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1.
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   

2.
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3.
Hummocky terrain composed of boulder gravel and a wavy contact between stratified till and sand are described and explained as products of subglacial meltwater activity beneath the Saginaw Lobe of the Laurentide Ice Sheet in south-central Michigan. Exposures and geophysical investigations of hummocky terrain in a tunnel channel reveal that hummocks (˜100m diameter) are glaciofluvial bedforms with a supraglacial melt-out till or till flow veneer. The hummocky terrain is interpreted as a subglacial glaciofluvial landscape rather than one of stagnant ice processes commonly assumed for hummocky landscapes. Sandy bedforms at another site are in-phase with a wavy contact at the base of a stratified till exposed for 50m along the margin of a tunnel channel. The 0.4m thick stratified till is overlain by up to 5m of compact, pebble-rich, sandy subglacial melt-out till. The contact between the till and sand has a wave form with a 0.5m amplitude and 3-5m wavelength. Bedding within the stratified till, sandy bedforms and melt-out till are mostly in-phase with each other. Clasts from the overlying stratified till penetrate and deform the underlying sand recording recoupling of the ice to its bed. Ice ripples cut into the base of river ice have a similar morphology and are considered analogs for cavities cut into the base of the glacier and subsequently filled with sand. Subglacial meltwater activity was not coeval at each study site, indicating that subglacial meltwater played important roles in the evolution of the subglacial environment beneath the Saginaw Lobe at different times.  相似文献   

4.
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   

5.
The foreground of Elisebreen, a retreating valley glacier in West Svalbard, exhibits a well-preserved assemblage of subglacial landforms including ice-flow parallel ridges (flutings), ice-flow oblique ridges (crevasse-fill features), and meandering ridges (infill of basal meltwater conduits). Other landforms are thrust-block moraine, hummocky terrain, and drumlinoid hills. We argue in agreement with geomorphological models that this landform assemblage was generated by ice-flow instability, possibly a surge, which took place in the past when the ice was thicker and the bed warmer. The surge likely occurred due to elevated pore-water pressure in a thin layer of thawed and water-saturated till that separated glacier ice from a frozen substratum. Termination may have been caused by a combination of water drainage and loss of lubricating sediment. Sedimentological investigations indicate that key landforms may be formed by weak till oozing into basal cavities and crevasses, opening in response to accelerated ice flow, and into water conduits abandoned during rearrangement of the basal water system. Today, Elisebreen may no longer have surge potential due to its diminished size. The ability to identify ice-flow instability from geomorphological criteria is important in deglaciated terrain as well as in regions where ice dynamics are adapting to climate change.  相似文献   

6.
A typical stratigraphy below a streamlined till plain in Northumberland, England, consists of cross-cutting lodgement till units, within and between which occur repeated shoestring interbeds of ‘cut and fill’ channels. Till units have erosional lower contacts; in certain cases marked changes in erratic content and local ice flow direction are evident from one till unit to another. These lodgement till complexes have hitherto been described by ‘tripartite’ schemes of lower grey till (s) and upper reddened till (s) identified with respect to ‘middle’ fluvial horizons; regional correlation proceeding on the basis of matching ‘middle’ horizons, with the whole sequence commonly interpreted as evidence for multiple glaciation. Data indicates, by way of contrast, that these lodgement till complexes were deposited during a single phase of subglacial deposition. Till deposition was not continuous but was interrupted by erosional episodes. Changes in the mix of bedrock lithologies transported by the glacier down a single flow line or by lateral displacement of basal ice flow units within the glacier result in till units of different facies to be emplaced when deposition recommences, a process referred to as ‘unconformable facies superimposition’. Subglacial meltwater flow was also a characteristic of the glacier bed; channeled glaciofluvial sediment bodies are found as ribbon-like inclusions in the till and appear to have been deposited rapidly. These so-called ‘middle’ fluvial horizons occur repeatedly in section, their lateral extent at any given exposure being dependent upon the orientation of the exposure with respect to former ice flow direction. These lenses act as internal drainage blankets and have accelerated postglacial soil formation in the drier climate of eastern Britain accounting for the reddened colour of upper till(s). It is suggested that this model of subglacial deposition can be employed in other areas of northern England characterized by subglacial (lodgement till plain) terrains.  相似文献   

