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)
- actice subglacial transport
- 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
- Ablation dominant moraine (AD) – which emerge at the surface as a result of the melt out of subglacial debris
- Ice-stream interaction moraine (ISI) – which find immediate surface expression downstream from glacier confluences, often by merging of two supraglacial lateral moraine
- 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
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.
- 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
- 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
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
Decoupled Ice Margin
In smaller glacier & arid mountain ranges, inefficient outwash discharge from glacial to fluvial systems, examples in Ngozumpa Glacier (
