MEANDERING FLUVIAL SYSTEMS

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Chapter 4
DEPOSITIONAL ENVIRONMENTS
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DEPOSITIONAL ENVIRONMENTS

• Sedimentary processes all leave their impress in
  some way on final sedimentary rock product.
• A sedimentary depositional environment is defined
  as a part of the earths surface which is
  physically, chemically, and biologically distinct
  from adjacent areas.
• A sedimentary environment is characterized by an
  unique set of physically, chemically, and
  biologically processes at a specified rate and
  intensity.
• A change in one variable may cause changes in all
  the others.

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• ENVIRONMENTS OF DEPOSITION
• Physical envionment
• Chemical envionment
• Biological envionment
• The physical environment is characterized by
  static and dynamic elements.
• Static physical elements include basin geometry
  depositional materials such as siliciclastic
  gravel, sand and mud water depth and
  temperature.
• Dynamic physical elements include factors such as
  energy and flow direction of wind, water and ice
  rainfall and snowfall, all of which influence
  currents and waves.

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• ENVIRONMENTS OF DEPOSITION
• The chemical characteristics of the environment
  (salinity, pH, Eh, and carbon dioxide and oxygen
  content of waters) control chemical processes
  such as mineral precipitation and solution.
• The biological aspects of the environment can be
  considered to encompass both the activities of
  organisms (plant growth, burrowing, boring,
  sediment ingestion, and extraction of silica
  and calcium carbonate from water to form
  skeletal materials) and the presence of organic
  remains as depositional materials
• (Boggs, 2001).

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Where would you find different sedimentary rocks?
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DEPOSITIONAL ENVIRONMENTS

• Non-Marine Environments
• Fluvial Environment
• Meandering River Deposits
• Braided River Deposits
• Alluvial Fan Deposits
• Desert Environment
• Lake / Lacustrine Environment
• Glacial Environment
• 2. Marine Environments
• Neritic (Continental-shelf) Environment
• Oceanic (Pelagic) Environment
• 3. Mixed Environments
• Beach (Low-open shore) Environment
• Shore-Lagoon Environment
• Delta Environment
• Reef Environment

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• Non-Marine Environments
• Fluvial Environment
• Meandering River Deposits
• Braided River Deposits
• Alluvial Fan Deposits
• Fluvial Associations

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FLUVIAL ENVIRONMENTS

• Most important agent of transport
• Characteristics
• Gradient,
• Discharge,
• Sediment supply and type,
• Setting (climate and tectonics)
• Types
• Meandering streams, Braided streams, Alluvial
  fans
• A continuum

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FLUVIAL ENVIRONMENTS
Gradient
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Alluvial Fan Deposits
Small fan emerging from a wineglass canyon. 
Death Valley, CA.
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Braided River Deposits
Resurrection River, Kenai Peninsula, Alaska
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Meandering River Deposits
Owens River, CA.
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Principal types of rivers
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MEANDERING RIVER DEPOSITS
Sinuous single channel
Meandering River in northern Owens Valley, CA
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Meandering river deposits

• Sinuous single channel
• Low gradient - fine sediment load (mixed and
  suspended load)
• Moderate to high discharge
• Periods of increased flow erosion and
  deposition
• Straight-line helical / transverse spiral flow
• Lateral and downstream migration of meanders
• Channel cut-offs result in oxbow lakes
• Avulsion results from channel breach
• End in delta systems

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Meandering River Channel
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Where is current going to be fastest? Answer
look at channel cross sections. Start with
straight section of a river.
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Then, what happens when this fast current hits
the corner? Answer it is carried by its
momentum to outside of bend. Overall, as current
moves along a meandering river, fastest part of
current goes from outside corner to outside
corner, crossing over in the middle between bends.
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• Two results
• First, deepest parts of channels are along bends.
  These are termed "pools". Cross-over points are
  called "riffles". The line that traces the
  deepest parts along a given river is called the
  "thalweg".
• Second result fastest currents shift from
  side-to-side in a river. This ends up creating a
  secondary cork-screw current within the river
  called a transverse or helical flow.

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Channel Cross Sections How do fast currents
affect shape of channel ?
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Channel Cross Sections How do fast currents
affect shape of channel ?
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The helical flow pushes the highest velocity
currents down along outer bank. Outer bank thus
sees strongest flow parallel and perpendicular to
main river flow. This causes lots of scouring
(erosion). The helical flow also helps carry
previously eroded sediment toward the inside of
the bend, where water velocities are lower and
thus allow sediment deposition.
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Meandering river deposits

• Channel deposits They are sediment deposits
  formed mainly from the activity of river
  channels.
• Channel lag deposits
• Point bar deposits
• Channel-fill deposits
• Bank deposits They are sediment deposits formed
  on the river banks and are produced during flood
  periods.
• Natural levee deposits
• Crevasse-splay deposits
• Flood plains They are fine-grained sediment
  deposits formed during heavy floods when river
  water flows over the levees into the flood plain.

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The morphological elements of meandering river
environments
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The morphological elements of meandering river
environments
Boggs, 2001
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PLAN VIEW
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• Channel deposits They are sediment deposits
  formed mainly from the activity of river
  channels.
• Channel lag deposits
• Point bar deposits
• Channel-fill deposits

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Channel Lag Deposits

• In rivers, the coarser sediments (gravels and
  pebbles) lag behind while the sand moves as bed
  loads.
• These coarse sediments represent residual
  concentration and accumulate as discontinuous
  lenticular patches in the deeper parts of the
  channel.
• Such deposits never form thick layers and they
  are discontinuous.
• They occupy the lowest part of a channel or
  point-bar sequence.

