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1.
Subglacial and subaqueous sediments deposited near the margin of a Late-glacial ice-dammed lake near Achnasheen, northern Scotland, are described and interpreted. The subglacial sediments consist of deformation tills and glacitectonites derived from pre-existing glaciolacustrine deposits, and the subaqueous sediments consist of ice-proximal outwash and sediment flow deposits, and distal turbidites. Sediment was delivered from the glacier to the lake by two main processes: (1) subglacial till deformation, which fed debris flows at the grounding line; and (2) meltwater transport, which fed sediment-gravity flows on prograding outwash fans. Beyond the ice-marginal environment, deposition was from turbidity currents, ice-rafting and settling of suspended sediments. The exposures support the conclusion that the presence of a subglacial deforming layer can exert an important influence on sedimentation at the grounding lines of calving glaciers.  相似文献   

2.
An assemblage of subglacial, ice-terminal and proglacial landforms and sediments provides evidence for the relationship between ice-marginal glacitectonics, sedimentary processes and subglacial and proglacial hydraulic processes at a retreating late Devensian ice margin in north-central Ireland. Deltas were deposited in glacial lakes impounded between the retreating ice margin and the southern Sperrin Mountains, followed by outwash and end moraine formation as the ice margin retreated south. Sediments within the moraines show evidence for ice margin oscillation from two opposing ice margins, including subglacial bedrock rafts and breccias which are separated by glacitectonic shears with silty partings. In adjacent outwash, vertically-disturbed proglacial sands, gravels and silts located in front of moraine positions attest to high hydraulic pressure and subsurface water flow during ice oscillation. The relationship between sedimentary and hydraulic processes in the ice margin region is described by a depositional model which links glacitectonic thrusting and subsurface water flow during ice oscillation to formation of subglacial, ice-terminal and proglacial sediments. The evidence presented in this paper shows that subglacial and proglacial morphosedimentary processes and patterns of sediment deposition are mediated by the presence of proglacial permafrost, which helps direct processes and patterns of groundwater flow.  相似文献   

3.
Along the south coast of Ireland, a shelly diamict facies, the Irish Sea Till, has been variously ascribed to subglacial deposition by a grounded Irish Sea glacier or to glacimarine sedimentation by suspension settling and iceberg rafting. Observations are presented here from five sites along the south coast to directly address this question. At these sites, sedimentary evidence is preserved for the onshore advance of a grounded Irish Sea glacier, which glacitectonically disturbed and eroded pre‐existing sediments and redeposited them as deformation till. Recession of this Irish Sea glacier resulted in the damming of ice‐marginal lakes in embayments along the south coast, into which glacilacustrine sedimentation then took place. These lake sediments were subsequently glacitectonised and reworked by overriding glacier ice of inland origin, which deposited deformation till on top of the succession. There is no evidence for deposition of the Irish Sea diamicts by glacimarine sedimentation at these sites. The widespread development of subglacial deforming bed conditions reflected the abundance of fine‐grained marine and lacustrine sediments available for subglacial erosion and reworking. Stratigraphical and chronological data suggest that the advance of a grounded Irish Sea glacier along the south coast occurred during the last glaciation, and this is regionally consistent with marine geological data from the Celtic Sea. These observations demonstrate extension of glacier ice far beyond its traditional limits in the Celtic Sea and on‐land in southern Ireland during the last glaciation, and remove the stratigraphical basis for chronological differentiation of surficial glacial drifts, and thus the Munsterian Glaciation, in southern Ireland. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

4.
The efficiency of subglacial drainage is known to have a profound influence on subglacial deformation and glacier dynamics with, in particular, high meltwater contents and/or pressures aiding glacier motion. The complex sequence of Middle Pleistocene tills and glacial outwash sediments exposed along the north Norfolk coast (Eastern England) were deposited in the ice-marginal zone of the British Ice Sheet and contain widespread evidence for subglacial deformation during repeated phases of ice advance and retreat. During a phase of easterly directed ice advance, the glacial and pre-glacial sequences were pervasively deformed leading to the development of a thick unit of glacitectonic mélange. Although the role of pressurised meltwater has been recognised in facilitating deformation and mélange formation, this paper provides evidence for the subsequent development of a channelised subglacial drainage system beneath this part of the British Ice Sheet filled by a complex assemblage of sands, gravels and mass flow deposits. The channels are relatively undeformed when compared to the host mélange, forming elongate, lenticular to U-shaped, flat-topped bodies (up to 20–30 m thick) located within the upper part of this highly deformed unit. This relatively stable channelised system led to an increase in the efficiency of subglacial drainage from beneath the British Ice Sheet and the collapse of the subglacial shear zone, potentially slowing or even arresting the easterly directed advance of the ice sheet.  相似文献   

