Sunday, June 28, 2009

GLACIATED VALLEY SYSTEMS

Incorporate icemarginal features including supraglcial, subglacial, progracial, (fluvial, glaciofluvial), periglacial & paraglacial landsystems

Two Factors:
- Influence of topography on glacier morphology, sediment transport paths and depositional basin

- Importance of debris from supraglacial sources in the glacial sediment budget

Clean glaciers – glaciers with limited supraglacial debris
Debris – covered glacier – glacier with substantial debris cover in ablation zones

Sediment sources

Process involved in delivering debris tto glacier surface – debris flow, snow avalanches, rockfalls, rock avalanches

Tectonically active regions – earthquake generated rock avalanche

Glaciers with high debris soncentration – rockfall rates are high, snowfall is low

Sediment Transport pathways

2 sediment transport pathways (Boulton)

  1. actice subglacial transport
  2. passive supraglacial or englacial transport

active because in basal shear zone, high particle collision undergoes significant abrasion, fracture or communition

sediment in upper layer – little modification, retains characteristics of parent debris

Moraine Classification

(By Eyles & Rogerson)

– based on relationship between debris supply and morphological development of the moraine

  1. Ablation dominant moraine (AD) – which emerge at the surface as a result of the melt out of subglacial debris
  2. Ice-stream interaction moraine (ISI) – which find immediate surface expression downstream from glacier confluences, often by merging of two supraglacial lateral moraine
  3. Avalanche type (AT) moraine – which are transient features formed by exceptional rockfall events onto a glacier

Dynamics of Debris covered glaciers

Thin debris cover – enhances ablation due to reduced albedo & increased absorption of long & short wave solar radiation

Thick cover – reduces ablation (due to low thermal conductivity

LAND SYSTEMS OF GLACIAL DEPOSITION

Ice marginal moraines & related landforms

Processes of moraine formation

Moraine formation at glacial margin with limited supraglacial debris

- Pushing (margin in buried by glacifluvial deposits/ Debris flow)

- Dumping of supraglacial debris

- Squeezing (fine grained saturated sediment present at the margin

Thrust moraine

Glaciers in contact with thick unconsolidated sediments such as glacimarine clays and silts

Moraines <10m>

Resistant crystalline rock

Lateral moraines and boulder lines

Lateral-Terminal Moraine Complex

Lateral Moraines – extend from equilibrium line as continuous sharp crested ridges increasing down glacier in cross section

Upper ablation Zone – moraine have little distal slope, a debris veneer accreted onto the valley wall

Down valley – moraines separated from valley wall forming lateral moraine trough

Lateral – frontal moraine – as debris fall, slumps, slides or flows down the ice edge and accumulates around glacier margin

Ice-proximal parts of lateral-frontal moraine – structurally complex because of widespread collapse & reworking following removal of ice support

Breach-lobe moraines

Inset loops or lobate moraines

Multi-lobed moraines – terminus morphology to a long term expansionary tendency and repeated breaching of main moraine barrier

Explanation based on morphology of glacier e.g. HATUNRAJU of Peru

Ablation Valleys – infills in lateral moraine trough

Separate lateral moraines from valley side slopes and act as gutters trapping slope debris transported by processes including rock fall, debris flow, snow avalanche, & fluvial transport

Within valley Asymmetry of lateral moraines

Large moraine volume on one side of the valley than on other

Causes of asymmetry

- Larger moraine occur on valley sides with extensive rock walls – increase debris supply

- Lateral moraine formed by pushing and thrusting of pre-existing material within valley asymmetry – result from differences in thickness & type of sediment on the foreland

- Cross valley differences in lithology or structure – influence debris supply to surface or bed

- Asymmetry may occur due to difference n glacier dynamic on either side of the valley.

Sub glacial sediments & Landforms

- Ice moulded bedrock – occupy upper part of former valley glaciers

- Striated roche motonees, whalebacks, ovedeepened rock basins – abrasion features

- Downvalley – rock outcrops, downstream of roche moutonees (lee side cavity frills)

Over consolidated lodgement or high strength deformation till – matrix support, fissile structure, abundant faceted striated clasts

Fluted moraines – down glacier, low preservation, may not survive more than few decades

Facies of Glacier retreat

Recessional or hummocky moraine

- Mark positions of annual readvances or more significant longer turn advances of the margin

- Common in low relief mountains

Hummocky moraine – products of widespread glacier stagnation (common in Scotland)

- Polygenetic

- Consisting of recessional moraines forming converging cross valley pair

- Flow-parallel drumlins and flutings

- Non-aligned mounds and ridges, recording uncontrolled ice-marginal deposition

Till sheets

3 facies of deposition based on association between activity of terminus, thickness of supraglacial cover and reworking of meltwater

Facies 1 – thick reworked accumulation of supraglaciall till deposited by back wasting & decay of melting ice cores buried beneath thick debris cover

Stationary ice terminus, predominance of meltwater process results in a chaotic disintegration of topography, final product does not reflect geometry of ice margin (Eyles)

Facies 2 – dispersed bouldary veneer by dumping from a retreating terminus, downglacier lineated pattern reflected deposition focused by structures such as gullies in the ice front. Thin debris cover, no relief inversion associated seasonal dump moraine – internal bedding due to gravity sorting

Facies 3 – supraglacial till complex comprising of interfingering lensate horizons of supraglacial meltout till & glaciofluvial sediment – areal extent greater at inactive, low gradient termini where meltwater streams & ponds occur within ice-cored terrain – distribution & relative development of facies may aid in reconstruction of ice margin dynamics during glacier retreat

Evolution of downwasting, debris cover glacier termini in Iceland – documented by kruger

Final deposition assemblage – low relief, hummocky topography underlain

Medial moraines

Supraglacial debris on valley glaciers, delievered to terminus – medial moraine

Seldom preserved after deglaciation

Contain small amount of debris & tend to undergo considerable reworking during glacier ablation

Deposition – longitudinal bands of facies 1 & 2

Rock Glaciers

Tongue like or lobate masses of ice & coarse debris that flow downslope by internal deformation

Ridges, furrows, lobes on surfaces, steep fronts down which debris collapses & overridden by advancing mass

Twofold genetic classification

- Periglacial rock glacier involve the slow deformation of ground ice below talus slopes

- Glaciel rock laciers

Form by the progressive burial and deformation of a core of glacier ice by a thick boundary debris mantle

In mountain environment

Rock, snow & ice are delivered to the base of slopes by avalanches & other mass movement processes

- Negligible rock component – clean glaciers will form where snow and ice con survive ablation over the balance year

- Snow & ice component zero – talus slope

- Rock component relatively high, debris accumulates as a lag on the ablation zone of the dirty ice mass – debris covered glacier

- Rock component much higher, avalanche snow and ice will occur as isolated but deformable lenses within a talus – rock glacier (g high mountain environment as Khumbu Himal, Karakoram, Lahul Himalaya)

Dependant on climate

Decreases precipitation/increase temperature- increased proportion of rock, formation of rock glacier

Glacial retreat – active rock glaciers as head of former rock glacier forms

Rock-glacierized moraines of Canadian Arctic

Coupled Ice Margin

Efficient transfer of sedimets between glacier & proglacial fluvial system

e.g. Humid mountain ranges of Alaska & new Zealand, from last glaciation in New Zealand & southern South America

Decoupled Ice Margin

In smaller glacier & arid mountain ranges, inefficient outwash discharge from glacial to fluvial systems, examples in Ngozumpa Glacier (Nepal), Hatunraju Glacier (Peru), Miage Glacier (Italy)