Exploring the San Rafael Swell, UT

Figure 1: Gigapan view of the eastside of the San Rafael Swell in Utah where I-70 cuts through it (Schott, 2009).

The San Rafael Swell (SRS) is a large monocline approximately 50 km long and 120 km wide and is in south-central Utah, just northeast of Capitol Reef National Park (Jeffery, 2011). The SRS region is an exemplary location for learning how to identify stratigraphic units because it exposes rocks from the Permian White Rim Sandstone to the Cretaceous Blue Gate Shale. The mapped SRS region is marked by the latitude and longitude pairs (38.976310°, -110.514526°), (38.976310°, -110.308717°), (38.869096°, -110.514526°), and (38.869096°, -110.308717°). This area was mapped using Google Earth, and most of the units were identifiable based on outcrop pattern, cliff formations, and changes in color. When issues arose in mapping the region, such as with the Kaibab Formation and Entrada Sandstone, the main solution was to map the known surrounding units. For example, the Kaibab Formation issue was resolved by searching for the overlying and cliff-forming Moenkopi Formation instead of the gray Kaibab Formation itself.

The geologic history of the SRS is also excellent for learning how to understand what tectonic events and paleoenvironments lead to the formation of the SRS and its geologic units. By constructing a cross section of the SRS, the monocline was shown as east facing. Cross section apparent dips were calculated based on strike and dip data provided by M. S. Wilkerson. Research into the SRS region also indicates that basement-involved faulting from the Laramide Orogeny created structures such as the SRS. In all, the stratigraphic and geologic history of the SRS region aided in constructing the geologic map and cross section of the SRS (Fig. 2a; Fig. 2b). The resulting cross section, geologic map, and stratigraphic column were constructed using Adobe Illustrator.

Geologic Field Experiences: Utah is a Geoscience course offered by DePauw University that teaches students basic field skills by traveling to the area of study. For the Spring 2020 term, the class planned to investigate the San Rafael Swell (SRS) monocline and the Moab regions in south-central Utah (Fig. 3). The course plan was to have pre-trip lectures for establishing necessary geologic background, with post-trip goals of synthesizing and understanding the regions by creating geologic maps, cross sections, and reports. While the field trip was cancelled due to the outbreak of COVID-19, the class still achieved its goal, and students learned how to interpret the evolution of a field outcrop through geologic maps, cross sections, research, and lectures. The course was modified, however, to focus only on the San Rafael Swell monocline, for which the professor, M. S. Wilkerson, provided data. While students were unable to collect their own field data, the given strike and dip data allowed students to calculated apparent dip and learn how to construct geologic cross sections utilizing Google Earth and Adobe Illustrator. The overarching goal of learning geologic literacy was achieved through written reports and mapping projects, with this StoryMap as the course’s capstone project.

The San Rafael Swell is a part of the Colorado Plateau region, which extends across Arizona, Colorado, Utah, and New Mexico (Carstens, 2012; Fig. 3) Through uplift and erosion, the Colorado Plateau exposes rocks as old as the Precambrian in its canyons, making it an excellent region for geology enthusiasts to explore.

From tropical marine oceans to large sand deserts, Utah’s geologic history is preserved in its rock and fossil records. Large-scale tectonic events, from subduction of the Farallon Plate to mountain-forming belts, helped create many of Utah’s famous geologic structures. In addition to describing the geologic and tectonic history of Utah, the stratigraphic units found at the SRS are also detailed in relation to their paleoenvironments. These rock descriptions helped in identifying units for the SRS geologic map and are summarized in a stratigraphic column (Fig. 4).

Following Kaibab Formation in the Permian Period, a large mass extinction occurred between the Paleozoic and Mesozoic (250-65 Ma) Eras.

The Moenkopi Formation (Trm) is the first of the Triassic rocks at the SRS and includes the Black Dragon Member, Sinbad Limestone Member, Torrey Member, and Moody Canyon Member. This formation unconformably overlies the Kaibab Formation and is a fine-grained mudstone approximately 690-1000 ft thick (Morris et al., 2010).

