Alluvial Fan.pdf

Environments of Deposition Continental

Fluvial alluvial fan, braided stream, meandering stream

Desert dunes, playa lakes, salinas

Lacustrine

Glacial

Deltaic delta plain, delta front, prodelta

Marginal- Marine Beach/ barrier island

Estuary/ lagoon

Tidal Flat

Neritic continental shelf, organic reef

Marine Pelagic continental slope and rise, deep- ocean floor

Continental (Terrigenous) Journey from source to sink

Plate tectonics lead to mountain building

Slope! Exposure! Produces rock

Climate causes weathering

Surface processes move sediments, sculpt landscape

Weathering, erosion, transport, deposition

Deposition Requires place to put sediments

Depression/ basin

Deep Marine is best

Lakes are good for short records

Terrestrial is difficult!

Glacial sediments are misleading

Abundant because they are recent

Deposition Gravity works to move sediments

Water

Wind

Change in slope or change in climate impacts sediment transport and abundance

Alluvial Fan Localized areas of enhanced sedimentation

downstream of points where laterally confined flows expand Narrow valley or gorge opens up, onto piedmont or

coastal plain

At basin margin Often structurally controlled (fault or mountain front)

Contribute greatly to the rock record! 10’s of meters - >100 km cone

“Fanglomerate”

Alluvial Fan Facies Models 3 types

Debris- flow dominated

Braided fluvial fans

Low- sinuosity/meandering fluvial fan

Generally lacking fossils

Fan shaped

Texturally immature

Death Valley

Martin Miller

http://wrgis.wr.usgs.gov/docs/parks/deva/galfan.html

Alluvial Fan Facies Models Morphology

Proximal, mid-fan, distal

Streamflow processes dominate Stream- channel sediments

Long narrow bodies of coarsest materials

Poorly sorted

Sheetflood deposits Sediments settle out of suspension

VERY high sediment load

Deposit gravels even!

Sieve deposits Very coarse grains are deposited (> sand)

Fines (and water) settle through

3 Alluvial Fan Facies Models Debris- flow dominated

Poorly sorted Pebbles, gravels, boulders dominate

No sedimentary structures Possibly reverse graded bedding in base

Flow is initiated when strength is exceeded Freezes after short distance

Although some travel up to 24 km (Sharp and Nobles, 1953)

Mud- flow dominated Similar but sand and muds dominate

Braided Fluvial Fan Meandering Fluvial Fan

Depositional Environments • Continental / Land (terrestrial and aqueous)

- alluvial fan, glacial, aeolian (desert), fluvial (rivers), lake (lacustrine), paludal (swamp), cave

• Marginal / Transitional / Coastal

- delta, beach, shoreline, estuary, barrier island and lagoon, littoral (intertidal)

• Marine

- shallow marine, reef, deep marine, pelagic

- Neritic (low tide – 200m), bathyl (200-2000 m), abyssal (> 2000m)

QAB 2023 Clastic Sedimentology & Petrography


Alluvial Fans Alluvial fans are sedimentary deposits that

typically form at the margins of a dry basin.

They typically contain coarse boulders and gravels and are poorly sorted.

Fine-grained sand and silt may be deposited near the margin of the fan in the valley, commonly in shallow lakes.

These lakes may periodically dry, and evaporite deposits may result.

http://www.uoregon.edu/~millerm/fan.html


Objectives of this lecture

An overall view of the different alluvial fan depositional environments and facies.

Description of diagnostic elements to identify depositional facies in rock sequences.


Recap • Sedimentology is concerned with the composition and

genesis of sediments and sedimentary rocks, and the creation of predictive models (Reading, 1996)


Outline of lecture • Modern alluvial fan environments

• Alluvial fan formation & processes

• Sedimentary structures, features and facies

• Facies association (s)

• Alluvial fan facies model


Alluvium - Loose material deposited by running water, typically streams. Usually a mixture of clay, silt, sand, and gravel

Alluvial – Pertaining to or composed of alluvium, or deposited by a stream or running water

Fluvial – Of or pertaining to rivers; produced by the action of a stream or river

Paludal – pertaining to a marsh


Modern alluvial fan environments Alluvial fans are depositional

landforms whose surface

forms a segment of a cone

radiating downslope from a

point where major drainages

leave mountains.

Fan-shaped deposit formed

where a fast flowing stream

flattens, slows and spread

typically at the exit of a

canyon onto a flatter plain.

Neighboring fans may

converge into a single apron

of deposits against a slope –

bajada or compound alluvial

fan


Alluvial fans are fan shaped deposits of water-transported material (alluvium).

They typically form at the base of topographic features where there is a marked break in slope.

Consequently, alluvial fans tend to be coarse-grained, especially at their mouths. At their edges, however, they can be relatively fine-grained.


Alluvial fan in Death Valley

A close-up view of the sediments show that alluvial fan deposits are generally coarse-pebbly-gravelly and poorly sorted


Alluvial fans at Andes Mountain. Local: Los Penitentes, Mendoza, Argentina - bajada


Alluvial fans, south of Cape York, Brodeur Peninsula.

Alluvial fan formation & processes

At the foot of mountains, valleys debouch their sediments into the plains. This mountain foot zone is characterized by small alluvial cones which are fed by gullies and build out onto valley surfaces cut sub-horizontally into the bedrock. Larger alluvial fans are fed by complex valley systems and grades out into the deposits of the alluvial plain.



ALLUVIAL FANS: Composed of flood-borne rock and debris, they are geologically and hydrologically active. Gravity and moving water sweep debris through canyons, dropping heavier matter first and carrying sand and silt farther, where it spreads outward in the shape of a fan. The fans here are gently sloped and may resemble hillsides to the untrained eye. Where canyons are close together their fans merge, making them more difficult to identify.



Depositional

gradients in the

piedmont zone

(mountain foot)

are steep, up to

30° in marginal

screes, but

diminish radially

down-fan. This

change in

gradient

correlates with

changes in

process and

sediment type.

Alluvial cones and the heads of

alluvial valleys are characterized by

boulder beds and conglomerates

deposited by gravity slides from the

adjacent mountain sides.


These grades down the fan into

conglomerates and massive or

crudely bedded argillaceous

pebbly sandstones and

siltstones – some are

‘diamictites’, deposits of mud

flows.

These grade down-fan into

poorly sorted massive or flat-

bedded pebbly sandstones, with

irregular scours and silt laminae.

These are deposits of flash

floods. They pass down slope

into the alluvium of braided

channel system. QAB 2023 Clastic Sedimentology & Petrography

Alluvial fan above Lake Louise, Alberta, Canada.


Alluvial fans in arid climates Alluvial fans are common in desert environment that are subjected to periodic flash floods from neighboring mountain range.

They are common around the margins of sedimentary basins of the Basin and Range province in SW North America.


Alluvial fans in humid climates Alluvial fans may also

developed in wetter climate.

In Nepal, the Koshi River has built a mega fan covering some 150,000 km2 below its exit from Himalayan foothills onto the nearly level plains the river traverses into India before joining the Ganges.


Alluvial fan in the French Pyrenees





Alluvial fan facies model


Alluvial fan deposits are characterized by extreme coarse grain size and poor sorting, by massive or sub-horizontal bedding and an absence of fossils.

Ancient alluvial fans are fairly common in fault-bounded intra-cratonic rifts.

Some examples are the Devonian Old Red Sandstone basins of Scotland and Norway


http://upload.wikimedia.org/wikipedia/commons/e/e2/Alluvial_fan_scheme.PNG



Thank you!


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Alluvial Fan.pdf

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