Seds - Fluvial and Alluvial Systems Flashcards
Braided River Systems
- Multi-thread channels, high energy, steep valley gradients (<0.5o), large and variable discharges, non-cohesive banks, bedload transport
- Grade from gravel to sandy-bed rivers
- Large and variable discharge = seasonality in braided rivers – large flux of sediment movement and then not much
- Bedload transport – bed of river does the transporting
- Bedforms
- Braid bars, Dunes, scours, bedload armouring
Braid Bars
• Creation
o 2 channels coming together creates eddy
o Causes scour
o Sediment is picked up and moved downstream quickly
o Acts as a point of nucleation
o Bifurcation will cause drag
o Will slow flow down and cause deposition as its not a flat smooth bed anymore
• Start of bar are lozenge shaped and flow bifurcates around it, causing deposition again
• Coarser in the middle – coarse material dropped first and then finer material placed on top – fine upward
• Bank bars are more restricted and do not move as much
Pebble Clusters
• Pebble Clusters:
o Pebble clusters are an excellent example in how drag can influence what is entrained in to a flow and also be used as a palaeocurrent indicator (palaeoflow of river is from right to left according to the pebble cluster).
o Pebble clusters
o Bedflow armouring – boulder or oversized clast moved in rare weather event – regular flow packs smaller clasts behind it – larger ‘v’ on upstream, smaller fine grained sediments on downstream - stabilizes river system
o Larger pebbles on opposite flow direction side
Trough cross bedding
o 3D bedform o Mostly found in braided systems o High lower flow regime o Undulation ‘ice-cream scoop’ on surface o Ancient structure
Overall Braided log?
• Lots of bars o Transverse bars or Longitudinal bars • Coarser grained • Straight multi-channeled • Grading up bars • Blocky grainsize profile o Waning into finer material • Little variation
Meandering stream
• Single channel, high sinuosity (channel length/channel reach length > 1.3)
• Migrate by selective bank erosion, point bar deposition, meander cut off and avulsion
• With bankfull discharge cross channel shear - helicoidal flow
Only time processes take place, during normal conditions sediment does not move
• Create terraces as they cut down through topography
• Important features
Fining up point bars (deposition)
channel avulsion
How the banks are selectively eroded and migrated
crevasse splays
Scroll bars – surface impressions – channel margin held up by sandy levees which doesn’t compact at same rate as the mud of the flood plain so it protrudes as the channels migrate without it – shows paleochannels
Helicoidal flow
• Helicoidal flow- faster erosion on the right and then the water ‘sloshes’ back and overturns to the right, causing deposition
• Outside of bend – faster, more erosion
o Leads to Dunes
o Ripples and bioturbation on inside bend
Cravasse splay
• Crevasse splay – breach in levee leads to deposition of sands
o Usually on outside of meander bend
Splays sand out into flood plain
Well sorted, mature sands
Can get sand in muddy flood plain environments therefore
Flash flood like – horizontal expansion and deposition of load
Thin and wedge away from channel
Small scale lamination within
Alluvial fan overview?
• Small discrete body of sediment – with apex and toe
• A semi-conical, downstream fining, sediment accumulation predominantly of alluvial origin, resulting from loss of transporting capacity due to horizontal flow expansion
• Stream emerges from a mountain belt and deposit broad cone shaped bodies of sediment
• Unconfined flow leads to rapid lateral expansion
o Friction causes fast deposition
• Radius typically 2-15 km
• Conglomerates, breccia, sandstones and mudstones
• Coarsest sediments found closest to mountain front (proximal) and finest further away (distal)
• alluvial from Latin “to wash against” vs. fluvial (Latin “river”)
• point-sourced from streams issuing from a drainage catchment.
• fan toe grades gently into a basin floor environment
• coalesced fans, particularly along faulted mountain fronts Bajada
• Source area: Mass wasting (e.g. mud flows, debris flows, rock falls) leads to gravity flows (mass flows). Stream water can later rework the deposits out across the fan.
• Slopes less than 10 degrees, <100km2
• Shape related to grain size
Two types of controls:
Allocyclic Controls • External controls - Tectonics - Climate - Sea level Autocyclic • Self-regulating • Internal controls - Gradient - Sediment grade - Sediment load
Graded River Profile:
• All depositional systems have an equilibrium/baselevel surface
• Surface separates erosion from deposition and may reflect several base levels
• e.g. lake level; nickpoints; sea level
Nickpoints – Where rivers will cut down or aggrade up to create a uniform equilibrium profile – never achieve it
• A River will react to stimuli and change in character: internal attributes - autocyclic mechanism
• e.g. channel width; roughness, gradient; discharge; sediment calibre; velocity; depth
• Long term changes - allocyclic changes
• River will attempt to equilibrate and Autocyclic and allocyclic processes overprint this attempt
Sed structures: Pebble Clusters
o Pebble clusters are an excellent example in how drag can influence what is entrained in to a flow and also be used as a palaeocurrent indicator (palaeoflow of river is from right to left according to the pebble cluster).
o Pebble clusters
o Bedflow armouring – boulder or oversized clast moved in rare weather event – regular flow packs smaller clasts behind it – larger ‘v’ on upstream, smaller fine grained sediments on downstream - stabilizes river system
o Larger pebbles on opposite flow direction sid
Pebble imbrication: in highly turbulent flows pebbles can be kept in suspension and preserve an angle to flow
Pebbles dip up stream
‘a’, ‘b’ & ‘c’ axes, best displayed in platy and discoidal pebbles
Sed structures: Trough Cross Bedding
o 3D bedform
o Mostly found in braided systems
o High lower flow regime
o Undulation ‘ice-cream scoop’ on surface
o Ancient structure
it indicates that the depositional environment contained a flowing medium (typically water or wind).
Outcrop detail of braided rivers:
- Blocky grain size profile; limited amount of fine-grained silts and muds; often with Txb, Pxb and some ripple xl. Large scale inclined foresets typical of bars (frontal and lateral)
- If got sandy terrain = on top of bar
- Lots of trough cross bedding + some gravel = In channel or in the core of the bar
Overall Braided Fluvial System:
• Lots of bars o Transverse bars or Longitudinal bars • Coarser grained • Straight multi-channeled • Grading up bars • Blocky grainsize profile o Waning into finer material • Little variation