In the Jurassic Period, Utah drifted northward into the 10-30°N latitude region, which is known to form deserts (Morris et al., 2016). The Glen Canyon Group (Tr-J), which includes the Wingate Sandstone (Jw), Kayenta Formation (Jk), and Navajo Sandstone (Jn) members developed in this dry climate.

The continued convergence of the oceanic plate and western edge of the North American Plate created forebulge and backbulge basins, which resulted in occasional marine conditions reflected in the San Rafael Group rocks (Morris et al., 2016). The mid-Jurassic San Rafael Group unconformably overlies the Glen Canyon Group, and at the SRS, its members include the Page Sandstone (Jp), Carmel Formation (Jc), Entrada Sandstone (Je), Curtis Formation (Jcs), and Summerville Formation (Js) members.

The Tertiary Period is the first in the Cenozoic Era, and between the Cretaceous to Tertiary, approximately 80-30 Ma, the Laramide Orogeny occurred (Wilkerson, 2020). During this event, the Farallon Plate subducted more rapidly into the North American Plate (Morris et al., 2016). As the Farallon Plate subducted, it also began to shallow into the continental interior, which reactivated old Precambrian normal faults (Wilkerson, 2020). In this “thick-skinned” deformation style, basement-involved reverse faults are the result of compressive stress, and younger layers of sedimentary rock “drape” above the fault (Morris et al., 2016; Fig. 47).

The uplifted rock results in structures such as the east-facing SRS monoline and the Water Pocket Fold (Fig. 48). During the Early Tertiary period, Utah had also moved north of the 10-30°N latitude region to a more temperate environment (Morris et al., 2016). 

Around 36 Ma in the Late Tertiary Period, the subduction of the Farallon Plate slowed until it eventually detached and the slab started to roll-back (Morris et al., 2016). This roll-back allowed magma to rise to the subsurface, which caused regional uplift and created volcanoes in southern Utah as part of the ignimbrite flareup (Morris et al., 2016). Large, intrusive granite and diorite laccoliths, such as Henry Mountains, also formed from this magma (Wilkerson, 2020). The regional uplift began entrenching the canyonlands as rivers were rejuvenated and began carving back down to sea level (Utah Geologic Survey, n.d). Basin and Range faulting also coincided with this regional uplift of the Colorado Plateau, which had an arid environment around 17 Ma (Morris et al., 2016).

Following the Tertiary Period, an ice age occurred due to fluctuations in earth’s orbit associated with the Milankovitch cycles (Morris et al., 2016). In this ice age, the freshwater Lake Bonneville extended over northwest Utah, and this lake is the ancestor of the Great Salt Lake in Utah (Utah Geologic Survey, n.d). During the Early Quaternary, humans also appeared (Utah Geologic Survey, n.d). As the ice age thawed and Earth entered a warm period, Lake Bonneville began to evaporate and accumulate salt, and this lull between ice ages is the current warm phase that the Earth is in today (Morris et al., 2016). Currently, Utah still maintains its moderately arid environment and depositional patterns that it did when the Great Salt Lake formed.

Although the Field Experiences: Utah course was unable to collect data directly from the SRS, “field” data was still gathered utilizing Google Earth, lectures, and literature on the SRS. The SRS field area is bounded by the four corners: (38.976310°, -110.514526°), (38.976310°, -110.308717°), (38.869096°, -110.514526°), and (38.869096°, -110.308717°).

The Geologic Map of the San Rafael Swell was constructed using Google Earth and Adobe Illustrator (Fig. 2a). By tracing the boundary for each unit in Google Earth, the contacts and satellite imagery was then transferred to Adobe Illustrator for coloring and annotation. These contacts were identified based on factors such as changes in color, outcrop pattern, and cliff facies, in which knowledge of each rock unit was gathered through lectures, hand samples, and literature to make informed decisions on the contacts.

