Till Wedges Research Articles

Submarginal debris transport and till formation in active temperate glacier systems: The southeast Iceland type locality

Exhaustive sedimentological analysis of freshly exposed subglacial surfaces and moraines in southern Iceland provides diagnostic sedimentological signatures of: a) debris transport pathways through active temperate glacier snouts; and b) till production in subglacial traction zones dominated by deforming layers. Three till end members are recognised based on stratigraphic architecture: 1) thin and patchy tills over eroded bedrock; 2) single push moraines and complexes; and 3) overridden moraines or outwash fans. Typical till thicknesses are 0.10–1.40 m, with each till relating to a deformation event driven by the seasonally tuned processes of glacier sub-marginal shearing, freeze-on, squeezing and bulldozing. Clast form trends demonstrate progressive modification towards mature forms in subglacial traction zones with till being clearly differentiated from scree and glacifluvial deposits. Clast macrofabric strengths are variable, rarely matching those of laboratory shearing experiments, except where obviously lodged clasts are abundant. They also consistently record former glacier flow directions. But localized variability is introduced by bedrock protuberances, cavity infill, clast interference and freshly imported plucked clasts. Within tills, macrofabrics strengthen from lower (B horizon) to upper (A horizon) tills but at the outer edges of sub-marginally thickening till wedges or push moraines, seasonally-driven cycles of squeezing/flowage, freeze-on/melt-out and bulldozing give rise to a range of clast macrofabric strengths as well as superimposed deformation signatures. This reflects two extremes of till emplacement including the more mobile, flowing and often liquefied matrixes in push/squeeze moraines and, in contrast, the lodgement, deformation and ploughing at the thin end of sub-marginal till wedges.

Phased occupation and retreat of the last British–Irish Ice Sheet in the southern North Sea; geomorphic and seismostratigraphic evidence of a dynamic ice lobe

Along the terrestrial margin of the southern North Sea, previous studies of the MIS 2 glaciation impacting eastern Britain have played a significant role in the development of principles relating to ice sheet dynamics (e.g. deformable beds), and the practice of reconstructing the style, timing, and spatial configuration of palaeo-ice sheets. These detailed terrestrially-based findings have however relied on observations made from only the outer edges of the former ice mass, as the North Sea Lobe (NSL) of the British-Irish Ice Sheet (BIIS) occupied an area that is now almost entirely submarine (c.21–15 ka). Compounded by the fact that marine-acquired data have been primarily of insufficient quality and density, the configuration and behaviour of the last BIIS in the southern North Sea remains surprisingly poorly constrained.This paper presents analysis of a new, integrated set of extensive seabed geomorphological and seismo-stratigraphic observations that both advances the principles developed previously onshore (e.g. multiple advance and retreat cycles), and provides a more detailed and accurate reconstruction of the BIIS at its southern-most extent in the North Sea. A new bathymetry compilation of the region reveals a series of broad sedimentary wedges and associated moraines that represent several terminal positions of the NSL. These former still-stand ice margins (1–4) are also found to relate to newly-identified architectural patterns (shallow stacked sedimentary wedges) in the region's seismic stratigraphy (previously mapped singularly as the Bolders Bank Formation). With ground-truthing constraint provided by sediment cores, these wedges are interpreted as sub-marginal till wedges, formed by complex subglacial accretionary processes that resulted in till thickening towards the former ice-sheet margins. The newly sub-divided shallow seismic stratigraphy (at least five units) also provides an indication of the relative event chronology of the NSL. While there is a general record of south-to-north retreat, seismic data also indicate episodes of ice-sheet re-advance suggestive of an oscillating margin (e.g. MIS 2 maximum not related to first incursion of ice into region). Demonstrating further landform interdependence, geographically-grouped sets of tunnel valleys are shown to be genetically related to these individual ice margins, providing clear insight into how meltwater drainage was organised at the evolving termini of this dynamic ice lobe. The newly reconstructed offshore ice margins are found to be well correlated with previously observed terrestrial limits in Lincolnshire and E. Yorkshire (Holderness) (e.g. MIS 2 maximum and Withernsea Till). This reconstruction will hopefully provide a useful framework for studies targeting the climatic, mass-balance, and external glaciological factors (i.e. Fennoscandian Ice Sheet) that influenced late-stage advance and deglaciation, important for accurately characterising both modern and palaeo-ice sheets.

