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1.
During the Younger Dryas cold event, the Scandinavian ice sheet readvanced in southwest Sweden and formed the Middle Swedish end-moraine zone (MSEMZ). Recent highway construction near Skara has created an exposure through the prominent ridge at Ledsjö. Through sketching and measurement of structural information, we have documented the internal character of the Ledsjö moraine. The moraine consists predominantly of clay with numerous sand pods and lenses, which show undeformed, brittle deformed, or fluidized structures. Based on geomorphology and structural geology, it is clear the moraine was made during two advances. As ice advanced, proglacial marine clay was subglacially mobilized by the ice and extruded at the ice margin forming a ramp of debris-flow sediment. Contemporaneously, subglacial meltwater transported sand to the margin, where the meltwater became a buoyant plume, and sand was deposited near the ice margin by currents moving away from as well as toward the ice margin. These processes resulted in interbedded sand and clay. Continued advance of the ice margin deformed this package and further pushed the assemblage into a ridge form with gravity sliding of portions of the ridge. Prior to the second advance, sand was deposited on the proximal side of the initial ridge. During readvance, this sand was thrust faulted and intruded by mobilized clay. Up ice of the intruded sands, subglacial, extensional deformation created a complex shear zone of faulted sand and clay. The Ledsjö moraine represents a subaerial example of submarine push moraines like the submerged moraines recently documented in Svalbard.  相似文献   

2.
We present results from three geophysical campaigns using high‐resolution sub‐bottom profiling to image sediments deposited in Loch Ness, Scotland. Sonar profiles show distinct packages of sediment, providing insight into the loch's deglacial history. A recessional moraine complex in the north of the loch indicates initial punctuated retreat. Subsequent retreat was rapid before stabilisation at Foyers Rise formed a large stillstand moraine. Here, the calving margin produced significant volumes of laminated sediments in a proglacial fjord‐like environment. Subsequent to this, ice retreated rapidly to the southern end of the loch, where it again deposited a sequence of proglacial laminated sediments. Sediment sequences were then disturbed by the deposition of a thick gravel layer and a large turbidite deposit as a result of a jökulhlaup from the Spean/Roy ice‐dammed lake. These sediments are overlain by a Holocene sheet drape. Data indicate: (i) a former tributary of the Moray Firth Ice Stream migrated back into Loch Ness as a major outlet glacier with a calving margin in a fjord‐like setting; (ii) there was significant sediment supply to the terminus of this outlet glacier in Loch Ness; and (iii) that jökulhlaups are important for sediment supply into proglacial fjord/lake environments and may compose >20% of proglacial sedimentary sequences. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

3.
Ice‐cored lateral and frontal moraine complexes, formed at the margin of the small, land‐based Rieperbreen glacier, central Svalbard, have been investigated through field observations and interpretations of aerial photographs (1936, 1961 and 1990). The main focus has been on the stratigraphical and dynamic development of these moraines as well as the disintegration processes. The glacier has been wasting down since the ‘Little Ice Age’ (LIA) maximum, and between 1936 and 1990 the glacier surface was lowered by 50–60 m and the front retreated by approximately 900 m. As the glacier wasted, three moraine ridges developed at the front, mainly as melting out of sediments from debris‐rich foliation and debris‐bands formed when the glacier was polythermal, probably during the LIA maximum. The disintegration of the moraines is dominated by wastage of buried ice, sediment gravity‐flows, meltwater activity and some frost weathering. A transverse glacier profile with a northward sloping surface has developed owing to the higher insolation along the south‐facing ice margin. This asymmetric geometry also strongly affects the supraglacial drainage pattern. Lateral moraines have formed along both sides of the glacier, although the insolation aspect of the glacier has resulted in the development of a moraine 60 m high along its northern margin. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

