Till Fabric Research Articles

Mapping, impacts, characterization and extent of acid sulfate soils in Finland

Acid sulfate soils (ASS) cause big problems worldwide due to their potential to form sulfuric acid during oxidation of sulfidic materials, resulting in very acid soil (pH <4.0). Impacts include acidification of soil and water, leaching of metals, decreased nutrient supply, deterioration of water fauna and flora, and corrosion of infrastructure. These soils also exhibit poor geotechnical properties. Finland has the largest occurrences of ASS in Europe, mainly along the coast of the Baltic Sea. The EU Water Framework Directive brought about wide co-operation to reduce the harmful impacts of ASS in Finland. One urgent step was to localize and characterize the occurrences of ASS. The more than 10 year-long programme, led by the Geological Survey of Finland (GTK), started in 2009 and field work was completed in 2021. During the programme observations, measurements, sampling and analyses were made at 23 000 sites in an area of 5 010 000 ha. Traditionally ASS in Finland have been considered to comprise fine-grained sulfidic sediments and/or their oxidized layers, occurring on agricultural land along the coast below the highest shoreline of the Littorina Sea transgression. This study recognized and classified significant occurrences of other types of potentially harmful ASS materials: (1) coarse-grained ASS (sand), (2) organic ASS (peat) and (3) unsorted ASS (till material). The methods, definition and classification of Finnish ASS have been revised. We calculated the extent of ASS along the coast to be about 1 000 000 ha corresponding to 21% of the area covered in the past by the Littorina Sea, and three to six times more than earlier estimates. In addition, some occurrences of ASS were recognized inland, mainly related to black shales and sulfidic ores. The mapping data can be accessed via the GTK map service (www.gtk.fi) providing information about the distribution and properties of ASS.

Assessment of the tribological performance of glass fibre reinforced polyamide 6 under harsh abrasive environments

High-performance thermoplastics exhibit significant potential to substitute metals in various applications, primarily due to their remarkable tribological, thermal and mechanical characteristics. Nevertheless, their characteristics require further refinement. This research aims to assess and compare the influence of distinct material types on the tribological characteristics of Polyamide 6 composites, as well as their reliance on the PV factor, which is the result of the velocity and pressure product.In the current research, the friction and wear performances of polyamide 6 (PA6) reinforced with glass fibres were examined in adverse PV factors till materials failed. Twin screw extrusion and subsequent injection moulding procedures were utilised to process the materials. The tribo-performance of the materials was assessed in ambient conditions using a pin-on-disc abrasive configuration. The morphological analysis of the abraded surfaces was conducted through field emission scanning electron microscopy (FESEM), and the plausible wear mechanisms contributing to wear are explained. Furthermore, the materials were thoroughly characterised for their tribological, mechanical and physical behaviours.The wear behaviour depended mainly on operating parameters and composition. PA6 proved to be beneficial till moderate PV conditions but not for harsh operating environments as compared to PA6 reinforced with 25% by volume of glass fibres (PA625GF). PA625GF showed PVlimit value of 1.04 MPa m/s which was decisively superior to PA6 (0.78 MPa m/s). Additionally, under a load of 50 N, PA625GF could sustain a higher speed of 800 rpm as compared to PA6 (600 rpm). The results showed that under all testing conditions glass fibres can minimise the coefficient of friction. Under low PV settings, the specific wear rate of PA625GF was remarkably reduced owing to the presence of glass fibres, reflecting superior anti-wear properties. Notably, the wear resistance of PA6 was considerably improved by glass fibres, particularly under high PV conditions, resulting in a PVlimit of 1.04 MPa m/s. The enhancement in PVlimit was attributed to the protective function of the fibres on the matrix and their superior mechanical properties. PA625GF proved to be tribo-potential materials in abrasive wear mode under extreme conditions in terms of low specific wear rate (8.484 ×10−3 mm3/N.m) and PVsafe value (0.78 MPa*m/s). This study is expected to pave the way for the development of novel self-lubrication composite materials for harsh service environments.

