstrate they are widespread in both facies. Their widespread occurrence downcore and distribution across the continental shelf provides sedimentological evidence of a deformable bed beneath an ice stream during the Last Glacial Maximum. Ultimately, better tools for discriminating between glacial deposits will allow for more detailed reconstructions of ice-sheet history, and thus determine the forcing mechanisms to which the ice is responding.

Deformation mechanisms and kinematics of a soft sedimentary bed beneath the Scandinavian Ice Sheet, north-central Poland, revealed by magnetic fabrics

Characterising deformable substrates of modern and past ice sheets is a key factor in understanding ice mass behaviour and dynamics. This is challenging when field-scale signatures of deformation in soft sedimentary beds are weakly developed or absent. To overcome these difficulties, in this study, we combine micro- and macroscale studies and quantitative analyses to decipher the nature of subglacial sediment deformation under a Pleistocene fast-flowing (streaming) ice sheet. We investigated a complex sedimentary succession at Nieszawa in north-central Poland. A multi-proxy approach involving till micromorphology in thin sections, ripple-cross lamination orientation, till fabric, clast microfabric and the anisotropy of magnetic susceptibility (AMS) data in a closely sampled vertical profile throughout the fluvial sand and till was used to reveal subglacial bed deformation mechanisms and kinematics. This records subglacial deformation of the sedimentary succession at Nieszawa under low basal shear stresses, where simple shear was the dominant bed deformation mechanism. Intergranular advection, decoupling at the ice-bed interface related to high subglacial water pressure, clast ploughing, and basal sliding also took place. The main finding of this study is that AMS reveals the shear strain development and ice flow direction in the structurally and texturally complex deposits under weak coupling conditions at the ice-bed interface beneath a fast flowing/streaming ice sheet, where strain and kinematic indicators other than AMS fabrics are vague or absent.

Till formation under a soft-bedded palaeo-ice stream of the Scandinavian Ice Sheet, constrained using qualitative and quantitative microstructural analyses

This study combines micro- and macroscale studies, laboratory experiments and quantitative analyses to decipher processes of till formation under a palaeo-ice stream and the nature of subglacial sediment deformation. Till micromorphology (grain lineations, grain stacks, turbate structures, crushed grains, intraclasts and domains), grain-size and till fabric data are used to investigate a basal till generated by the Vistula Ice Stream of the Scandinavian Ice Sheet during the last glaciation in north-central Poland. A comparison of microstructures from the in situ basal till and laboratory-sheared till experiments show statistical relationships between the number of grain lineations and grain stacks; and between the number of grain lineations and turbate structures. Microstructures in the in situ till document both brittle and ductile styles of deformation, possibly due to fluctuating basal water pressures beneath the ice stream. No systematic vertical and lateral trends are detected in the parameters investigated in the in situ till, which suggests a subglacial mosaic of relatively stable and unstable areas. This situation can be explained by an unscaled space-transgressive model of subglacial till formation whereby at any given point in time different processes operated in different places under the ice sheet, possibly related to the distance from the ice margin and water pressure at the ice–bed interface. A new quantitative measure reflecting the relationship between the number of grain lineations and grain stacks may be helpful in discriminating between pervasive and non-pervasive deformation and constraining the degree of stress heterogeneity within a deformed bed. Independent strain magnitude estimations revealed by a quantitative analysis of micro- and macro-particle data show low cumulative strain in the ice-stream till in the order of 10–102.

Ductile and brittle styles of subglacial sediment deformation: An example from western Ireland

In western Ireland, within a drumlin formed inside the margin of the late Pleistocene ice sheet, a range of different styles of ductile and brittle sediment deformation is observed within subglacial diamicton (till) and associated subglacial waterlain sediments. At Roonah Point, County Mayo, the structures associated with these different deformation styles include gravel clusters that have loaded subjacent sediments, glacitectonic shears, water escape structures, and clastic dikes. These structures are found for tens of metres laterally within the same diamicton units and the structures may cross-cut or be superimposed upon one another. Based on these properties and geometric relationships, the relative chronology and processes and patterns of subglacial deformation can be reconstructed, including an evaluation of subglacial conditions and pore water pressure regimes. This illustrates the relationship between glacitectonic forcing by ice–bed coupling, and the structural response within subglacial sediments to changing pore water pressure gradients.