4.
The youngest ice marginal zone between the White Sea and the Ural mountains is the W-E trending belt of moraines called the Varsh-Indiga-Markhida-Harbei-Halmer-Sopkay, here called the Markhida line. Glacial elements show that it was deposited by the Kara Ice Sheet, and in the west, by the Barents Ice Sheet. The Markhida moraine overlies Eemian marine sediments, and is therefore of Weichselian age. Distal to the moraine are Eemian marine sediments and three Palaeolithic sites with many C-14 dates in the range 16-37 ka not covered by till, proving that it represents the maximum ice sheet extension during the Weichselian. The Late Weichselian ice limit of M. G. Grosswald is about 400 km (near the Urals more than 700 km) too far south. Shorelines of ice dammed Lake Komi, probably dammed by the ice sheet ending at the Markhida line, predate 37 ka. We conclude that the Markhida line is of Middle/Early Weichselian age, implying that no ice sheet reached this part of Northern Russia during the Late Weichselian. This age is supported by a series of C-14 and OSL dates inside the Markhida line all of >45 ka. Two moraine loops protrude south of the Markhida line; the Laya-Adzva and Rogavaya moraines. These moraines are covered by Lake Komi sediments, and many C-14 dates on mammoth bones inside the moraines are 26-37 ka. The morphology indicates that the moraines are of Weichselian age, but a Saalian age cannot be excluded. No post-glacial emerged marine shorelines are found along the Barents Sea coast north of the Markhida line.  相似文献   

5.
Moraine sequences in front of seven relatively low‐altitude glaciers in the Breheimen region of central southern Norway are described and dated using a ‘multi‐proxy’ approach to moraine stratigraphy. Lichenometric dating, based on the Rhizocarpon subgenus, is used to construct a composite moraine chronology, which indicates eight phases of synchronous moraine formation: AD 1793–1799, 1807–1813, 1845–1852, 1859–1862, 1879–1885, 1897–1898, 1906–1908 and 1931–1933. Although the existence of a few cases of older moraines, possibly dating from earlier in the eighteenth or late in the seventeenth centuries cannot be ruled out by lichenometry, Schmidt hammer R‐values from boulders on outermost moraine ridges suggest an absence of Holocene moraines older than the Little Ice Age. Twenty‐three radiocarbon dates from buried soils and peat associated with outermost moraines at three glaciers—Tverreggibreen, Storegrovbreen and Greinbreen—also indicate that the ‘Little Ice Age’ glacier maximum was the Neoglacial maximum at most if not all glaciers. Several maximum age estimates for the Little Ice Age glacier maximum range between the fifteenth and seventeenth centuries, with the youngest from a buried soil being AD 1693. A pre‐Little Ice Age maximum cannot be ruled out at Greinbreen, however, where the age of buried peat suggests the outermost moraine dates from AD 981–1399 (at variance with the lichenometric evidence). Glaciofluvial stratigraphy at Tverreggibreen provides evidence for minor glacier advances about AD 655–963 and AD 1277–1396, respectively. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

6.
Until recently, little was known about the Quaternary marine sedimentary record in East Greenland. Geophysical and geological investigations in Scoresby Sund were undertaken to characterize the nature and chronology of this record. Seismic records show that almost 70% of the outer fjord system is covered by about 10 m of unlithified sediments, making direct correlation with the Quaternary records on land and the adjacent continental margin difficult. These acoustically unstratified sediments are scoured by icebergs above 550 m water depth. Almost 90% of core material is massive diamicton of Holocene age, deposited mainly from iceberg rafting and turbid meltwater. Sedimentation rates are 0.1 -0.3 m 1000 yr-1. Thicker accumulations of unlithified Quaternary sediments in Scoresby Sund occur as sediment ridges and in two other major depocentres. A low sediment ridge runs across the mouth of Scoresby Sund, and is interpreted as an end moraine of Late Weichselian Flakkerhuk stadial age. The very restricted sediment thickness suggests that grounded ice filled the fjord during the Flakkerhuk and an ice shelf was not present. High inputs of ice rafted debris to the continental margin at about 18 000 BP indicate this as a probable age for the moraine. During the Allerød Interstadial, ice probably retreated from the outer fjord system, since massive diamictons similar to those of Holocene age are present at the base of most cores. A major depocentre of acoustically stratified sediments at the head of Hall Bredning is interpreted to represent ice proximal deposits from a glacier margin extending across the fjord. It is adjacent to dated moraines on land and is inferred to be of Milne Land stadial age (about 10 000 BP). A similar age is interpreted for acoustically laminated sediments and a moraine at the entrance of Vikingebugt, on the south side of Scoresby Sund. Dated kame terraces in the inner fjord system indicate that ice retreated to its present position 6–7000 years ago.  相似文献   

