Alluvial fans are steeply sloped landforms found at the base of mountains that experience significant changes from debris flow deposits near the mountain to braided stream deposits in the middle fan and sheet flow deposits near the fan toe. Alluvial fan deposits include a variety of poorly sorted facies deposited by debris flows, braided streams, and sheet flows such as breccia, gravel, sand, and mud.

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.

6. Photo by W. W. Little
Bajadas
Photo by W. W. Little

7. 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.

8. 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.

10. Photo by W. W. LittlePhoto by W. W. Little
Matrix-supported Breccia

11. Photo by W. W. Little
Structureless Sand & Gravel
Clast-supported, matrix-rich, structureless (massive) conlomerate
deposited by braided streams.

12. Photo by W. W. Little

13. Photo by W. W. Little
Alluvial fan conglomerates are commonly imbricated.
Imbrication

14. Photo by W. W. Little
Horizontally-bedded Gravel
Horizontally-bedded gravel is deposited both on bar tops and as
sheet flow.

15. 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.

16. Photo by W. W. Little
Lens-shaped Gravel and Sand
Beds
Clast-supported, matrix-rich, structureless (massive) conlomerate
deposited in stream channels.

17. 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.

18. Photo by W. W. Little

20. 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.

21. Photo by W. W. Little
Transitional Flow Regime
Horizontally-bedded Sand
Horizontally-stratified sand deposited on bar tops and as sheet
flows. Commonly graded.

22. Facies Associations
Three common facies associations are found on alluvial fans and are
related to debris flows, braided streams, and sheet flow.

23. Large-scale Architecture
Kamal et al. (2007)