7.
《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   

8.
Glacial deposits and landforms, interpreted from the continuous seismic reflection data, have been used to reconstruct the Late Weichselian ice-sheet dynamics and the sedimentary environments in the northeastern Baltic Sea. The bedrock geology and topography played an important role in the glacial dynamics and subglacial meltwater drainage in the area. Drumlins suggest a south-southeasterly flow direction of the last ice sheet on the Ordovician Plateau. Eskers demonstrate that subglacial meltwater flow was focused mostly within bedrock valleys. The eskers have locally been overlain by a thin layer of till. Thick proximal outwash deposits occupy elongated depressions in the substratum, which often occur along the sides of esker ridges. Ice-marginal grounding-line deposit in the southern part of the area has a continuation on the adjacent Island of Saaremaa. Therefore, we assume that its formation took place during Palivere Stadial of the last deglaciation, whereas the moraine bank extending southwestward from the Serve Peninsula is tentatively correlated with the Pandivere Stadial. The wedge-shaped ice-marginal grounding-line deposit was locally fed by subglacial meltwater streams during a standstill or slight readvance of the ice margin. The thickness of the glacier at the grounding-line was estimated to reach approximately 180 m. In the western part of the area, terrace-like morphology of the ice-marginal deposit and series of small retreat moraines 10–20 km north of it suggest stepwise retreat of the ice margin. Therefore, a rather thin and mobile ice stream was probably covering the northeastern Baltic Sea during the last deglaciation.  相似文献   

9.
Murtoos are recently discovered triangular-shaped subglacial landforms that form under warm-based ice and in association with significant subglacial meltwater flow. They appear in distinct fields and commonly occur in the area that was covered by the Fennoscandian Ice Sheet during glacial periods. Murtoos potentially represent a transition form from non-channelized to channelized subglacial drainage networks. In the present study, we analyse and classify murtoos and murtoo-related landforms in the Finnish area of the Fennoscandian Ice Sheet based on their characteristics and appearance in LiDAR-based digital elevations models. Combined with morphometric analyses, the observations suggest that five types of murtoos and murtoo-related landforms are common and widespread in Finland: (i) triangle-type murtoos (TTMs), (ii) chevron-type murtoos (CTMs), (iii) lobate-type murtoos (LTMs), (iv) murtoo-related ridges and escarpments (MREs), and (v) other murtoo-related polymorphous landforms (PMRs) that look like small mounds and ridges. The morphometric characteristics of the different types are described here in detail, and it is shown that they are spatially and geomorphologically related. In addition, we provide examples of murtoos other than the TTMs to demonstrate that different murtoo types and murtoo-related landforms are composed of similar sediments and architectural characteristics. The diversity of murtoo landforms and the transition between distinct murtoo types indicate rapid and complicated variations in the configuration of subglacial hydrology at different spatial and temporal scales. This study emphasizes the essential role of subglacial meltwater in the shaping of glacial landscapes and the redistribution of large volumes of sediments during the deglaciation of the Fennoscandian Ice Sheet.  相似文献   

10.
Tunnel channels in southeast Alberta are attributed to erosion by channelized, subglacial meltwater flows. An anabranching tunnel channel network dissects the preglacial drainage divide of the ancestral Milk River. Channel morphology and landform associations are used to evaluate competing hypotheses of tunnel channel formation. Mechanisms that invoke subaerial channel incision, direct glacial erosion or steady state, time-transgressive erosion at the ice margin cannot explain convex-up longitudinal channel profiles, anabranching channel networks or confinement to the preglacial drainage divide. Results conclude that the tunnel channel network in southeast Alberta represents late-stage erosion by a channelised subglacial flow of catastrophic dimensions. Interpretations for this tunnel channel network are in agreement with conclusions obtained for the regional subglacial landform assemblage.  相似文献   

