Geology of the San Rafael Swell and Moab Areas, Utah

This report deals with the San Rafael Swell and Moab areas of Utah. The purpose of this project was to describe and understand the stratigraphic units in our field area and how they relate to the tectonic history of Utah. For this project, we created a geologic map and cross section of the East limb of the San Rafael Swell using Google Earth imagery and data provided by Wilkerson (2020). This map and cross section highlight the structure of the San Rafael Swell, an asymmetric anticline, and the erosion habits of the area's stratigraphy. The swell itself was created during the Laramide Orogeny. This orogeny generated compressive forces that faulted more brittle basement rock and folded the more ductile overlying units. A number of flatirons can be found along the rim of the San Rafael Swell. These flatirons manifest as v-shaped stratigraphy on the geologic map and are a result of differential erosion. The stratigraphic units that make up these v-shapes are the more resistant, cliff forming units. These units were deposited over a period of time ranging from the Permian through Cretaceous periods. As time progressed, Utah was subjected to orogenies, changing sea levels, and differing latitudinal and longitudinal locations. As a result, the depositional environments of the area’s stratigraphic units changed over time from fluvial, to marrine, and even desert systems

Cover photo from Google Earth imagery (2019).

The purpose of this report is to describe the stratigraphy and geologic history of the San Rafael Swell and Moab areas of Utah. Originally, this project was meant to be completed using data gathered during a week-long trip to our field area; however, with the development of COVID-19, this trip could not be carried out. In order to address this complication, we instead completed our “field” work using Google Earth imagery and data provided by Wilkerson (2020).  

In addition to this imagery, we utilized literature research to gather information about the rock units and geologic history of our field area. For some of these rock units, we were able to describe the lithologies using hand samples provided in class. Using this research and provided data, we constructed a geologic map of our San Rafael Swell field area through Google Earth imagery and Adobe Illustrator. After constructing this map, we put together a cross section using similar techniques. Using our literature research, constructed map, and cross section, we compiled a description of the stratigraphy and geologic history of the San Rafael Swell and Moab Areas of Utah. Figure 1 provides an outline of the stratigraphy of this area.

Utah is known for its spectacular geology and array of national parks, monuments, and state parks. This geology, though, is the result of millions of years of tectonic and depositional history. Although this history spans a vast period of time, the geologic units found in our field area were deposited from the Permian through Cretaceous periods; thus, this report will primarily focus on this timeframe. These units, along with their depositional time frames, are laid out in the stratigraphic column of Figure 1. Over the course of its history, Utah was subjected to a number of different depositional environments that produced the stratigraphy we see today. This section will explore the geologic history of Utah and its corresponding stratigraphy.

As water levels fluctuated, basins created during compressional uplift experienced alternating periods of high and low water levels (Morris et al., 2016). As a result, many basins accumulated evaporite mineral deposits (Morris et al., 2016). In our general field area, deposition during the Permian was primarily eolian, though transgressive periods brought in alluvial fans (Morris et al., 2016). In areas closer to the Uncompahgre Highlands, fluvial and deltaic deposits were common. The transgressive and regressive periods during this time were responsible for the deposition of the Cutler Group, which includes both eolian and marine deposits Morris et al., (2010). Cross-beds found in the Cutler Group suggest eolian deposition, whereas the lower limestone beds indicate marine deposits.

To the West, the Kaibab Sea generated marine deposits late in the Permian (Figure 3). These marine deposits would form the Kaibab Formation. This unit is dominated by oolitic dolostones and limestones that provide evidence for the seas that once covered Utah. Given that the dolostone is oolitic, the Kaibab sea was likely shallow, as oolites form in wave-agitated waters. That being said, the waters that produced the Kaibab Formation were still some of the highest that the Kaibab Sea saw (Condon, 1997). Between the sea and highlands, an eolian erg likely produced the White Rim Sandstone (Baars, 2010). For many Permian deposits, sediment accumulated as the Uncompahgre Highlands eroded into surrounding basins (Utah: A Geologic History, N.d). Capping off the end of the Permian was a mass extinction event.

