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1.
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   

2.
Englacial debris structures, morphology and sediment distribution at the frontal part and at the proglacial area of the Scott Turnerbreen glacier have been studied through fieldwork and aerial photograph interpretation. The main emphasis has been on processes controlling the morphological development of the proglacial area. Three types of supraglacial ridges have been related to different types of englacial debris bands. We suggest that the sediments were transported in thrusts, along flow lines and in englacial meltwater channels prior to, and during a surge in, the 1930s, before the glacier turned cold. Melting-out of englacial debris and debris that flows down the glacier front has formed an isolating debris cover on the glacier surface, preventing further melting. As the glacier wasted, the stagnant, debris-covered front became separated from the glacier and formed icecored moraine ridges. Three moraine ridges were formed outside the present ice-front. The further glacier wastage formed a low-relief proglacial area with debris-flow deposits resting directly on glacier ice. Melting of this buried ice initiated a second phase of slides and debris flows with a flow direction independent of the present glacier surface. The rapid disintegration of the proglacial morphology is mainly caused by slides and stream erosion that uncover buried ice and often cause sediments to be transported into the main river and out of the proglacial area. Inactive stream channels are probably one of the morphological elements that have the best potential for preservation in a wasting ice-cored moraine complex and may indicate former ice-front positions.  相似文献   

3.
《Quaternary Science Reviews》1999,18(10-11):1213-1246
This paper describes the landform and sediment assemblage produced by a surge (in 1948) of the Kongsvegen/Kronebreen tidewater glacier complex in northwest Spitsbergen. The main geomorphological products of this advance are two large thrustmoraine complexes on opposite sides of the fjord, and a system of geometrical ridges revealed on glacier decay. The thrust-moraines are composed largely of diamicton, sandy and muddy gravel, gravelly sand, sand and mud, with minor laminites. All of these appear to be derived from the fjord floor and represent both fine fjord basin sediments and coarse grounding-line fan deposits. Thrusting was the principal mode of emplacement of the sediment onto the adjacent land areas during the 1948 advance. However, the geomorphology of the thrust-moraine complexes on either side of the fjord is quite different, reflecting a transpressive regime on the southwest side (mainly long ridges) and a normal compressive regime on the northeast side (short ridges and pinnacles of a ‘hummocky’ nature). The advance which produced the moraine complex has previously been attributed to a surge of Kongsvegen, but the glaciological and geomorphological evidence suggests that the advance involved both Kongsvegen and Kronebreen. Comparison of the landform assemblage produced by this event with that produced by other tidewater glacier surges demonstrates the diverse range of landform assemblages associated with glacier surges, or other episodes of rapid flow, within glaciomarine environments.  相似文献   

4.
Quaternary sedimentary successions are described from the Linda Valley, a small valley in western Tasmania that was dammed by ice during Early and Middle Pleistocene glaciations. Mapping and logging of exposures suggest that an orderly sequence of deposits formed during ice incursion, occupation and withdrawal from tributary valleys. Four principal sediment assemblages record different stages of ice occupation in the valley. As the glacier advanced, a proglacial, lacustrine sediment assemblage dominated by laminated silts and muds deposited from suspension accumulated in front of the glacier. A subglacial sediment assemblage consisting of deformed lacustrine deposits and lodgement till records the overriding of lake-bottom sediments as the glacier advanced up the valley into the proglacial lake. As the glacier withdrew from the valley, a supraglacial sediment assemblage of diamict, gravel, sand and silt facies formed on melting ice in the upper part of the valley. A lacustrine regression in the supraglacial assemblage is inferred on the basis of a change from deposits mainly resulting from suspension in a subaqueous setting to relatively thin and laterally discontinuous laminated sediments, occurrence of clastic dykes, and increasing complexity of the geometry of deposits that indicate deposition in a subaerial setting. A deltaic sediment assemblage deposited during the final stage of ice withdrawal from the valley consists of steeply dipping diamict and normally graded gravel facies formed on delta foresets by subaqueous sediment gravity flows. The sediment source for the delta, which prograded toward the retreating ice margin, was the supraglacial sediment assemblage previously deposited in the upper part of the valley. A depositional model developed from the study of the Linda Valley may be applicable to other alpine glaciated areas where glaciers flowed through or terminated in medium- to high-relief topography.  相似文献   