Former dynamic behaviour of a cold-based valley glacier on Svalbard revealed by basal ice and structural glaciology investigations

AbstractLarge numbers of small valley glaciers on Svalbard were thicker and more extensive during the Little Ice Age (LIA), demonstrated by prominent ice-cored moraines up to several kilometres beyond present-day margins. The majority of these glaciers have since experienced a long period of strongly negative mass balance during the 20th century and are now largely frozen to their beds, indicating they are likely to have undergone a thermal transition from a polythermal to a cold-based regime. We present evidence for such a switch by reconstructing the former flow dynamics and thermal regime of Tellbreen, a small cold-based valley glacier in central Spitsbergen, based on its basal sequence and glaciological structures. Within the basal sequence, the underlying matrix-supported diamict is interpreted as saturated subglacial traction till which has frozen at the bed, indicating that the thermal switch has resulted in a cessation of subglacial sediment deformation due to freezing of the former deforming layer. This is overlain by debris-poor dispersed facies ice, interpreted to have formed through strain-induced metamorphism of englacial ice. The sequential development of structures includes arcuate fracture traces, interpreted as shear planes formed in a compressive/transpressive stress regime; and fracture traces, interpreted as healed extensional crevasses. The formation of these sediment/ice facies and structures is indicative of dynamic, warm-based flow, most likely during the LIA when the glacier was significantly thicker.

A comparative case study of subglacial bedforms in northern Lithuania and south-eastern Iceland

This paper aims to compare the dynamics of the subglacial environment as determined by an analysis of the structure and sedimentology of both Pleistocene and modern glacial deposits. Investigations focused on subglacial sediments in areas of streamlined relief in northern Lithuania (Ruopiškiai megascale subglacial lineations - MSGL) and south-eastern Iceland (drumlinised terrain exposed by the recent retreat of Skeiðarárjökull glacier). Sedimentological analyses involved granulometry, till macrofabrics, and the anisotropy of magnetic susceptibility (AMS) of micro-clasts. Comparative investigations of subglacial bedforms exposed at Ruopiškiai and Skeiðarárjökull highlighted differences in their formation and post-genetic transformation. In both cases, their initiation was associated with ice advance and subglacial sediment deformation. However, subsequent dynamics were different.

Formation of mega-scale glacial lineations on the Dubawnt Lake Ice Stream bed: 2. Sedimentology and stratigraphy

Mega-scale glacial lineations (MSGLs) are highly elongate, subglacial landforms produced beneath zones of fast-flowing ice. While qualitative data on their morphology have existed for several decades, studies of their composition and sedimentology are comparatively rare. Sediment exposures along the course of the Finnie River in Nunavut, northern Canada, provide a window into the internal stratigraphy and sedimentology of MSGLs formed by the Dubawnt Lake Palaeo-Ice Stream during regional deglaciation of the Laurentide Ice Sheet. Stratigraphic sections record evidence for an initial advance of ice into the study area followed by ice sheet recession and deposition of glacifluvial and glacilacustrine outwash. Subsequently, the Dubawnt Lake Palaeo-Ice Stream overrode and reworked this outwash subglacially forming an ‘MSGL till’. This till comprises a sandy, red diamicton facies, forming the core of the MSGL ridges and containing variably deformed lenses, stringers and rafts of outwash. The sedimentology of this diamicton is consistent with an origin as a glacitectonite and hybrid till formed by a combination of non-pervasive subglacial sediment deformation and lodgement. Facies variations from stratified to massive diamicton reflect, in turn, variations in strain and subglacial transport distance. The occurrence of stratified glacifluvial sediments within these ridges and the well-preserved nature of many of the sandy inclusions within the diamicton imply relatively short transport distances and incomplete mixing. MSGLs under the Dubawnt Lake Palaeo-Ice Stream formed through a combination of subglacial erosion and deposition. This included non-pervasive, subglacial sediment deformation and the reworking of pre-existing sediment depocentres during streaming flow. These results highlight the importance of sediment supply to MSGL formation with the presence of abundant pre-existing sediments which were subsequently overridden being critical to lineation formation.

