fluvial sediments “as a young man, my fondest dream was to become a geographer. however, while...

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- Albert Einstein (unpublished letters)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