fluvial sediments “as a young man, my fondest dream was to become a geographer. however, while...
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Fluvial Sediments
“As a young man, my fondest dream was to become a geographer. However, while working in the customs office I thought deeply about the matter and concluded it was far too difficult a subject. With some reluctance, I then turned to Physics as a substitute.”
- Albert Einstein (unpublished letters)
http://www.geocities.com/sogodbay/Images/SDK/Inecar03.jpg
• 2 Main types:– Bedrock channels: – sediment transport capacity > sediment supply
Types of rivers
http://www.fs.fed.us/eng/pubs/html/98771207/98771207.html
Types of rivers
• 2 Main types:– Alluvial channels
• Channels formed in and by sediment transport• Sediment transport capacity <= sediment supply
http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/fluvial_systems/alluvial_landforms_page_1.html
What to rivers do?
• They..– Erode material– Transport eroded
material– Deposit transported
eroded material -> sediments– Move in space over time
River erosion
Leads to first few characteristics of fluvial sediments
1/ larger clasts than eolian sediments
2/ Not necessarily well sorted
3/ Often rounded movie
River transport
• Transported clasts dependent on flow “strength”
• “strength” proportional to density, flow velocity, flow depth, slope
• => deeper or faster or steeper flows can carry larger clasts
Leads to next characteristics of fluvial sediments
4/ clast size can be an indication of river “strength”
River sediment deposition
• Sediments are deposited following change is river dynamics
- If river slows down, shallows (slope and depth)
-most dramatic example: Alluvial fans
http://lang.sbsun.com/projects/fireflood/articles/alluvialfan.asp http://geology.about.com/library/bl/images/blalluvfan.htm
Types of alluvial rivers
• Straight: sinuosity (path length / length covered) < 1.3
• Meandering -> side to side oscillations
• Braided: flow divides into more than one thread
• Anabranching: stable braid
Braided channels
Virgin River [north fork]Zion National Park Compton (1985)
Meandering rivers
“superelevation”
Centrifugal force
Hydrostatic pressure
“Primary circulation”:
“Secondary circulation”:
Net Result: channel migration
Stratigraphic x-section
Channel x-section
Idealized fluvial fining-upward sequence[Walker]
Summary1/ larger clasts than eolian sediments
2/ Not necessarily well sorted
3/ Often rounded
4/ Clast size can be linked to channel “strength”
Shinarump conglomerate
Example of alluvial fan conglomerate [fanglomerate] shed from the Sevier-Laramide upliftsPaleocene Echo Canyon conglomerate, Echo Canyon, Utah
Blocks of Shinarump, Coal Pits wash, Zion National Park
Triassic Chinle Formation [painted desert, mostly paleosols] overlain by Jurassic fluvial and eolian seds – ledge and cliff formers]
Lower Jurassic Moenave Fm.Zion National Park
Ledge formers--fluvial channelsand related facies
Slope formers--paleosols
Eocene Claron Formation in Bryce National Park – mostly fluvial and floodplain
Eocene Claron Formation in Bryce National Park – mostly fluvial and floodplain
channel facies
Shinarump Conglomerate member of the Triassic Chinle Formation[basal member]
Permian paleosols[soft, badlands topography, colorful]
fluvial channel deposit[resistant cap, ledge former]
Shinarump conglomerate member of the Chinle Formation, Capitol Reef
Shinarump – cross-bedded coarse sandstone with some soft-sediment deformation
Shinarump conglomerate member of the Chinle Formation, outside Zion NP
Lower Jurassic Moenave Fm.Zion National Park
Lower Jurassic Moenave Formation, Zion National Park[trail above campground]
Lower Jurassic Moenave Fm.Zion National Park
Lower Jurassic Moenave Fm.Zion National Park
Lower Jurassic Moenave Fm.Zion National Park
Lower Jurassic Moenave Fm.Zion National Park