Unlike earlier Paleozoic periods, tectonics were not a major player during Triassic Utah. Although Pangea was still together, it would not be long before it began splitting into Gondwana and Laurasia. At this time, Utah was located near the equator (Morris et al., 2016). Throughout the early Triassic, conditions were hot and humid. Western seas brought periods of flooding, and mudflats, rivers, lakes, and swamps were common (Utah Geologic Survey, N.d). Although the Kaibab Sea could no longer be found in Utah, the Moenkopi Sea had taken its place (Figure 10). As this sea regressed, it would give way to deltaic and fluvial systems. Although deposition of the Moenkopi Formation consisted of some shallow marine deposition, deposition in our field area primarily occurred during these periods of regression. Sedimentary structures found in the Moenkopi Formation support this interpretation of a shallow marine or fluvial environment, as ripple marks, mudcracks, burrows, and marine fossils are common.

By the time that the Chinle Formation was being deposited (Figure 11), deposition in our field area was primarily fluvial floodplains influenced by rivers and deltas coming off of the Uncompahgre Highlands (Dubiel, 1992). Although the Uncompahgre Highlands were significantly eroded during the Permian, it produced sediments for a period of time during deposition of the Chinle Formation in the late Triassic (Condon, 1997). Lithics in the Chinle Formation indicate that it was deposited over a river valley. River valley deposition likely accounts for the range of rock types found in the Chinle, as they would vary according to fluvial changes in the river valley. This interpretation is supported by Doelling (2010).

For most of the Triassic, Utah was dominated by water-laden environments. As this period came to a close, these systems began to dramatically shift.  These shifts occurred as Utah was pulled northward and moved into the much drier trade-winds belt (Morris et al., 2016). As a result, late Triassic deposits produced the arid deserts and dunes of the Wingate Sandstone. The Wingate Sandstone was likely deposited in an eolian desert environment, with sizable cross beds and frosted grains serving as evidence for this interpretation. Doelling (2010) suggests that this environment was an erg, likely being similar to the depositional environment of the Navajo Sandstone. Cross-beds were likely formed during the building of windblown dunes, with grains becoming frosted as they flew through the air and collided with one another. This dry climate would continue into the Jurassic.

With the end of the Triassic Period and beginning of the Jurassic Period came a vast period of eolian deposition. At this time, Utah continued moving through the Trade Winds belt and the Wingate Sandstone and Navajo Sandstones were deposited (Morris et al., 2016). Between the deposition of the Wingate Sandstone and Navajo Sandstone, though, was a period of erosion as the rivers and deltas of the Kayenta criss-crossed the land (Figure 18). The depositional environment of the Kayenta Formation likely included areas of both eolian and fluvial deposition. As the contact between the Wingate Sandstone and Kayenta Formation is gradational, the transition from eolian to fluvial environments was likely relatively continuous. In the Kayenta Formation, small cross-beds, in addition to other fluvial structures, suggest stream and river deposition (Morris et al., 2010).

After this period of fluvial deposition came the vast eolian dunes of the Navajo Sandstone. The sedimentary structures and frosted grains of the Navajo Sandstone indicate that it was deposited in a desert environment. The large, sweeping cross-beds of this unit provide evidence for the windblown dunes that once covered Utah.

This desert would give way to a shallow sea and deposition of the Carmel Formation, which was likely deposited in a sabka-like environment. Based on the evaporite minerals found in the formation, such as gypsum and calcite, the Carmel Formation was deposited in an area where wet periods along with dry periods of evaporation were common. Limestones found in the Carmel Formation likely formed in the shallow marine sea Doelling (2010).