Fláajökull (north lobe), Iceland: active temperate piedmont lobe glacial landsystem

ABSTRACTA 1:6250 map of the foreland of Fláajökull's north lobe as it appeared in 1989, together with a 1:350 scale map of a sample area of recently exposed glacial landforms from 2014, enables an assessment of the spatial and temporal evolution of glacial landform assemblages at the margin of an active temperate piedmont lobe terminating at ice-marginal thickening till wedges. The pattern of landform development captured in these maps indicates that the glacier margin developed strong longitudinal crevassing and well-developed ice-marginal pecten (three-dimensional crenulations) during its historical recession. This is recorded by early recessional phase linear push moraines on well-drained distal slopes of the foreland and the later development of interrelated sawtooth moraines, crevasse squeeze ridges and till eskers, indicative of extending ice flow and poorly drained submarginal conditions. This landform record is a palaeoglaciological signature of a changing process–form regime inherent within the active temperate piedmont lobe landsystem model.

Widespread, multi-source glacial erosion on the Chukchi margin, Arctic Ocean

Multibeam bathymetry and sub-bottom profiler data acquired in 2011 from R/V Marcus Langseth in a broad grid over the Chukchi Sea margin reveal multiple glacigenic features on the top and slopes of the outer Chukchi Shelf/Rise and adjacent Borderland. Glacial lineations record a complex pattern of erosion likely formed by both local glaciation and far-traveled ice shelves/streams sourced from the Laurentide, and possibly East Siberian ice sheets. Multiple till units and stacked debris flows indicate recurrent glacial grounding events. Composite till wedges of several hundred meters thick extend the shelf edge by 10–20 km in places. Distribution of ice-marginal features on the Chukchi Rise suggests stepwise deglacial retreat towards the shelf, backing up the broad bathymetric trough at the eastern side of the Rise. Glacigenic features other than extensive iceberg scouring cannot be identified above 350-m depth, and no glacigenic bedforms are present on the current-swept shallow shelf. Despite the resulting uncertainty with the southern extent of the glaciation, the data suggest a widespread grounded-ice presence on the northern Chukchi Shelf, which makes it an important, previously underestimated component of the Arctic paleo-glacial system.

Till stratigraphy and sedimentology at the margins of terrestrially terminating ice streams: case study of the western Canadian prairies and high plains

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

New insights on the post-rift seismic stratigraphic architecture and sedimentary evolution of the Antarctic Peninsula margin (Central Bransfield Basin)

A seismic stratigraphic study of the Antarctic Peninsula margin of the Central Bransfield Basin was conducted using seismic and bathymetric data. The study focused on seismic sequences and units deposited after the basin opened, and yield a record of the evolution of the northern-most Antarctic Peninsula Ice Sheet. Two seismic sequences compose the post-rift stratigraphic succession on the Central Bransfield Basin slope platforms. Both sequences are composed of seismic units that record glacial/interglacial cycles and show evidence of the Antarctic Peninsula Ice Sheet having grounded on the continental shelf and slope platform on several occasions in the past. This evidence includes glacial and glaciomarine related sedimentary and erosional features within the seismic units (glacial unconformities, glacial troughs, till wedges, prograded wedges, trough mouth fans, glaciomarine sheet deposits and moraines), which are bounded by glacial unconformities. The Antarctic Peninsula lower slope also registered in its eastern sector the deposition of gravity flow deposits during ice sheet retreat and interglacial stages. The deposition of seismic units on the middle slope produced the present-day step-like profile of the margin composed of wide flat slope platforms and narrow and steep upper and lower slopes. Seismic units show changes in thickness and distribution that record the effect of the physiography and climatic changes on margin sedimentation. These variations produced an overall change from more line-sourced deposition to more point sourced deposition as ice streams evolved. The stacking of units shows a change in the stratigraphic architecture through time, from dominantly progradational (S2) to progradational/aggradational (S1). This change is correlated with a change in the frequency of ice sheet grounding events which was probably driven by a higher rate of eustatic rise and fall in relation with the Mid Pleistocene global climatic change.

Till deposition by glacier submarginal, incremental thickening

AbstractMacro‐ and micro‐scale sedimentological analyses of recently deposited tills and complex push/squeeze moraines on the forelands of Icelandic glaciers and in a stacked till sequence at the former Younger Dryas margin of the Loch Lomond glacier lobe in Scotland are used to assess the depositional processes involved in glacier submarginal emplacement of sediment. Where subglacial meltwater is unable to flush out subglacial sediment or construct thick debris‐rich basal ice by cumulative freeze‐on processes, glacier submarginal processes are dictated by seasonal cycles of refreezing and melt‐out of tills advected from up‐ice by a combination of lodgement, deformation and ice keel and clast ploughing. Although individual till layers may display typical A and B horizon deformation characteristics, the spatially and temporally variable mosaic of subglacial processes will overprint sedimentary and structural signatures on till sequences to the extent that they would be almost impossible to classify genetically in the ancient sediment record. At the macro‐scale, Icelandic tills display moderately strong clast fabrics that conform to the ice flow directions documented by surface flutings; very strong fabrics typify unequivocally lodged clasts. Despite previous interpretations of these tills as subglacial deforming layers, micro‐morphological analysis reveals that shearing played only a partial role in the emplacement of till matrixes, and water escape and sediment flowage features are widespread. A model of submarginal incremental thickening is presented as an explanation of these data, involving till slab emplacement over several seasonal cycles. Each cycle involves: (1) late summer subglacial lodgement, bedrock and sediment plucking, subglacial deformation and ice keel ploughing; (2) early winter freeze‐on of subglacial sediment to the thin outer snout; (3) late winter readvance and failure along a decollement plane within the till, resulting in the carriage of till onto the proximal side of the previous year's push moraine; (4) early summer melt‐out of the till slab, initiating porewater migration, water escape and sediment flow and extrusion. Repeated reworking of the thin end of submarginal till wedges produces overprinted strain signatures and clast pavements. Copyright © 2005 John Wiley & Sons, Ltd.

