Geomorphological and seismostratigraphic evidence for multidirectional polyphase glaciation of the northern Celtic Sea

High-resolution seismic and bathymetric data offshore southeast Ireland and LIDaR data in County Waterford are presented that partially overlap previous studies. The observed Quaternary stratigraphic succession offshore southeast Ireland (between Dungarvan and Kilmore Quay) records a sequence of depositional and erosional events that supports regional glacial models derived from nearby coastal sediment stratigraphies and landforms. A regionally widespread, acoustically massive facies interpreted as the ‘Irish Sea Till’ infills an uneven, channelized bedrock surface overlying irregular mounds and deposits in bedrock lows that are probably earlier Pleistocene diamicts. The till is truncated and overlain by a thin, stratified facies, suggesting the development of a regional palaeolake following ice recession of the Irish Sea Ice Stream. A north–south oriented seabed ridge to the north is interpreted as an esker, representing southward flowing subglacial drainage associated with a restricted ice sheet advance of the Irish Ice Sheet onto the Celtic Sea shelf. Onshore topographic data reveal streamlined bedforms that corroborate a southerly advance of ice onto the shelf across County Waterford. The combined evidence supports previous palaeoglaciological models. Significantly, for the first time, this study defines a southern limit for a Late Midlandian Irish Ice Sheet advance onto the Celtic Sea shelf. © 2020 John Wiley & Sons, Ltd.     

Radiocarbon constraints on the age of the maximum advance of the British–Irish Ice Sheet in the Celtic Sea

Reconstructions of the British–Irish Ice Sheet (BIIS) during the Last Glacial Maximum (LGM) in the Celtic Sea and southern Ireland have been hampered by a paucity of well-dated stratigraphic records. As a result, the timing of the last advance of the largest outlet of the BIIS, the Irish Sea Ice Stream, to its maximum limit in the Celtic Sea has been variously proposed as being pre-last glaciation, Early Devensian and LGM. The Irish Sea Till was deposited by the Irish Sea Ice Stream during its last advance into the Celtic Sea. We present 26, stratigraphically well constrained, new AMS radiocarbon dates on glacially transported marine shells from the Irish Sea Till in southern Ireland, which constrain the maximum age of this advance. The youngest of these dates indicate that the BIIS advanced to its overall maximum limit in the Celtic Sea after 26,000–20,000 ¹⁴C yr BP, thus during the last glaciation. The most extensive phase of BIIS growth therefore appears to have occurred during the LGM, at least along the Celtic Sea and Irish margins. These data further demonstrate that the uppermost inland glacial tills, from the area of supposed “older drift” in southern Ireland, a region previously regarded as having been unglaciated during the LGM also date from the last glaciation. Thus most of southern Ireland was ice covered at the LGM. Advance of the BIIS to its maximum southern limit in the Celtic Sea may have been a short-lived glaciodynamic response facilitated by subglacial bed conditions, rather than a steady-state response to climate forcing alone.     

The glacigenic deposits of Western Lleyn,North Wales: Terrestrial or marine?

This paper describes a complex sequence of glacigenic sediments occupying a faultbounded depression at Aberdaron Bay, western Lleyn. The sequence offers an insight into sedimentary environments during deglaciation of the Irish Sea Basin. A lower stratified diamict association (LDA) containing contorted units of fine sand/silt and displaying strong, consistent clast fabrics, is overlain by an upper diamict association (UDA) with weaker fabrics and extensive sand and gravel layers and channel fills. Certain characteristics of the sequence can be explained by a glacimarine depositional model, but there are several problems. In particular, the geometry of the sediments is difficult to explain without recourse to the melting of buried ice. An alternative model that overcomes these problems involves the decay of a terrestrial glacier containing reworked marine sediments. Supraglacial sediment flows released during decay of thinner ice covering the surrounding hills (UDA) would have rapidly buried a thick stagnant ice mass in the Aberdaron depression, facilitating slow melting and release of basal meltout till (LDA). A model is presented that accounts for the stagnation and in situ decay of a debris-rich Irish Sea glacier, and which could explain many of the deposits and landforms surrounding the Irish Sea Basin without recourse to high relative sea-levels.     