Soon, this sea would again recede and eolian deposits and sabkas would dominate the coming years, as well as some shallow marine deposition. The depositional environment of the Entrada Sandstone was likely a combination of both tidal and coastal dunes. In areas of the Entrada where large cross-beds of well-sorted sandstone may be found, deposition was likely eolian (Morris et al., 2010). Areas where planar beds are common were likely tidal in deposition (Morris et al., 2010). The Curtis Formation was likely deposited in a shallow marine environment. Glauconitic sandstone is an immediate indicator of marine deposition, and because the Curtis Formation is highly glauconitic, it is reasonable to interpret its deposition as marine. From there came the deposition of the Summerville Formation, which was likely deposited over a tidal flat, an idea which is supported by Morris et al., (2010). Ripples, small cross-beds, and siltstones found in the Curtis Formation are all consistent with tidal flat deposition.

While these events were taking place, the North American continental crust was converging with that of the Pacific Ocean (Figure 19), thus resulting in the Cordilleran volcanic arc (Morris et al., 2016). This convergence would create the Cordilleran highlands as well as basins, and would eventually lead to a fold-thrust belt in central Utah (Morris et al., 2016). The Jurassic period ended with the deposition of the Morrison. The depositional environment of the Morrison Formation was likely a floodplain containing both lakes and streams (Figure 20). The Brushy Basin Member of the Morrison Formation was likely fluvial and lacustrine, as explained by Doellin, et al. (2010). The Salt Wash Member was likely fluvial, being a part of a braided river system as supported by cross-bedding found in this unit (Doelling, 2010). Deposition of the Tidwell Member was likely similar to that of the Brushy Basin Member, with fluvial flood plains and bodies of water (Doelling et al., 2010).

The most recent units found in our field area were deposited during the Cretaceous Period. With this period came an influx of the tectonic and orogenic activity. As the Farallon and North American plates converged, east-west compression created an expansive mountain belt (Morris et al., 2016). This activity, known as the Sevier Orogeny, placed Utah between the Cordilleran Thrust Belt and the Cordilleran Basin System (Figure 34). As this compression began, a combination of thrust loading and movement of a clastic wedge towards the Cordilleran Basin generated lithospheric flexure (Titus, 2013). In the Late cretaceous, flexural loading was coupled with exceptionally high sea levels (Titus, 2013). With the formation of this belt also came the creation of anticlines and synclines (Morris et al., 2016). Morris et al. (2016) states that at least five major thrust faults accompanied this orogeny, with successive thrusts arising from preexisting thrusts. In addition to mountain ranges, this orogeny produced a large basin that would eventually become home to the Western Interior Seaway (Morris et al., 2016). As sea levels rose, deposits moved from fluvial to shoreline to marine. This seaway transgression produced the Dakota Sandstone and Cedar Mountain Formations.  Fossil evidence supports this interpretation, as lower units contain brackish water fossils and upper units contain marine fossils (Morris et al., 2010). Doelling et al. (2010) points out that lower units of the Dakota Sandstone contain fluvial structures, but as you progress upwards through the Dakota Sandstone, deposits become almost exclusively marine (Doelling, et al., 2010). Deposition of the Cedar Mountain Formation is believed to have been fluvial (Doelling, et al., 2010).

As the Western Interior Seaway reached its maximum transgression, deposition of the Mancos Shale began. The depositional environment of the Mancos Shale was likely primarily marine, though the cross-bedded Ferron Sandstone Member was likely more shallow than other members. In order to form cross-beds, waves or fluvial systems would have to have been involved. As the fossils found in the Mancos Shale are marine, they provide further evidence that the depositional environment was marine. Morris et al., (2010) supports this conclusion.

Towards the end of the Cretaceous period, the Sevier Orogeny gave way to the Laramide Orogeny. The Laramide Orogeny would continue until well after the Cretacious had ended. Between the Sevier and Laramide Orogenies, the angle of subduction between the North American and Farallon plates became shallower (Morris, et al., 2016). In some areas, this angle change produced thin-skinned compression, whereas in other areas it pushed up high-angle reverse faults in thick-skinned compression (Morris et al., 2016). Compression from the Laramide Orogeny also produced a number of anticlines and monoclines throughout Utah.

Although geologic events continued to shape Utah well after the Cretaceous, the units in our field area end with this time Period. Between the Permian and Cretaceous periods, Utah experienced a number of dramatic changes in its geology and depositional environments. From orogenies and uplifts, to basins, seas, and vast deserts, Utah’s history was certainly eventful. The events that shaped Utah are responsible for the spectacular geology that we see today.