7.
Eyles, N., Eyles, C., Menzies, J. & Boyce, J. 2010: End moraine construction by incremental till deposition below the Laurentide Ice Sheet: Southern Ontario, Canada. Boreas, 10.1111/j.1502‐3885.2010.00171.x. ISSN 0300‐9483. Just after 13 300 14C a BP in central Canada, the retreating Ontario lobe of the Laurentide Ice Sheet briefly re‐advanced westwards through the Lake Ontario basin to build a large end moraine. The Trafalgar Moraine (27 km long, 4 km wide) is composed of a distinctly red‐coloured silt‐rich till (Wildfield Till, up to 16.5 m thick) formed by the reworking of proglacial lake deposits and soft shale bedrock. The moraine has a pronounced ramp‐like longitudinal form passing upglacier into fluted till resting on exposed shale. Analysis of water well stratigraphic data, drilled sediment cores, downhole gamma‐ray logs and exposures in deep test pits shows that within the moraine the Wildfield Till is built of superposed beds up to 7 m in thickness. These are inferred to result from the repeated incremental deposition of fine‐grained debris being moved towards the ice margin as a deforming bed such as identified at modern glaciers. A total till volume of 0.81 km3 was produced in a very brief time‐span along a transport path probably no greater than 10 km in length. Subglacial mixing of pre‐existing sediment and soft shale was clearly a very effective process for generating and moving large volumes of till to the ice margin. Similar till‐dominated end moraines occur widely around the margins of the Great Lake basins, where the markedly lobate margin of the retreating Laurentide Ice Sheet re‐advanced repeatedly into proglacial lakes and over fine‐grained sediment. This suggests the wider applicability of the till transport and incremental depositional model presented here.  相似文献   

8.
The geomorphic, stratigraphic and sedimentological characteristics of glaciolacustrine sediments in the metropolitan Detroit, Michigan area were studied to determine environments of deposition and make paleogeographic reconstructions. Nine lithofacies were identified and paleoenvironments interpreted based on their morphostratigraphic relationships with relict landforms. The sediments studied are found southeast of the Defiance and Birmingham moraines lying beneath a lowland characterized by a low morainal swell (Detroit moraine) and a series of lacustrine terraces that descend progressively in elevation southeastward. The glaciolacustrine sediments were deposited approximately 14.3–12.4 kA BP during the Port Bruce and Port Huron glacial phases of late Wisconsinan time, and are related to proglacial paleolakes Maumee, Arkona, Whittlesey, Warren, Wayne, Grassmere, Lundy and Rouge. The glaciolacustrine section is typically 2–4 m thick and consists of a basal unit of wavy-bedded clayey diamicton overlain by a surficial deposit of stratified and cross-stratified sand and gravel. The basal unit is comprised of subaqueous debris flow deposits that accumulated as subaqueous moraine in paleolake Maumee along the retreating front of the Huron lobe. The surficial deposits of sand and gravel were formed by traction, resulting from lacustrine wave activity and fluvial processes, in lakebed plain, beach ridge and deltaic depositional settings. Much of the lake-margin sand and gravel was derived from clayey diamicton by lacustrine wave action and winnowing, and that associated with paleolakes of the Port Huron phase is largely reworked Port Bruce sediment. Paleogeographic reconstructions show that the Defiance, Birmingham and Detroit moraines, Defiance and Rochester channels, and the Rochester delta, were deposited penecontemporaneously as paleolake Maumee expanded northward across the map area. A unique type of wavy bedform is characteristic of clayey diamicton deposited by subaqueous mass flow in the study area that is useful for differentiating sediment: 1) deposited by mass flow in subaqueous vs. subaerial settings, and 2) deposited by subaqueous mass flow vs. basal till. These bedforms are a useful tool for identifying subglacial meltwater deposits, and facilitate the mapping and correlation of glacial sediments based on till sheets. The map area provides a continental record of ice sheet dynamics along the southern margin of the Laurentide ice sheet during Heinrich event H-1. The record reveals rapid glacial retreat (～ 0.8 km/yr) contemporaneous with the discharge of a large volume of meltwater. Evidence in the study area for subglacial meltwater is problematic, but indications that periglacial conditions persisted in the map area until ～ 12.7 kA BP, and extended for 200 km or more south of the ice front suggest that a frozen substrate may have contributed to instability of the LIS.  相似文献   

