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 diminishesFlows 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 viewNote 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

And afterwards…….

So, what will the stratigraphy look like?

Channels near apex of fan

Channel gravels

And suspended load deposits on distal portions of the fan

With bioturbation….

Downslope fining of alluvial fan deposits

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

Pleasant Valley, Nevada

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