Presentation on theme: "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."— Presentation transcript:
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.
Narrow outlet (valley or gorge) Radial spreading alluvial fan Fan apex An alluvial fan in Death Valley
Sediments become finer- grained Coarse, channelized debris An alluvial fan in Death Valley
Ok, back to alluvial fans….. The evolutionary stages of alluvial fans
Alluvial fans: Steeper gradient than “normal” river profiles, thus are often dominated by debris flow processes
Kosi megafan, northern India
Migrating fluvial channels
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
Losing water and velocity caused the debris flow to become more viscous and stop, creating a very steep front
Debris flow “channel” on a talus in eastern Sierra Nevada
A closer view Note the existence of the “levees”
The radial deposition of alluvial fan Active depositional lobe
Period of active deposition, massive floods and debris flows
Debris flow fan caused by typhoon, southern Taiwan, 2005
So, what will the stratigraphy look like?
Channels near apex of fan
And suspended load deposits on distal portions of the fan
Downslope fining of alluvial fan deposits
Facies relationships, a Devonian fan in Sweden, showing characteristic interfingering of depositional facies
Pleasant Valley, Nevada
Three small alluvial fans on Soda Lake Road Is there anything strange of these fans?
Yes! They are offset 6.5 m by the 1857 earthquake