Week 9: Glacial Sediments Flashcards
Components of the debris cascade
SOURCE
TRANSPORT
DEPOSITION
diagram
Primary sources
Subglacial
Extraglacial
Aeolian
Secondary sources
Pre-existing
What does transport type depend on?
Clast morphology
Size
Fabric
What does deposition type depend on?
Bedding
Structure
Grain size
Fabric =
Relationship between clast orientation in response to direct imparting of strain/stress by ice
Proglacial transport path =
In front of ice
Paraglacial transport path =
Valley side or system/floor interacting with glacial processes
Subglacial transport path =
Transports actively, reworked, often deformed
Different transport paths give different signals on clast e.g.
Striae/faceting/grounding
High level transport =
DEBRIS NOT IN CONTACT WITH BED
Above ELA debris buried beneath successive layers of snow
Below ELA debris stays on surface until reaches snout
= minimal alteration
Low level transport =
BED CONTACT
Material from subglacial erosion/crevasse fall/downward ice flow
Debris deposited subglacial or to glacier surface at snout brought by compression
= basal debris is modified
diagram
How can debris be brought to surface at snout by compression?
At glacier margine = switch from extensional to compressional states = elevates sediment to sub/englacial
Primary glacigenic deposits (tills)
Glacitectonite
Subglacial traction till
Subglacial melt out till
Glacifluvial deposits
Plane bed deposits
Ripple cross-laminated facies
Cross-bedded facies
Gravel sheets
Hyperconcentrated flow
Silt and mud drapes
Gravitational mass movement (subaerial and subaqueous)
Scree/debris fall deposits
Slide and slump
Debris flow deposits
Turbidites
Suspension settling and ice bergs
Cyclopsams/cyclopels
Varves
Iceberg rafted debris/diamicton
Dropstone diamicton
Under-melt diamicton
Iceberg turbate
Is subglacial often:
a) primary
b) secondary
sediment
Primary
- others get reworked and follow second pathway
Glacitectonite =
Originally something else, deformed but recognisable (unlike till)
I.E. HAS SIGNAL OF PRE-EXISTING MATERIAL WITHIN
= rock/sediment deformed by subglacial shearing (deformation) but retains some of structural characteristics of parent material which may consist of ig/met/sed rock or unlithified sediments (Benn and Evans 1996)
Sedimentary signal reflects…
The energy of the system
- action of water sorts the material
“Mass flow deposits spectrum”
Material that has flowed (poor sorting) –> flows with high water content –> flows themselves –> turbidity flows
Diamict =
mixture of different grain size
e.g. cobbles/gravel/clay/silt
Subglacial tills tend to be diamicts
N.B. difficult to differentiate subglacial till from debris flow
Till =
sediment deposited DIRECTLY from glacier ice which has not undergone significant disaggregation but may have undergone glacially induced deformation
PRIMARY GLACIAL DEPOSIT
“…is a sediment and is perhaps more variable than any sediment known by a single name” (Flint 1957)
What sediment is deposited indirectly from glacier ice?
Sediment settled through water/mass movement
Relating tills and diamictons
All tills are diamictons but not all diamictons are tills
How does till form?
Lodgement
Deformation
Ploughing
Melt-out
N.B. Most tills are hybrids with complex interactions between these processes
Subtraction till =
Encompasses all processes that may result in the production of till through time
Lodgement =
plastering of debris from base of sliding glacier to the rigid/semi-rigid bed
Frictional drag between clast and bed > shear stress due to moving ice = clast lodged and ice flows around
Lodgement in soft beds
Clast ploughs through substratum
Once ploughed sediment provides sufficient resistance to retard forward movement = lodgement
Preferential lodgement
- Of large (>10cm) clasts?
- When thicker > deforming layer + clast log jams + boulder pavements
= bridge between ice and more rigid substratum
How do you transition from lodgement till to deformation till?
Occurs as you thicken sediment
Controlled by water trapped in layer of till
More saturated = more able to deform
Melt-out =
passive subglacial release of debris by melting of glacier ice
= ‘thaw consolidation’/volume reduction due to debris content of ice (high = slight vol reduction = delicate englacial structures preserved)
Melt-out preservation potential
Balance between meltwater production and drainage away from site
High pore P in sediment pile/slurries = low preservation potential
If sediment structures complete suggests PASSIVELY let out
Secondary processes: - slope processes - stream reworking sediment - winds reworking sediment = lower
Effect of foliation on melt-out preservation
Horizontal foliation = foliation preserved
Dipping foliation = foliation dip modified
diagram
Subglacial mass foliage (cavity fill) =
common where ice flows over irregular bedrock - deposition due to gravity - slurry - fall - flow ... in subglacial cavities
Subglacial deformation =
When applied shear stress > material yield strength = sediments form mobile deforming layer beneath glaciers flowing over SOFT substrates
What is the deforming layer in subglacial deformation made of?
