Alluvial Fans

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Alluvial Fans
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Depositional Model
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The upper Fan EnvironmentErosion by streams and
glaciers
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The Mid-Fan EnvironmentBraided Streams and
debris flows
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Debris FlowsMore commonly known as mud slides or
flows. These are sediment gravity flows, meaning
that they flow as a mass of sediment under the
influence of gravity.The fine-grained matrix
(silt and clay) move as a highly viscous fluid
capable of transporting very large material in
suspension.
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Stratigraphic Succession of the mid-fan consists
of interbedded braided stream deposits, debris
flows and sieve deposits
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Distal Fan Deposits
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Factors Controlling Fan Development-Fan
Progradation versus Retrogradation

• Climate-stream discharge
• Tectonics-uplift of source
• Basin Subsidence

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River Geomorphology
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Factors Controlling Stream Geomorphology

• Climate-stream discharge
• Gradient
• Substrate Conditions

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Braided Rivers

• High gradients
• Flashy discharge
• Minimal vegetation (arid)
• High bedload

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Little to know floodplainEphemeral channels
with low width to depth ratiosLow preservation
of barsGenerally coarse, poorly sorted deposits
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Braided Stream Deposits
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Mials Classifiction
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Platte-Type Braided Rivers
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Scott Type Braided Rivers
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Troughs and Forsets are dominant
sedimentarystructures
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Grain Size
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Braided fan-delta complexes