5.
Traditionally, geometrical ridge networks are interpreted as the product of the flow of subglacial sediment into open basal crevasses at the cessation of a glacier surge (‘crevasse-fill’ ridges). They are widely regarded as a characteristic landform of glacier surges. Understanding the range of processes by which these ridge networks form is therefore of importance in the recognition of palaeosurges within the landform record. The geometrical ridge network at the surge-type glacier Kongsvegen in Svalbard, does not form by crevasse filling. The networks consist of transverse and longitudinal ridges that can be seen forming at the current ice margin. The transverse ridges form as a result of the incorporation of basal debris along thrust planes within the ice. The thrusts were apparently formed during a glacier surge in 1948. Longitudinal ridges form through the meltout of elongated pods of debris, which on the glacier surface are subparallel to the ice foliation and pre-date the surge. This work adds to the range of landforms associated with glacier surges.  相似文献   

6.
The impact of modern cold glaciers on arid periglacial landscapes has received little attention compared with other glacial regimes, and there is a widely held assumption that cold glaciers are not effective geomorphological agents, despite recent studies to the contrary. This paper focuses on the processes operating at the margins of a number of glaciers in the Dry Valleys of Victoria Land, notably the Wright Lower Glacier. The glaciers are entraining primarily older drift deposits and highly weathered regolith which texturally are sandy gravels, as well as well‐sorted sands of fluvial origin. Despite basal temperatures of the order of ?16°C, frozen layers and blocks of sand and gravel are being incorporated into the base of the glaciers by folding and thrusting. The sedimentary products are ridges and aprons several metres high within which the principal lithofacies are sand, gravel, foliated glacier ice, lake ice and snow. These facies are glaciotectonized strongly. Draped over these landforms is a veneer of well‐sorted aeolian sand up to half a metre thick. Supraglacial streams flowing off the glaciers incise these landforms and the sediment is redeposited as alluvial fans, lake deltas and lake‐bottomset deposits. Overall the sediment/landform association differs markedly from those of other glacial regimes, with sand and gravel being the dominant facies, while the usual indicators of glacier working (such as facets and striations on clasts) are lacking. The preservation potential for these landforms on a thousand‐year time scale is high, as modification in this arid regime by slope processes and running water is limited. Sublimation of buried ice is so slow that ridge features are likely to remain ice‐cored almost indefinitely, modified only by wind transport and weathering.  相似文献   

7.
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   

8.
The Mount Cook area in the Southern Alps of New Zealand is heavily glacierized with numerous peaks over 3000 m a.s.l. feeding several large valley glaciers. The region is subject to rapid tectonic uplift and heavy precipitation (up to 15 m per year). This paper describes the clast roundness, clast shape and textural characteristics associated with five glaciers (Fox, Franz Josef, Hooker, Mueller and Tasman) in terms of inputs to the glacier system, transport by the glaciers and reworking following glacial deposition. Inputs include rockfall, alluvial fan and avalanche material delivered to the surface of valley glaciers. Basal debris, where observed at the terminus of two glaciers, consists mainly of incorporated fluvial material. Following deposition, reworking is mainly by subglacial and proglacial streams. The dominant facies are (i) boulder gravel with mainly angular clasts on the steep slopes above the glaciers, (ii) sandy boulder gravel, with mainly angular and subangular clasts, forming lateral and end moraines, and (iii) sandy boulder/cobble gravel with mainly subrounded clasts, and sand, which represent glacially transported sediment reworked by braided rivers. Diamicton is rare in the contemporary glacial environment. Since most sediment associated with glaciers in the Southern Alps lacks unambiguous indications of glacial transport, interpretation of similar sediments in the geological record should not necessarily exclude the involvement of glacial processes.  相似文献   

9.
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10.
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   

