Sedimentation and Stratigraphy Geology 5142 Dr' Thieme

1
Sedimentation and StratigraphyGeology 5142Dr.
Thieme

• Lecture 16 Meandering, Braided, and Anastomosing
  Stream Facies

2
Fluvial Transport and Alluvial Sediment

• Fluvial - anything associated with rivers or
  stream energy
• Alluvial - water-laid sediment
• specific to the sediment in river valleys
• more general in that it also applies to
  water-laid deposits in other settings
• controlled by
• supply of detritus
• gradient of the river
• discharge magnitude and seasonal variations

3
Stream Longitudinal Profile
Gradient decreases along the downstream axis.
4
Hydraulic Geometry

• channel width, depth,
• and velocity all increase
• in the downstream
• direction
• Depth increases at
• the fastest rate
• Why does velocity increase?

5

• Mountain streams have
• turbulent flow
• Water molecules and
• sediment particles are moving
• at many angles which are not
• parallel to streamflow
• Boulders or large
• woody debris in a
• channel introduce more
• frictional drag

6
Turbulent and Laminar Flow

• Deep, narrow channel at upstream end.
• More of the water comes in contact with channel
  walls.
• Velocity is decreased by frictional drag.
• Wide, shallow channel at downstream end has
  laminar flow.
• Most of the resistance to flow comes from shear
  between water planes.

7
(No Transcript)
8
(No Transcript)
9
Continuity Equation

• Similar to Principle of Conservation of Mass and
  Energy in Physics

Q w d v
discharge width depth velocity
10
Base Level
A Peneplain in the Davis Cycle of Erosion

• the lowest elevation to which a stream can erode
  its channel

11
Graded Stream

• has just the velocity required for the load
  supplied from the drainage basin
• Stream adjustments toward equilibrium or graded
  condition
• Raising base level causes deposition
• Lowering base level causes erosion
• Increasing gradient causes meandering

12
Sinuosity

• ratio of stream length (along the thalweg) to
  valley length
• sinuosity gt 1.5 implies meandering
• meandering of rivers dissipates stream power over
  low relief surfaces

13
Types of river

• Straight - single, low sinuosity (lt 1.5) channel
  without dividing bars
• Meandering - high sinuosity (gt 1.5) channel
  formed as one bank is eroded and its sediment
  deposited downstream on the opposite bank
• Anastomosing - a number of separate channels
  which divide and join along the river
• Braided - a number of separate channels, each
  having low sinuosity, separated by bars that are
  emergent when the river level is low

14
Braided channels become straight during floods
that submerge the longitudinal bars.
Anastomosing channels are more permanently
separated by islands built of river alluvium.
Maximum meander amplitude is a threshold above
which meanders become cutoff to form oxbow lakes.
15
(No Transcript)
16
(No Transcript)
17
(No Transcript)
18

• Meandering Stream
• predominantly transport sediment in suspension
• channel bedload is deposited at the base of each
  point bar
• cross-bedded and cross-laminated sands are
  laterally accreted as the channel builds the
  point bar
• column fines upward
• suspended load predominates toward the top of the
  river
• "overbank" floods transport sediment into the
  floodplain

19
(No Transcript)
20
(No Transcript)
21
Controlling Factors

• The following factors favor the type of river,
  favoring the braided or anastomosing channel
  patterns
• steep slope (gt 1º)
• erodible banks
• abundant coarse sediment supply
• rapid fluctuations in discharge

22
(No Transcript)
23

• Braided Stream
• predominantly transport sediment as bedload
• gravel or coarse sandy basal lags of wide,
  shallow channels
• trough and planar cross-bedded sands will
  alternate in distinct sets
• crossbed set size and mean grain size decrease
  upward
• may have a thin "overbank" topstratum of
  suspended sediment

24

• Braided Stream
• rivers fed by glacial meltwater provide some of
  the most typical sections
• both gravel lag and bedload sand beds
• ripple-bedded sands (Sr) represent small bars
• thinner beds of laminated fine sand or mud (Fi)
  from slackwater between meltwater pulses

25

• Braided Stream
• rivers in arid to semiarid climates where coarse
  clastic detritus is provided by
• steep slope
• erodible bedrock
• planar crossbeds represent longitudinal bars that
  give way to transverse ("linguoid") bars up
  section

26

• Braided Stream
• gravel bed rivers
• debris flows or alluvial fans supplying detritus
  along the channel margins
• bar deposits show slightly greater sand fraction
  or better sorting
• little or now slackwater stage deposition

27
Youth - relief is steepest - V shaped
channels Mature - wider valleys -
streams meander Old Age gently rolling hills
- low gradient channel -
meandering or braided Peneplain -
flat, level plain at base
level Rejuvenation starts the cycle
over
28
Davis Cycle of Erosion

• stages are universal
• cycles were identified with erosion surfaces or
  peneplains in New Jersey and Pennsylvania
• Schooley (gt1000 ft)
• Harrisburg (200-500 ft)
• Somerville (lt200 ft)

29
Peneplains of Mid-Atlantic

• Schooley
• Harrisburg
• Somerville

30
Objections to the Davis Cycle

• no intact peneplains exist
• stream erosion rates vary with
• bedrock resistance
• climate
• tectonics probably has no regular cycle
• glacial/interglacial cycle affects erosion
• emphasis on time over process