Terrestrial sedimentary environments I - alluvial fans

Facies Models:

When sedimentologists interpret rock units, they do so using a genetic approach based on characteristics of depositional environments, as opposed to simple descriptions of rocks. This viewpoint allows us to predict what types of sediments and stratigraphic sequences would be formed in a given depositional setting. It is important to ask what processes are involved in depositional systems. On a global scale, facies models generalize sedimentary features in modern depositional systems, but distill out local variability.

We begin our survey of depositional facies:

Non-marine environments are poorly preserved because they sit above the base level (usually sea level) of streams flowing across them and often above their graded longitudinal profile, the level on Earth's surface above which sediments must eventually erode, and below which they are deposited. Major continental environments include:


Q: What environments are thus preferentially preserved in the geologic record?

Q: How are terrestrial environments then preserved at all?

Alluvial fans

The most proximal (close to sediment source) and coarse grained of water-transported sedimentary environments is the alluvial fan. These cone shaped deposits form where high gradient narrow canyon streams disgorge onto flat plains at the mouths of canyons. At this point, streams typically lose the competence to transport framework clasts. Note, when applied to a stream: Consequently, the sediment they contain is recently derived from local sources. Alluvial fans are generally restricted in area, typically being no more than 1-10 km from their sediment's source rock.


Joshua Tree National Park, CA
These are found next to mountain belts and are the products of two main depositional processes: Secondary processes include:

Flow regime: In the modern world, alluvial fans are bodies of very course grained sediment. They have steep upper surfaces, ranging from 16 deg.- 1.5 deg., with the slope decreasing towards the basin. Slope angle increases with sediment size. Thus, the slope magnitude depends on: Due to this steep slope, alluvial fans are always upper flow regime (i.e. turbulent - Re > 2000) and commonly supercritical (i.e. rapid - Fr > 1). In contrast, river (fluvial) environments have gradients of 0.5deg.- 0.01.

Tectonic setting: Because they typically form so far above the graded stream profiles of the streams that form them, it is a wonder that any alluvial fans make it into the rock record. The ones that do form adjacent to regions undergoing rapid uplift. We tend to see them in:

Fan deposit types: Depositionally, alluvial fans are complex, and their character changes over time. First, we consider the major deposit types that make them up:


Alluvial fan sedimentation model:


The schematic above shows the distribution of the four fan deposits in plan, longitudinal cross-section, and transverse cross-section. The thing to note:

This simpler schematic shows how the fan tends to: As a result, at any given locality, there is a general coarsening upward trend.

Why do we care? Three reasons:

On a practical level, alluvial fans represent a high risk of natural hazards in the form of landslides and debris flows and are studied for this reason.

Variations:


Mescal Bajada, Anza Borrego State Park, CA

Bajadas: In arid regions, alluvial fans covering the pediments of mountain ranges can coalesce to form bajadas, broad aprons of alluvial sediment.

Fan deltas: Typically a fan forms when a canyon stream loses competence by flowing onto a plain. In some cases, the deltas formed by high-gradient stream abruptly reaching a body of water take on many of the characteristics of alluvial fans.

Identifying alluvial fans in the rock record:

Do not confuse with:


Q: What type of sandstone would be predicted to form in alluvial fans?