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

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

Devensian glacigenic sedimentation and landscape evolution in the Cardigan area of southwest Wales

The depositional processes associated with late Devensian ice in areas bordering the Irish Sea basin have been the subject of considerable debate. Among the key areas around the Irish Sea, southwest Wales occupies a particularly crucial position because it is here that ice flowing from the north impinged upon the coast orthogonally and encroached inland. Two main hypotheses have emerged concerning deglaciation of the Irish Sea basin. The traditional hypothesis holds that sedimentation was ice‐marginal or subglacial, whereas an alternative hypothesis that emerged in the 1980s argued that sedimentation was glaciomarine. Southwest Wales is well‐placed to contribute to this debate. However, few detailed sedimentological studies, linked to topography, have been made previously in order to reconstruct glacial environments in this area. In this paper, evidence is presented from four boreholes drilled recently in the Cardigan area, combined with data from coastal and inland exposures in the lower Teifi valley and adjacent areas. A complex history of glaciation has emerged: (i) subglacial drainage channel formation in pre‐Devensian time, (ii) deposition of iron‐cemented breccias and conglomerates possibly during the last interglacial (or in the early/mid‐Devensian interstadial), (iii) late Devensian ice advance across the region, during which a glaciolacustrine sequence over 75 m thick accumulated, within a glacial lake known as Llyn Teifi, (iv) a second high‐level glaciolacustrine succession formed near Llandudoch, (v) outside the Teifi valley, ice‐marginal, subglacial and glaciofluvial sediments were also laid down, providing a near‐continuous cover of drift throughout the area. Glacial advance was characterized by reworking, deformation and sometimes erosion of the underlying sediments. The glaciomarine hypothesis is thus rejected for southwest Wales. Copyright © 2001 John Wiley & Sons, Ltd.     

A critical review and re-investigation of the Pleistocene deposits between Cranfield Point and Kilkeel,Northern Ireland: Implications for regional sea-level models and glacial reconstructions of the northern Irish Sea basin

The coastline of County Down includes sites that are pivotal to understanding the history of the last glaciation of the northern Irish Sea Basin in relation to relative sea level and regional glacial readvances. The cliff sections display evidence that has been used to underpin controversial models of glaciomarine sedimentation in isostatically-depressed basins followed by emergent marine and littoral environments. They also provide crucial evidence claimed to constrain millennial-scale ice sheet oscillations associated with uniquely large and rapid sea-level fluctuations. This paper reviews previous work and reports new findings that generally supports the ‘terrestrial’ model of glaciation, involving subglacial accretion and deformation of sediment beneath grounded ice. Deep troughs were incised into the till sheet during a post Late Glacial Maximum draw-down of ice into the Irish Sea Basin. Ice retreat was accompanied by glaciomarine accretion of mud in the troughs during a period of high relative sea level. The trough-fills were over-ridden, compacted, deformed and truncated during a glacial re-advance that is correlated with the Clogher Head Readvance. Grounding-line retreat accompanied by rapid subaqueous ice-proximal sedimentation preserved a widespread subglacial stone pavement. Raised beach gravels cap the sequence. The evidence supports an uninterrupted fall in relative sea level from c. 30?m that is consistent with sea level curves predicted by current glacio-isostatic adjustment modelling. Critical evidence previously cited in support of subaerial dissection of the troughs, and hence rapid fall and rise in relative sea level prior to the deposition of the glaciomarine muds, is not justified.     

Provenance and depositional environments of Quaternary sediments from the western North Sea Basin

