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