5.
A model for sedimentation by surging glaciers is developed from analysis of the debris load, sedimentary processes, and proglacial stratigraphy observed at the Icelandic surging glacier, Eyjabakkajökull. Three aspects of the behavior of surging glaciers explain the distinctive landformsediment associations which they may produce: (a) sudden loading of proglacial sediments during rapid glacier advances results in the buildup of excess pore pressures, failure, and glacitectonic deformation of the overridden sediments; (b) reactivation of stagnant marginal ice by the downglacier propagation of surges is associated with large longitudinal compressive stresses. These induce intense folding and thrusting during which basal debris-rich ice is elevated into an englacial position in a narrow marginal zone. As the terminal area of the glacier stagnates between surges, debris from this ice is released supraglacially and deposited by meltout and sediment flows; (c) local variations in overburden pressure beneath stagnant, crevassed ice cause subglacial lodgement tills, which are sheared during surges, to flow into open crevasses and form “crevasse-fill” ridges.  相似文献   

6.
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.  相似文献   

7.
A Pleistocene drift sequence in hummocky terrain along part of the southern Avalon Peninsula of Newfoundland is interpreted to comprise complexly interrelated lodgement till, melt-out till, flow till, supraglacial and proglacial outwash, and supraglacial rhythmites. The gray and tan melt-out tills are stacked in imbricate fashion, giving rise to exceptionally thick stratigraphic sections. Contacts between melt-out tills are interpreted as remnants of shear planes because they are sharp, they dip in the up-ice direction, and they converge toward valley margins. Overlying flow tills interdigitate with supraglacial outwash. The drift sequence was deposited during a single episode of glaciation, rather than by repeated glacier advance, as previously proposed. It is the product of thrusting of englacial debris along ice-marginal shear planes, subsequent melting-out of englacial debris, and formation of supraglacial flow till and outwash. Preservation of this sequence probably is due to high content of englacial debris within the Wisconsinan ice. The sedimentary, glacitectonic, and morphologic features of this sequence are similar to those found at the margins of certain Arctic glaciers of subpolar thermal regime which have recently been the subject of Pleistocene glacial sedimentation models for west-central Canada and Great Britain. Recognition of these distinct elements indicates wisconsinan glacier lobes were of the cold Arctic type in southeastern Newfoundland. Alternative explanations for this sequence, such as deposition by glaciers of temperate thermal regime or by surging glaciers, are discounted. Because the features described here are complex and difficult to recognize, they may be more widespread in Pleistocene drift than has previously been interpreted.  相似文献   

8.
The Chuos Formation is a diamictite-dominated succession of Cryogenian age, variously interpreted as the product of glaciomarine deposition, glacially related mass movement, or rift-related sediment remobilisation in a non-glacial environment. These interpretations have wide ranging implications for the extent of ice cover during the supposedly pan-global Neoproterozoic icehouse. In the Otavi Mountainland, northern Namibia, detailed analysis of soft-sediment deformation structures on the macro- and micro-scale support glacitectonic derivation in response to overriding ice from the south/south-east. Overall, the upward increase in strain intensity, predominance of ductile deformation features (e.g. asymmetric folds, rotational turbates and necking structures, clast boudinage, unistrial plasmic fabrics) and pervasive glacitectonic lamination support subglacial deformation under high and sustained porewater pressures. In contrast, soft-sediment structures indicative of mass movements, including flow noses, tile structures, and basal shear zones, are not present. The close association of subglacial deformation, abundant ice-rafted debris and ice-contact fan deposits indicate subaqueous deposition in an ice-proximal setting, subject to secondary subglacial deformation during oscillation of the ice margin. These structures thus reveal evidence of dynamic grounded ice sheets in the Neoproterozoic, demonstrating their key palaeoclimatic significance within ancient sedimentary successions.  相似文献   

