1. Lecture 4 Alluvial 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

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

10. Kosi megafan, northern India

11. Migrating fluvial channels

12. 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

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

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

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

16. The radial deposition of alluvial fan
Active depositional lobe

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

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

20. And afterwards…….

21. So, what will the stratigraphy look like?

22. Channels near apex of fan

23. Channel gravels

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

25. With bioturbation….

26. Downslope fining of alluvial fan deposits

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

28. Pleasant Valley, Nevada

29. Pleasant Valley, Nevada

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

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