Evidence against pervasive bed deformation during the surge of an Icelandic glacier

Abstract Hagafellsjökull Vestari, Iceland, surged in 1980 and advanced about 700 m. Subsequent retreat of the ice front has exposed a suite of streamlined bedforms and former subglacial sediments. Investigations were undertaken on the recently deglaciated forefield to explore the degree of ice-bed coupling during the surge. The morphometry of flutes at this site suggest that the ice was relatively well coupled to its bed. Widespread till wedges show that ploughing was a dominant process. These observations suggest that pervasive deformation of the sedimentary bed at high water pressures in surge was a strong possibility. However, evidence from the micromorphology of drumlin sediments shows laterally continuous clay bands preserved between till layers within a drumlin. Thus pervasive deformation of the whole sediment thickness did not occur. There is evidence only of a thin layer of deformation (<16 cm). We propose that at low effective pressures there was a reduction in the strength of the ice-bed interface rather than a reduction in sediment strength. Motion was concentrated at the sole with localized and shallow deformation where clasts ploughed through the substrate.

Groundwater flow beneath ice sheets: Part II — Its impact on glacier tectonic structures and moraine formation

Meltwater flowing as groundwater from beneath the margin of an ice sheet determines the distribution of sub-surface heads and effective pressures. A subglacial groundwater flow model is used, together with an ice sheet loading model, to compute the magnitudes and directions of the principal effective stresses in the subsurface, from which the distribution of different types of sub-surface failure in the subglacial and proglacial zones are deduced. Zones of hydrofracturing, shear fracture and pervasive shear failure are distinguished. Beneath the ice sheet divide area, intact rocks of high tensile strength may fail. Hydrofracturing and liquefaction are two coupled processes which lead to the formation of upward-filled and downward-filled sediment dykes and till wedges. Quicksand conditions are developed where strong vertical seepage pressures occur, producing sediment diapirism. It is suggested that subglacial permeability magnitude may be the product of a self organising process. Certain types of moraine (extrusion moraines) are suggested to be a consequence of upward movement and surface extrusion of sediment driven by rising groundwater. It is suggested that groundwater over pressure associated with narrow proglacial permafrost plates are conductive to the formation of large push moraines, and that many large ancient and modern examples are produced in this setting.

Small to medium-sized glacitectonic structures in till and in its substratum and their comparison with mass movement structures

Ductile and brittle deformation structures have been observed in till of various origins, particularly in subglacial till and flowtill, and in the substratum beneath subglacial till. Although some of the glacitectonic structures have been claimed to be palimpsest structures from debris-rich glacier ice, such a preservation is not proven to be very common. However, deformation structures are found in megablocks and other clasts that occur in melt-out till and in deformation till. Most glacitectonic deformation structures are formed in the substratum or in the already deposited subglacial till by an actively moving glacier, for instance recumbent folds, shears, boudins, thrusts, hook folds, diapirs, tension fractures, till wedges, fissility, glacitectonic breccia, conjugate sets of fractures. Structures similar to those reported from subglacial primary till and its substratum occur also in flowtill and other mass movement deposits, but deformations are less common in the substratum beneath them. The orientation of the deformations in flowtill is unrelated to the direction of glacial movement. The shapes of some shears and folds are different in flowtill, when compared with those in subglacial till and its substratum. Still, the flow and shear structures alone are not sufficient to distinguish certain varieties of till (fir instance subglacial primary till from flowtill) but they are useful as local directional indicators of stress and as genetic criteria in combination with other features, particularly with directional glacial erosion marks and fabric.