9.
The glacial geomorphology of Teesdale and the North Pennines uplands is analysed in order to decipher: a) the operation of easterly flowing palaeo-ice streams in the British-Irish Ice Sheet; and b) the style of regional deglaciation. Six landform categories are: i) bedrock controlled features, including glacitectonic bedrock megablocks or ‘rubble moraine’; ii) discrete mounds and hills, often of unknown composition, interpreted as weakly streamlined moraines and potential ‘rubble moraine’; iii) non-streamlined drift mounds and ridges, representing lateral, frontal and inter-ice stream/interlobate moraines; iv) streamlined landforms, including drumlins of various elongation ratios and bedrock controlled lineations; v) glacifluvial outwash and depositional ridges; and vi) relict channels and valleys, related to glacial meltwater incision or meltwater re-occupation of preglacial fluvial features. Multiple tills in valley-floor drumlin exposures indicate that the subglacial bedform record is a blend of flow directions typical of areas of discontinuous till cover and extensive bedrock erosional landforms. Arcuate assemblages of partially streamlined drift mounds are likely to be glacially overridden latero-frontal moraines related to phases of “average glacial conditions” (palimpsests). Deglacial oscillations of a glacier lobe in mid-Teesdale are marked by five inset assemblages of moraines and associated drift and meltwater channels, named the Glacial Lake Eggleshope, Mill Hill, Gueswick, Hayberries and Lonton stages. The Lonton stage moraines are thought to be coeval with bedrock-cored moraines in the central Stainmore Gap and likely record the temporary development of cold-based or polythermal ice conditions around the margins of a plateau-based icefield during the Scottish Readvance.  相似文献   

10.
Evidence for former fast glacier flow (ice streaming) in the southwest Laurentide Ice Sheet is identified on the basis of regional glacial geomorphology and sedimentology, highlighting the depositional processes associated with the margin of a terrestrial terminating ice stream. Preliminary mapping from a digital elevation model of Alberta identifies corridors of smoothed topography and corridor‐parallel streamlined landforms (megaflutes to mega‐lineations) that display high levels of spatial coherency. Ridges that lie transverse to the dominant streamlining patterns are interpreted as: (a) series of minor recessional push moraines; (b) thrust block moraines or composite ridges/hill–hole pairs constructed during readvances/surges; and (c) overridden moraines (cupola hills), apparently of thrust origin. Together these landforms demarcate the beds and margins of former fast ice flow trunks or ice streams that terminated as lobate forms. Localised cross‐cutting and/or misalignment of flow sets indicates temporal separation and the overprinting of ice streams/lobes. The fast‐flow tracks are separated by areas of interlobate or inter‐stream terrain in which moraines have been constructed at the margins of neighbouring (competing) ice streams/outlet glaciers; this inter‐stream terrain was covered by more sluggish, non‐streaming ice during full glacial conditions. Thin tills at the centres of the fast‐flow corridors, in many places unconformably overlying stratified sediments, suggest that widespread till deformation may have been subordinate to basal sliding in driving fast ice flow but the general thickening of tills towards the lobate terminal margins of ice streams/outlet glaciers is consistent with subglacial deformation theory. In this area of relatively low relief we speculate that fast glacier flow or streaming was highly dynamic and transitory, sometimes with fast‐flowing trunks topographically fixed in their onset zones and with the terminus migrating laterally. The occurrence of minor push moraines and flutings and associated landforms, because of their similarity to modern active temperate glacial landsystems, are interpreted as indicative of ice lobe marginal oscillations, possibly in response to seasonal climatic forcing, in locations where meltwater was more effectively drained from the glacier bed. Further north, the occurrence of surging glacier landsystems suggests that persistent fast glacier flow gave way to more transitory surging, possibly in response to the decreasing size of ice reservoir areas in dispersal centres and also locally facilitated by ice‐bed decoupling and drawdown initiated by the development of ice‐dammed lakes. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

11.
The Brampton kame belt represents one of the largest glaciofluvial complexes within the UK. It is composed of an array of landform-sediment assemblages, associated with a suite of meltwater channels and situated within a palimpsest landscape of glacial features in the heart of one of the most dynamic parts of the British-Irish Ice Sheet. Glacial geomorphological mapping and sedimentological analysis have allowed a detailed reconstruction of both the morphological features and the temporal evolution of the Brampton kame belt, with processes informed by analogues from modern ice margins. The kame belt demonstrates the development of a complex glacier karst typified by the evolution of subglacial meltwater tunnels into an englacial and supraglacial meltwater system dominated by ice-walled lakes and migrating ice-contact drainage networks. Topographic inversion led to the extensive reworking of sediments, with vertical collapse and debris flows causing partial disintegration of the morphology. The resultant landform comprises a series of kettle holes, discontinuous ridges and flat-topped hills. The Pennine escarpment meltwater network, which fed the Brampton kame belt, is composed of an anastomosing subglacial channel system and flights of lateral channels. The Brampton kame belt is envisaged to have formed during the stagnation of ice in the lee of the Pennines as ice retreated westwards into the Solway Lowlands. The formation of the Brampton kame belt also has particular conceptual resonance in terms of constraining the nature of kame genesis, whereby an evolving glacier karst is a key mechanism in the spatial and temporal development of ice-contact sediment-landform associations.  相似文献   