9.
In the west-central part of Lago Argentino, the Puerto Bandera moraines are clearly detached from longer, more prominent moraines of the last glaciation and from shorter and smaller Neoglacial moraines. Scientists have long speculated about the age of the Puerto Bandera moraines. Detailed geomorphologic studies in the western area of Lago Argentino, including stratigraphic profiles at Bahía del Quemado in the northern branch (Brazo Norte), indicate that the Puerto Bandera moraines were deposited by three pulses of ice. Each of the three pulses is represented by single moraine ridges and belts of tightly arranged ridges. The timing of the three glacier advances was established by radiocarbon dating, including data published by John Mercer. The oldest moraine system, formed during the Puerto Bandera I substade, was deposited ca. 13,000 14C yr B.P. Moraines of the Puerto Bandera II substade were deposited ca. 11,000 14C yr B.P. The youngest moraine system was deposited during a minor readvance, shortly before 10,390 C14 yr B.P., and thus appears to have occurred some time during the European Younger Dryas interval. After this third substade, the ice tongues retreated into the interior branches of Lago Argentino and have remained there since. Evidence found at Bahía del Quemado, together with data provided by other authors, attests to a significant climatic change by the middle Holocene, which we believe occurred during the Herminita advance, the first Holocene glacial readvance recognized within the area.  相似文献   

10.
We measured 10Be concentrations in boulders collected from the Orsha and Braslav moraines, associated with the Last Glacial Maximum extent and a recessional stage of the Scandinavian Ice Sheet (SIS), respectively, providing a direct dating of the southeastern sector of the ice-sheet margin in Belarus. By combining these data with selected existing radiocarbon ages, we developed a chronology for the last deglaciation of Belarus. The northeastern part of the country remained ice free until at least 19.2±0.2 cal. kyr BP, whereas the northwestern part of the country was ice free until 22.3±1.5 cal. kyr BP. A lobate ice margin subsequently advanced to its maximum extent and deposited the Orsha Moraine. The ice margin retreated from this moraine at 17.7±2.0 10Be kyr to a position in the northern part of the country, where it deposited the Braslav Moraine. Subsequent ice-margin retreat from that moraine at 13.1±0.5 10Be kyr represented the final deglaciation of Belarus. Direct dating of these moraines better constrains the relation of ice-margin positions in Belarus to those in adjacent countries as well as the SIS response to climate change.  相似文献   

