In the rain shadow of the supercontinent Pangea, the western U.S. was an arid climate in the Mesozoic era. As the break up of this supercontinent began, a series of eolian controlled deserts covered the region through out the Jurassic period. The Wingate and Navajo sandstones, shaped by erosion, are some of the most impressive and expansive forns of this time period.

Wingate and Navajo Sandstone

Both of these are eolian desert deposits:

Tectonic Setting

• Deposits are found between 10-30 degrees in consistent high pressure regions. They are also found in rain shadows.give eX This can be used to indicate paleolatitude and past climates. They are often found with playa lakes, alluvial fans, and other desert deposits. (Prothero,1990)

Geometry and Composition

• Sand dunes are common. These are made up of very well sorted sand which is mostly quartz in composition. The dunes form tabular bodies in sets. These sets can reach 30m to 35m in thickness. These sets form overlapping cross beds which are generally constant through out the set. Ripples occur laterally showing that they are not from flows but from wind. Other deposits found are mud-stone and processes of ephemeral lakes. (Prothero,1990)

• The Wingate is capped by the Kayenta Formation, which was a brief period of wetter climates in the region resulting in ephemeral braided streams. Fluvial flows from the west provided new silicates for the extended basin (Lawton .......) as this region (the Colorado Plateau) was pushed up vertically. The Wingate forms the large cliff faces commonly found in Canyonlands and Island in the Sky.The Navajo is a homogeneous, well sorted, and rounded quartz and is more dune like in appearance. It is found in forms such as the Slick rock (pic.) in southern Utah. Arches National Park is a stunning example of Navajo sandstone. The terrain is not "petrified" or "fossil" dunes, but rather preserved. These type of formations cover a vast portion of the western states. For pics. see Geology,Utah State Uni.

See Canyonlands State Park Geology page for detailed description of formation processes.

Signs of Ancient Life

• Ostracods, bipedal dinosaurs and mammal like reptiles have been found in this area (Prothero,1990). Most of the tracks in this region are found in Mesozoic sediments. It is important to note that this is partly due to the long time period of the Mesozoic, and the ideal desert conditions. There are currently around 300 vertebrate footprint sites throughout the Mesozoic of the west (consistently more than body fossils).These sites are being used to make inferences about the population size and behavior of the fauna of the time (Lockley and Hunt, 1994) Mesozoic Life on Land

Moenkopi Formation

• The Moenkopi is an ancient mud flat very much like those of today. The exception is that they covered an expansion of more than 100 miles from the coast. The shapes of the depositional environment (ripples, cracks and such) have been very well preserved in the brown shales found today. (Baars, 1989 p. 55-57)

Chinle Formation

• The Chinle is a fluvial formation which covered the Moenkopi with stream gravel and sands. Rocks found in this formation range from shale, siltstone, and sandstone. Petrified wood is also common. This formation was a prime site for uranium prospecting in the past. (Baars, 1989 P. 57) The formation reaches a thickness in excess of 500 m in the Four Corners area of Colorado, Arizona, New Mexico, and Utah (Dubiel, 1994, p.145).

• Baars, Don. Geology of Canyonland Country. Lawrence, KS: Canyon Pud. Ltd, 1989.

• Dubiel, Russell F. Triassic Deposystems, peleogeagraphy, and Paleoclimate of the Western Interior. C.O.: C&M Press, 1994.

• Lockley, M.G., and A.P. Hunt. A Review of Mesozoic Verterate Ichnofaunes of the Western Interior United States: Evidence and Amplications of a Superior Track Record. C.O.: C&M Press, 1994

• Prothero, Donald R. Interpreting the Stratigraphic Record. New York: W.H. Freeman & Co.,1990.