San Rafael Swell – A fake but Spectacular Reef!

Last couple of weeks have been a bit crazy! Plenty of things to do and a wonderful visitor to take care of but I am back! Today I thought I would share some pictures that I took as part of the MYRES field trip. This time I am showing images from the San Rafael Swell near highway I-70 in Utah. I highly recommend stopping at this location if you happen to be visiting Utah (see maps). 

Click image to enlarge 

Click image to enlarge 

Here I include the description from the MYRES field trip guide (copy and paste because I can’t describe it better than they did!):

“The San Rafael Swell is an elongated anticline that formed during the Laramide Orogeny; it is 80 miles long and 30 miles wide at its widest point. The steeply dipping eastern limb of the anticline forms a monocline that offers some of the most dramatic tectonic expression in this area; the white cliffs of Jurassic Navajo Sandstone that dominate the skyline as you drive west on I-70 are locally referred to as the San Rafael “Reef”, though this is used as a geomorphic term and does not imply carbonate structure.

This is indeed a dramatic tectonic view! The high dips to the west (Navajo Sandstone) sharply contrast with the almost horizontal beds to the east (Morrison Formation?). How do you accommodate all this deformation in such a small distance – from almost flat to almost vertical? How did that happen? I am just a humble stratigrapher! Photo by Lorena Moscardelli  [LINK to higher resolution]

Traveling west, the highway crosses flat terrain underlain by the Mancos Shale. The highway cuts almost perpendicular to the strike of the monocline going downsection as you proceed west; near mile post 150 the base of the Mancos shale is crossed. A sandstone ridge of the underlying Cretaceous Dakota Sandstone is visible near mile 149.5, and west of that are sandstones of the Cedar Mountain Formation. Stratigraphically below the yellow and brown Cedar Mountain Formation sandstones, red, gray, and green sandstones and shales of the Jurassic Morrison Formation begin to outcrop. The Morrison Formation is exposed along I-70 between miles ~147.5 and ~149. West of mile post 147, sandstone and shale outcrops of the Entrada Sandstone and Carmel Formation tilt sharply upward with beds dipping to the east, defining the monocline at the eastern margin of the San Rafael Swell.

Inset 1 above: In this picture, I am standing on top of the Entrada Sandstone. On the background, you can see the monocline and the reddish Carmel Formation on top of the Navajo Sandstone. Photo by Penn State graduate student

At The Rest Stop (see maps)

This rest stop offers a view of the eastern side of the San Rafael Swell, with its abrupt monocline forming the steep “reef” topography. The hill of red sandstone and shale at the rest area is part of the Jurassic Entrada Sandstone, the same unit from which the arches of Arches National Park are formed. In this area, however, the Entrada Sandstone is less resistant than it is to the southeast, and has eroded to encircle the main topography of the San Rafael Swell at a lower elevation. Beneath the Entrada Sandstone are sandstone beds of the Carmel Formation, Jurassic near-shore marine and tidal-flat sediments, which occur as purple, red, and tan sandy beds that dip steeply to the east immediately above the white, cliff-forming Navajo Sandstone.

Inset 2 above: In this picture the contrast between the steeply dipping beds associated with the San Rafael Swell to the west and the almost horizontal beds of the Morrison Formation (?) to the east is astonishing 

As you continue west on I-70; the road leads you downsection into the center of the San Rafael Swell. The road passes through the Jurassic Carmel Formation, Navajo Sandstone, Kayenta Formation, and desert-vanished sandstone cliffs of the Wingate Formation, then through bright red sandstones and shales of the Triassic Chinle and Moenkopi Formations. Just east of mile post 144, the road crosses into limestone of the Permian Kaibab Formation (immediately west of the Triassic red beds)”

So if you are in that part of the continental USA…go check it out! It is really spectacular!