Although this class originally intended to personally visit our field area, the rise of COVID-19 prevented us from doing so. Despite this setback, we were able to work with our field area virtually using Google Earth imagery and location data provided by Wilkerson (2020). With this data, we constructed a geologic map (Figure 39) of our field area. From our map, we constructed a geologic cross section (Figure 44). This map and cross section work to highlight the features of the San Rafeal and its relationship to the tectonic history of Utah.

The San Rafael Swell stands out topographically as a large, dome-shaped feature. This swell is a northeast trending, asymmetrical, doubly plunging anticline (Hawley et al., 1968). The West limb of the San Rafael Swell dips rather gently, whereas the East limb is far more steep (Doelling and Hylland, 2002). The anticline was likely created between 70 and 40 million years ago during the Laramide Orogeny (Doelling and Hylland, 2002). With this orgeny came compression and uplift. This compression then caused brittle basement rocks to fault, while more ductile overlying strata draped over top (Morris et al., 2010). Through these events, the San Rafael Swell was created.

The field area imagery used to create Figure 39 is pictured in Figure 40. The construction of this map was completed through field and contact data provided by Wilkerson (2020). Rock unit descriptions obtained through literature research and hand sample analysis supplemented the interpretation of this field area. These units can be distinguished from each other based off of their colors and textures. For example, the Morrison Formation (see Figure 39) is far lighter the the Summersville Formation to the West and the Cedar Mountain Formation to the East (see figure 40). The units in this map are based off of those discussed in the stratigraphy section of this report and the corresponding stratigraphic column. In this area, units dip generally eastward. These dips reflect the shape of the San Rafael Swell, as the "peak" of the swell lies to the west; thus, units dip east as you move down the Eastern limb.