Subglacial deforming bed conditions recorded by late Quaternary sediments exposed in Vineland Quarry, Ontario, Canada

There has been considerable interest in recent years in the development of theoretical models of subglacial transport and deposition of sediment, but relatively few studies report field documentation of the resultant sediment stratigraphies. This paper presents detailed sedimentological description and analysis of a succession of late Quaternary deposits interpreted to record subglacial overriding and deformation of previously deposited lacustrine sediments exposed in the Vineland Quarry that sits close to the crest of the Niagara Escarpment within the Lake Ontario basin. The predominately fine-grained sediments record deposition under glaciolacustrine conditions followed by deformation and deposition by overriding glacial ice. Laminated silt and clay deposits overlie the striated bedrock surface and were deposited within a lake that formed as the Ontario Lobe of the Laurentide Ice Sheet advanced during the Port Huron stadial and ponded water against the Niagara Escarpment. The laminated silt and clay facies show increasing amounts of deformation up-section, passing from planar through ductile to brittle deformation. This succession of deformed facies is overlain by a macroscopically massive clay-rich diamict that caps the section. This pattern of sediment deposition and deformation is consistent with that proposed by current models of subglacial sediment deformation with the disrupted laminated silts and clays representing ‘glacitectonites’ resulting from downward penetrating stresses imparted by an overriding ice sheet. The uppermost massive diamict unit represents full macroscopic homogenization of the overridden sediment and is classified as a subglacial ‘traction till’. The gradual transition from undisturbed laminated deposits through increasingly deformed sediment to structureless, diamict suggests that these deposits record a single episode of ice advance across the region. This ice advance was probably the short-lived advance of the Ontario Lobe of the Laurentide Ice Sheet that occurred at approximately 13,000 ybp.

Insights into subglacial processes inferred from the micromorphological analyses of complex diamicton stratigraphy near Illmensee-Lichtenegg, Höchsten, Germany

Menzies, J. & Ellwanger, D. 2010: Insights into subglacial processes inferred from the micromorphological analyses of complex diamicton stratigraphy near Illmensee-Lichtenegg, Hochsten, Germany. Boreas, 10.1111/j.1502-3885.2010.00194.x. ISSN 0300-9483. Investigations of a 30-m-high section of Pleistocene sediments at Illmensee-Lichtenegg, Hochsten in Baden-Wurttemberg provide detailed information on subglacial conditions beneath the Rhine Glacier outlet of the Alpine ice sheet in southern Germany. The sediment exposure extends from an upper cemented sand and gravel (Deckenschotter) into diamictic units that extend down to weathered Molasse bedrock. The exposure reveals sediments symptomatic of active syndepositional stress/strain processes ongoing beneath the ice sheet. Macrosedimentology reveals diamicton subfacies units and a strong uni-direction of ice motion based on clast fabric analyses. At the microscale level, thin-section analyses provide a substantially clearer picture of the dynamics of subglacial sediment deformation and till emplacement. Evidence based on detailed micromorphological analyses reveals microstructural strain and depositional markers that indicate a subglacial environment of ongoing soft bed deformation in which the diamictons can be readily identified as subglacial tills. Within this subglacial environment, distinct changes in pore-water pressure and sediment rheology can be detected. These changes reveal fluctuating conditions of progressive, non-pervasive deformation associated with rapid changes in effective stress and shear strain leading to till emplacement. This site, through the application of micromorphology, increases our understanding of localized subglacial conditions and till formation.

Neogene glacigenic debris flows on James Ross Island, northern Antarctic Peninsula, and their implications for regional climate history