As the majority of the data on Quaternary sediments from the North Sea Basin are seismostratigraphical, we analysed the Elsterian Swarte Bank Formation, the Late Saalian Fisher Formation and the Late Weichselian (Dimlington Stadial) Bolders Bank Formation in order to determine genesis and provenance. The Swarte Bank Formation is a subglacial till containing palynomorphs from the Moray Forth and the northeastern North Sea, and metamorphic heavy minerals from the Scottish Highlands. The Fisher Formation was sampled from the northern and central North Sea. In the north, it is interpreted as a subglacial till, with glaciomarine sediments cropping out further south. These sediments exhibit a provenance signature consistent with the Midland Valley of Scotland, the Eocene of the North Sea Basin, the Grampian Highlands and northeast Scotland. The Bolders Bank Formation is a subglacial till containing palynomorphs from the Midland Valley of Scotland, northern Britain, and a metamorphic heavy‐mineral suite indicative of the Grampian Highlands, Southern Uplands and northeast Scotland. These data demonstrate that there was repeated glaciation of the North Sea Basin during the Middle and Late Pleistocene, with ice sheets originating in northern Scotland. There was no evidence for a Scandinavian ice sheet in the western North Sea basin. Copyright © 2011 John Wiley & Sons, Ltd.     

Sedimentation and Iithostratigraphy of the North Sea Drift and Lowestoft Till Formations in the Coastal cliffs of northeast Norfolk,England

The most complete terrestrial sequence of Anglian (Elsterian) glacial sediments in western Europe was investigated in northeast Norfolk, England in order to reconstruct the evolution of the contemporary palaeoenvironments. Lithostratigraphically the glacial sediments in the northeast Norfolk coastal cliffs can be divided into the Northn Sea Drift and Lowestoft Till Formations. Three of the diamicton members of the North Sea Drift Formation (Happisburgh, Walcott and Cromer Diamictons) were deposited as lodgement and/or subglacial deformation till by grounded ice, but one, the Mundesley Diamicton, is waterlain and was deposited in an extensive glacial lake. Sands and fine sediments interbedded between the diamictons represent deltaic sands and glaciolacustrine sediments derived not solely from the melting ice in the north but also from extra-marginal rivers in the south. The Lowestoft Till Formation is not well preserved in the cliffs but includes lodgement till (Marly Drift till) and, most probably, associated meltwater deposits. Extensive glaciotectonism in the northern part of the area is shown to relate to oscillating ice that deposited the Cromer Diamicton and also partially to the ice sheet that deposited the Marly Drift till. It is suggested that during the Anglian Stage the present day northeast Norfolk coast was situated on the northwestern margin of an extensive glaciolacustrine basin. This basin was dammed by the Scandinavian ice sheet in the north and northeast. Because the grounding line of this ice sheet oscillated in space and time, part of the North Sea Drift diamictons were deposited directly by this ice. However, during ice retreat phases glaciolacustrine deposition comprised waterlain diamicton, sands and fines. When the Scandinavian ice sheet was situated in northernmost Norfolk, the British ice sheet (responsible for depositing the Marly Drift facies) entered the area from the west. This ice sheet partially deformed the North Sea Drift Formation sediments in the northern part of the area but not in the south, where the British ice sheet apparently terminated in water. The interplay of these two ice sheets on the northern and western margins of the glacial lake is thought to be the major determining factor for the accumulation of thick glacial deposits in this area during the Anglian glaciation.     

Ice directions in western lleyn and the status of the gwynedd readvance of the last irish sea glacier

A ‘Gwynedd readvance’ of the last Irish Sea glacier, terminating on the north coast of the Lleyn peninsula in northwest Wales, has been proposed on the basis of differences in ice flow direction and in drift landforms on either side of the proposed limit. However, detailed drift mapping of western Lleyn reveals erosional and depositional evidence of ice flow direction which are consistent with a single advance and retreat. Differences in drift landforms are explained by the influence of topography on glacier dynamics and sedimentation. There is, therefore, no reason to invoke a ‘Gwynedd readvance’ of the last Irish Sea glacier and the marked climatic deterioration which a substantial readvance of such a large glacier would imply.     