Pre-existing sediment
Soft bedrock
Sediment deposited in syn-sediment context then sheared
“MOSAIC”
Continuum of glacitectonite
From lightly deformed parent material –> heavily sheared deposits
Type A glacitectonite
Penetrative
Original structures truncated by glacitectonic elements (banded/laminated)
Type B glacitectonite
Non-penetrative
Small cumulative strains
Preserve pre-deformational structures
Subglacial traction till =
sediment deposited by glacier sole sliding over +/ deforming its bed, having been released directly from ice by P melting +/ liberated from substrate then disaggregated and completely/largely homogenised by shearing (Evans et al 2006)
Completely homogenised by subglacial shear = uniform till appearance
Subglacial tills =
hybrids; lodgement/deformation/melt-out occur across glacier bed at various spatial and temporal scales
Melt-out till =
Sediment released by melting/sublimation of stagnant/slowly moving debris-rich glacier ice and directly deposited without subsequent transport/deformation (Benn and Evans 1998)
Melt-out till at Matanuska Glacier, Alaska layers
Lawson 1981
a) Structureless pebbly, sandy silt
b) Discontinuous laminae, stratified lenses and pods of texturally distinct sediment in massive pebbly silt
c) Layers of texturally-, compositionally- or colour-contrasted sediment
Layers may drape clasts
Is sediment in hollows affected by stress?
No = preserved
Communition =
Slow break down and mixing of one material into another
“commented gradation”
Subglacial till mosaic horizontal info
DIAGRAM
Evans et al 2006
Area of pre-existing lake sediments
Area of till sliding (striated/polished bedrock)
Lee-side cavity
Descending hydro fracture tills
Melt out till hollow re-coupled with bed
Overridden glacilacustrine sediments
Ascending hydro fracture tills
Glacifluvial system controls
Topographic setting
Buried ice
Total sediment in transport
Buried ice effect on glacifluvial system
Melts = smooth, flat braided landscape –> chaotic environment i.e. kame and kettle
What determines the total sediment in transport?
1) DISCHARGE
2) SEDIMENT SUPPLY (cut and fill)
Typical landform-sediment assemblages
SANDUR
KAME AND KETTLE TOPOGRAPHY
ESKERS
Sandur =
braided outwash plain
Esker =
sinuous looking ridge close to margin/sub-margin
Coarse/fine sediment and where its found
Coarse = proximal = cut and fill/scour = channelise landscape
Fine = distal = low energy channels and deposition dominates
Bedform definition
Grain size and flow velocity give rise to form, wavelength and height of bedform
diagram
Hyperconcentrated flow =
complete mixture of sediment in suspension from boulders to clay, en mass laminar flow
How do bedforms form?
- Suspended load, hyper concentrated flows, v high discharge
- Bedload (sliding, rolling, saltating particles)
Froude no. >1
= supercritical flow (fast and shallow)
Froude no. <1 =
Subcritical flow (deep and slow)
Higher energy bedforms are…
Closer to ice
Main types of bedform
Ripple cross-lamination (current ripples)
Planar and trough cross cross bedded gravels/sands
Gravel sheets
Hyperconcentrated flow (debris slurry)
Silt and mud drapes (waning discharge)
Nature of subaqueous glacial sediments dictated by:
- Water density and stratification
- Bathymetry
- Density of inflowing meltwater vs absent lake/seawater
Stratification occurs due to
T/salinity
Bathymetry =
glacier/water interface
Indicates whether glacier fed/ice contact
Position of inflow in water common due to…
Relative densities
Greater difference = less mixing = PLUME
Overflow
Most common in glacimarine
Low density plume
Warm meltwater
Low levels debris
Interflow
Glacimarine
Similar densities
Underflow
High density plume (due to low T)
High concentration suspended sediment
Sinks = turbidity current
Ice berg drop, dump and grounding structures e.g. ice berg dump
Ice berg tips, sediment on top falls and creates cone on sea floor
Deltas =
Sediment masses built into standing water by fluvial transported debris delivered to shore
Tool for when ice terminated in water body and fixing former SL because always trades into UPPER body of water
Delta graded sequence
Fluvial gravels (topsets)
Dipping sand/gravel (foresets)
Merge distally
Finer grained (bottomsets)
Deltas; water level =
Fluvial aggradation
Deltas; delta front =
Progradation
Grounding line fans =
aka subaqueous outwash fans
= fan shaped depocentres where sub and englacial streams debouch subaqueously
Debouch =
Emerge from confined space into wide, open area
Processes in grounding line fans
Meltwater processes/facies dominate
Mass flow also important for remobilisation of unstable ice proximal sediments
Streams exiting grounding line immediately dump bedload
Sediment gravity flow = underflow = turbidity current (starts to sort)
Also rising turbulent plume:
1) settling layer/suspension settling
2) turbid overflow plume
Decrease in flow competence/sedimentation rate away from inflow points
Proximal sediments in grounding line fans
Coarse gravelly subaqueous outwash
Gravelly mass flows
Distal sediments in grounding line fans
Settling out of suspension = laminar stratified silts/clays
Proximal cyclopsams and distal cyclopels
Bioturbated massive mud
Glacilacustrine varves
Cyclopsams/cyclopels =
couplets of graded sediments related to tide
Grounding line fans vs turbidity currents
Difficult to distinguish
qSubaqueous and subaerial mass movement =
Fall Slide Slump Debris flow Turbidity currents
Important agents of sediment transfer
Matrix cohesion effects
Strongly influences resulting flow behaviour/deposits
Debris flows vs turbidity currents
Debris flows = high concentration plastic slurries
Turbidity currents = rapidly moving turbulent underflows
Transitions between flow types due to changes in water content/strength during flow
Creep =
Slow inter granular frictional sliding with quasi-static grain contacts
Slide =
Transiational/rotational; coherent mass with minor internal deformation
Slump =
Coherent mass with considerable internal deformation
Flow (with plastic behaviour) =
Remoulded mass
Non-turbulent bus possibly with transient large scale turbulent churning
Flow (with fluid behaviour) =
Fully turbulent
Fall =
Solitary grains/loose grain assemblages