Detailed sedimentological and microtextural analyses of newly-discovered late Neogene diamictites and other coarse-grained facies, mostly sandwiched between hyaloclastite of the James Ross Island Volcanic Group and Cretaceous sandstone and mudstone, indicate deposition mainly by glacigenic debris flows. The deposits on James Ross Island (northern Antarctic Peninsula) constrain the depositional setting, ice–bed dynamics and regional palaeoclimate. The sequences on James Ross Island vary in age but are mainly late Miocene and Pliocene. Unlike Neogene sedimentary sequences elsewhere in Antarctica, those on James Ross Island are unusually well-dated by a combination of 40Ar/ 39Ar and 87Sr/ 86Sr analyses on fresh interbedded lavas and pristine bivalve molluscs, respectively. The Sr isotopic ages of the debris flows cluster around 4.74, 4.89, 5.44, 5.78, and 6.31 Ma and probably date relatively warm periods in the northern Antarctic Peninsula region, when the bivalves lived under ice-poor or seasonally ice-free conditions. The bivalves are often remarkably well-preserved, lack adhering lithified sediment and, in at least two locations, are large, mainly unfragmented and sometimes articulated, suggesting that they were alive immediately prior to their incorporation in subaqueous debris flows at the margins of an advancing glacier. These fossiliferous glacigenic debris flows signify episodes of ice expansion during relatively warm periods, or “interglacials”, of the late Miocene and Pliocene. The James Ross Island glacigenic sedimentary successions attain thicknesses of up to 150 m and extend over 4 km laterally. The high volume of glacigenic sediment delivery implicit in the James Ross Island successions indicates that a series of dynamic ice fronts crossed the region during the late Miocene and Pliocene epochs. Associated evidence, in the form of clast abrasion (including striations and faceting) and bedrock erosion, is indicative of basal sliding and subglacial sediment deformation active at the ice–bed interface and wet-based temperate or polythermal regimes, prior to remobilisation. The evidence further suggests two local ice caps on James Ross Island during the warm periods, as well as ice-overriding by the Antarctic Peninsula Ice Sheet from the west and northwest.

Integrated summer insolation forcing and 40,000‐year glacial cycles: The perspective from an ice‐sheet/energy‐balance model

Although the origins of the 40,000‐year glacial cycles during the early Pleistocene are readily attributed to changes in Earth's obliquity (also having a 40,000‐year period), the lack of ice‐volume variability at precession periods (20,000 years) is difficult to reconcile with most parameterizations of the insolation forcing. It was recently proposed that precession's influence on glaciation is muted because variations in the intensity of summer insolation are counterbalanced by changes in the duration of the summertime, but no climate model has yet been shown to generate obliquity period glacial cycles in response to the seasonal insolation forcing. Here we present a coupled ice‐sheet/energy‐balance model that reproduces the seasonal cycle and, when run over long time periods, generates glacial variability in response to changes in Earth's orbital configuration. The model is forced by the full seasonal cycle in insolation, and its response can be understood within the context of the integrated summer insolation forcing. The simple fact that obliquity's period is roughly twice as long as that of precession results in a larger amplitude glacial response to obliquity. However, for the model to generate almost exclusively obliquity period glacial variability, two other conditions must be met. First, the ice sheet's ablation zone must reside poleward of ∼60°N because insolation intensity is more sensitive to changes in Earth's obliquity at high latitudes. Second, the ablation season must be long enough for precession's opposing influences on summer and fall insolation intensity to counterbalance one another. These conditions are consistent with a warm climate and a thin ice sheet, where the latter is simulated as a response to subglacial sediment deformation. If a colder climate is prescribed, or in the absence of basal motion, ice sheets tend to be larger and undergo greater precession period variability, in keeping with proxy observations of late Pleistocene glaciation.

Subglacial bedforms and conditions associated with the 1991 surge of Skeiðarárjökull, Iceland

Much previous research at surge‐type glaciers has sought to identify features diagnostic of surge‐type behaviour. However, in comparatively little work have subglacial landform–sediment characteristics been used to reconstruct changing basal processes and conditions during surge events. Subglacial bedforms described in this article are associated with the 1991 surge of Skeiðarárjökull, Iceland, and include a series of drumlins with superimposed flutes and basal crevasse‐fill ridges. The drumlins were formed by the subglacial erosion of ice‐contact fans. Sedimentary evidence indicates a shift from rigid‐bed to soft‐bed conditions during the surge. The presence of eroded but undeformed fan sediments suggests that they acted as a rigid bed when initially overridden. Subsequent deposition of a layer of deformation till resulted in a change to soft‐bed conditions and the generation of flutes and subglacial crevasse‐fill ridges. The lack of mixing between this till and the underlying stratified sediments indicates that subglacial sediment deformation was restricted to a thin layer and that its deposition resulted in a cessation of subglacial erosion. The drumlin is therefore a composite of both rigid‐bed and soft‐bed processes that illustrates changes in basal conditions and processes during the course of the event. The limited time frame in which the drumlin formed and the presence of kettleholes across its surface are distinctive features that may warrant further investigation in the search for features diagnostic of past surge events.