11.
Thick deposits of glaciomarine clay and silt overlain by Holocene marine sediments in Norwegian fjord valleys have been, and still are, subject to erosional processes such as river incision, ravine formation and slide activity. In Buvika, Mid‐Norway, these land‐forming processes have been highly influenced by the valley‐fill stratigraphy. Glaciomarine and marine clay sediments dominate this 8 km long hanging valley south of the Gaulosen fjord, with local occurrences of coarser‐grained sediments. Studies of sediments and structures in road excavations together with 14C ages indicate at least one, possibly two, minor glacier readvances in late Allerød/early Younger Dryas (YD) time. This implies a more dynamic ice sheet with more minor ice‐front oscillations than earlier documented in this region. Glacioisostatic rebound resulted in groundwater leaching of marine clay and quick‐clay formation in certain layers or zones. The relative sea‐level fall led to incision by rivers accompanied by numerous slides involving quick clay, which completely liquefies when remoulded. To the east, permeInger‐Lise Solberg (e‐mail: inger‐lise.solberg@sintef.no ), Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology (NTNU). Present address: SINTEF Building and Infrastructure, Høgskoleringen 7a, NO‐7465 Trondheim, Norway; Kåre Rokoengen, Department of Geology and Mineral Resources Engineering, NTNU, Sem Sælands veg 1, NO‐7491 Trondheim, Norway; Louise Hansen, Lars Olsen and Harald Sveian, Geological Survey of Norway, NO‐7491 Trondheim, Norwayable layers of northwesterly dipping sand and gravel generally originate from a former ice‐marginal delta. These relatively thick and frequent layers of interbedded sand and gravel in the clay‐dominated deposits drain groundwater in the slopes, leading to the development of deeply incised ravines. To the south and north, thinner layers of coarse material in the clay lead to pore‐pressure build‐ups and quick‐clay development, resulting in numerous slide scars. Knowledge of the morphology, stratigraphy and erosion pattern of areas prone to formation of quick clay is important in order to understand the landscape development and evaluate risk areas.  相似文献   

12.
At least five Middle to Late Pleistocene advances of the northern Cordilleran Ice Sheet are preserved at Silver Creek, on the northeastern edge of the St Elias Mountains in southwest Yukon, Canada. Silver Creek is located 100 km up‐ice of the Marine Isotope Stage (MIS) 2 McConnell glacial limit of the St Elias lobe. This site contains ~3 km of nearly continuous lateral exposure of glacial and non‐glacial sediments, including multiple tills separated by thick gravel, loess and tilted lake beds. Infrared‐stimulated luminescence (IRSL) and AMS radiocarbon dating constrain the glacial deposits to MIS 2, 4, either MIS 6 or mid‐MIS 7, and two older Middle Pleistocene advances. This chronology and the tilt of the lake beds suggest Pleistocene uplift rates of up to 1.9 mm a?1 along the Denali Fault since MIS 7. The non‐glacial sediment consists of sand, gravel, loess and organic beds from MIS 7, MIS 3 and the early Holocene. The MIS 3 deposits date to between 30–36 14C ka BP, making Silver Creek one of the few well‐constrained MIS 3‐aged sites in Yukon. This confirms that ice receded close to modern limits in MIS 3. Pollen and macrofossil analyses show that a meadow‐tundra to steppe‐tundra mosaic with abundant herbs and forbs and few shrubs or trees, dominated the environment at this time. The stratigraphy at Silver Creek provides a palaeoclimatic record since at least MIS 8 and comprises the oldest direct record of Pleistocene glaciation in southwest Yukon.  相似文献   

