Flash Flood Deposits
When it rains in deserts, it often floods because there is little vegetation to trap water in soils and slow the runoff. Two environment types dominated by flash flood sediment transport are common: valleys with ephemeral rivers (wadis) and alluvial fans. Alluvial fans form in areas with a steep gradient from a drainage catchment to the basin floor whereas wadis in valleys form where the gradients are much lower. Tectonic activity it 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. Less structurally active deserts where deposition is dominated by flash floods, such as eastern Egypt, tend to have wadis (which is an Arabic word).
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 steeps (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.
Wadis - Wadis are similar to braided river deposits, which we will talk about next week. They have a high sediment load for the amount of water.
Flow types - Three types of flows are common: 1) debris flows, 2) sheet flows, and 3) channelized flows.
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. Debris flows 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.
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 later and farther down the fan and later in time. 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 flood deposits 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.
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.
Alluvial Fan Facies
- Poorly sorted beds (diamictites) that are of an approximately uniform thickness but of limited lateral extent, deposited by debris flows;
- 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;
- Normally graded sandstone beds that are laterally extensive deposited by sheet flows;
- 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”.
- Dawn Sumner (Department of Earth and Planetary Sciences, UC Davis)