Till characteristics, genesis and transport beneath Antarctic paleo‐ice streams

Marine geophysical data from the Antarctic continental shelf indicate the former presence of ice streams that drained through bathymetric troughs during the last glacial cycle. Streaming flow is recorded by elongate subglacial bedforms, formed in the upper part of an acoustically transparent sediment layer. Cores from the layer demonstrate that it is a weak and porous subglacial till (“soft till”). Shear within the soft till was concentrated in zones that were 0.1–0.9 m thick. The soft till is a “hybrid” till, formed by a combination of subglacial sediment deformation and lodgment. The base of the soft till is marked by a strong reflector which represents the top of a denser and stronger “stiff till.” The form of this reflector ranges from flat and continuous to deeply grooved and irregular, implying the operation of different mechanisms of sediment mobilization and incorporation into the soft till layer. The irregular form of the basal reflector is consistent with grooving, whereas the flat and continuous form is more consistent with sediment mobilization as a subglacial traction carpet related to changes in basal effective stress. Geophysical evidence for large‐scale advection of the soft till, combined with evidence for deformation partitioning into relatively thin layers up to 0.9 m thick, indicates that localized shear zones integrated to transport significant volumes of sediment beneath Antarctic paleo‐ice streams.

Reversal of ice motion during the outburst of a glacier-dammed lake on Gornergletscher, Switzerland

AbstractDuring the outburst flood of a glacier-dammed lake on Gornergletscher, Switzerland, in July 2004, the drained lake water triggered anomalous glacier motion. At the onset of the outburst, the ice-flow direction in the vicinity of the lake became closer to the central flowline. When the lake discharge magnitude decreased, the flow direction altered such that the ice moved back to the azimuth of the initial motion. At one of the survey points, where the ice flows parallel to the central flowline, the ice accelerated along the pre-event flow direction followed by a 180° backward motion that lasted over 2 days. These observations indicate the impact of the lake outburst on the subglacial and englacial stress conditions; however, the reversal in the flow direction is difficult to explain by drawing on our current understanding of glacier mechanics. The timing and the timescale of the flow-direction changes suggest that the elastic glacier motion and its rebound played a role under the rapidly changing stress conditions, but the Young’s modulus of ice is too large to cause the observed ice motion. Other processes, including basal separation and subglacial sediment deformation, are discussed as possible mechanisms for the reversal of the ice motion.

Microearthquakes and subglacial conditions

Ten passive seismic recording stations were deployed in a 9 km by 6 km array on Rutford Ice Stream, West Antarctica, to detect microearthquakes from the ice stream bed. The array covered an area where a varying distribution of basal sedimentary conditions had been identified from seismic reflection surveys and interpreted as areas of both basal sliding and subglacial sediment deformation. The passive seismic experiment was designed to give an independent confirmation of this interpretation, as basal sliding is believed to be associated with the release of more acoustic energy than a deforming bed. The results show that this is the case, basal sliding producing six times more events. This confirms that the spatial pattern of basal conditions does indeed reflect the distribution of different ice flow mechanisms. It also shows that microearthquake monitoring can be a valuable new technique for mapping ice stream basal conditions over wide areas.

Extent and dynamics of the West Antarctic Ice Sheet on the outer continental shelf of Pine Island Bay during the last glaciation

Swath bathymetry and TOPAS sub-bottom profiler acoustic data reveal the presence of a major cross-shelf bathymetric trough containing streamlined subglacial bedforms formed in soft till that extends to the continental shelf edge of outer Pine Island Bay, Amundsen Sea, West Antarctica. The new data indicate that the West Antarctic Ice Sheet (WAIS) extended to the shelf edge of Pine Island Bay during the last glaciation, and was drained by a grounded, fast-flowing palaeo-ice stream through this outer-shelf trough. Topography and subglacial geology are likely to have exerted a strong control on the development and location of palaeo-ice streams in Pine Island Bay. TOPAS records show that mega-scale glacial lineations are formed in an acoustically transparent sediment layer interpreted to be soft till and are inferred to be the product of subglacial sediment deformation beneath the palaeo-ice stream. The flat to irregular nature of the basal reflector of the acoustically transparent layer of soft till implies that a combination of groove-ploughing of the substrate and subglacial sediment deformation were important processes beneath the palaeo-ice stream in the outer trough of Pine Island Bay, as well as palaeo-ice streams elsewhere in Antarctica. Therefore, the formation of the soft till layer, and possibly associated subglacial bedforms, are likely to be a function of these palaeo-ice stream processes. In a regional context, the WAIS extended to the continental shelf edge of the Bellingshausen, Amundsen and Ross Seas, and was drained by a number of palaeo-ice streams during the last glaciation.