Effect of Various Aspects of Joints on Permeability of Jointed Glacial Till, Northeastern Ohio

ABSTRACT Glacial till bluffs along Lake Erie's shoreline, northeastern Ohio, contain three sets of joints that influence the hydraulic behavior of till mass like joints in rock influence the hydraulic behavior of rock mass. According to the Unified Soil Classification System, the till classifies as low plasticity silt to low plasticity clay, with a clay content of 36–41 percent. Mineralogically, it consists of quartz, illite, and kaolinite. The joints in till are either open or filled with sand, silt, or both from the overlying lacustrine material or with disintegrated till from the joint walls. We used double-ring infiltration and dye tests to estimate field permeability. Laboratory permeability tests were performed on dry and saturated samples of till material, with manually created joints that were either kept open or filled with sand or disintegrated till material to simulate field conditions. Field and laboratory test results showed that permeability of jointed till depended upon whether the joints were open or filled, the nature of the joint-filling material, and the water content of the till (dry versus saturated) and that permeability generally decreased with increasing test time. Dry samples exhibited a substantially larger decrease in permeability than saturated samples, with the largest decrease occurring during the first 24 hours. Open joints in dry samples collapsed during the test procedure, and the permeability of these samples was generally similar to the permeability of the collapsed till material. Permeability tests on saturated samples with open joints were inconclusive. We present a comparison of field and laboratory test results.

Origin of glacial curvilineations by subglacial meltwater erosion: Evidence from the Stargard drumlin field, Poland

AbstractDeciphering the origin of subglacial landforms is crucial for understanding processes operating under ice sheets. Due to the inaccessibility of the ice/bed interface limiting real‐time observations, the origin of these landforms is typically interpreted from the morphological and sedimentological record exposed after ice retreat, which makes the conclusions conjectural. Here, we focus on a recently discovered new type of subglacial landform called glacial curvilineations (GCLs), whose formation generates some controversy. We use LiDAR imagery and geological data to constrain the origin of GCLs found in a tunnel valley in the Stargard drumlin field, Poland. The tunnel valley is about 3800 m long, 520 m wide, and it occupies about 2 km2. A total of 61 individual GCLs have been mapped there; they are up to 550 m long and typically 0.5–1.5 m high, but some reach a height of up to 5 m. The GCLs are composed of mostly massive till represented by either a single or several diamicton units occasionally separated by thin layers of meltwater sand. Till macrofabrics in the GCLs and in the nearby till plain are distinctly consistent, and have high and uniform clustering strengths. At all locations, the dominant till fabric direction broadly corresponds with the ice movement direction given by the drumlin orientations, but it is oblique to the orientation of the tunnel valley and independent of the orientations of the individual GCLs. The lack of correspondence between the orientation of the GCLs and the till fabrics in them suggests that the GCLs consist of an antecedent till deposited during the drumlinizing ice advance and subsequently carved by subglacial meltwater erosion in the tunnel valley. These results substantiate the hypothesis that GCLs are erosional features postdating the material constituting them, and emphasize the role of subglacial meltwater flows as a landforming agent.

Influence of soil-water characteristic curve on the safe height and stand-up time of unsupported vertical trenches in unsaturated soils

ABSTRACT Geotechnical projects are typically initiated with an unsupported vertical trench (UVT) in unsaturated soils. In this case, it is fundamental to consider the contribution of soil suction toward the shear strength and hydraulic conductivity of soils when analyzing the stability of UVTs. The soil–water characteristic curve (SWCC) is a vital tool for estimating the variations of shear strength and hydraulic conductivity of soils with soil suction. In this study, numerical analyses were conducted to investigate the influence of SWCC on the safe height and stand-up time of UVTs in sandy and glacial till materials. SWCCs were established using three SWCC fitting models based on experimental data. The analysis results demonstrate that it is essential to understand the critical differences between the SWCCs obtained using different fitting models and their influence on the safe height and stand-up time of UVTs, rather than simply comparing the estimates.

