Terrestrial sedimentary environments IV - Deserts and Eolian
But first, Outcrop du jour:
Eolian environments are those where the wind is the primary mode of sediment transport. This fact limits their geographic distribution to regions where water driven transport is rare because:
Water is 700 times more viscous than air. Thus, Stokes' Law indicates that particles should sink much more rapidly in it. This is, indeed, the case. The wind can usually transport nothing coarser than sand as bed load. Any wind capable of moving sand, however, is able to loft finer particles long distances. Thus: eolian environments tend to be sand dominated.
Deserts: Continental environments characterized by low precipitation. Generally:
- Low precipitation: < 25 cm rain/year (<10")
- Lacking permanent streams
- With less than 15% of surface area covered by vegetation.
- Generally that precipitation that occurs happens in very short intense bursts. As a result:
- there are high rates of runoff and low infiltration rates.
- regolith - generally undersaturated, dry.
- Soils are poorly developed - typically pedocals.
- In deserts, most sediment is moved by water albeit during brief, rare, intense intervals.
- On a day to day basis, however, eolian transport and deposition predominates.
- To encounter a truly pure eolian environment (at least for the last 3 Ga) you would have to visit Mars.
Eolian flow dynamics:
- Grain size: Well sorted sands, due to Stokes' law and the viscosity of air.
- Unconstrained dimensions: Unlike in channelized water, there is little constraint on the dimensions of eolian deposits:
- Saltation, as in streams. Clasts that are too heavy to move in suspension can occasionally be lifted off the ground. When they fall, they bounce, but also kick up other clasts that repeat the process in a cascade. Typically, only sand-sized grains enter saltation.
- Surface creep: Unlike water, air rarely has enough inertia to roll or push large objects along. Instead, we have surface creep, in which saltating objects impact larger grains and nudge them along.
Kansas dust storm from Symon sez
Suspended load: Silt & clay sized particles resist being taken into suspension, but once they are up, they tend to remain suspended for long periods, often removing them from the environment of origin. Air, like water, experiences friction with the ground. Therefore the layer of air in the few centimeters next to the ground is moving slowly. Saltating sand grains smashing into clay and silt particles kick them up into regions of faster moving air. From here on, they can be kept in suspension indefinitely.
- in large dust storms 1 km3 may carry up to 1000 tons
- if a large dust storm moves over 100's of square km - it may move 100 million tons of dust.
Frosted grains from Through the Sandglass
- Typically of fine grained structureless rock like chert or quartzite.
- planar faces that meet at sharp ridges
- Facets eroded on windward side, but storms roll or rotate, expose new side.
Note: Mars is a good place to find ventifacts.
Desert varnish: Sediments are often oxidized or coated with iron oxides, largely because the products of weathering reactions aren't removed from the environment of weathering by water. One interesting consequence is desert varnish, the dark shiny patina of clay minerals and manganese and iron oxides that forms as a result of a combination of action of windblown dust, chemical weathering, and water in dew. Desert varnish forms very slowly. A thin coat may be 2000 years old.
- Mud and silt are lifted away by wind but framework clasts aren't.
- gradually erodes lowers the ground surface, concentrating framework clasts.
- Eventually, concentrated framework clasts protect underlying fine sediments from wind erosion.
Eolian dunes are like subaqueous dunes, only different. Any topographic irregularity can create a wind shadow in which sand will be deposited, adding to the irregularity and provoking more deposition.
- Stoss face [German Stossen - "to push or bump"]: the upwind face. Generally low angle face with ripples.
- Slip face: The lee or downwind face. Typically a high angle face down which sand slides or cascades.
Dunes tend to form in topographic pockets where the sand is confined. The White Sands dunefield (right) in New Mexico, for instance, is trapped against a range of high mountains by prevailing winds.
Dune morphology: Depend mainly on 3 factors
- strength and consistentcy of winds
- amount of sand source
- presence of vegetation.
- Barchan (crescent) dunes [Kazakh - crescent shaped dune]
- solitary, cresent shaped dune whose horns point downwind.
- forms where sand supply is limited and wind is stong and constant
- Transverse dunes:
- Imagine a barchan environment but with much more sand, so that the barchans grew to where they fused together into long wavy ridges perpendicular to prevailing wind.
- Form in sand rich environments with strong steady prevailing winds.
- Parabolic dunes:
- Common in costal areas with moderate to abundant sand and winds and some vegetation cover. Picture transverse dunes that become anchored at points by vegetation so that parts of the dune migrate while others are held in place.
- Cresent shape but tips point up wind
- Longitudinal (or seif) dunes [Seif = Arabic - sword]
- long straight ridges more or less parallel to the wind
- moderate to low sand supply and strong prevailing winds that change direction frequently. As a result, the dune effectively "doesn't know which way to migrate," and its orientation represents an average of the various wind directions it experiences. Slip and stoss faces change identity frequently.
- Saturn's moon Titan is the Solar System's "longitudinal dune central."
- Aggradation in eolian environments: Dunes tend to form on flat surfaces. This could be bedrock, but more often is soil or regolith that is held in place by groundwater that diffuses upward from the water table by capillary action.
Dunes in the rock record: So what happens if the water table rises beneath a dune field? The dunes can be immobilized and, eventually lithified by the precipitation of cements. The result is an eolian sandstone.
- Diagnostic dune features:
- Supermature sands with frosted grains.
- Absence of matrix clasts
- High angle (20-35¡) cross beds.
- Bedforms tend to be very large compared to subaqueous counterparts, up to 35 m. thick.
- Characterized by climbing ripples marks
- Reverse grading: smaller particles blown across the dune crest tend to travel farther than large ones. The result is that although well sorted, eolian sand deposits show a coarsening upward sequence.
- Playa lake deposits
- Alluvial fan deposits
- Stream deposits
Distinguishing form other supermature sands:
Eolian sandstones resemble beach deposits in being:
- Supermature quartzarenites
Where they differ:
- Size of bedforms
- Eolian sands interfinger with other desert facies (playa, alluvial, stream), beach sands interfinger with marginal marine environments
- Eolian sands contain fossils of terrestrial animals, if any. Beach sands contain copious marine fossils.