Deformation till in the Great Lakes region: implications for rapid flow along the south-central margin of the Laurentide Ice Sheet

Structural and lithologic data indicate that, while deposited under actively moving ice, considerable portions of three muddy calcareous subglacial tills in the Great Lakes region probably experienced some component of ductile deformation. Viscous till flow and ductile shear are invoked to explain a combination of features such as recumbent isoclinal folds, unlithified sediment clasts, mixed ostracode shells, reversed stone lee ends, girdle and transverse fabrics, irregular stone pavements, fine striae following stone curves, and inconsistent stone striae and a-axes. Deforming, fine-textured subglacial till is considered as a subhorizontal shear zone, rheologically layered with associated structures (in descending order): ductile (e.g., isoclinal folds), brittle–ductile (e.g., fissility), and brittle (e.g., till wedges). Rheology would be controlled mainly by till pore water content, matrix texture, and stone content. Spatial and temporal superposition of rheologies and subglacial processes probably occurred while some fine tills were forming. Fine deformation till may be especially common around areas of the Great Lakes region where proglacial mud and weak bedrock were remoulded as ice travelled along major basins and troughs. In such areas, under a wet-based glacier, resulting till would have been too weak to sustain a large shear stress or inhibit rapid ice flow over it. Instead, in these places, the till was probably water saturated, accounting for most of the glacial flow, and protected the substrate from extensive deformation while effectively acting as a lubricant to overriding ice. Areas of fine deformation till probably represent areas of former low subglacial fluid conductivity and rapid glacial flow. In other areas, subglacial sheet flow of meltwater may have accelerated glacial flow. These two types of areas may have been connected at times under zones of ice streaming and (or) surging.

Clastic dikes formed beneath an active glacier

Abstract Clastic dikes (dykes) consisting of diamicton and sand were observed in a diamicton bed and in an underlying bed of gravel at a site in the province of Skåne, southern Sweden. In the diamicton bed, a clastic sill made up of laminated sand and gravel also occurs. Features resembling the observed dikes, but made up of only diamicton have been named till wedges. The dikes dip 45–90° and are parallel to each other in the horizontal plane. The largest dikes are 0.8–2.5 m wide and at least 4.0–5.5 m deep. The dikes and the sill were formed beneath an active glacier which fractured the bed. The dikes became infilled with drift from above. Although the dikes and the sill may have been produced when the glacier bed was unfrozen, it seems more likely that they were formed when frozen-bed conditions prevailed.

A large till wedge in Denmark: implications for the subglacial thermal regime

A subglacial till intrusion in meltwater sediments is described from a locality in southern Denmark. The structure is interpreted as a large till wedge, developed during glacier flow from the southeast. The intrusion seems to have occurred at a time when the glaciofluvial sediments were unfrozen. The possibility of development of till wedges in frozen sediments is discussed.

Further discussion of the ‘till wedge’ concept in the light of replies from Professor Dreimanis and Dr. Mörner

Abstract The ‘cracking and filling’ mechanism as applied to the formation of ‘till wedges’ developed in frozen materials is not entirely consistent with the physical properties of ice-sediment bonds below the freezing point. Momer's dismissal of the ice-contact superglacial depositional environment for the Forslovsholm sequence is rejected on the basis of good field evidence. At that locality the structures are patently load phenomena and therefore cannot be used as palaeoglacier flow indices.

The first report on “till wedges” in Europe: A reply

Abstract The eriticism by Worsley of my paper on till wedges is based on mistakes and misunderstandings. A close examination of the argumentation reveals its weaknesses. After having been properly named and discussed by Dreimanis and by Mörner, “till wedge” (in Swedish moränkil) is now a specific geological term covering both morphology and genesis. Till wedges are formed by cracking-and-filling or squeczing-in.

A new find of till wedges in Nova Scotia, Canada

Abstract Till wedges are reported from the Milford Gypsum quarry in Nova Scotia, Canada. The till wedges are believed to have been formed by the cracking-and-filling mechanism. Till wedges have previously only been reported from Ontario, Connecticut and southern Sweden.

The first report on “till wedges” in Europe — a discussion

Abstract The significance of “till wedges”, as recently described by N.-A. Morner, is questioned. It is suggested that they are not unique features and that the evidence presented could equally well be interpreted as density re-adjustments in sediments deposited supraglacially. Thus it is doubtful whether they can be used as palaeoglacier flow vectors.

The first report on “till wedges” in Europe — a reply

(1973). The first report on “till wedges” in Europe — a reply. Geologiska Foreningen i Stockholm Forhandlingar: Vol. 95, No. 1, pp. 156-157.

The First Report on Till Wedges in Europe and Late Weichselian Ice Flows over Southern Sweden

Abstract Till wedges are reported from a gravel pit in southern Sweden. They have previously only been described from North America Till wedges are intrusions into Underlying stratified deposits, formed by cracking and filling (in frozen ground) and squeezing in (in unfrozen ground). Their orientation is an important and reliable indicator of the direction of glacial movement. Till wedges (in Swedish morankilar) therefore provide a new tool for the establishment of the directions of ice flows. The till wedges here described indicate that the first Late Weichselian ice flow over southwesternmost Sweden came from the northwest.