Swell Geology: The San Rafael Group by Allyson Mathis
The San Rafael Group is a set of rock layers (formations to geologists) that were first described near the San Rafael Swell west of Moab. Most people know the Swell as the upwelling (anticline) of sandstone layers that makes a prominent ridge that I-70 cuts through in one of the most dramatic changes in scenery in all of canyon country. Suddenly the land transforms from a wide open plain to deep canyons and sheer cliffs.
The rock layers in the San Rafael Group are not the older rocks that make up the core of the Swell but are those that are exposed along its flanks. These rocks are present in the flats of the San Rafael Desert, in the approach to the Swell, and in Goblin Valley State Park.
Although most of the rock layers of the San Rafael Group are also in the Moab area, they are different here than in central Utah because the ancient environments varied from place-to-place.
The San Rafael Group was deposited in and around the arm of a shallow seaway or trough that reached down through central Utah from the north. The trough variously experienced shallow marine or tidal flat conditions depending on slight changes in sea level through time. To the east near Moab (e.g., away from the trough), sediments were deposited mostly under continental conditions that varied between a coastal flat and active sand dunes. These variations in paleogeography and depositional environments caused sand to be deposited in dunes near Moab while sediments were deposited in marine or tidal conditions where the Swell is now. The ancient environments in Utah during the Jurassic when the San Rafael Group was deposited.
Overall the San Rafael Group is thinner near Moab because Moab was further away from the central part of its depositional basin near the Swell. In central Utah, the group also contains a rock layer (the Page Sandstone) at its base that is absent here. There are also differences in lithology (rock type) in the layers found in different places. As a general rule, the San Rafael Group’s rocks in southeastern Utah are coarser grained (e.g., contain more sand) than those in and around the Swell itself.
The differences in lithology between Moab and central Utah is why three of San Rafael Group layers have distinct members in the Moab area because of facies changes from one place to another. A member is a distinct part of a formation, usually restricted to a certain geographic area and for rocks with a different rock type. A facies is a distinct part of a rock layer that differs from other areas, usually based on its appearance or lithology. An example of a facies change would be the change from siltstone to sandstone, or of a unit having thin bedding in one place and massive bedding in another.
The Entrada Sandstone undergoes a facies change from the San Rafael Swell area to the Moab region where the Slick Rock Member is. The “earthy facies” of the Entrada in Goblin Valley (no formal member has been named) was deposited at the margin of the tidal flat area. The Slick Rock Member was deposited in the heart of an ancient sand dune field and today is prominent in Arches National Park where it hosts most of the park’s famous rock openings.
The Dewey Bridge Member is a distinct dark red facies of the Carmel Formation. Elsewhere the Carmel is mostly drab gray to reddish siltstone that contains a lot of gypsum (an evaporite mineral that was deposited in tidal areas with high salinity). The Dewey Bridge Member is also thinner, at most a few hundred feet thick, versus up to the 1,000 feet thick Carmel Formation in Capitol Reef National Park.
The rocks of the San Rafael Group are relatively soft so they aren’t cliff-formers like the underlying Navajo Sandstone that holds up the crest of the San Rafael Swell. The softer San Rafael Group rocks make subdued flat irons on the edge of the Swell and the rocks in the flat areas. James St. John photo.
The Curtis Formation also has a distinct member in southeastern Utah versus central Utah. The Moab Member was deposited as sand dunes versus the marine facies to the west.
The distinct lithology of the Slick Rock and Moab Members is what makes them be such prominent cliff-forming layers in Arches National Park. The white Moab Member is a particularly hard cap rock that tops some of the fins (narrow sandstone ridges) in Arches National Park. Both the Slick Rock and Moab members are hard sandstones, unlike the softer facies of Entrada and Curtis exposed in Goblin Valley and elsewhere near the Swell.
The Summerville Formation is the uppermost formation in the San Rafael Group. It is very thin near Moab, and in places is even absent. Like the other San Rafael Group formations, it is much thicker near the Swell.
The San Rafael Group was deposited in the Jurassic (the age of dinosaurs) between about 172 and 156 million years ago. With the exception of some dinosaur tracks mostly in the Moab Member, there are few fossils in the San Rafael Group. But the rock types that make up each formation as well as how they vary across distances allow geologists to understand the variations in the ancient environments.
The Entrada Sandstone (reddish orange) overlain by the Curtis Formation (white) in Goblin Valley (left) and in Arches National Park (right). Arches National Park contains the Slick Rock and Moab Members of the two formations respectively.
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.
To learn more about Moab’s geology, visit the Geology Happenings archive online at https://www.moabhappenings.com/Archives/000archiveindex.htm#geology
The San Rafael Group is a set of rock layers (formations to geologists) that were first described near the San Rafael Swell west of Moab. Most people know the Swell as the upwelling (anticline) of sandstone layers that makes a prominent ridge that I-70 cuts through in one of the most dramatic changes in scenery in all of canyon country. Suddenly the land transforms from a wide open plain to deep canyons and sheer cliffs.
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.