11.
It is proposed that a lake, here named “Subglacial Lake McGregor”, existed beneath the Laurentide Ice Sheet at, or near, the last glacial maximum. The lake resided in the ancient buried McGregor and Tee Pee preglacial valleys, which are now mostly filled with glacigenic deposits. The greatest thickness of sediment in the valleys is in the form of chaotically deposited lake beds that were laid down in a subaqueous environment by a number of process: gravity flow, water transport, and suspension settling. Topographic, sedimentary, and stratigraphic evidence point to a subglacial, not a proglacial, origin for the beds. During the early stages of lake existence, ice movement was significant as there are numerous sets of shear planes in the sedimentary beds. This indicates that the lake filled (lake sedimentation) and drained (shearing of the beds by overlying ice when ice contacted the bed) often. Thus, early in its history, the lake(s) was/were ephemeral. During the later stages of lake existence, the lake was relatively stable with no rapid draining or influx of sediment. Gradual drainage of the lake resulted in lowering of the ice onto the lake beds resulting in subglacial till deposition. Drainage was not a single continuous event. Rather it was characterized by multiple phases of near total drainage (till deposition), followed by water accumulation (lake sedimentation). Water accumulation events became successively less significant reflected by thinning of lake beds and thickening of till beds higher in the stratigraphic sequence. Since subglacial lake sedimentation appears to be restricted to the subglacial valleys, it is suggested that the valleys acted as a large-scale interconnected cavity system that both stored and transported water. It is also suggested that these acted as the main routes of water flow beneath the Laurentide Ice Sheet.  相似文献   

12.
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   

13.
An excellent section in the Welzow-Süd open-cast lignite mine in Lower Lusatia, eastern Germany, provided a rare opportunity to study a small (5 m deep), buried subglacial meltwater channel of Saalian age. The channel is steep-sided and distinctly U-shaped. It is separated from undeformed outwash deposits in which it is incised by a sharp erosional contact and it is filled with meltwater sand and till. The till was possibly squeezed into the channel from the adjacent ice/bed interface. Directly beneath the channel, there is a partly truncated diapir of clayey silt, evidencing sediment intrusion into the channel from below. During channel formation, the pressure gradient was oriented from the surrounding sediments into the channel, so that the channel served as a drainage conduit for groundwater from the adjacent subglacial aquifer. The substratum consists largely of sandy aquifers with a total thickness of about 100 m, separated by two aquitards. Channel formation was initiated when hydraulic transmissivity of the bed did not suffice to evacuate all the subglacial meltwater as groundwater flow. As the Welzow-Süd channel belongs to a dense network of subglacial channels in eastern Germany, temporary ice-sheet instability in this region prior to channel formation seems possible.  相似文献   

14.
《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   

15.
Subglacial erosional forms are commonly found on bedrock substrates inside the Late Weichselian ice margin in County Donegal, northwest Ireland, and can be used to provide detailed information on subglacial processes and environments. The erosional forms occur on spatial scales from whalebacks (tens of metres in scale), to asymmetric and channelized bedrock-cut scours (tens of cm in scale) and striations (mm scale). Processes responsible for development of subglacial erosional forms occur along a continuum, from free meltwater existing as a laterally extensive sheet at the ice-bed interface, to abrasion by basal ice. Channelized bedrock-cut scours are particularly common in County Donegal, and show asymmetric and meandering thalwegs, U-shaped cross-profiles and steep lateral margins. Innermost parts of the scours are highly polished and have striations that follow thalweg direction. In places, bedrock surfaces are overlain by a delicate polish and thin calcite cement, and are buried beneath glacial till. Based on their morphology, the bedrock scours are interpreted as s-forms caused by high-pressure subglacial meltwater erosion. Striations within the scoured channels reflect periods of ice-bed coupling and subglacial abrasion. The range of features observed here was used to consider relationships between subglacial topography, hydraulic processes and ice-bed coupling. Precipitation of calcite cement took place in depressions on the bedrock surface by CO2 degassing. Infilling of depressions by glacial till formed a new type of 'sticky spot' related to spatial variations in subglacial water pressure. The temporal evolution of sticky spots reflects interactions within the subglacial environment between subglacial relief, hydraulic regime and ice-bed coupling.  相似文献   

