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Photo by W. W. Little

Alluvial Fan System

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.


Bajadas



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.


Clast-supported Breccia

Both clast- and matrix-supported, structureless breccia with random clast orientation are common as debris flow deposits.

Photo by W. W. LittlePhoto by W. W. Little

Matrix-supported Breccia


Structureless Sand & Gravel

Clast-supported, matrix-rich, structureless (massive) conlomerate deposited by braided streams.



Alluvial fan conglomerates are commonly imbricated.

Imbrication


Horizontally-bedded Gravel

Horizontally-bedded gravel is deposited both on bar tops and as sheet flow.


Clinoforms

Clinoforms in both sand and gravel are common features associated with deposition off the downstream end of braided stream bars.


Lens-shaped Gravel and Sand Beds

Clast-supported, matrix-rich, structureless (massive) conlomerate deposited in stream channels.


Cut-and-Fill Structures

Flash-flooding is a common process on alluvial fans, resulting in channel-form deposits with erosional bases and graded bedding.



Planar Cross-bedded Sand & Gravel

Planar cross-bedding associated with foreset deposition off the front of intrachannel bars of braided streams.


Transitional Flow Regime Horizontally-bedded Sand

Horizontally-stratified sand deposited on bar tops and as sheet flows. Commonly graded.

Facies Associations

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

Large-scale Architecture

Kamal et al. (2007)