Lecture Thirty - River (fluvial) environments, dynamics and successions Flashcards
Summeries the different environments found in different depositional settings.
Continental:
Fluvial - alluvial fan, braided stream and meandering river.
Desert.
Lacustrine.
Glacial.
Marginal marine:
Deltic - delta plain, delta front, prodelta.
Beach/barrier bar.
Estuarine/lagoonal.
Tidal flat.
Marine:
Neritic - shelf, organic reef.
Oceanic - slope, deep ocean floor.
What is geomorphology?
Geomorphology = the surface shape of the Earth (or any planet).
What are regional source influences of river dynamics?
Long term tectonic stability leads to lowering of relief and development of low energy river systems.
Tectonic activity leads to increased relief and higher energy river systems.
Wet climate and high relief produces consistently high energy flow conditions (e.g. high rain fall, snow meltwater settings).
Dry climate produces epesodic high energy flow events/conditions.
Sudden events in source area cause disequilibrium in river systems, even in low gradient areas.
–> Erosive and channel geometry change.
E.g. Regional tectonic uplift causes long term activity level increase in river system because erosion rates are increased.
E.g. Major volcanic events cause shorter term disequilibrium.
- Rivers become chocked with volcanic sediment.
- Very rapid discharge rate.
Explain sediment transport by running water.
Erosion of sediment:
Removal of sediment from source by gravity, ice, wind, or force of running water.
Transportation of sediment occurs during turbulent flow through:
Bed load (rolling, sliding).
Saltation (bouncing).
Suspension.
Dissolved ionic material.
Downstream deposition of sediment:
Channel fill.
Avulsion (of ‘channel switching’)/
Formation of bars.
Explain alluvial fans.
Alluvial fan:
At margins of tectonically acitve mountain ranges.
Fans of course sediments up to 100’s of m thick.
Radius - kms to 10’s of km.
Form in all environments, mostly common in arid to temperate settings, and this is where sediments can condense without being carried away by water. Therefore when a little water does come it can carry a large amount of settled sediments.
Form where mountain rivers spread onto adjacent plains.
Common in tectonically active areas.
Fans that build into standing water are called fan deltas.
Multiple shallow braided channels on fan surface.
Gradient and flow velocities decrease from apex to edges.
- Course deposits near fan apex, and fine distal deposits.
Facies belts prograde (ance towards the sea as a result of the accumulation of waterborne sediment) outwards as fan grows.
Proximal fan deposits:
High flow velocities = torrent flow conditions.
–> Massive deposits (upper flow regime).
Deposits:
Distal fan deposits:
Stream flow deposits.
–> Stratified and cross-bedded = upper and lower flow regime.
Debris flow deposits:
Massive deposits.
Fan grades into normal braided river, lake or transverse trunk river beyond distal margin.
Depositional model:
Thick (100’s m) conglomerate-sandstone succession in stacked multiple channels.
High velocity structures, mostly tractional.
Radial grain size fining.
Radial paeleocurrent patterns.
Can be found in Zagros mountains (Iran), Death Valley (USA) and Svalbard (Norway).
Explain plains rivers.
Include:
Braided river.
Meandering rivers.
Straight rivers - bedrock controlled.
Anastomosing rivers - several linked stable channels, in well vegetated, clay filled flood plain.
Form on lower gradient plains than alluvial fans.
Sediment depositions dominates over erosion.
- Erosion during peak floods.
Deposition during waning floods and normal flow.
Factors determinaing braided vs meandering:
High water discharge, slow/gradient, sediment load and sediment size = braided river.
Low water discharge, slow/gradient, sediment load and sediment size = meandering river.
Explain braided rivers.
Braided rivers:
On high gradient alluvial plains adjacent to or within margins of mountain ranges.
Multiple, unstable, interconnected straight to slightly sinuous channels.
Occur in tectonically active regions (e.g. NZ, Himalayas and Alaska).
Form broad channelized plains up to km’s wide.
Sand and gravel bars ebtween channels (braidede river bars, or just bars).
High flow velocitices.
- Course sediment (gravels and dand) deposited in channels.
- Fine sediment carried through in suspension.
High velocity sedimentary structures:
Massive beds, planar lamination.
–> High velocity flood flow.
Dunes and cross beds.
–> wanning, normal flow conditions.
Channels short lived:
Choke with sediment and relocate during floods.
Depositional model:
Stakced, cross cutting multiple channels and bars.
Course sediment dominant, muds minor.
Sudden increase in flow rate.
–> Erosional surfaces.
High velocity sedimentary structures.
Explain meandering rivers.
On low graident alluvial plains over extensive, tectonically stable continental areas (fairly low topographical regions).
Channel is asymmetrical in cross section.
1) Steep bank:
- Outside of the corner.
- Erosion by helical flow of stream.
- Highest flow velocities.
2) Shallow bank:
- Inside of the corner.
- Depositional (as this is where the flow is least strong).
- Point bar.
- Lowest flow velocities.
With continuing sedimentation on bars sinusity increases.
Sediment grain size:
Coursest in channel axis (sand, gravel).
Finer in point bar (fine sand, silt).
Finest on flood plain (silt, mud).
Sedimentary strucuture:
Steep, deep out side bank and shallow, inside bank (when looking at a cross section of a meandering river).
Flood plain:
Wide, vegetated.
- Rich in organic carbon.
- Often fossiliferous (plants).
Periodically inundated by floods.
Deposits layers of silt/mud, sand only near channel on levee banks.
Depositional model:
Discontinuous sand lenses (‘shoestrings’ in transverse section).
+
Sheets of thick, widespread flood plain mudstones.
Typical of tectonically stable continental plains.
E.g. Murray River, Mississippi river.