13.
The Vastiansky Kon' is the largest exposure of Quaternary deposits in the Pechora lowland, northern Russia. Morphologically the site belongs to the so-called Markhida Moraine; a complex, east–west trending zone of ice-marginal landforms deposited by the Kara Sea Ice Sheet during the last glaciation. The site exhibits a succession of sediments more than 100 m thick that, according to previous studies, covers the interval from the end of the Elsterian to the beginning of the Holocene. Unfortunately both the strong glaciotectonic deformation of the sedimentary succession and few absolute dates have made the chronological interpretation of the section difficult. The present paper reviews previous studies of the site published in Russian, and presents the results of a reinvestigation focusing on the post-Eemian stratigraphy. A marine Eemian clay more than 8 m thick is overlain erosionally by 20 m of fluvial deposits of Late Eemain or Early Weichselian age. The fluvial succession is overlain by a till and a marine clay, which, according to one interpretation, may represent an Early or Middle Weichselian advance of the Kara Ice Sheet followed by a transgression. The clay shows a transition into 15 m of estuarine and fluvial sediments overlain by more than 12 m of tundra–floodplain deposits. The whole succession has been upthrusted glaciotectonically by the last ice advance, which deposited a more than 12 m thick till on top of the section. Based on a number of subtill radiocarbon age-estimates from the site, in the range 25–32 ka BP, the youngest ice advance is considered to be of late Weichselian age, although a Middle Weichselian age cannot be excluded. © 1998 John Wiley & Sons, Ltd.  相似文献   

14.
Physical evidence for the drainage of glacial lakes remains relatively rare in depositional records, giving rise to much debate on the location of outlets and discharge pathways, as well as on the climate impact of the attendant meltwater forcing. Lake Ojibway developed following the withdrawal of the Laurentide Ice Sheet in northern Ontario and Quebec, Canada. The late‐stage evolution of this large ice‐dammed lake was influenced by the complex dynamics of the retreating ice margin, which highly complicates the identification of the termination of Lake Ojibway in glaciolacustrine sediment records. Here, we document the composition of sections of rhythmites that contain in their upper part an anomalously thick and whitish bed (10–15 cm) that is in turn overlain by ～1 m of faintly bedded rhythmites. Grain‐size analyses showed that the thick whitish bed consists primarily of fine to coarse silt (2–63 μm), contrasting with the lower and upper rhythmites that are largely dominated by clay (<2 μm). The detrital carbonate content of the thick silt bed is characterized by consistently high values (2.5 to 2.8%), whereas the bounding rhythmites show lower and highly variable values. Oxygen isotope measurements further show a marked change going from typical glacial meltwater values (～ ?29.6 to ?27.7‰; VSMOW) for the lower rhythmites and the silt bed to modern‐like meteoric values (?18.4 to ?14.6‰) for the uppermost rhythmites. These data suggest that this marker bed may be associated with a major drawdown event that possibly corresponds to the final drainage of Lake Ojibway. AMS radiocarbon dating of ostracods extracted from the drainage bed also documents an important hardwater effect within the Ojibway basin.  相似文献   

15.
Lakes developed on progressively younger end moraines of the Klutlan Glacier were initially assumed to have originated shortly after moraine emplacement and to have persisted to the present. Limnological differences between lakes on old vs young moraines were thought to result from limnological maturation within the lakes and ponds themselves and in response to the development of soils and vegetation on moraine surfaces. This study represents a paleolimnological test of this hypothesis. If true, the first-formed sediments of lakes on old moraines should be comparable to sediments presently forming in lakes on young moraines. Geochemical and paleontological studies of surface sediment to a series of lakes on progressively older moraines provide baseline information for comparing successive levels of lake sediment cores from older moraines. Results indicate that the time of lake initiation seldom reflects moraine age. Even on the oldest moraine (Harris Creek), lake basins are presently forming. Their sediment character more closely relates to the rapidity of basin formation due to melting of buried ice than to age of the lake itself or of the moraine on which it is situated. Vegetation and soil development play an important but secondary role in determining the character of lake sediments; rapid subsidence can convert humic-water lakes surrounded by second-generation spruce forests into turbid-water lakes with unstable, slumping margins. A detailed paleolimnological study of two lakes, one on the unglaciated upland and another in an outwash channel penetrating the oldest moraine, revealed progressive limnologic changes through time, suggesting that their basins were stable for 1200 and 400 yr, respectively. The changes in diatom stratigraphy of these lakes appear to relate to natural limnological changes associated with lake maturation and accumulation of nutrients as well as to changes in the surrounding vegetation and soils.  相似文献   