Multidecadal delay between deglaciation and formation of a proglacial lake sediment record

Basal ages obtained from proglacial lake sediments are often used to constrain deglaciation histories, based on the assumption that fine-grained stratigraphic records start forming immediately after glacier retreat. Here, we test this assumption by studying the onset of sedimentation in Calluqueo Lake, Chilean Patagonia, which progressively deglaciated between the 1940s and the end of the 20th century. Although the glacier-proximal basin has been ice-free for at least three decades, it does not yet contain a fine-grained stratigraphic record, despite the modern sedimentation rate of ∼3 cm yr−1. By comparison, the distal basin contains a fine-grained stratigraphic record starting in 1997 ± 2 CE, i.e., 20–50 years after it was deglaciated. Based on these results, we show that several decades are required for ∼1 m of fine-grained sediments to accumulate between the coarse till material and sufficiently smoothen the uneven lake floor to start forming a fine-grained stratigraphic record. Although the exact timing depends on lake floor morphology and sediment accumulation rates, our results suggest that proglacial lake sediment records lack the first decades of sedimentation. This delay is mostly negligible when using radiocarbon ages from basal sediments to date glacier retreat since it falls within the range of radiocarbon uncertainties, though its importance increases throughout the Holocene. It is however significant for chronologies entirely based on varve-counting. Therefore, our results support the use of basal ages to establish deglaciation chronologies but they call for attention when using them to reconstruct Holocene ice retreat, especially with varve chronologies.

Contrasted geomorphological and limnological properties of thermokarst lakes formed in buried glacier ice and ice-wedge polygon terrain

Abstract. In formerly glaciated permafrost regions, extensive areas are still underlain by a considerable amount of glacier ice buried by glacigenic sediments. It is expected that large parts of glacier ice buried in the permafrost will melt in the near future, although the intensity and timing will depend on local terrain conditions and the magnitude and rate of future climate trends in different Arctic regions. The impact of these ice bodies on landscape evolution remains uncertain since the extent and volume of undisturbed relict glacier ice are unknown. These remnants of glacier ice buried and preserved in the permafrost contribute to the high spatial variability in ground ice condition of these landscapes, leading to the formation of lakes with diverse origins and morphometric and limnological properties. This study focuses on thermokarst lake initiation and development in response to varying ground ice conditions in a glacial valley on Bylot Island (Nunavut). We studied a lake-rich area using lake sediment cores, detailed bathymetric data, remotely sensed data and observations of buried glacier ice exposures. Our results suggest that initiation of thermokarst lakes in the valley was triggered from the melting of either buried glacier ice or intrasedimental ice and ice wedges. Over time, all lakes enlarged through thermal and mechanical shoreline erosion, as well as vertically through thaw consolidation and subsidence. Some of them coalesced with neighbouring water bodies to develop larger lakes. These glacial thermokarst lakes formed in buried glacier ice now evolve as “classic” thermokarst lakes that expand in area and volume as a result of the melting of intrasedimental ground ice in the surrounding material and the underlying glaciofluvial and till material. It is expected that the deepening of thaw bulbs (taliks) and the enlargement of Arctic lakes in response to global warming will reach undisturbed buried glacier ice where it is still present, which in turn will substantially alter lake bathymetry, geochemistry and greenhouse gas emissions from Arctic lowlands.

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.

The isotopic geochemistry of CaCO3 encrustations in Taylor Valley, Antarctica: Implications for their origin

Calcium carbonate (CaCO<sub>3</sub>) encrustations occur in most desert soils, including polar ones, and such encrustations preserve records of geochemical, hydrological, and atmosphere processes affecting these soils. We have collected a series of CaCO3 encrustations found underneath surface rocks in the soils and tills of Taylor Valley, McMurdo Dry Valleys (~78°S lat.), Antarctica. These encrustations were analyzed for 87Sr/86S and δ18O and δ13C to determine what relation they have with the underlying soils, and the material in which they are in contact, and to identify the processes that control their formation. In all but one case, the isotopic data indicate that the source of Sr to these encrustations is not from the rock on which it is associated. The primary source of Sr (and by analogy Ca) is either from dust that has been deposited through aeolian processes or from the aggregate of till material within the soils. The δ13C values for Taylor Valley encrustations ranged from 5.7 to 11.0‰, and are consistent with a carbon source from atmospheric CO<sub>2</sub>. The δ18O values range from –8.1 to –11.2‰ and are heavier than expected for equilibrium calcite precipitation from Taylor Valley meteoric water. Taken together these results indicate that the CaCO<sub>3</sub> was formed by rapid evaporation of films beneath clasts that had become supersaturated with respect to CaCO<sub>3</sub>.

