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Lecture 4 Alluvial fans

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. An alluvial fan in Death Valley. Narrow outlet (valley or gorge). Fan apex. Radial spreading alluvial fan. An alluvial fan in Death Valley.

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Lecture 4 Alluvial fans

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  1. Lecture 4Alluvial fans

  2. What is an alluvial fan? … a fan-shaped deposit of alluvium found where a stream flows out of a mountain onto flatter terrain.

  3. An alluvial fan in Death Valley Narrow outlet (valley or gorge) Fan apex Radial spreading alluvial fan

  4. An alluvial fan in Death Valley Coarse, channelized debris Sediments become finer-grained

  5. Ok, back to alluvial fans….. The evolutionary stages of alluvial fans

  6. Alluvial fans: Steeper gradient than “normal” river profiles, thus are often dominated by debris flow processes

  7. Kosi megafan, northern India

  8. Migrating fluvial channels

  9. Debris flows Shear stress vs. strain So flow is plug like and ceases as it reaches shallower surface, hence the velocity diminishes Flows also stop as they thin or lose water

  10. Losing water and velocity caused the debris flow to become more viscous and stop, creating a very steep front

  11. Debris flow “channel” on a talus in eastern Sierra Nevada

  12. A closer view Note the existence of the “levees”

  13. The radial deposition of alluvial fan Active depositional lobe

  14. Period of active deposition, massive floods and debris flows

  15. Debris flow fan caused by typhoon, southern Taiwan, 2005

  16. And afterwards…….

  17. So, what will the stratigraphy look like?

  18. Channels near apex of fan

  19. Channel gravels

  20. And suspended load deposits on distal portions of the fan

  21. With bioturbation….

  22. Downslope fining of alluvial fan deposits

  23. Facies relationships, a Devonian fan in Sweden, showing characteristic interfingering of depositional facies

  24. Pleasant Valley, Nevada

  25. Pleasant Valley, Nevada

  26. Three small alluvial fans on Soda Lake Road Is there anything strange of these fans?

  27. Yes! They are offset 6.5 m by the 1857 earthquake

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