16.
In the Omagh Basin, north central Ireland, subglacial diamict ridges lie transverse to southwestward Late Devensian (ca. 23–13 ka) ice flow. These ridges (0.5–2.5 km long, 100–450 m wide, 15–35 m high), are similar morphologically to Rogen moraines, which have not been described previously from the British Isles. The crests of some transverse ridges are streamlined, cross-cut or overprinted by drumlins, whereas other ridges are unmodified and were not affected by later drumlinisation. At Kilskeery, west–east trending eskers overlying unmodified transverse ridges post-date drumlinisation (17–14 14C ka). Esker formation shows that the subglacial thermal regime changed from cold-based, favouring bedform preservation, to warm-based with meltwater flowing through enclosed subglacial channels. Patterns of flow-transverse-ridges and spatial variations in the degree of bedform modification record dynamic changes in regional subglacial environments during the last deglacial cycle. This ice-mass variability cannot be reconciled with current Irish glacial models, which are based on immobile ice centres and ordered stages of ice retreat. In a wider context, these changes in bedform patterns and basal ice regimes have a similar signature to millennial-scale ice-mass oscillations recorded by dated proxy evidence elsewhere in the amphi-North Atlantic. © 1997 John Wiley & Sons, Ltd.  相似文献   

17.
Streamlined subglacial landforms that include drumlins in three study areas, the upper Chandra valley around Chandra Tal, the upper Spiti Valley and the middle Yunam Valley of the NW Himalaya of India were mapped and studied using geomorphic, sedimentological and geochronological methods. These streamlined subglacial landforms include a variety of morphological types, including: (i) half egg‐shaped forms; (ii) complex superimposed forms; (iii) dome‐shaped forms; (iv) inverse forms; and (v) flat‐topped symmetrical forms. Sedimentological data indicate that subglacial deformational processes are responsible for the formation of the streamlined subglacial landforms in the Chandra Tal and upper Spiti Valley study areas. In contrast, streamlined landforms in the middle Yunam Valley are the result of melt‐out and subglacial erosional processes. In the Yunam Valley study area, 11 new cosmogenic 10Be surface exposure ages were obtained for boulders inset into the crests of streamlined subglacial landforms and moraines, and also for a bedrock surface. The streamlined landforms date to 8–7 ka, providing evidence of an early Holocene valley glaciation, and older moraines date to ～17–15 and 79–52 ka, representing other significant valley glacial advances in the middle Yunam Valley. The subglacial landforms in the Chandra Valley provide evidence for a ≥300‐m‐thick Lateglacial glacier that advanced southeast, overtopping the Kunzum Range, and advancing into the upper Spiti Valley. The streamlined subglacial landforms in these study areas of the NW Himalaya highlight the usefulness of such landforms in developing glacial chronostratigraphy and for understanding the dynamics of Himalayan glaciation.  相似文献   

18.
The distribution of basal drag zones (sticky spots) underneath palaeo‐ice streams or lobes is largely unknown. We investigated the centre of the large (300 km long and up to 400 km wide) deglacial Hayes Lobe in NE Manitoba, Canada, by focusing on surficial till and its composition to get insights into dispersal patterns and their potential relationships to areas of basal drag. Subglacial bed roughness is a good criterion to identify areas of basal drag, but till composition may provide important insights across smoother beds. The onset zone of the Hayes Lobe overlies Palaeozoic Carbonate Platform rocks, whereas the majority of the lobe overlies the low‐lying Canadian Shield. We show that, within a 3500‐km2 central area of this lobe, calcareous detritus within the till has been transported over 100 km within subglacial environments of reduced ice‐bed coupling and fast ice flow. Six per cent of samples (n = 782), however, outline 0.2 to 4 km wide spots with a dominantly local composition. The glacial history and composition indicate that the till within these spots contains high inheritance from a pre‐Late Wisconsinan ice‐flow phase, which we suggest was protected beneath sticky spots (low erosion, high strength) during transport of substantial calcareous detritus to the area. Furthermore, our findings show that local till spots are present within streamlined landforms, as well as till blankets or veneers over bedrock. This diverse geomorphology indicates that the process of drumlinization within the deglacial Hayes Lobe does not appear to have been responsible for significant sediment transport or deposition across the study area. The overall record thus indicates potentially complex spatiotemporal shifts between calcareous till deposition, sticky conditions, erosion and drumlinization – which supports the subglacial bed mosaic model.  相似文献   