11.
Two kinds of buried structures are described from Dzirżenin, north-east of Warsaw, where they occur within a glaciofluvial landform: (1) narrow till ridges, showing vertically oriented structures, excavated from stratified gravel and sands; and (2) a narrow vertical zone of massive gravelly/sandy material, involving vertically oriented lens-like layers composed of massive sand with pebbles, or of diamicton. The gravelly/sandy zone is also closely surrounded by stratified glaciofluvial sediments. In spite of their vertical position and internal deformation, the till ridges and gravelly/sandy zone show non-tectonic contacts with the surrounding, stratified, undisturbed sediments. The glaciofluvial sediments that occur immediately next to the structures under discussion are characterized by the occurrence of comparatively coarse material and interbeddings of diamicton, which wedge out away from these structures. The gravelly/sandy zone separates different kinds of water-laid deposits. The buried structures are interpreted as former debris-laden bands, thrust upwards within the frontal part of the ice sheet and then transformed into still-frozen debris ridges projecting over the already dead ice. Further melting of the decaying ice resulted in abundant glaciofluvial sedimentation, and the debris ridges also supplied material for the deposition of the neighbouring stratified deposits. One of the ridges separated different glaciofluvial environments. The glaciofluvial sediments completely buried the ice-cemented ridges, which were finally transformed by a melting-out process into the till ridges and the gravelly/sandy zone. The former are interpreted as having been transformed from upturned debris-laden bands with a high concentration of debris or from the bands composed of frozen-up sediment slabs. The gravelly/sandy zone is interpreted as having (most probably) been deposited from upturned bands characterized by a lesser concentration of debris.  相似文献   

12.
A push moraine deposited by the surging tidewater glacier Paulabreen (Svalbard) was investigated using 2D resistivity profiling. Six longitudinal and transverse profiles were obtained on the moraine and the resistivities were compared with data from three boreholes. Four profiles indicate that the inner part of the moraine is ice-cored and that the buried glacier ice is more than 30 m thick. A transverse profile shows evidence of basal crevasses near the former glacier margin. Three profiles cross the former glacier margin and onto a proglacial plain which dips slightly away from the former glacier margin. Low resistivities were encountered where borehole and field observations indicate that the plain consists of marine muds with a high salt content. This landform has previously been interpreted as a slab of seabed pushed up in front of the surging glacier, possibly facilitated by permafrost in the seabed. We suggest, alternatively, that the landform originated from sediments extruded from below (or pushed in front of) the glacier at the surge terminus and deposited as a debrisflow. Ground penetrating radar can reveal small-scale structures, but larger structures and overall composition are better imaged by resistivity measurements.  相似文献   

13.
This paper presents the sediment, landform and dynamic context of four avalanche-fed valley glaciers (Khumbu, Imja, Lhotse and Chukhung) in the Mount Everest (Sagarmatha) region of Nepal. All four glaciers have a mantle of debris dominated by sandy boulder-gravel that suppresses melting to an increasing degree towards the snout, leading to a progressive reduction in the overall slope of their longitudinal profile. Prominent lateral–terminal moraine complexes, also comprising sandy bouldergravel, enclose the glaciers. These terminal moraines originally grew by accretion of multiple sedimentary facies of basal glacial and supraglacial origin, probably by folding and thrusting when the glaciers were more dynamic during the Little Ice Age. The four glaciers are in various stages of recession, and demonstrate a range of scenarios from down-wasting of the glacier tongue, through morainedammed lake development, to post-moraine-dam breaching. Khumbu Glacier is at the earliest stage of supraglacial pond formation and shows no sign yet of developing a major lake, although one is likely to develop behind its >250 m high composite terminal moraine. Imja Glacier terminates in a substantial body of water behind a partially ice-cored moraine dam (as determined from geophysical surveys), but morphologically appears unlikely to be an immediate threat. Chukhung Glacier already has a breached moraine and a connected debris fan, and therefore no longer poses a threat. Lhotse Glacier has an inclined, free-draining tongue that precludes hazardous lake development. From the data assembled, a conceptual model, applicable to other Himalayan glaciers, is proposed to explain the development of large, lateral-terminal moraine complexes and associated potentially hazardous moraine dams. – 2008 Elsevier Ltd. All rights reserved.  相似文献   