Weichselian sedimentary record and ice-flow patterns in the Sodankylä area, central Finnish Lapland

Three different till units separated by interstadial fluvial deposits were observed in the Sodankylä area in the River Kitinen valley, northern Finland. The interbedded glaciofluvial sediments and palaeosol were dated by OSL to the Early (79±12 to 67±13 ka) and Middle (41±9 ka) Weichselian. A LiDAR DEM, glacial lineations, the flow direction of till fabrics, esker chains and striations were applied to investigate the glacial flow patterns of the Sodankylä, Kittilä and Salla areas. The analysis revealed that the youngest movement of the Scandinavian Ice Sheet is not visible as DEM lineations within the studied areas. The modern morphology in Kittilä and Salla shows streamlined landforms of various dimensions mainly oriented from the NW and NNW, respectively, corresponding to the Early/Middle Weichselian ice-flow directions inferred from till fabrics. The Late Weichselian ice flow has produced an insignificant imprint on the landforms. This study suggests a northern location for the ice-divide zone during the Early/Middle Weichselian, and a more western–southwestern position during the Late Weichselian. The OSL ages of 14±3.3 ka from the aeolian deposits may indicate ice-free areas during the Bølling–Allerod warm period in the vicinity of the River Kitinen.

Stratigraphic Position of Tills in the Orłowo Cliff Section (Northern Poland): A New Approach

Abstract The site at Orłowo Cliff was used to analyse the stratigraphic position and palaeogeographic interpretations of the properties and depositional conditions of two basal tills from the Late Pleistocene. A multi-proxy approach involved lithofacies, petrographic analysis of the fine gravel fraction, analyses of indicator erratics and till fabric. TL dating of intra-moraine deposits was used to determine depositional time frames of tills. The sediment profile at Orłowo Cliff shows a distinct reduction in number of Pleistocene units. Obtained dating results suggest the presence of Middle and Late Pleistocene fluvial units. The main issue discussed is the stratigraphic position of the older till (Unit O-4). It can be assumed that this till was deposited probably during the Middle Weichselian (MIS4). At Orłowo Horn the till of Unit O-4 reveals incorporation of the erratic material derived from an older till in the surrounded area (according to petrographic composition - probably from MIS 8). The younger till (Unit O-6) was deposited in the Late Weichselian (MIS 2). Moreover, the till of Unit O-6 is characterised by a significant shift towards the south-west in terms of the erratic origin in Unit O-4.

Ice-flow directions of the last Scandinavian Ice Sheet in central Poland

The article presents the results of studies on the ice-flow directions of the last Scandinavian Ice Sheet (SIS) in central Poland, at the southern periphery of the SIS, and particularly within the Płock lobe. The ice-flow pattern was derived mainly from direct indices, such as: till fabric; vergence of glaciotectonic compression structures; orientation of small-scale subglacial deformations formed beneath the flowing ice; orientation of pavement stone longitudinal axes; stone lee ends; and clast surface striae. The study is supplemented with a reconstruction of ice-movement directions based on orientation analysis of both concave (tunnel valley) and convex (eskers) glacial morpholineaments (linear glacial landforms). Till age was also determined, using luminescence (OSL) datings from sand deposits. These were collected directly at numerous sites, from under and above the last SIS till unit. Combined with previously published data, the results of the present investigation of the last ice-sheet advance during the Late Weichselian (ca 23–18 ka) indicate that there were two different ice-flow phases and that the maximum extent was asynchronous. The western part of the said area experienced two advances during the Leszno (Brandenburg) and Poznań (Frankfurt) phases, while during the earlier one the ice masses came from the NW, presumably along the eastern limb of the Odranian (B2 after Punkari 1997) ice stream, and then from the N, along the Vistulian (B3) stream. The last SIS occupied the eastern sector of the area only once, during the younger Poznań phase, building the Płock lobe. The lobe-shaped ice margin with a fan-like flow pattern within the lobe may be indicative of a palaeo-ice stream terminal zone.