12.
Glacigenic sediments exposed in coastal cliffs cut through undulatory terrain fronting the Last Glacial Maximum laterofrontal moraine at Waterville on the Iveragh Peninsula, southwest Ireland, comprise three lithofacies. Lithofacies 1 and 2 consist of interdigitated, offlapping and superimposed ice‐proximal subaqueous outwash and stacked sequences of cohesionless and cohesive subaqueous debris flows, winnowed lag gravels and coarse‐grained suspension deposits. These are indicative of sedimentation in and around small grounding line fans that prograded from an oscillating glacier margin into a proglacial, interlobate lake. Lithofacies 3 comprises braided river deposits that have undergone significant syn‐sedimentary soft‐sediment deformation. Deposition was likely related to proglacial outwash activity and records the reduction of accommodation space for subaqueous sedimentation, either through the lowering of proglacial water levels or due to basin infilling. The stratigraphic architecture and sedimentology of the moraine at Waterville highlight the role of ice‐marginal depositional processes in the construction of morphostratigraphically significant ‘end moraine’ complexes in Great Britain and Ireland. Traditional ‘tills’ in these moraines are often crudely stratified diamictons and gravelly clinoforms deposited in ice‐proximal subaqueous and subaerial fans. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

13.
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.  相似文献   

14.
The Kuannersuit Glacier surged 11 km between 1995 and 1998. The surge resulted in the formation of an ice cored thrust moraine complex constructed by subglacial and proglacial glaciotectonic processes. Four main thrust zones are evident in the glacier snout area with phases of compressional folding and thrusting followed by hydrofracture in response to the build-up of compressional stresses and the aquicludal nature of submarginal permafrost and naled. Various types of stratified debris-rich ice facies occur within the marginal zone: The first (Facies I) comprises laterally continuous strata of ice with sorted sediment accumulations, and is reworked and thrust naled ice. The second is laterally discontinuous stratified debris-rich ice with distinct tectonic structures, and is derived through subglacial extensional deformation and localised regelation (Facies II), whilst the third type is characterised by reworked and brecciated ice associated with the reworking and entrainment of meteoric ice (Facies III). Hydrofracture dykes and sills (Facies IV) cross-cut the marginal ice cored thrust moraines, with their sub-vertically frozen internal contact boundaries and sedimentary structures, suggesting supercooling operated as high-pressure evacuation of water occurred during thrusting, but this is not related to the formation of basal stratified debris-rich ice. Linear distributions of sorted fines transverse to ice flow, and small stratified sediment ridges that vertically cross-cut the ice surface up-ice of the thrust zone relate to sediment migration along crevasse traces and fluvial infilling of crevasses. From a palaeoglaciological viewpoint, marginal glacier tectonics, ice sediment content and sediment delivery mechanisms combine to control the development of this polythermal surge valley landsystem. The bulldozing of proglacial sediments and the folding and thrusting of naled leads to the initial development of the outer zone of the moraine complex. This becomes buried in bulldozed outwash sediment and well-sorted fines through surface ablation of naled. Up-ice of this, the heavily thrust margin becomes buried in sediment melted out from basal debris-rich ice and subglacial diamicts routed along thrusts. These mechanisms combine to deliver sediment to supraglacial localities, and promote the initial preservation of structurally controlled moraines through insulation, and the later development of kettled dead ice terrain.  相似文献   