The Late Devensian (<22,000 BP) Irish Sea Basin: The sedimentary record of a collapsed ice sheet margin

The Late Devensian (<20 ka BP) glacial geology of the Irish Sea Basin (4000 km²) is an event stratigraphy recording the entry of marine waters into a glacio-isostatically-depressed basin, and the rapid retreat of the Irish Sea Glacier as a tidewater ice margin. Marine limits occur up to 140 m O.D. Across much of the central basin, the ice margin was uncoupled from its bed exposing a subglacially-scoured topography to glaciomarine processes. The Irish Sea Glacier was a major drainage conduit of the last British Ice Sheet; calving of the marine ice margin resulted in fast flow (surging) of ice streams recorded by drumlin fields around the northern basin margin and tunnel valleys. Rapid evacuation of the basin may have stranded large areas of dead ice in peripheral zones (e.g. Cheshire/Shropshire Lowlands) and initiated the collapse of the ice sheet.Thick wedges of ice-contact glaciomarine sediments were deposited during ice retreat as morainal bank complexes by successive tidewater ice margins stabilized at pinning points around the Irish Sea coast. Where morainal banks occur on the seaward side of drumlin swarms there is a clear sequential relationship between rapid ice loss from calving ice margins, the development of fast flowing ice streams, drumlinization and the pumping of subglacial sediment to tidewater. Raised delta complexes are locally associated with marine limits along the high relief coastal margins of Wales, east central Ireland, and the Lake District. Associated valley infill complexes record downslope resedimentation of heterogenous sediments into the marine environment during ice retreat. Co-eval offshore deposits are represented by well-stratified glaciomarine complexes that infill a subglacially-scoured topography that shows networks of tunnel valleys. Glaciomarine mud drapes occur well to the south of the maximum limit of grounded ice in the basin (e.g. North Devon, Scilly Islands, Southern Ireland). The age of these distal sediments, previously mapped as pre-Devensian tills, is constrained by amino acid ratios.Basin rebound following deglaciation was rapid, with over 100 m recovery in 3 ka, and was followed by a low marine still stand. Peat, accumulating in offshore areas now as much as 55 m below sea level has been drowned by the postglacial eustatic rise in sea level.The glacio-sedimentary model identified in this paper, involving rapid ice retreat and related sedimentation triggered by rising relative sea level, suggests that isotatic downwarping is an important mechanism for deglaciating continental shelves.     

Glacial deposits at Wylfa Head,Anglesey, North Wales: Evidence for Late Devensian deposition in a non-marine environment

The nature and origin of glacial sediments at Wylfa Head are described, and their significance with regard to sedimentary environments during Late Devensian deglaciation of the Irish Sea Basin is discussed. Recent models of deglaciation under glaciomarine conditions are challenged. The Quaternary sequence at Wylfa consists of eroded and glaciotectonically deformed bedrock, locally derived lodgement till, calcareous silt-rich lodgement till containing northern erratics, discontinuous units of orange-brown silty sand of possible aeolian origin, and grey laminated freshwater silts filling a small kettle hole. The till units thicken to the south where the surface is drumlinised. It is concluded that the landforms and deposits result from a warm-based Irish Sea glacier, which moved towards the southwest. Spatial variation in basal water pressure resulted from localised drainage through zones of more heavily jointed bedrock. Rapid glacial erosion occurred in areas where subglacial water pressure was relatively high, while deposition of the resulting basal sediment took place where water pressures were reduced. The glacier also carried basal calcareous silty till onshore, which was deposited by lodgement processes. None of the deposits at Wylfa are interpreted as glaciomarine in origin, and there is no evidence at this site for an isostatically induced marine transgression prior to deglaciation.     

Sea‐level change in the Irish Sea since the Last Glacial Maximum: constraints from isostatic modelling

Models of glacio‐hydroisostatic sea‐level change have been published for the British Isles that are broadly consistent with the observational evidence, as well as with glaciological constraints. It has been argued, however, that the models fail to represent sea‐level change along the Irish Sea margins and in southern Ireland for the post‐deglaciation period. The argument rests on the interpretation of the depositional environment of the elevated ‘Irish Sea Drift’ on both sides of the Irish Sea: whether this is terrestrial or glaciomarine. The isostatic models for the British Isles are consistent with the former interpretation in that sea‐levels on either side of the Irish Sea, south of about the Isle of Man, are not predicted to have risen above present sea‐level at any time since the deglaciation of the Irish Sea. This implies that ice over both the Irish Sea and Ireland was relatively thin (ca. 600–700 m over Ireland). If the glaciomarine interpretation of the elevated Irish Sea Drift is correct, then the maximum ice thickness over central and southern Ireland would have to reach 2000 m, exceeding that over Scotland. Furthermore, for the resulting sea‐level change to be consistent with the Holocene evidence, this thick ice sheet could not have extended to the eastern side of the Irish Sea. Nor could it have been very thick at its northern and western limits. If such an ice model is extreme and incompatible with glaciological observations then the alternative is to accept the interpretation of the Irish Sea Drift as terrestrial in origin. Copyright © 2001 John Wiley & Sons, Ltd.     

