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Glaciofluvial Environments • Landforms Assemblages

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Glaciofluvial Environments • Landforms Assemblages Glaciofluvial environments • Landforms assemblages: All glaciofluvial forms/sediments occupy positions on a spatial and temporal continuum. Drainage networks in receding glaciers (constantly deforming) evolve by: - enlarging drainage pathways; expand so much that tunnel system is larger than glacier system itself; spatial distribution changes (size continuum) and temporal (move) - combining drainage pathways; tunnels beneath others may collapse creating larger tunnels - changing from hydrostatic>atmospheric pressures; e.g. meltwater still present in advancing glaciers, just moves to subglacial under hydrostatic pressure and so water forced through tunnels due to over flow pressure of ice; as ice melts may cause switch to atmospheric. - R channel esker: glacier drainage network; water cutting up into ice rather than down into the bed, fills with esker sediment; atmospheric pressure produces sediments to be deposited. - H channel esker: due to pressure change; move from hydrostatic to atmospheric channel, produces widening of the tunnel with flat topped profile due to AP. - Ice walled: due to tunnel collapse. Opens up to atmosphere producing supraglacial stream running across. - Kame and kettle topography: due to ice surface drainage; ice continuously melting so anything that fills a hole is constantly changing shape and position until full melt. - Proglacial sandur progradation: final stages of meltwater produce growth of sand and gravel out in front of ice. • ESKERS: sit within low linear landscapes, esker runs through the middle where ice would have been located. Can have lots of branches, tunnels breached; find 2 types of morphology. As tunnel collapse, create ice channel fills, then glacier ice retreats and water retreats back. - R channel fills: under hydrostatic pressure. Hummocky long profiles. H channel fills: can’t move uphill and so are aligned with bed slope as ice doesn’t determine direction of travel. Includes valley eskers: subglacially engorged askers. Beaded esker: sediment deposited in large part of the tunnel, ability to change pressure as approach margin. - Characteristics: a) single continuous ridge, very rare as water normally breaches tunnels; b) single ridges with variable height and width (beaded); c) single low ridges linking numerous mounds/beads; d) complex braided systems: bumps throughout landscape. ALIGNMENT: water in tunnels follow pressure gradient and so are parallel to direction of glacier flow; transverse to flow caused by atmospheric pressure as ice is not moving the water e.g. valley eskers – moves over ridges, creates moraines. - Regional patterns (planforms): found eskers that are 100skm long; deposited in segments during retreat in the ablation zone as needed meltwater; channel system migrates towards ice sheet centre. When mapping can see long ridges with gaps between where likely to see a moraine (ice marginal features) e.g. Laurentide ice sheet divide. - Regional distribution: Storrar (2014): mapped 20,000 eskers on Laurentide ice sheet bed; found eskers became more frequent during deglaciation due to increased rates of ice margin recession and climate warming – more meltwater and so more channel infills. Meltwater becomes channelised, getting water into channels becomes increasingly important during deglaciation. Towards ice divide (of ice moving in diff directions), channels more numerous, denser network as recedes. - Incremental deposition of long eskers: - 1) Boulton (2007): eskers are complex sequences in corridors back towards ice margin. Each esker fed by a drainage basin; groundwater drains above bed, driving distribution of tunnels. Axis of tunnel remains stable showing strong coupling between groundwater and summer melt. - 2) Hooke & Fastook (2007): Laurentide ice sheet. Found eskers in straight tadpole shape segments due to: a) separate deposition of each segment in sub marginal tunnel, towards the margin tunnel gets wider due to more atmospheric pressure and so deposition; b) melt and sedimentation rates increase with increasing glacier margin surface slope as picks up energy to transport sediment; c) melt rates exceed tunnel closure rates by increasing amount towards margin and reduction in water flow deposition (less ice pressure, higher melt) – under hydrostatic pressure trying to close but closes in winter due to