16.
This paper focuses on the structural glaciology, dynamics, debris transport paths and sedimentology of the forefield of Soler Glacier, a temperate outlet glacier of the North Patagonian Icefield in southern Chile. The glacier is fed by an icefall from the icefield and by snow and ice avalanches from surrounding mountain slopes. The dominant structures in the glacier are ogives, crevasses and crevasse traces. Thrusts and recumbent folds are developed where the glacier encounters a reverse slope, elevating basal and englacial material to the ice surface. Other debris sources for the glacier include avalanche and rockfall material, some of which is ingested in marginal crevasses. Debris incorporated in the ice and on its surface controls both the distribution of sedimentary facies on the forefield and moraine ridge morphology. Lithofacies in moraine ridges on the glacier forefield include large isolated boulders, diamictons, gravel, sand and fine-grained facies. In relative abundance terms, the dominant lithofacies and their interpretation are sandy boulder gravel (ice-marginal), sandy gravel (glaciofluvial), angular gravel (supraglacial) and diamicton (basal glacial). Proglacial water bodies are currently developing between the receding glacier and its frontal and lateral moraines. The presence of folded sand and laminites in moraine ridges in front of the glacier suggests that, during a previous advance, Soler Glacier over-rode a former proglacial lake, reworking lacustrine deposits. Post-depositional modification of the landform/sediment assemblage includes melting of the ice-core beneath the sediment cover, redistribution of finer material across the proglacial area by aeolian processes and fluvial reworking. Overall, the preservation potential of this landform/sediment assemblage is high on the centennial to millennial timescale.  相似文献   

17.
The morphology, sedimentology and architecture of an end moraine formed by a ～9 km surge of Brúarjökull in 1963–64 are described and related to ice‐marginal conditions at surge termination. Field observations and accurate mapping using digital elevation models and high‐resolution aerial photographs recorded at surge termination and after the surge show that commonly the surge end moraine was positioned underneath the glacier snout by the termination of the surge. Ground‐penetrating radar profiles and sedimentological data reveal 4–5 m thick deformed sediments consisting of a top layer of till overlying gravel and fine‐grained sediments, and structural geological investigations show that the end moraine is dominated by thrust sheets. A sequential model explaining the formation of submarginal end moraines is proposed. The hydraulic conductivity of the bed had a major influence on the subglacial drainage efficiency and associated porewater pressure at the end of the surge, thereby affecting the rates of subglacial deformation. High porewater pressure in the till decreased its shear strength and raised its strain rate, while low porewater pressure in the underlying gravel had the opposite effect, such that the gravel deformed more slowly than the till. The principal velocity component was therefore located within the till, allowing the glacier to override the gravel thrust sheets that constitute the end moraine. The model suggests that the processes responsible for the formation of submarginal end moraines are different from those operating during the formation of proglacial end moraines.  相似文献   

18.
This paper examines marine geophysical and geological data, and new multibeam bathymetry data to describe the Pleistocene sediment and landform record of a large ice‐stream system that drained ～3% of the entire British?Irish Ice Sheet at its maximum extent. Starting on the outer continental shelf NW of Scotland we describe: the ice‐stream terminus environment and depocentre on the outer shelf and continental slope; sediment architecture and subglacial landforms on the mid‐shelf and in a large marine embayment (the Minch); moraines and grounding line features on the inner shelf and in the fjordic zone. We identify new soft‐bed (sediment) and hard‐bed (bedrock) subglacial landform assemblages in the central and inner parts of the Minch that confirm the spatial distribution, coherence and trajectory of a grounded fast‐flowing ice‐sheet corridor. These include strongly streamlined bedrock forms and megagrooves indicating a high degree of ice‐bed coupling in a zone of flow convergence associated with ice‐stream onset; and a downstream bedform evolution (short drumlins to km‐scale glacial lineations) suggesting an ice‐flow velocity transition associated with a bed substrate and roughness change in the ice‐stream trunk. Chronology is still lacking for the timing of ice‐stream demise; however, the seismic stratigraphy, absence of moraines or grounding‐line features, and presence of well‐preserved subglacial bedforms and iceberg scours, combined with the landward deepening bathymetry, all suggest that frontal retreat in the Minch was probably rapid, via widespread calving, before stabilization in the nearshore zone. Large moraine complexes recording a coherent, apparently long‐lived, ice‐sheet margin position only 5–15 km offshore strongly support this model. Reconstructed ice‐discharge values for the Minch ice stream (12–20 Gt a?1) are comparable to high mass‐flux ice streams today, underlining it as an excellent palaeo‐analogue for recent rapid change at the margins of the Greenland and West Antarctic Ice Sheets.  相似文献   