Novel volumetric Helmholtz free energy function accounting for isotropic cavitation at finite strains

Cavitation in rubber-like materials is commonly known as sudden void growth under hydrostatic tension till material failure occurs. Experimental investigations of adhesives, e.g. structural silicones accounting for cavitation in combination with the numerical treatment of this phenomenon are rare. Accordingly, this paper presents a micro-mechanically motivated constitutive model accounting for isotropic void growth. It was developed based on numerical homogenization schemes of a cube with an incompressible matrix containing a single, vacuous void in the center. To differentiate whether an initial void is growing, a new developed void growth criterion is presented. The void growth criterion was coupled with the new developed volumetric Helmholtz free energy function to extend the classical volumetric-deviatoric split to large volume strains. Experiments were performed with a structural silicone under uniaxial tension as well as so-called pancake tests and compared with the new developed volumetric constitutive model accounting for isotropic cavitation. To observe qualitatively cavitation during experiments, a new testing device was developed, which enables one to characterize this damaging effect of the adhesive bonding. The numerical validation shows a good approximation of the experimental results. In order to improve the simulation results, an optimization study on the constitutive parameters was conducted.

Directional properties of glacial relief and sediments as an effect of multi-stage evolution: Case study of the Tczew Hump, northern Poland

This paper focuses on the upland territory south-west of the Gulf of Gdańsk characterized by the existence of numerous hills with distinct ordering of their crest-line orientation. Characteristic features of relief in the Tczew Hump were analysed using GIS-based tools. Directional properties and morphometric characteristics are not uniform throughout the study area. Therefore, subsequent detailed analyses were performed after dividing it into three zones: north-western (NW), central and south-eastern (SE). The highest hills are situated in the central part of the Hump. The landforms analysed are mostly elongated, but they show varying elongation ratios (1.0–4.05). In general a NE-SW orientation is observed, however, the azimuthal distribution of their crest-lines shows considerable variation (from N-S to E-W). Two main systems of the crest-line orientation are distinguished: System A (NNE-SSW) and System B (NE-SW/ENE-WSW).The landforms considered are covered with till consisting of two subunits, what reflects two subsequent ice-sheet movement periods in the Late Weichselian but without releasing the area from ice-sheet cover. The subunits can be identified by changes in their petrographic composition – up to ten times more erratics of local provenance in the upper one than in the lower one. Ice flow direction during deposition of both till subunits, based on the till fabric of each of the three analysed sections, coincides with System A of the crest-lines. However, most of the till-covered hills fit System B, which suggests that they retained their previous (older than Late Weichselian) orientation. The complex relief of the Tczew Hump has a palimpsest nature and its arrangement emerged in several stages: (1) formation of the primeval relief before the Late Weichselian (hills built of sediments of heterogeneous ages); (2) partial relief reshaping in subglacial conditions during the two, successive, ice-flow events in the Late Weichselian (till-covered hills, clear bipartition of the till); (3) development of additional landforms during deglaciation − typically not covered with till (or with flow till on the slopes only), showing an orientation determined by the crevasse system developed in the ice.

Elsterian ice sheet dynamics in a topographically varied area (southern part of the Racibórz–Oświęcim Basin and its vicinity, southern Poland)

During the Elsterian Glaciation, the Scandinavian Ice Sheet, before reaching its maximum extent, advanced over the mountainous foreland of Southern Poland, which was characterized by a substrate of increasingly varied relief. One such area was the hilly Rybnik Plateau of the southern part of the Raciborz–Oświecim Basin, located directly north of the Ostrava Basin and Moravian Gate, where the ice sheet advanced far south and formed a large lobe. In this study, till from the Łaziska site, located in southern part of the Rybnik Plateau, was analysed and interpreted. The till was deposited at the top of an W–E oriented ridge composed of Neogene clays. The study indicates that a high basal water pressure occurred at the base of the ice sheet, favouring a basal mechanism of ice sheet movement. Till fabric and kinematic structures indicate that ice flowed easily from the west over the ridge, parallel to its axis. Reconstructed patterns of ice flow direction reflect an ice sheet that was characterized by a strong spatially varied dynamics. The Odra Valley was the main corridor of fast flowing ice to the Ostrava Basin, from where ice was distributed radially in different directions. The second corridor of actively flowing ice was probably located in the Ruda palaeovalley to the north of the Rybnik Plateau. In the central part of the Rybnik Plateau, ice flow was restricted. This less dynamic ice sheet behaviour resulted mostly from the much higher location of the area. This study indicates that the large morphological and lithological variations of the ice sheet substrate induced varied frictional resistance, producing a spatially varied stress field within the ice sheet. This generated active zones of streaming ice adjacent to more passive zones. Potential switching between neighbouring active zones transporting ice towards the ice sheet margin is postulated.

