8.2: Arid Environments
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Review of Details of Cross Lamination/Stratification Formation
There are several important details of cross lamination formation that I’d like to emphasize before the midterm. First, the geometry of the laminae within the cross stratification reflects the geometry of the surface during deposition. A layers of sediment is deposited on a surface, and the geometry of the bottom of the layer preserves the geometry of the depositional surface. If there is a change in slope on the surface, there will be a change of slope at the bottom of the laminae that mimics the surface. However, layers can vary in thickness. On the lee size of a ripple or dune crest, there is often more deposition on the steep slope because that is where the sediment lands when it is washed over the crest. It is the high depositional rate here that makes the slope steep. If the slope gets too steep, the grains avalanche downslope, and it is no longer quite as steep. The geometry of the ripple or dune is directly related to the distribution of deposition.
The geometry of laminae on the downstream side, e.g. where they merge with the erosion surface that forms on the stoss side of the next ripple/dune downstream, is also interesting. The laminae thin to zero thickness right where they “downlap” onto this surface. If deposition occurs beyond the steep slope of the ripple/dune, the laminae curve and gradually thin. If deposition only occurs on the steep slope, the laminae end more abruptly again the erosional surface. The geometry of the laminae tells you very precisely where deposition occurred relative to the ripple/dune trough.
These videos review these concepts:
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.
Types of Deposits Typical of Arid Environments - 1) Wind blown sand (well sorted, texturally mature medium or finer sand); 2) Flash flood deposits (poorly sorted breccia, including debris flows); and 3) 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.
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 in Death Valley: http://g.co/maps/n2udd
- A wadi in Egypt: http://g.co/maps/369c4
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 - Two types of flows are common: 1) debris flows and 2) sheet 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. 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.
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.
Characteristics of Alluvial Fan Deposits
- Poorly sorted beds that are of an approximately uniform thickness but of limited lateral extent, deposited by debris flows;
- Moderately to well sorted 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 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.
The viscosity of air is low so it is typically a turbulent flow (Re=ulρ/µ). Density is also low, but viscosity is a larger effect. Think back to the Bernoulli Effect. The lift force has to overcome gravity, but it also depends on the difference in density between the fluid and the grains. The lower the density of the fluid, the harder it is to lift the grains against gravity. Thus, wind speeds must be very high to transport grains, and wind tends to lift only medium sand or smaller grains into saltation or suspension even at 30 m/s (about 60 miles/hour). Larger grains can only roll along the ground, mostly due to impacts from saltating grains.
Saltation - Wind transports sand as bedload (traction and saltation) and in suspension, like water. The traction and saltation transport are slightly different, however, because the impacts between grains are more forceful. Water dampens the impacts by limiting grain speed by friction between the water and grains and the effects of viscosity. Air does much less so because of both lower density and much lower viscosity. Thus, impacts when saltating grains land are very forceful. This has 2 effects: 1) more grains are launched into the saltating layer than the fluid could lift. This leads to a positive feedback - once saltation starts, the number of saltating grains increases rapidly. 2) The landing grains push the grains they hit, leading to surface creep of grains. This processes can push much bigger grains up slopes than could be transported by the wind alone, even with traction transport.
Ripple Formation - As in water ripples, wind ripples form as an initial pile of grains once saltation has started. However, the mechanisms of growth are different. There is no separation of a laminar flow sublayer or back eddy as seen in water ripples, in part because air flow is very turbulent. Rather, the impacts from saltating grains push coarser grains up the back sides of ripples to the crests where they eventually avalanche off the lee slope. Most of the smaller grains get transported off the crests where wind speeds are the greatest. Some small grains may also accumulate in troughs, especially between the larger grains. This leads to one of the rare cases of reverse grading: The larger grains are more concentrated at the tops of ripples and smaller grains are more concentrated at the bases. If there is significant accumulation of sand, thin reversely graded layers can be preserved in the rock record and are indicative of aeolian transport.
Cross Lamination - Migration of ripples occurs due to erosion on the windward side and deposition on the leeward side, as in water ripples. However, cross lamination is rare because the sand in dunes tends to be well sorted, especially the sand that gets transported down the lee slope - which is where sediment accumulates. Sometimes cross lamination is preserved due to fluctuations in wind speed resulting in different grain sizes being deposited at different times.
Dunes and Draas - Larger bedforms also form due to wind transport of sand. Unlike water transported sand, dunes and draas (huge bedforms really observable only from the air) commonly have abundant smaller bedforms developed on them. Basic sand transport on both is the saltation of sand up their windward sides and avalanching down their leeward sides. Cross stratification is common and large scale. Meter-thick beds are common even though the tops of the bedforms are not preserved.
Sand Characteristics - Another aspect of strong collisions between grains is that they are commonly rounded very quickly and commonly have frosted surfaces due to collision damage. The collisions also break down softer grains, particularly lithic fragments. Thus, most dune sand consists of well rounded, well sorted quartz sand. Rare exceptions, such as the gypsum sands at White Sands National Park can persist due to a lack of hard dense grains to abrade the softer gypsum grains.
Characteristics of Eolian Deposits -
- Well sorted, rounded grains;
- Little clay or silt sized grains;
- Large bedforms, thus thick sets of cross strata;
- Ripple stratification is rare;
- Some reversely graded laminae (not beds);
- Dawn Sumner (Department of Earth and Planetary Sciences, UC Davis)