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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.
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