Alluvial Fan

A view of Mackay Valley located in Central Idaho. The large alluvial Fan in the background came off the Lost River Mountain Range.
An alluvial... more

by Robert Bales

Title

Alluvial Fan

Artist

Robert Bales

Medium

Photograph - Photo

Description

A view of Mackay Valley located in Central Idaho. The large alluvial Fan in the background came off the Lost River Mountain Range.
An alluvial fan is a fan- or cone-shaped deposit of sediment crossed and built up by streams. If a fan is built up by debris flows it is properly called a debris cone or colluvial fan. These flows come from a single point source at the apex of the fan, and over time move to occupy many positions on the fan surface. Fans are typically found where a canyon draining from mountainous terrain emerges out onto a flatter plain, and especially along fault-bounded mountain fronts.
The formation of this alluvial Fan:
Small-radius (<2 km), steep (8-17�), Holocene debris flow dominant alluvial fans are depositing on top of large-radius (~5 km), shallow (2-3�) inactive Pleistocene sheetflood dominant alluvial fans along the western flank of the Lost River Range, Idaho. Channel ways, with and without backfilling, have developed within the large sheetflood dominant alluvial fans. The sheetflood dominant alluvial fans are being dissected. Three wedge shaped gravel packages were identified by field mapping and measured sections in four alluvial fans along the active extensional Lost River fault. These alluvial fan deposits have all of the characteristics of growth strata (progressive unconformities) observed in extensional tectonic settings. The ages of the surfaces were calculated from carbonate coat thicknesses on clasts in the soil and are between 42,000 � 18,000 yr and the present. Each of the four alluvial fans has different stratal patterns even though they are all situated on the active Mackay fault segment. This lateral variability has implications to rock record interpretations. The younger fan depositing on top of the older fan exhibits the interaction of neighboring alluvial fans, and there is a changing of the sediment transport process over time on an individual alluvial fan. Complexly interacting controls on alluvial fan development include: 1) temporal change in the locus of maximum displacement on the Mackay fault segment, and 2) changes in Pleistocene and Holocene discharge.

Uploaded

July 30th, 2014