Micromorphological evidence supporting Late Weichselian glaciation of the Northern North Sea

Thirteen samples from three cores and boreholes are examined using micromorphology to test existing interpretations of Late Quaternary sedimentary sequences from the Norwegian Channel, North Sea Fan and the North Sea Plateau. Previous studies have interpreted these sediments using arbitrary parameters as reflecting Late Weichselian subglacial and glacimarine conditions associated with the Scandinavian Ice Sheet and Norwegian Channel ice stream. This study develops existing micromorphological criteria to interpret the samples as reflecting specific processes of subglacial deformation and proximal and distal glacimarine sedimentation during and subsequent to the Last Glacial Maximum. The study concludes by outlining diagnostic criteria for the identification of these sediment types from core and borehole samples of other Quaternary sediments.     

The Late Devensian glaciation along the western margin of the Cheshire-Shroshire Iowland

The glacial succession in the western part of the Cheshire-Shropshire lowland records the advance, coalescence and subsequent uncoupling of Irish Sea and Welsh ice-sheets during the Late Devensian stage. During advance a discontinuous sheet of basal till was emplaced across the floor of the region by subglacial lodgement. On retreat, compression of the Irish Sea ice sheet against bedrock obstruction generated a zone of supraglacial sedimentation resulting in the creation of the Wrexham-Ellesmere-Wem-Whitchurch moraine system, and the formation of a wide range of sedimentary environments, including ice-marginal sandur troughs, ice-front alluvial fans, proglacial ribbon sandur, and subglacial, ice-contact and proglacial lakes. The geometry of sedimentary units, and their lithologic and geomorphic characteristics, display spatially ordered patterns of sediment-landform assemblage which show that the statigraphic succession is a response to rapidly changing depositional conditions at a retreating supraglacial ice-margin punctuated by minor still-stands and ice-front oscillations.     

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

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

Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea

Passchier, S., Laban, C., Mesdag, C.S. & Rijsdijk, K.F. 2010: Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea. Boreas, Vol. 39, pp. 633–647. 10.1111/j.1502‐3885.2009.00138.x. ISSN 0300‐9483. Changes in subglacial bed conditions through multiple glaciations and their effect on ice dynamics are addressed through an analysis of glacigenic sequences in the Upper Pleistocene stratigraphy of the southern North Sea basin. During Elsterian (MIS 12) ice growth, till deposition was subdued when ice became stagnant over a permeable substrate of fluvial sediments, and meltwater infiltrated into the bed. Headward erosion during glacial retreat produced a dense network of glacial valleys up to several hundreds of metres deep. A Saalian (MIS 6) glacial advance phase resulted in the deposition of a sheet of stiff sandy tills and terminal moraines. Meltwater was at least partially evacuated through the till layer, resulting in the development of a rigid bed. During the later part of the Saalian glaciation, ice‐stream inception can be related to the development of a glacial lake to the north and west of the study area. The presence of meltwater channels incised into the floors of glacial troughs is indicative of high subglacial water pressures, which may have played a role in the onset of ice streaming. We speculate that streaming ice flow in the later part of the Saalian glaciation caused the relatively early deglaciation, as recorded in the Amsterdam Terminal borehole. These results suggest that changing subglacial bed conditions through glacial cycles could have a strong impact on ice dynamics and require consideration in ice‐sheet reconstructions.     

