Non-marine environments

Alluvial Fans

The Start of the Sedimentary Cycle

• Bedrock weathered away from uplifted areas (mountain ranges)

• Carried away in mountain streams• Start the process of building up

sedimentary deposits.• First of these deposits: Alluvial fans

How do alluvial fans form?

• When a narrow (confined) canyon stream disgorges onto a valley floor

• Sudden deceleration in flow and in gradient– Decreased ability in the stream to carry

coarser material: this is dropped.• Results in a cone-shaped deposit of

coarse stream sediments, sheet flood deposits and debris flows: Alluvial Fan

• Alluvial fans best known from arid environments, where periodic flow occurs in the canyons but also occur in humid environments.– Usually triangular in map view and wedge-shaped in

cross section.– Slopes range from 1 – 25°, average 5-10°.– The larger the particle size, the steeper the slope.

• Described as “active” when the fan is building or “inactive” when it is not.– To be active: must be continued uplift and erosion of

highlands to supply sediment: fault scarps are common sites of alluvial fans.

Typical structure of an alluvial fan

RADIAL FAN SECTION

FAN SURFACE

RADIAL PROFILE

Transport of material on alluvial fans

• Three methods:– Stream flow– Debris flow– Mud flow

Stream Flow• In arid environments• Flash floods in canyons: extreme erosional power.• As flow velocity decreases, bounders, cobbles and

pebbles are dropped.• Results in a flow that is choked with more sediment than

it can carry: braided streams form on the fan (dry up quickly)

• Each flood cuts new channels, filling old ones with gravel.

• At high flood levels, sand and gravel-rich flow covers the mid-fan (N.B. no fine material): Sheetflood Deposits

Stream Flow: Sheetflood deposits

• Typically well sorted, stratified and cross-bedded.• Commonly form lobes than emerge from the channel at

the intersection point of the fan surface & the channel profile

• Little silt/ clay so water flows freely through these deposits without blocking the pores so these lobe deposits are commonly called sieve deposits

• These become progressively coarser towards the front of the lobe, where gravel accumulates.

• Sieve deposits are normally proximal/ upper mid-fan deposits.

Schematic profile of a sieve lobe

deposit

Debris Flow on Alluvial Fans• When sediment becomes saturated with water:

flows as a viscous plastic mass: behaves like quicksand

• Debris flow can carry very large boulders andalso clays and fine particles

• Results in very poorly sorted deposits with little or no stratification

• Sometimes the base of a debris flow shows inverse grading (grain size increases upwards)

• Generally form lobes in the upper reaches of the fan.

Mud Flow deposits on alluvial fans

• Where the debris flow is primarily fine particles

• Forms restricted narrow lobes like debris flows

• Mudflows that are more fluid can form enormous sheetflood deposits (>10km/h)

• Very fast moving, very dangerous deposits.

Typical depositional structure in alluvial fans

• Require rapid uplift: commonly found in– Rapidly downdropping grabens– Foreland basins– Strike-slip basins

• Typical profile:– Mixture of unsorted debris flows– Stream channel conglomerates (fanglomerates)– Cross-bedded sandstones– Sieve deposits– Commonly coarsen up in the stratigraphic record

• Sequence:– Cross-bedded ssts of distal fan at base– Overlain by coarser proximal fan deposits as uplift continues– Thin fining up sequence of fan decay on top

Can be very thick: 9000m of fanglomerates at margin of San Andreas Fault.