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Navajo Sandstone

(in Glen Canyon Group)

 

 

Age: Early Jurassic

Figure 1: Paleogeographic map of the Early Jurassic, Wingate Sandstone, Navajo Sandstone, and Kayenta Formation. (Blakey, 2008)

The Navajo Sandstone is dated as Early Jurassic, although precise dating is typically difficult due to a lack of age diagnostic fossils, a common problem in eolian deposits.

 

Depositional Environment: Eolian (wind blown)

The Navajo Sandstone was deposited in an eolian environment composed of large sand dunes, similar to portions of the modern Sahara Desert.  In an eolian environment there are two primary types of deposits: 1) dunes, typified by large-scale trough cross stratification; and  2) interdunes, which are the flat lying areas between dunes.

 

Paleogeography:

The Navajo Sandstone represents an enormous erg, a large sand sea.  This sand sea extended over most of Utah as well as parts of New Mexico, Arizona, Colorado and Wyoming.  Though the deposits are known by different names in different areas, they were all a part of this major erg system.  At this time, the modern Colorado Plateau region was at very low latitude, approximately 10o north of the equator (Blakey 2008).  The Colorado Plateau region was located near the western edge of Laurentia, the western-most portion of North America (not having accreted to the rest of the continent by then).  By the Early Jurassic, Pangaea had begun to break up.

   

Detrital zircon geochronology indicates that the Navajo erg received some sediment from the Appalachian Mountains via a continental scale river system similar to the modern Mississippi River. (Dickinson and Gehrels 2003; Rahl et al.2003)  To the south and west of the erg were mountains of the nascent Cordilleran Arc, while to the east lay to the platform of central North America and the remnants of the Ancestral Rocky Mountains.  Directly adjacent to the south and west of the erg lay the fluvial facies of the Kayenta Formation.

 

Tectonics:

    Although the Navajo Sandstone was not deformed by the active tectonics, it did form in a basin that was a result of the regional tectonics.  As the mountains to the south and west were uplifting a flexural basin was formed from the added mass of the new mountain range.  The subsidence of this basin created room for the sand to be deposited in.  This also caused deceleration of the regional winds due to a decrease in the pressure gradient, which caused the sand being transported by the wind to be deposited in the erg (Kocurek 2003).

 

Climate: arid

    The climate in the Colorado Plateau region during deposition of the Navajo Sandstone was very dry (classified as hyper-arid).  Due to the mechanics of global atmospheric circulation, large desert, such as the Arabian Desert, are typically located around 25-30o north and south of the equator in the trade wind belt.  Because of the Navajo location on the western side of the Laurentian landmass, easterly trade winds were very dry by the time that they reached the Colorado Plateau region at approximately 10o north latitude, delivering little rain all to the region (Kocurek and Dott 1983, Loope et al. 2004).

    Dunes require strong winds to form.  Winter monsoon winds blowing from the northwest seem to be counter to the northeasterly winds typically present at the low latitudes at which the Navajo Sandstone was deposited.  Studies of modern low latitude atmospheric circulation shows that low latitude, low-pressure systems can cause monsoon winds that undergo a 900 change in direction as they approach latitudes within 100 of the equator.  These observations explain how dune fields formed by northwesterly winds could develop at a latitude where northeasterly winds are expected.  During the summer a lighter cross equatorial monsoon wind blew from the southwest modifying the dune shapes (Loope et al. 2004).

 

Features:

    The Navajo Sandstone is most notable for its excellently preserved, large-scale trough cross strata recording lee-face deposition on the subareial sand dunes (Kocurek and Dott 1983).  Two types of internal stratification are common in the cross strata: grain flow strata and wind ripple strata.  Grain flow strata form as avalanches of sand grains slump down the lee faces of the dunes.  They primarily form during periods when the wind is blowing in the dominant dune forming direction.  These strata can be recognized most easily by their downslope pinch outs towards the toe of the dune.  Wind ripple strata leave thin, inversely graded “pin stripe” laminae, formed by ripples superimposed on the much larger sand dunes.  In some cases, wind ripples at the toe of the dune and form aprons or plinths of reworked sand.

     In Capitol Reef National Park there are two primary eolian deposits, the Navajo Sandstone and the Wingate Sandstone.  Since they were formed in the same depositional environment the two formations on might think these should look fairly similar.  However, the two formations weather quite differently.  The Navajo tends to weather into smooth rounded domes and cliffs, whereas the Wingate tends to from very blocky, vertical cliffs.  The Navajo also has a tendency to sometimes have weathered pockets from a process called honeycomb weathering.  In general, the Wingate tends to be red in color, and the Navajo is typically more white in the field trip area.  This is most likely the result of higher permeability in the Navajo, permitting higher fluid flow and diagenetic bleaching of the rock.