• AMES Geology, N.d., The Navajo Sandstone:  Navajo-sandstone.html  (Accessed May 2020)
• Baars, Donalds L. “Geology of Canyonlands National Park, Utah” in “Sprinkel, Douglas A, Thomas C. Chidsey, and Paul B. Anderson. Geology of Utah's Parks and Monuments. , 2010. Print. P. 61-84
• Budge, Kent G. “Geology of the Jemez Area, Chapter 4: The Mesozoic.” Jemez.kgdudge.com, 2014-2017, jemez.kgbudge.com/dinosaurs.html. (Accessed May 2020)
• Bylund, Kevin., 2019, Lower Ferron Sandstone and Juana Lopez Members of the Mancos Shale around the north side of the San Rafael Swell:  http://ammonoidea.blogspot.com/2019/03/lower-ferron-sandstone-and-juana-lopez_26.html  (Accessed May 2020)
• Condon, Steven M. 1997. Geology of the Pennsylvanian and Permian cutler group and Permian Kaibab limestone in the Paradox Basin, southeastern Utah and southwestern Colorado. No. 2000. US Government Printing Office.
• Doelling, Hellmut H., Blackettm, Robert E., Hamblin, Alden H., Pollock, Gayle L., Powell, J. Douglas. 2010. “Geology of Grand Staircase-Escalante National Monument, Utah” in “Sprinkel, Douglas A, Thomas C. Chidsey, and Paul B. Anderson. Geology of Utah's Parks and Monuments. , Print. P. 161-192
• Doelling, Hellmut H. 2010. “Geology of Arches National Park” in “Sprinkel, Douglas A, Thomas C. Chidsey, and Paul B. Anderson. Geology of Utah's Parks and Monuments. , Print. P. 11-36
• Doelling, Hellmut H. and Hylland, Michael D., 2002, San Rafael Swell Proposed as Site of New National Monument:  https://geology.utah.gov/map-pub/survey-notes/san-rafael-swell-proposed-as-new-national-monument/  (Accessed May 2020)
• Dubiel, Russell F. 1992. Sedimentology and depositional history of the Upper Triassic Chinle Formation in the Uinta, Piceance, and Eagle basins, northwestern Colorado and northeastern Utah. No. 1787. US Department of the Interior, US Geological Survey.
• Fillmore, R., 2000, The geology of the parks, monuments, and wildlands of southern Utah, Univ. of Utah Press, 268 p. (accessed via Wilkerson (2020) lecture slides).
• Fitzgerald, Adrienne., 2015, Kayenta Formation:  https://www.nps.gov/zion/learn/nature/kayenta.htm  
• Fitzgerald, Adrienne., 2015, Kaibab Formation:  https://www.nps.gov/zion/learn/nature/kaibab.htm  (Accessed May 2020)
• Geology and Geophysics Department at the University of Utah. Outcrops along the side of Hwy 24. University of Utah. Sed.utah.edu  2020.
• Geology and Geophysics Department at the University of Utah.  University of Utah. Sed.utah.edu  2020.
• Grazul, Kendall R., 2015, "FOSSIL PLANTS AND PALYNOMORPHS FROM THE LATE PALEOZOIC CUTLER FORMATION, EASTERN PARADOX BASIN", Master's report, Michigan Technological University.  https://digitalcommons.mtu.edu/etds/910  
• Hawley, C. C., Robeck, R. C., & Dyer, H. B. (1968). Geology, altered rocks and ore deposits of the San Rafael Swell, Emery County, Utah. US Government Printing Office.
• Madsen, Scott. "The Lower Cretaceous Cedar Mountain Formation, eastern Utah: the view up an always interesting learning curve : [papers from a symposium of the Geological Society of America, at the annual meeting in St. George, on May 4-6, 2007] /".
• Manning, Vicky Wood., Morris, Thomas H., Ritter, Scott M. 2010. “Geology of Capitol Reef National Park” in “Sprinkel, Douglas A, Thomas C. Chidsey, and Paul B. Anderson. Geology of Utah's Parks and Monuments. Print. P. 85-108
• Morris, T.H., Spiel, K.G., Cook, P.S., and Bonner, H.M., 2016, Landscapes of Utah’s Geologic Past, BYU Press, 80 p.
• Petrie, Elizabeth., N.d., Outcrop in Southeastern Utah’s Carmel Formation (IMAGE): https://www.eurekalert.org/multimedia/pub/120788.php  (Accessed May 2020)
• Ross, Charles A., and June R.P. Ross. “Permian Period.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 16 Oct. 2018,  www.britannica.com/science/Permian-Period .
• “San Rafael Swell.” 38°49’25.39” N and 110°40’37.87” E. Google Earth Pro, 2019. Accessed May 2020.
• Sprinkel, Douglas A. 2010. “Geology of Flaming Gorge National Recreational Area, Utah-Wyoming” in “Sprinkel, Douglas A, Thomas C. Chidsey, and Paul B. Anderson. Geology of Utah's Parks and Monuments. Print. P. 285-308
• Titus, Alan L., et al. 2013 “Geologic Overview.” At the Top of the Grand Staircase: The Late Cretaceous of Southern Utah, edited by ALAN L. TITUS and MARK A. LOEWEN, Indiana University Press, 2013, pp. 13–41. JSTOR, www.jstor.org/stable/j.ctt16gzhs1.8. Accessed 9 Apr. 2020.
• Titus, Alan & Powell, J.D. & Roberts, Eric & Sampson, Scott & Pollock, S.L. & Kirkland, James & Albright, L.. (2005). Late Cretaceous stratigraphy, depositional environments, and macrovertebrate paleontology of the Kaiparowits Plateau, Grand Staircase-Escalante National Monument, Utah. 10.1130/2005. 
• “Utah: A Geologic History.” Utah Geological Survey, geology.utah.gov/popular/general-geology/geologic-history/utah-a-geologic-history/#toggle-id-12-closed. (Accessed April. 2020)
• Wilkerson, Scott., “GEOS 220.” DePauw University, Greencastle Indiana. 2020.
• Wonderly, Chris., 2019, White Rim Sandstone:  whiterim.htm  (Accessed May 2020)