Weichselian stratigraphy and palaeoenvironments at Bellsund, western Svalbard

A coastal cliff facing the ocean at the west coast of Spitsbergen has been studied, and seven formations of Weichselian and Holocene age have been identified. A reconstruction of the palaeoenvironment and glacial history shows that most of the sediments cover isotope stage 5. From the base of the section, the formation 1 and 2 tills show a regional glaciation that reached the continental shelf shortly after the Eemian. Formation 3 consists of glacimarine to marine sediments dated to 105,000–90,000 BP. Amino acid diagenesis indicates that they were deposited during a c . 10,000-year period of continuous isostatic depression, which indicates contemporaneous glacial loading in the Barents Sea. Foraminifera and molluscs show influx of Atlantic water masses along the west coast of Svalbard at the same time. Local glaciers advanced during the latter part of this period, probably due to the penetration of moist air masses, and deposited formation 4. A widespread weathering horizon shows that the glacial retreat was succeeded by subaerial conditions during the Middle Weichselian. Formation 5 is a till deposited during the Late Weichselian glacial maximum in this area. The glaciation was dominated by ice streams from a dome over southern Spitsbergen, and the last deglaciation of the outer coast is dated to 13,000 BP. A correlation of the events with other areas on Svalbard is discussed, and at least two periods of glaciation in the Barents Sea during the Weichselian are suggested.     

Sedimentological and deformational criteria for discriminating subglaciofluvial deposits from subaqueous ice‐contact fan deposits: A Pleistocene example (Ireland)

A pit located near Ballyhorsey, 28 km south of Dublin (eastern Ireland), displays subglacially deposited glaciofluvial sediments passing upwards into proglacial subaqueous ice‐contact fan deposits. The coexistence of these two different depositional environments at the same location will help with differentiation between two very similar and easily confused glacial lithofacies. The lowermost sediments show aggrading subglacial deposits indicating a constrained accommodation space, mainly controlled by the position of an overlying ice roof during ice‐bed decoupling. These sediments are characterized by vertically stacked tills with large lenses of tabular to channelized sorted sediments. The sorted sediments consist of fine‐grained laminated facies, cross‐laminated sand and channelized gravels, and are interpreted as subglaciofluvial sediments deposited within a subglacial de‐coupled space. The subglaciofluvial sequence is characterized by glaciotectonic deformation structures within discrete beds, triggered by fluid overpressure and shear stress during episodes of ice/bed recoupling (clastic dykes and folds). The upper deposits correspond to the deposition of successive hyperpycnal flows in a proximal proglacial lake, forming a thick sedimentary wedge erosively overlying the subglacial deposits. Gravel facies and large‐scale trough bedding sand are observed within this proximal wedge, while normally graded sand beds with developed bedforms are observed further downflow. The building of the prograding ice‐contact subaqueous fan implies an unrestricted accommodation space and is associated with deformation structures related to gravity destabilization during fan spreading (normal faults). This study facilitates the recognition of subglacial/submarginal depositional environments formed, in part, during localized ice/bed coupling episodes in the sedimentary record. The sedimentary sequence exposed in Ballyhorsey permits characterization of the temporal framework of meltwater production during deglaciation, the impact on the subglacial drainage system and the consequences on the Irish Sea Ice Stream flow mechanisms.     

Deforming bed conditions on the Dánischer Wohld Peninsula, northern Germany

At the Dänischer Wohld Peninsula coastal sections (North West Germany), subglacial deformation was found at three scales. At the smallest scale, features typical of deforming bed tills were found, i.e. small boudins, tectonic laminations and low fabric strength till. At an intermediate scale, large lenses of glaciolacustrine sediments were found within subglacially deformed till. At the largest scale, there were large (over 5 m high) subglacial folds. We suggest that these styles of sedimentation/deformation were associated with a series of readvances during overall glacial retreat: subglacial deformation occurred during each advance and glaciolacus trine sedimentation occurred during each retreat. This led to glaciolacustrine sediments and deforming bed tills being folded together during subsequent readvances. Where the rheology was relatively weak, the lacustrine sediments were totally incorporated into the diamicton and lost their previous identity. However, where the glaciolacustrine sediments were relatively strong, they survived. We suggest that this style of deformation is typical of the conditions just upglacier from the ice margin and is associated with a relatively thick deforming layer and a high input of subglacial sediment. We conclude that the evidence found at this site provides further indications that the southern margins of the Fenno-Scandinavian ice sheet were coupled with the glacier bed and underwent deforming bed conditions.