 

Sites Best to See it:

Stop 2-3 - Navajo Waterfall

Stop 2-4 - Navajo Sandstone Soft Sediment Deformation

Stop 3-3 - Grand Wash Trail

Figure 2: A modern dune field at White Sands National Monument shows analogous eolian features to the Navajo Sandstone.  Note the smaller wind ripples superimposed on the larger dunes in the foreground.  The flat, lightly vegetated low spot in the middle of the picture is an interdune area.

Figure 3: This paleogeographic reconstruction of the western US during the Early Jurassic (Kocurek and Dott,1983 p. 106) shows the Navajo Sandstone and correlative eolian units, the Nugget and the Aztec, covering parts of Utah, Wyoming, Colorado, Arizona, Nevada, New Mexico and California.  To the south and west lie mountains of the nascent Cordilleran Arc and the Mogollon Highlands, while to the east lay the North American platform and the remnants of the Ancestral Rocky Mountains. 

 

Figure 4: Seasonal climate reconstructions (Loope et al., 2004 p. 317) showing the primary wind direction for the Navajo Sandstone.

 

 

Figure 5: Figure from Loope et al., 2004 (p. 318) showing reorientation of trade winds to westerlies at low latitudes due to regional low-pressure regimes driving monsoon systems.

 

 

 

Figure 6: This image shows the large-scale trough cross bedding that is characteristic of the Navajo.   These cross strata are the preserved lee faces of sand dunes that were present in the erg when the Navajo was deposited.  Trough cross strata can be seen to cut down into lower layers, indicating that the wind that formed the dunes also caused them to scour into previous dune deposits. 

        It is also possible to see the pock marked weathering pattern, common to the Navajo sandstone, called honeycomb weathering.

Figure 7: This is a picture of grain flow strata in the Navajo Sandstone.  As you look along the bedding you can see how individual layers expand and pinch out.  This is due to the avalanche nature of the grain flow strata.  Laterally some parts of the avalanche will be thicker than others, while at their edges and in a down slope direction the individual flows must pinch out where they come to an end.

 

 

Figure 9: Since both the Wingate and Navajo Sandstones are eolian formations present in Capital Reef National Park, and are nearly adjacent formations with only the Kayenta formation separating them, it is important to be able to tell them apart.  This photo shows the Wingate in the midground with the Navajo in the center background, on the skyline, and is good for highlighting the differences between the two formations.  The Wingate tends to weather in shear, vertically jointed cliffs, while the Navajo tends to weather in rounded domes.  The Wingate tends to be red from iron oxide staining, though as can be seen in the center midground the Wingate can also be bleached, while the Navajo is almost always a light, bleached color. 

Figure 10: This is a picture of wind ripple strata in the Navajo Sandstone.  Wind ripple strata are typically very thin, parallel laminated strata that are formed as the wind moves sand up and over the dune.  If the wind ripples are being blown by winds in the primary dune forming direction then they will collect at the crest of the dune until they have built up a steep enough slope to avalanche down the lee face of the dune.  This process forms grain flow strata but no direct evidence of the wind ripples are preserved.  If the wind is blowing in a direction where the wind ripples do not collect at a dune crest and become grain flows then wind ripples can be preserved.  This can happen if the wind is blowing in a secondary direction and wind ripples are carrying sand across the face that is normally the lee slope.  This can also happen when the wind is blowing in the primary direction on dune faces that are not on the lee slope of the dune.  See Kocurek et al., 2007 for a good explanation of where to expect to find wind ripple strata preserved on a dune.  If wind ripples are being preserved over the course of a full year then seasonal variations in wind strength will lead to slight variations in the grain size of the sand carried.  This alternation in grain size is reflected in the eolian deposit as slight variations in the way the outcrop weathers, giving a ribbed texture to the weathered rock.

 

 

 

For a complete list of references please go to the References page.

 


 

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Disclaimer: The information is property of the University of Utah. Unless cited, images and files found on this site have been taken or created by the Geology and Geophysics Department at the University of Utah. Any use of these images should be cited appropriately. The stratigraphic column is from: Mathis, A. C. 2000. Capitol Reef National Park and Vicinity Geologic Road Logs, Utah, in: P.B. Anderson and D.A. Sprinkel (eds.) Geologic Road, Trail, and Lake Guides to Utah’s Parks and Monuments Utah Geological Association Publication 29. http://www.utahgeology.org/uga29Titles.htm

Copyright (c) 2010, Geology and Geophysics Department, The University of Utah

 

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