ice creep. - Esker sediment: Silts to boulders and diamictons. Tills also squeeze up into tunnel as less pressure there. Cut and fill sequences produced by variable discharge i.e. laminated then cut and filled with coarser material. Sediment demonstrate glacier melt patterns/tunnel constriction e.g. increased velocity of expansion of tunnel. - Formation: single tunnel shown as sinuous ridge. Become infilled with sediment after opening up of tunnel but pressure produces cut and fill sequences. Cavities: valves - water leads into causing drainage of channel, discharge drops and sediment deposited over wider area. These can be open or closed due to water pressure causing floating or decoupling of glacier = beaded esker. Mostly englacial but can be supraglacial. - Supraglacial eskers/ interlobate moraines: initially sub/englacial, evolve in supraglacial ridges (trough fills) and widening produces supraglacial lakes. At ice divide, ice downwastes leaving ice wall channel, get infill between leaving delta like feature. Later evolution: leaves kame and ice walled lake plain, chaotic areas of sediment. - Surge eskers: 1996: glacier sits over volcano, building large reservoirs, lets go every 11 years; water drains leaving esker and ice walled lake years later. - 1964 Iceland surge: Produced high catastrophic discharges; water escapes onto snout surface due to tunnels unable to carry rapidly rising volumes through crevasses on surface causing sediment to be deposited near these, can overlap; forms zig zag eskers which are crevasse infills. - Esker tunnel/lake interface: where esker enters lake, splaying of sediment into cavities due to flotation of tunnel mouth. Water loses velocity, drops sediment into deepwater column, sediment splays into centre e.g. beaded; e.g. Hooker glacier: debris covered, many tunnels full of sediment, underneath proglacial lake causes constant deposition of complex sand and gravel deposits; e.g. Oak ridge lake: moraine in deep water due to constantly filled with sediment (Barnett, 1998). • Kames: found at margins as glacier downwastes. Where large holes in ice, fill with lakes which connect to the bed (kame plateau). Tunnels where there are eskers, kame etc which collapse. • KAME AND KETTLE: fluvial deposition on wasting glacial surface. Guided by controlled ridges of debris charged ice and infilling of glacier karst (eroded limestone) but difficult to identify from pitted outwash. Heavily disturbed stratified sediments. - Kame terraces: ice marginal stream beds, accumulate sediment in a gutter between glacier and valley slope – deposited on valley slopes as more stable but collapses as melting occurs producing hummocks with flat tops. Lots of faulting and kettle holes. • Ice walled lake plains: circular plateaux (flat topped hills); surrounded by chaotic kames, eskers and hummocky moraine. Sediment is locally disturbed lake and glaciofluvial - Formation: infills of glacier karst holes, sediment accumulates in big holes, ice melts surrounding leaving upstanding plateaux with collapsed sides due to atmospheric pressure. Stable (debris charged ice) and unstable (debris poor ice). • OUTWASH SYSTEMS - Sandur: proglacial outwash fan with braided channel network e.g. Ellesmere Island, Canada - Morphology: 1) Proximal zone: few deep and narrow channels due to higher energy carrying large sediment; 2) Intermediate: complex wide and shallow braids due to less energy; 3) Distal: very shallow, ill defined channels due to low energy and so just sand and silt deposits. - Pitted sandar: Pits on surface. proximal outwash buries glacier snout forming overlap with kame and kettle (differentiation difficult); or ice blocks buried on jokulhlaup sandur. 1918 example: outwash extended coastline by 8km due to so much sediment produced, catastrophic impacts across Europe. Type of kettle relates to amount of sediment within iceberg as it melts (diagram). - Valley trains: deposition outwash confined by valleys; this is then incised producing terrace flights; individual terrace levels can be related to and paired with moraines - Architectural elements: relate to different processes within a stream and so provide evidence of discharge history e.g. braided channels. Coarser material located in proximal areas of outwash; finer located intermediate/distal. - Vertical profile models: glaciofluvial successions represent a continuum of variation (Miall, 1977). • INTERGRATING GLACIOFLUVIAL SEDIMENTS/LANDFORMS: temporal and spatial evolution. .
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