Alluvial Fan Systems

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

1. Photo by W. W. Little Alluvial Fan System

2. Simplified Model Alluvial fans are highly complex, steeply-sloping (>1 – 2 ) 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. Little Photo 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)