White Rim Sandstone and Kaibab Formation Contact

Wilkerson (2020) noted that the White Rim Sandstone (Pwr) of the Cutler Group is the oldest exposed rock at the SRS. This gray and tan sandstone was in the western portion of the site, where streams had cut into it. By locating these streams outlined in light gray, the White Rim Sandstone was traced, and it generally followed a dendritic outcrop pattern (Fig. 49).

Moenkopi Formation and Chinle Formation Contact

The contact between the Moenkopi Formation and younger Chinle Formation (Trc) relied on multiple cues because the white color indicative of the Chinle’s youngest member, the Shinarump Conglomerate, was not always present (Fig. 52).

To map the Trm-Trc contact, the best method was to find and follow the cliff right underneath the White Shinarump Conglomerate (Fig. 52). While the Chinle Formation also contains beds that are slightly more red than the Moenkopi Formation, this was not as easily traced because both formations can take on a rusty-red color. Therefore, to ensure consistency, the cliff below the Shinarump Conglomerate was followed over color.

Wingate Sandstone and Kayenta Formation Contact

With the distinctive coloring of the Wingate Sandstone, the moment the tans and reds transitioned into light-gray, the boundary was marked. Additionally, the Kayenta Formation (Jk) has a "ledgey" appearance that helped separate the two units (Fig. 55). This contact also followed the overall flatiron pattern.

Entrada Sandstone and Curtis Formation Contact

Similar to the Jc-Je contact, the Entrada Sandstone to Curtis Formation (Jct) contact relied on mapping the alluvium first to realize where it was cut off. The Curtis Formation is a marine, glauconite-bearing deposit, and while the green sheen of the glauconite could not be discerned on Google Earth, the gray hue to this formation helped distinguish it from the reddish-tan Entrada Sandstone (Fig. 59).

Additionally, the the Curtis Formation is cliff-forming, in which the Je-Jct contact followed along a base of the gray Jct cliff outlined in red in Figure 59.

Curtis Formation and Summerville Formation Contact

The Summerville Formation (Js) is a reddish-brown siltstone that clearly differs from the gray Curtis Formation (Fig. 60). The stark change in color was consist throughout the mapped area, and is marked in orange on Google Earth (Fig. 60).

Morrison Formation and Cedar Mountain Formation Contact

The youngest member of the Morrison Formation is the Brushy Basin Member, which can vary from red, light purple, and tan, and also has a distinctive “popcorn” texture. The Cedar Mountain Formation (Kcm) then unconformably overlies the Morrison Formation, creating a sharp surface to follow at the base of the Cedar Mountain Formation (Fig. 62).

Although some of the Brushy Basin Member's light-red is exposed, talus and debris from the Cedar Mountain Formation has fallen onto it. Additionally, the Cedar Mountain Formation was also slightly red; therefore, in order to keep consistency, the eroded surface was followed over color.

After identifying and mapping the stratigraphic units at the SRS, a cross section from A-A' was constructed using strike and true dip data provided M. S. Wilkerson. Only data points close to the cross section were selected to calculate apparent dip (Table 1).

Topography for the cross section was taken from Google Earth and adjusted for vertical exaggeration. The rock contacts identified for the geologic map were used to created the stratigraphic layers, which were then dipped to their apparent dip (Fig. 2b). Morris et al. (2010) provided information on the fault, Precambrian Basement thickness, and Undifferentiated Paleozoic thickness added to the cross section. An additional scale bar marking sea level was included based on placing the SRS approximately 5000 ft above sea level (Hamel, 2015). Overall, a cross section for the SRS was created from the data provided by M. S. Wilkerson, Google Earth, and supporting literature on the SRS.

Despite the outbreak of COVID-19, the San Rafael Swell in Utah was still investigated through the Geologic Field Experiences: Utah course offered by DePauw University. The stratigraphic and geologic history of the region explained how the Laramide Orogeny formed the east-facing San Rafael Swell Monocline in south-central Utah. By constructing a stratigraphic column, geologic map, and cross section of the San Rafael Swell, skills for describing rocks in the field were still developed and each unit was researched and scrutinized "in the field" through Google Earth.