19.
Glaciated terrains in east-central Alberta and south-central Michigan contain channels that have hummocks and transverse ridges separating depressions along their floors. This association imparts a linked pothole appearance. Similar channels are often interpreted as tunnel channels or subaerial channels, partly filled with sediment from a subsequent glacial advance, a stagnating ice roof, or slumped sediment from the channel margins. However, the truncation of sedimentary packages in the channel walls and intrachannel hummocks indicates that they are erosional landforms, cut into glacial sediments (till), bedrock, or gravel. Eskers overlie and are found within a few channels, indicating that these channels formed before the final stagnation that produced the eskers. These two characteristics, combined with the observation that many channels have convex-up long profiles, indicate that the channels were eroded by pressurized, subglacial water. Because the formative mechanisms for this type of channel are not clear, and modern environments that could produce this type of landform are inaccessible, we draw on several morphologic analogues to propose mechanisms for channel erosion. We conclude that the erosion of these linked pothole channels (incipient tunnel channels) was the product of the complex interaction between complex turbulent flow structures and various scales of roughness elements.  相似文献   

20.
Nick Eyles   《Sedimentary Geology》2006,190(1-4):257-268
Water plays a dominant role in many glacial processes and the erosional, depositional and climatic significance of meltwaters and associated fluvioglacial processes cannot be overemphasized. At its maximum extent c. 20,000 years ago, the volume of the Laurentide ice sheet was 33 × 106 km3 (about the same as the volume of all ice present today on planet Earth). The bulk of this was released as water in little more than 10,000 years. Pulses of meltwater flowing to the Atlantic Ocean from large ice dammed lakes altered thermohaline circulation of the world's oceans and global climate. One such discharge event via Hudson Bay at 8200 years BP released 160,000 km3 of water in 12 months. Global sea levels recovered from glacial maximum low stands reached at about 20,000 years ago at an average rate of 15 m per thousand years but estimates of shorter term rates suggest as much as 20 m sea level rise in 1000 years and for short periods, rates as high as 4 m per hundred years. Meltwaters played a key role in lubricating ice sheet motion (and thus areal abrasion) across the inner portions of the ice sheet where it slid over rigid crystalline bedrock of the Canadian Shield. The recharge of meltwater into the ice sheets bed was instrumental in generating poorly sorted diamict sediments (till) by sliding-induced shearing and deformation of overpressured sediment and soft rock. The transformation of overpressured till into hyperconcentrated slurries in subglacial channels may have generated a highly effective erosional tool for selective overdeepening and sculpting of bedrock substrates. Some workers credit catastrophic subglacial ‘megafloods’ with the formation of drumlins and flutes on till surfaces. Subglacial melt river systems were instrumental in reworking large volumes of glaciclastic sediment to marine basins; it has been estimated that less than 6% of the total volume of glaciclastic sediment produced during the Pleistocene remains on land. Fluvioglacial and glaciolacustrine sediments and landforms dominate large tracts of the ‘glacial’ landscape in North America. The recharge of subglacial meltwater into underlying bedrock and sediment aquifers created transient reversals in the long-term equilibrium flow directions of basinal fluids. With regard to pre-Pleistocene glacial record, meltwaters moved enormous volumes of terrestrial ‘glaciclastic’ sediment to marine basins and thus played a key role in preserving a record of glaciation, a record otherwise almost entirely lost on land.  相似文献   

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