15.
Till is of common occurrence in the marine environment and can be both subglacial and proglacial in origin. Former glacial margins are often represented by till tongues, that are interbedded with stratified glaciomarine sediment, and the relationship is readily mappable using seismic reflection techniques. It is inferred that within individual till tongues, sediment-gravity flow deposits form transitional contacts with till of subglacial origin, but these contacts cannot be distinguished on seismic profiles. These unstratified, gravity-induced deposits formed in close proximity to the glacial source at the grounding line are considered to be secondary tills (flow-till complex) in terms of the INQUA classification of tills. Till-tongue successions and till deltas are large, ice-marginal depocentres associated with floating front ice-margins, and appear to be closely related in origin. Tabular and hummocky moraines of the mid-Norwegian Shelf also are considered to be products of floating-front margins and represent a large proportion of the retreat tills formed during ice recession. Seismostratigraphic evidence for channelisation in ice-marginal deposits appears to provide a means of distinguishing between former tidewater and floating-front margins.  相似文献   

16.
De Geer moraine ridges occur in abundance in the coastal zone of northern Sweden, preferentially in areas with proglacial water depths in excess of 150 m at deglaciation. From detailed sedimentological and structural investigations in machine‐dug trenches across De Geer ridges it is concluded that the moraines formed due to subglacial sediment advection to the ice margin during temporary halts in grounding‐line retreat, forming gradually thickening sediment wedges. The proximal part of the moraines were built up in submarginal position as stacked sequences of deforming bed diamictons, intercalated with glaciofluvial canal‐infill sediments, whereas the distal parts were built up from the grounding line by prograding sediment gravity‐flow deposits, distally interfingering with glaciolacustrine sediments. The rapid grounding‐line retreat (ca. 400 m yr?1) was driven by rapid calving, in turn enhanced by fast iceflow and marginal thinning of ice due to deforming bed conditions. The spatial distribution of the moraine ridges indicates stepwise retreat of the grounding line. It is suggested that this is due to slab and flake calving of the ice cliff above the waterline, forming a gradually widening subaqueous ice ledge which eventually breaks off to a new grounding line, followed by regained sediment delivery and ridge build‐up. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

17.
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.  相似文献   

18.
《Quaternary Science Reviews》2007,26(19-21):2375-2405
Late Devensian glacigenic sediments and landforms along the north-west coast of Wales document the advance and subsequent retreat of the eastern margin of an Irish Sea Ice Stream that met, coalesced and ultimately uncoupled from ice radiating outwards from the adjacent Welsh Ice Cap centred over Snowdonia. Across the boundary between the two former ice masses is a set of sediment–landform assemblages that reflect rapidly changing erosional and depositional conditions during ice interaction. From the inner part of the ice-stream the assemblages range outwards, from a subglacial depositional assemblage, characterised by drumlin swarms; through a subglacial erosional assemblage, marked by prominent bedrock scours and large subglacial rock channels; through an ice-marginal assemblage, identified by closely spaced, glaciotectonised push moraines and intervening marginal sandur troughs; into a freely expanding proglacial sandur and lacustrine delta assemblage. The ice-marginal assemblage provides evidence for numerous oscillatory episodes during retreat and at least 20 ice-marginal limits can be identified. At least 11 of these display multiple criteria for identifying readvance and, in the ideal case, is characterised by a moraine form built by localised tectonic stacking of diamict to the rear, fronted by a clastic wedge of ice-front alluvial fan gravel and intercalated flow till. The distribution of sediment–landform assemblages suggests a highly dynamic, convergent ice-stream flow pattern, with high ice velocity, a sharply delineated lateral shear margin, pervasive ice-marginal glaciotectonic deformation and a tightly focused ice-marginal sediment delivery system; all signature characteristics of contemporary ice streams.  相似文献   

19.
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.  相似文献   

20.
Flutes are a distinctive type of glacial landform and comprise closely-spaced, streamlined ridges and furrows usually developed on till surfaces and aligned parallel to ice movement direction. Several models proposed to explain their formation involve, at least in part, post-depositional deformation or transfer of subglacial sediments; others involve primary deposition or erosion. The flutes on several glacier forelands in the mountains of the southern Lyngen peninsula in North Norway are associated with glaciers with cold-based margins. To explain the formation of the flutes three main sets of variables are investigated: 1, the landforms and their shapes, dimensions and field relationships; 2, the physical properties of the materials comprising the flutes; and 3, the glacier properties, and in particular, the basal thermal regime and ice-debris relationships at the glacier margin. Existing models of flute formation which involve post-depositional deformation or transfer of subglacial sediments do not explain satisfactorily several aspects of the flutes found in Lyngen. Instead, a model is proposed in which the flutes are primary features formed by deformation of the basal ice layer around subglacial boulders or other obstacles.  相似文献   

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