Alluvial fans form where a narrow confined valley emerges into a broad flat area.

• radiate out from a point source
• may coalesce to give a broad apron of sediment (bajada)

Modern fans

both allocyclic and autocyclic processes

• faults (tectonic uplift)
• sea level changes
• channel abandonment

Sedimentation on alluvial fans

• debris flows
• sheet flood
• stream flow
  

Debris flows

• move en masse down slope with little sorting or abrasion of particles
• matrix strength -mud sized particles

Deposits are:

• poorly sorted
• ungraded (reverse grading near base)
• gravel dominated
• large clasts may protrude from the top

Debris flows tend to freeze in place

• slopes are too low
• their contained water has infiltrated into the underlying deposits
  

sheet flood and stream flow

• streams flow across fan surfaces
• deposit sediment and rework earlier deposits
• streams may migrate across the fan
• allocyclic vs autocyclic
  

• water-laid deposits show a progressive change down fan
• planar cross stratified beds increase down slope
• grain size reduction

Cycles in alluvial fan deposits

3 main allocyclic trends:

A. Coarsening and thickening upward due to periodic fault uplift of source or subsidence of the basin;

B. Coarsening and thickening upward followed by fining and thinning in asymmetric cycles reflecting a time interval between uplift events; and

C. Fining/thinning upward due to back-stepping of boundary faults.

Autocyclic processes produce thinner (a few metres vs 10-25 m) cycles that tend to fine upward and are related to channel abandonment.

Large areas of the fan are inactive for long periods

• development of a soil profile

Cross-section and facies distribution