19.
《Sedimentary Geology》1999,123(3-4):163-174
Over large areas of the western interior plains of North America, hummocky moraine (HM) formed at the margins of Laurentide Ice Sheet (LIS) lobes that flowed upslope against topographic highs. Current depositional models argue that HM was deposited supraglacially from stagnant debris-rich ice (`disintegration moraine'). Across southern Alberta, Canada, map and outcrop data show that HM is composed of fine-grained till as much as 25 m thick containing rafts of soft, glaciotectonized bedrock and sediment. Chaotic, non-oriented HM commonly passes downslope into weakly-oriented hummocks (`washboard moraine') that are transitional to drumlins in topographic lows; the same subsurface stratigraphy and till facies is present throughout. These landforms, and others such as doughnut-like `rim ridges', flat-topped `moraine plateaux' and linear disintegration ridges, are identified as belonging to subglacially-deposited soft-bed terrain. This terrain is the record of ice lobes moving over deformation till derived from weakly-lithified, bentonite-rich shale. Drumlins record continued active ice flow in topographic lows during deglaciation whereas HM was produced below the outer stagnant margins of ice lobes by gravitational loading (`pressing') of remnant dead ice blocks into wet, plastic till. Intervening zones of washboard moraine mark the former boundary of active and stagnant ice and show `hybrid' drumlins whose streamlined form has been altered by subglacial pressing (`humdrums') below dead ice. The presence of hummocky moraine over a very large area of interior North America provides additional support for glaciological models of a soft-bedded Laurentide Ice Sheet.  相似文献   

20.
Glacier thermal regime is shown to have a significant influence on the formation of ice‐marginal moraines. Annual moraines at the margin of Midtdalsbreen are asymmetrical and contain sorted fine sediment and diamicton layers dipping gently up‐glacier. The sorted fine sediments include sands and gravels that were initially deposited fluvially directly in front of the glacier. Clast‐form data indicate that the diamictons have a mixed subglacial and fluvial origin. Winter cold is able to penetrate through the thin (<10 m) ice margin and freeze these sediments to the glacier sole. During winter, sediment becomes elevated along the wedge‐shaped advancing glacier snout before melting out and being deposited as asymmetrical ridges. These annual moraines have a limited preservation potential of ～40 years, and this is reflected in the evolution of landforms across the glacier foreland. Despite changing climatic conditions since the Little Ice Age and particularly within the last 10 years when frontal retreat has significantly speeded up, glacier dynamics have remained relatively constant with moraines deposited via basal freeze‐on, which requires stable glacier geometry. While the annual moraines on the eastern side of Midtdalsbreen indicate a slow steady retreat, the western foreland contains contrasting ice‐stagnation topography, highlighting the importance of local forcing factors such as shielding, aspect and debris cover in addition to changing climate. This study indicates that, even in temperate glacial environments, restricted or localised areas of cold‐based ice can have a significant impact on the geomorphic imprint of the glacier system and may actually be more widespread within both modern and ancient glacial environments than previously thought.  相似文献   

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