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Channel Deposits

• When seen in cross-section, channels (red arrow)
  have elliptical profiles and are filled with
  sediment that is generally coarser than the
  surrounding rock. They also cut into (by erosion)
  the underlying rock. Hammer (circled) shows scale.

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Channel conglomerate in the Pass Peak formation
(Eocene), Hoback Basin Divide, about 30 miles
northwest of Pinedale, Wyoming. (Dunbars
Rogers, 1957)
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Point Bar Deposits

• Arcuate swells swales known as scroll bars,
  chutes, chute bars
• Capped by fine mud (flood plain mud)
• Low-flow velocity sand ripples
• Planar lamination and large-scale trough
  cross-bedding
• Channel Lag of gravel and/or coarse sand

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PLAN VIEW
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As it flows, it deposits sediment on banks that
lie on the insides of curves (point bar
deposits), and erode the banks on the outside of
curves.
Point bar

• Sacramento River, CA.

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Point Bar
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Erosion Cut-bank
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Swales
Point bar
Owens River, CA.  Note the different generations
of old meanders.
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Channel Migration (along outside of bends in the
river)
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Lateral Migration of Channel
Trough
Deposition
Erosion
Cross Section of Channel
Erosion at one side, plus deposition on the
other! The accumulating sediment on the inside
of a bend forms the point bar. The point bar is
most often exposed during drier parts of the
year, and is under water during floods.
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Lateral Migration of Channel
Trough
Deposition
Erosion
Cross Section of Channel
How should grain-size vary as a function of depth
along the point bar? Fastest currents in deepest
water near outside of bend gravels or coarsest
sands. Then fine sediments in shallower waters.
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Cross beds in Cambrian Prospect Mtn. Quartzite,
E. Nevada. The photo above shows how cross beds
can indicate the "up" direction in tilted rocks.
The red arrow points to younger rocks. Notice
that bedding in the younger rocks cuts the cross
beds of the older rocks.
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Channel-fill Deposits

• Channel-fill deposits represent sedimentation in
  stream channels that have been abandonned by a
  stream because of cut-off processes and
  avulsions, that means by sudden abandonment of a
  part of or the whole of a channel course, or by
  filling due to extreme increase in rate of
  sedimentation and reduction in depth.

Figure 73.
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Cut-off processes
Neck cut-off
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Neck cut-off
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Cut-off processes
Neck cut-off
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Chute cut-off
Cut-off processes
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Avulsion
Boggs, 2001
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Channel-fill Deposits

• The abandonned channel are slowly alluviated and
  sealed at both ends, isolating the old channel
  loop in the form of a cut-off lake, or
  oxbow-lake.
• Once it is isolated by formation of cut-off
  channel, point bar sands and gravels will be
  buried by silts and muds carried in by river
  floods and by sediment slumping in from sides as
  rain fills up lake.
• Commonly well-laminated and may contain plant
  remains and various other shells.

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Oxbow lake and the Chippewa River. Eau Claire,
Wisconsin.
As the channel migrates, parts of it may become
abandonned and left behind as Oxbow Lakes. These
lakes have a characteristic horse-shoe shape.
They become sites for deposition of fine-grained
lake sediment.
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• Bank Deposits
• They are sediment deposits formed on the river
  banks and are produced during flood periods.
• Natural levee deposits
• Crevasse-splay deposit

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• Bank deposits
• Natural levee deposits When a river floods its
  banks, the water passes from a deep channel to
  shallow flow. Friction with the bottom becomes
  relatively a lot more important and thus
  currently velocities drop rapidly. As a result,
  coarser sediment carried in suspension in the
  turbulent river channel is abruptly dropped on
  banks.
• This creates a ridge or levee of silt-to-sands
  along the river banks. Sands fall nearer the
  river banks, while silts and muds are deposited
  further away (size-sorting).

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• Bank deposits
• Crevasse-splay deposits Floods can cut small
  channels in levees. Rapid erosion of levee, plus
  rush of flood waters produces fan-shaped lobe of
  sand. Both bedload and suspended sediments are
  often coarser than levee deposits and are poorly
  sorted.

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PLAN VIEW
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• Flood Plain Deposits
• They are fine-grained sediment deposits formed
  during heavy floods when river water flows over
  the levees into the flood plain.

Lateral size grading
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Meandering and Floodplains
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• Flood Plain Deposits
• Away from levees and chutes, flow velocities are
  very slow due to shallow flow and friction with
  all sorts of trees, bushes, houses, tractors,
  etc. Thus, flood plains are rich in silt, mud,
  and organic matter (Fertile soil).
• Lateral size grading occurs in the flood plain
  deposits.

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Meandering river deposits
Flood Plain Vertical Accretion
Point Bar Lateral Accretion
Channel Lag deposits
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Typicalcolumnar section ofMeanderingriver
deposits
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Diagnostic Features of MEANDERING FLUVIAL SYSTEMS

• Geometry
• Long, ribbon-like sand bodies (shoe-string sands)
• Sequence
• Fining upwards from channel lag through point-bar
  deposits high amount of fine-grained sediments
• Sedimentology
• Gravels to muds planar trough X-beds, ripple
  X-lamination floodplain deposits fossiliferous