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Alluvial fans

  • Page ID
    10864
  • Introduction

    When it rains in arid environments, such as deserts, it often floods because there is little vegetation to trap water in soils and slow the runoff.  Alluvial fans form in areas with a steep gradient from a drainage catchment to the basin floor. Tectonic activity is typically required to maintain steep slopes because they erode to lower slopes through time. The Basin and Range Province in eastern California and Nevada is an area with abundant examples of alluvial fans.

    Alluvial fans are cone shaped accumulations of coarse sediment deposited at the transition from confined flow in a canyon to unconfined flow in a basin. This also corresponds to a break in slope. As the slope shallows and the flows spread out, the flows slow down and deposit much of the sediment that they were able to transport in the canyon. (Think about the Hjulstrom diagram). Fan geometry is determined by the rate of deposition. At the canyon mouth, it is steep (up to 15°) due to rapid deposition of coarse sediment. It shallows to about 5° over the main part of the fan and shallows even more to 1-2° at the toe. Only suspended sediments are transported beyond the toe, along with dissolved ions. If the water can pond, the fine grains settle out and the water evaporates forming minerals like gypsum and halite, and creating playa lake deposits. Deposition on a given alluvial fan is very rare - one event occurs about every 300 years on most fans in the southwestern US.

    clipboard_eb907def3c384f543d1f30a0e73122074.png
    Figure \(\PageIndex{1}\): Alluvial fan deposited in a glacial valley. Note the conical shape and surficial flows actively depositing sediment.

     Alluvial fans in Death Valley: http://g.co/maps/n2udd

    • The fluids involved are water, usually in the form of precipitation.
    • Three types of flows are common: 1) debris flows, 2) sheet flows, and 3) channelized flows.

    Sediment Transport Processes

    Debris flows are slurries of mud, rock debris, and just enough water to make the sediment into a viscous flow. Due to the high viscosity, the flow is laminar, like a glacier, and like a glacier, there is no significant sorting of grain sizes. Debris flows can transport very large blocks and continue to move until the internal friction of the flow due to viscosity exceeds the flow’s momentum when it freezes into place. This can occur due to either the loss of water or lower slope. The resulting deposits show little sorting and would be classified as a mud supported breccia or a diamictite. (Diamictites are defined as very poorly sorted sedimentary rocks with no grain size sorting within them. They are characteristics of laminar flow deposits.) In most cases, debris flow deposits are unsorted and lack any form of stratification. They are laterally restricted because they do not spread out too much, and they are commonly an even thickness throughout, with steep edges to the flows.

    Debris flow
    Figure \(\PageIndex{2}\): The front of a debris flow of sediment containing very large blocks. Note that most of the flow is restricted to the channel and the lateral variability in grain size due to energy loss on the flow boundary.

    Sheet flows are turbulent flows with significantly more water and less mud than debris flows. Since the flows are turbulent, there is significant grain sorting and normally graded, fining upward deposits are common. Once a flow reaches the mouth of the canyon, the flow spreads out and the coarsest rocks are deposited first. Finer grains are deposited last as they require the least amount of energy to keep in suspension. This produces normally graded beds, but deposition is very rapid and the grading is commonly poor. The suspended load may make it to the toe of the fan if the water doesn’t filter into the fan first. Sheet flows produce broad deposits that are clast supported, with some imbrication of clasts. Unlike a debris flow, sheet flows commonly cover 1/3 to 1/2 of a fan surface.

    sheet_flow.png
    Figure \(\PageIndex{3}\): Example of what a sheet flow may look like. This example is not on an alluvial fan; however, it does demonstrate the idea that sheet flows are not restricted to channels and typically flow over large surface areas.

    Channelized and other types of flows - A number of other flow types are also common on fans. For example, if there is insufficient rain to produce a sheet flow, ephemeral rivers can flow down the surface of the fan - which is more common. This produces braided river type deposits, which we will talk about later. There is also a significant gradation between debris flows and sheet floods. They represent two end members, and there are lots of variations in mud content and water content which variously affect the viscosity of the flow and thus the resulting sedimentary deposits.

    channels.png
    Figure \(\PageIndex{4}\): Braided streams cut into the alluvial fan deposit sediments. Larger channels may form episodically, resulting in fluctuating grain size distributions in a typical vertical facies.
    Screen Shot 2020-04-29 at 2.34.57 PM.png
    Figure \(\PageIndex{5}\): Representation of what to expect in vertically in an alluvial fan deposit. Note the episodic gravel deposits between thicker, more consistent sand deposits. As mentioned before, the finer sediments are deposited most distally away from the inlet of the alluvial fan.

    Characteristics of Deposited Sediment

    Characteristics of Alluvial Fan Deposits

    1. Poorly sorted beds (diamictites) that are of an approximately uniform thickness but of limited lateral extent, deposited by debris flows;
    2. Moderately to well sorted sandstone beds, often normally graded with pebbles at the base deposited in ephemeral channels; these show some cross stratification due to turbulent flow dynamics;
    3. Normally graded sandstone beds that are laterally extensive deposited by sheet flows;
    4. Average grain size decreases down slope and the abundance of debris flow deposits decreases down slope.

    Side note about facies: Each of list items 1-3 above could be described as a subfacies of the Alluvial Fan Facies, with its own grain sizes, characteristics, sedimentary structures, etc. Each of those could also be considered a facies, and the overall alluvial fan deposit could be assigned to an “Alluvial Fan Facies Assemblage”. If I was studying the sedimentation in all of Death Valley, I would probably be most interested in distinguishing between alluvial fan facies, playa deposits, and eolian deposits, so I would use the “Alluvial Fan Facies” I defined above. In contrast, if I wanted to highlight variations in the depositional processes on one fan in Death Valley, I would probably define the different types of alluvial fan deposits as different facies, so that my facies would emphasize the differences in debris flows, sheet flows, and channelized flows. I would then have an “Alluvial Fan Facies Assemblage” that would be distinct from my “Playa Facies”.

    Types of Deposits Typical of Arid Environments

    • Wind blown sand (well sorted, texturally mature medium or finer sand)
    • Flash flood deposits (poorly sorted breccia, including debris flows)
    • Playa lake deposits (silt, mud and evaporites). There are also very cold desert environments, such as the McMurdo Dry Valleys, Antarctica. These environments have glacial  deposits left by glaciers that flow in from areas with higher precipitation (e.g. higher elevations) or the ice cap.

    Typical Vertical Sequence of Facies Representing this Environment

    Screen Shot 2020-04-29 at 3.02.37 PM.png

    (A) is a debris flow dominated alluvial fan (B) is a sheet flood dominated alluvial fan.

    Key Words

    • Aridity - Aridity defines a desert, not temperature. An arid region gets less than 250 mm of rain/year. Our average in Davis is about 480 mm. That puts us barely in a semi-arid climate, which has 250-500 mm of rain/year on average. Arid environments are characterized by little vegetation.

    References