14.
Despite a long history of glaciological research, the palaeo‐environmental significance of moraine systems in the Kebnekaise Mountains, Sweden, has remained uncertain. These landforms offer the potential to elucidate glacier response prior to the period of direct monitoring and provide an insight into the ice‐marginal processes operating at polythermal valley glaciers. This study set out to test existing interpretations of Scandinavian ice‐marginal moraines, which invoke ice stagnation, pushing, stacking/dumping and push‐deformation as important moraine forming processes. Moraines at Isfallsglaciären were investigated using ground‐penetrating radar to document the internal structural characteristics of the landform assemblage. Radar surveys revealed a range of substrate composition and reflectors, indicating a debris‐ice interface and bounding surfaces within the moraine. The moraine is demonstrated to contain both ice‐rich and debris‐rich zones, reflecting a complex depositional history and a polygenetic origin. As a consequence of glacier overriding, the morphology of these landforms provides a misleading indicator of glacial history. Traditional geochronological methods are unlikely to be effective on this type of landform as the fresh surface may post‐date the formation of the landform following reoccupation of the moraine rampart by the glacier. This research highlights that the interpretation of geochronological data sets from similar moraine systems should be undertaken with caution.  相似文献   

15.
《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   

16.
Four major sedimentary facies are present in coarse-grained, ice-marginal deposits from central East Jylland, Denmark. Facies A and B are matrix-supported gravels deposited by subaerial sediment gravity flows as mudflows (facies A) and debris flows (facies B). Facies C consists of clast-supported, water-laid gravels and facies D are cross-bedded sand and granules. The facies can be grouped into three facies associations related to the supraglacial and proglacial environments: (1) the flow-till association is made up of alternating beds of remobilized glacial mixton (facies A) and well-sorted cross-bedded sand (facies D); (2) the outwash apron association resembles the sediments of alluvial fans in containing coarse-grained debris-flow deposits (facies B), water-laid gravel deposited by sheet floods (facies C) and cross-bedded sand and granules (facies D) from braided distributaries; (3) the valley sandur association comprises water-laid gravel (facies C) interpreted as sheet bars and longitudinal bars interbedded with cross-bedded sand and granules (facies D) deposited in channels between bars in a braided environment.The general coarsening-upward trend of the sedimentary sequences caused by the transition of bars and channel-dominated facies to debris-flow-dominated facies indicate an increasing proximality of the outwash deposits, picturing the advance and still stand of a large continental lowland ice-sheet. The depositional properties suggest that sedimentation was caused by melting along a relatively steep, active glacier margin as a first step towards the final vanishing of the Late Weichselian icesheet (the East Jylland ice) covering eastern Denmark.  相似文献   

17.
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18.
《Quaternary Science Reviews》2007,26(5-6):743-758
Detailed examination of the Tekapo Formation in the Tasman Valley, New Zealand has identified 20 facies, and five facies associations. These associations are delta foresets and bottomsets, sediment density flows, ice-contact lake sediments with ice-rafted debris and resedimentation deposits, and outwash gravels. Interpretation of the sediment-landform associations informed by observations at modern glacier termini suggests that the Late Pleistocene Tekapo Formation moraines have been formed by downwasting of a more expanded Tasman Glacier. During the early stages of glacier retreat, ponds on the glacier surface develop into thermokarst lakes which enlarge and coalesce to form a large supraglacial lake. Continued downwasting causes the lake outlet river to entrench into the impounding latero-frontal ice-cored moraine, lowering the lake level. This exposes lake-bottom sediments and forms shorelines on the proximal slopes of the ice-cored moraine. As the ice-cored moraine melts, these lake sediments are deformed and deposited against the Mt. John moraine. The observations and interpretations reported here suggest the Late Pleistocene end moraine is a constructional feature not a structural (glaciotectonic) feature as suggested by previous studies.  相似文献   

19.
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   

20.
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   

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