Microstructures and microshears as proxy for strain in subglacial diamicts: Implications for basal till formation

Ring-shear experiments were used to study subglacial sediment deformation and the development of S-matrix microstructures and microshears to constrain glacially induced deformation in diamicts. The experiments approximated subglacial conditions, and the diamict was sheared and sampled incrementally to strains of 0, 7, 18, 33, 57, and 107. At strains between 7 and 18, a steady association of S-matrix microstructures developed, which did not evolve further despite continued shearing. The I L -index, a measure of microshear-orientation strength parallel to the shearing direction correlates log-linearly with strain, which indicates its potential use as strain proxy. The ring-shear data were compared with the characteristics of Quaternary basal tills from Denmark, Poland, and Svalbard. Based on I L -index values, we conclude that these tills only experienced strain of ∼10 1 , which is orders of magnitude lower than expected for deformation tills. This suggests that pervasive subglacial deformation and sediment advection in the mobile layer at the ice-bed interface may be less significant than previously assumed.

Flow dynamics and till genesis associated with a marine-based Antarctic palaeo-ice stream

Geophysical and geological data indicate that during the last glacial cycle a palaeo-ice stream drained the Antarctic Peninsula Ice Sheet (APIS) through Marguerite Bay to the edge of the continental shelf via a bathymetric trough (Marguerite Trough). Mega-scale glacial lineations (MSGL) in outer Marguerite Trough are formed in an acoustically transparent sediment unit, consisting of a soft deformation till. The association of MSGL with deformation till indicates that bed deformation occurred beneath the ice stream, and the thickness of the subglacial deforming layer was on the order of several metres. MSGL within outer Marguerite Trough are related to a pre-existing thick sequence of soft sediments on the outer shelf and are inferred to have formed by a combination of subglacial sediment deformation by attenuation from a point source and groove-ploughing. Morphologically, the MSGL differ in several respects from those documented elsewhere and this variation suggests a polygenetic origin for this subglacial landform. An initial advance of the APIS across the Marguerite Bay continental shelf at the LGM deposited a stiff subglacial till. Following this advance, the ice margin within Marguerite Trough retreated 70–100 km before stabilising and depositing a grounding zone wedge. An ice stream then developed in the trough and extended to the shelf edge forming a soft deformation till. Development of the ice stream may have been a glacio-dynamic response to regional deglaciation. Subsequent retreat of the Marguerite Trough ice stream was underway by 13,490 (uncorrected) 14C years before present. Based on the geophysical and core data, ice-stream retreat appears to have been rapid and it contrasts with reconstructions of palaeo-ice streams from other locations in Antarctica. This implies marked regional variations in Antarctic ice-stream dynamics during Late Quaternary deglaciation.

Till genesis and glacier motion inferred from sedimentological evidence associated with the surge-type glacier, Brúarjökull, Iceland

AbstractA study employing macro- and micro-sedimentological techniques was conducted at three sites with recently deglaciated sediments in the proglacial area of Brúarjökull, a surge-type outlet glacier of the Vatnajökull ice cap, Iceland. Tills at these sites were likely deposited and deformed during the 1963/64 surge. At the height of the last surge, these sediments were beneath 90-120 m of ice, and associated basal shear stresses would have been 24-32 kPa. Tills associated with the surge at these sites formed by a combination of subglacial sediment deformation and lodgement and are thus regarded as ‘hybrid tills’. The tills show evidence of both ductile and brittle deformation. Discontinuous clay lenses within the tills, indicating local ice-bed decoupling and sliding, imply that subglacial water pressures were spatially and temporally variable during the surge. The thickness of the subglacial deforming-till layer was 50-90 cm.