lecture 4 alluvial fans. what is an alluvial fan? … a fan-shaped deposit of alluvium found where a...
TRANSCRIPT
![Page 1: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/1.jpg)
Lecture 4
Alluvial fans
![Page 2: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/2.jpg)
What is an alluvial fan?
… a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain.
![Page 3: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/3.jpg)
Narrow outlet (valley or gorge)
Radial spreading alluvial fan
Fan apex
An alluvial fan in Death Valley
![Page 4: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/4.jpg)
Sediments become finer-grained
Coarse, channelized debris
An alluvial fan in Death Valley
![Page 5: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/5.jpg)
Ok, back to alluvial fans…..
The evolutionary stages of alluvial fans
![Page 6: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/6.jpg)
![Page 7: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/7.jpg)
![Page 8: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/8.jpg)
![Page 9: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/9.jpg)
Alluvial fans:Steeper gradient than “normal” river profiles, thus are often dominated by debris flow processes
![Page 10: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/10.jpg)
Kosi megafan, northern India
![Page 11: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/11.jpg)
Migrating fluvial channels
![Page 12: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/12.jpg)
Debris flows
Shear stress vs. strain
So flow is plug like and ceases as it reaches shallower surface, hence the velocity diminishesFlows also stop as they thin or lose water
![Page 13: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/13.jpg)
Losing water and velocity caused the debris flow to become more viscous and stop, creating a very steep front
![Page 14: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/14.jpg)
Debris flow “channel” on a talus in eastern Sierra Nevada
![Page 15: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/15.jpg)
A closer viewNote the existence of the “levees”
![Page 16: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/16.jpg)
The radial deposition of alluvial fan
Active depositional lobe
![Page 17: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/17.jpg)
Period of active deposition, massive floods and debris flows
![Page 18: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/18.jpg)
Debris flow fan caused by typhoon, southern Taiwan, 2005
![Page 19: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/19.jpg)
![Page 20: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/20.jpg)
And afterwards…….
![Page 21: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/21.jpg)
So, what will the stratigraphy look like?
![Page 22: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/22.jpg)
Channels near apex of fan
![Page 23: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/23.jpg)
Channel gravels
![Page 24: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/24.jpg)
And suspended load deposits on distal portions of the fan
![Page 25: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/25.jpg)
With bioturbation….
![Page 26: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/26.jpg)
Downslope fining of alluvial fan deposits
![Page 27: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/27.jpg)
Facies relationships, a Devonian fan in Sweden, showing characteristic interfingering of depositional facies
![Page 28: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/28.jpg)
Pleasant Valley, Nevada
![Page 29: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/29.jpg)
Pleasant Valley, Nevada
![Page 30: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/30.jpg)
![Page 31: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/31.jpg)
Three small alluvial fans on Soda Lake Road
Is there anything strange of these fans?
![Page 32: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/32.jpg)
Yes! They are offset 6.5 m by the 1857 earthquake
![Page 33: Lecture 4 Alluvial fans. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain](https://reader037.vdocument.in/reader037/viewer/2022110205/56649ccb5503460f94995157/html5/thumbnails/33.jpg)