Summary
Alluvial fans are cone-shaped deposits of gravel, sand, and silt sized particles. When part of an alluvial fan, these detrital materials are referred to as alluvium. Alluvial fans commonly form along the edges of mountains, cliffs, and steep hills of arid-semiarid environments. Movement of sediment begins at the apex of the cone, and is then transported radially outward by either water or gravity driven processes. Sediment transport occurs infrequently, but with great force. Alluvial fans are generally divided into 2 major types; debris-flow-dominated, and stream-flow-dominated fans (Boggs, 2014). Alluvium deposits are typically coarse-grained and poor-sorted due to their relatively short transport distance. Deposits are mainly comprised of virgin sediment, meaning sediment that has been produced by the weathering and erosion of uplifted bedrock (Blair et al., 2009).
Geographic Dimensions
Between 0.5km to 10km, with larger fans extending up to ~20km. (Blair et al., 2009)
Key Depositional Processes
Sediment transport begins at the transitional area between the confined flow of a channel to the unconfined flow of a open basin. Sediment is driven through this channel mainly by water-driven processes, however after emerging from the mouth of the channel, sediment movement is primarily caused by gravity-driven processes (Hooke, 1967).
Water-driven
Sheetfloods are large amounts of sediment-rich runoff water that move outward and downslope. Sediment concentration can range from 20-45%. This can create finely laminate layers of fine particles. Incised-channel flow occurs in the confined channels at the head of the alluvial fan. Water flow carries sediment down and out of the channel, depositing it at the apex of the alluvial fan's cone (Boggs, 2014).
Gravity-Driven
Debris-flows are large , fast moving masses of rock, sand, and other debris. Debris-flow deposits are characterized by poorly-sorted clasts, and possibly reverse grading (Boggs, 2014). Mud-flows are a specific type of debris-flow, dominated by sand and silt sized particles, which can exhibit more plastic behavior than a typical debris-flow. Landslides are another common form of deposition in alluvial fans, carrying gravel-sized to boulder-sized clasts.
Principle Sedimentary Structures & Distinguishing Characteristics
Debris-flow deposits
Poorly sorted, coarse sediment with muddy matrix
Reverse-grading possible
Stream-flow deposits
Relatively well-sorted, sheetlike deposits of gravel, sand, and silt
Cross-bedding, lamination, and possible structureless appearance