Alluvial Fan Systems

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Geol 370: Sedimentology and Stratigraphy Topic 10e: Alluvial Fan Systems

Text of Alluvial Fan Systems

  • 1. Photo by W. W. Little Alluvial Fan System
  • 2. Simplified Model Alluvial fans are highly complex, steeply-sloping (>1 20) fluvial systems found at the base of active mountain ranges. They show significant changes down fan from proximal debris flow deposits to mid-fan braided stream deposits to distal fan sheet flow and playa deposits beyond the fan toe.
  • 3. Photo by W. W. Little Bajadas Photo by W. W. Little
  • 4. Photo by W. W. Little Common Facies A wide variety of facies, mostly poorly-sorted, make up alluvial fan deposits, including matrix-rich breccia, clast-supported gravel, trough and planar sand and gravel, and laminated mud.
  • 5. Photo by W. W. Little Clast-supported Breccia Both clast- and matrix-supported, structureless breccia with random clast orientation are common as debris flow deposits.
  • 6. Photo by W. W. LittlePhoto by W. W. Little Matrix-supported Breccia
  • 7. Photo by W. W. Little Structureless Sand & Gravel Clast-supported, matrix-rich, structureless (massive) conlomerate deposited by braided streams.
  • 8. Photo by W. W. Little
  • 9. Photo by W. W. Little Alluvial fan conglomerates are commonly imbricated. Imbrication
  • 10. Photo by W. W. Little Horizontally-bedded Gravel Horizontally-bedded gravel is deposited both on bar tops and as sheet flow.
  • 11. Photo by W. W. Little Clinoforms Clinoforms in both sand and gravel are common features associated with deposition off the downstream end of braided stream bars.
  • 12. Photo by W. W. Little Lens-shaped Gravel and Sand Beds Clast-supported, matrix-rich, structureless (massive) conlomerate deposited in stream channels.
  • 13. Photo by W. W. Little Cut-and-Fill Structures Flash-flooding is a common process on alluvial fans, resulting in channel-form deposits with erosional bases and graded bedding.
  • 14. Photo by W. W. Little
  • 15. Photo by W. W. Little Planar Cross-bedded Sand & Gravel Planar cross-bedding associated with foreset deposition off the front of intrachannel bars of braided streams.
  • 16. Photo by W. W. Little Transitional Flow Regime Horizontally-bedded Sand Horizontally-stratified sand deposited on bar tops and as sheet flows. Commonly graded.
  • 17. Facies Associations Three common facies associations are found on alluvial fans and are related to debris flows, braided streams, and sheet flow.
  • 18. Large-scale Architecture Kamal et al. (2007)