Metal speciation in soil at a contaminated glassworks site in southeastern Sweden

Risk assessments of metal contaminated land are often based on total metal concentrations of the soil material, although it is well known that the potential for leaching is determined by the fractionation between labile metal forms and forms that are not to be assessed as geochemically active. In the present study, a 4-step sequential leaching procedure was applied to solid material from five subareas of a glass waste site in southeastern Sweden (including, for example, landfill areas and natural till material). The aim was to assess whether the major fraction of the total Cd and Pb content should be considered labile (easily mobilizable) or relatively stably sorbed, which is a question that is motivated by the fact that previous site-specific investigations have shown strongly elevated metal concentrations in soil, but very little impact on local groundwater. Although the total metal concentrations as well as the amount of visable glass particles differed between the subareas, the fractions extracted in the different steps of the sequential leach were similar for all areas. Only a minor part of the total content of both Cd and Pb was released in the final strong acid residual leach (9-19 %), which indicates that there has been a significant release of metal ions from the glass waste material in the past, followed by a transition to secondary phases. Of the two metals, Cd was assessed to be the most labile one. The first extraction, targeting the exchangeable and carbonate bound fraction, released 43-59 % of the total Cd content. So, although the relatively small effect seen on groundwater quality indicates an effective retention of these metal cations by the soil solid matrix, the relatively high fraction of potentially mobile forms highlights the need of further assessments of the stability in major retention mechanisms.

An anisotropy of magnetic susceptibility (AMS) fabric record of till kinematics within a Late Weichselian low Baltic till, southern Sweden

Herein we report on the results of an anisotropy of magnetic susceptibility (AMS) fabric case‐study of two Late Weichselian tills exposed in a bedrock quarry in Dalby, Skåne, southern Sweden. The region possesses a complex glacial history, reflecting alternating and interacting advances of the main body of the Scandinavian Ice Sheet (SIS) and its ice lobes from the Baltic basin, perhaps driven by streaming ice. AMS till fabrics are robust indicators of ice‐flow history and till kinematics, and provide a unique tool to investigate till kinematics within and amongst till units. The till section investigated here contains ~8 m of the Dalby Till – a dark grey silt‐clay rich till deposited during one or more Baltic advance – overlain by ~1.5 m of the regional surface diamicton. AMS fabrics within the lower part of the Dalby Till conform to the regional surface fluting, and reflect sustained flow from the ENE with progressive increases in basal strain. A boulder‐rich horizon approximately 3 m from the base of the till marks a restricted excursion in till fabric direction, fabric strength and style of strain. Ice flow is from the SW and W in the upper section. We interpret these fabrics to record shifting ice flow and bed conditions at the margins of the Young Baltic Advance ice lobe in southern Sweden, prior to a short‐lived re‐advance of the main body of the SIS over mainland Sweden recorded by the surface diamicton.

Multiple subglacial till deposition: A modern exemplar for Quaternary palaeoglaciology

The sedimentology of a vertical succession of alternating beds of massive and fissile diamictons on a þorisjökull plateau icefield outlet foreland is employed to assess the evolution of subglacial traction tills at the margins of active temperate glaciers with deformable substrates, where tills appear to form sub-marginal incrementally thickened wedges that display multiple till emplacement events, potentially of annual scale. Lodged boulders display strong A-axes and surface striae alignments which parallel surface flutings, indicating that fluting construction and till emplacement were related to moulding by consistent glacier flow from the SSW during the Little Ice Age. In contrast, clast macrofabrics at the sub-boulder size, not unlike those reported from other Icelandic tills, are not as strong as would be expected in a subglacially sheared medium, indicating shear strains too low for a steady state strain signature. This separation of fabric data has isolated the strain signatures of the lodgement and deformation components of subglacial traction till, whereby the orientations of the largest, lodged clasts record high cumulative shear strains and those of the sub-boulder sized clasts record greater susceptibility to deformation of their enclosing matrix. This is likely due to the effect of clast collisions in clast rich till and the perturbations set up by the numerous large boulders, consistent with observations on till fabrics in flutings and around lodged clasts. A/B plane macrofabric data display unusually high degrees of isotropy, reflective of the more variable orientations of A/B planes, which are thought to reflect A/B plane susceptibility to dip parallel or anastomosing shear planes. A wide range of clast angularity values reflects the localized input of freshly plucked and hence relatively highly angular blocks to the deforming layer, a characteristic of stepped bedrock profiles beneath the snouts of mountain glaciers. Finally, we hypothesize that the massive and fissile units may represent A and B horizons of subglacial deforming layer couplets, and that each couplet could record seasonal emplacement and partial inter-couplet modification near the ice margin. If the latter is the case, then less than 1 m of subglacial till is advected to the glacier margin per (annual?) deformation event.

Unravelling the complex nature of the Upper Weichselian till section at Gdynia Babie Doły, northern Poland

Abstract The complexity of glacial sequences may increase when these formed underneath ice sheets despite subsequent changes in their extent that are accompanied by alterations in the direction of the ice flow. Our aim was to determine whether or not changes in ice sheet dynamics during the Late Weichselian are also recorded in sediments formed north of the area of its fluctuating margin (i.e., where the ice sheet prevailed independent of such fluctuations). It is shown that in these areas such a record could have occurred, as documented by results of till studies at Babie Doły. The examination was carried out using several analyses: lithofacies properties of sediments, petrographic till composition (fine gravel fraction, indicator erratics), till matrix CaCO3 content, till fabric, as well as orientation of striae on the top surfaces of large clasts. In parallel, datings of sub- and supra-till sediments using the TL method were carried out. The basal till at Babie Doły represents almost the entire Upper Weichselian, but it can be divided into subunits whose features indicate different ice flow directions and debris supply. The lower subunit developed as a result of the palaeo-ice stream along the main axis of the Baltic Sea (from the north), expanding to areas adjacent to the depression of the Gulf of Gdańsk. The upper subunit developed when the influence of the palaeo-ice stream in the study area decreased, the main role having been taken over by the ice flowing from the northwest. The till analysed also shows considerable lateral variation, indicative of the mosaic nature of subglacial sedimentation. We consider the diversity of permeability of deposits over which the ice sheet extended to be the prime factor that determined such a situation.

An anisotropy of magnetic susceptibility (AMS) investigation of the till fabric of drumlins: support for an accretionary origin

This paper describes the results of a spatially dense anisotropy of magnetic susceptibility (AMS) till fabric study of a single drumlin in the Weedsport Drumlin Field, New York State, USA. AMS till fabrics provide a robust, quantitative and unbiased approach to assess subglacial till kinematics and infer ice‐flow dynamics. The drumlin selected for this detailed investigation was systematically sampled at 18 locations to evaluate the patterns of ice flow and associated till kinematics within a drumlin and to test erosional vs. depositional models for its formation. AMS till fabric analysis yielded strong fabrics that increase in strength towards the drumlin crest, indicating that bed deformation occurred during till deposition and that deformation within the drumlin was greater than that in the interdrumlin low. Fabric orientations reveal drumlin convergent, divergent and parallel ice‐flow paths that illustrate a complex interaction between ice flow and the drumlin form; fabric strength and shape reveal systematic differences in bed deformation between the interdrumlin and drumlin regions. These observations are inconsistent with purely erosional models of drumlin genesis; instead, these observations are more consistent with syndepositional streamlining of till transported, probably locally as a deforming bed, from the interdrumlin low towards the drumlin locality.