The Geology of the San Rafael Swell, Utah

The Cutler Group includes two primary geologic units in the field area, the Organ Rock Shale and the White Rim Sandstone, both dating to the Permian Period (298 - 251 Ma). The reddish-brown Organ Rock Shale is from the Uncompahgre Uplift to the east, and is extremely arkosic. This unit was deposited in a swampy, slow-moving hydrous environment, where fine-grained sediments accumulated and were compacted over time. The quartz-rich White Rim Sandstone comes from the west, and represents a coastal dune or beach depositional environment. This sandstone layer includes large cross-beds, with individual beds reaching 5 - 75 feet in thickness (Morris et al., 2010). In areas such as Canyonlands National Park and Dead Horse Point State Park, the Organ Rock Shale and White Rim Sandstone of the Cutler Group are seen together in outcrop, in an interfingered "candy-striped" pattern.

The Kaibab Formation includes a limestone that is pale yellow-white when dry and yellow-tan in hand sample. There is a range in color for the crystals, from clear and colorless to white and opaque quartz. These grains are subangular, medium- to fine-grained, and well-cemented in their matrix. With the addition of dilute HCl, the hand sample effervesces. Upon microscopic inspection, the grains themselves do not appear to react with the acid, but rather the cement in the matrix is effervescing. This reaction to dilute acid indicates the quartz grains are likely held together by a matrix composed of calcite. Overall, the Kaibab Formation hand sample is a moderately sorted rock. No distinct bedding or sedimentary structures are present (Fig. 6).

As the Kaibab Formation is a limestone, the depositional environment of this unit was likely a shallow marine environment. The fine grains are sorted moderately well, so there may have been enough movement of water to bring in the occasional larger grain, but overall the current was consistent enough that grains were sorted well. This unit dates to the Early Permian (around 270 Ma), and is known for having fossils from organisms that lived in shallow Permian seas (National Park Service, 2015).

In hand sample, the Moenkopi Formation has very fine grains, which are rounded and very well sorted. These grains are well cemented by a matrix likely made of calcite or carbonate, as it effervesces with dilute HCl. The sample is either a very fine-grained sandstone or siltstone. The sample is an orange-tan when dry, and a darker caramel-brown when wet. Macroscopically, the hand sample shows evidence of trough cross-laminations (Fig. 7).

The trough cross-beds in the Moenkopi Formation indicate a depositional environment of a water current, like a stream or river. The Moenkopi Formation is associated with the Lower to early Middle Triassic (about 251 - 241 Ma), and the formation includes a sandstone, siltstone, mudstone, and chert-rich conglomerate (Huntoon et al., 2010). The sorting, sphericity, and size of the grains also imply weathering or transportation over great distances, which further supports this idea of fluvial and shallow marine environments.

The Chinle Formation primarily appears in three variations of colors and textures. The first of these is a rusty - dark rust-brown mudstone, with a slightly gritty texture and very fine grains. The mudstone is very well sorted with subrounded grains, where the grains are contained in lenticular beds. With the addition of dilute HCl, the hand sample effervesces between the lenticular beds, as well as in thin beds that are lighter in color than the rest of the sample. The Chinle Formation can also be a lighter, rose-brown, poorly sorted sandstone. This sandstone contains both clear, colorless, subrounded quartz grains, as well as white, opaque, subangular grains of feldspar or lithics. With the possibility of quartz, feldspars, and lithics present, the sample is likely a lithic arenite. The cement holding these grains together effervesces in dilute HCl, indicating a cement of calcite. The third and final hand sample of the Chinle includes a tan, very fine-grained sandstone. This sample has lenticular beds that nearly resemble herringbone cross-stratification, in which the grains are well sorted and subangular. Overall, the sample is fissile, fizzes in HCl, and is micaceous. All three hand samples are visible in Figure 8.

The Chinle Formation dates to the Upper Triassic, around 237 - 201 m.y. (Huntoon et al., 2010). With sandstones and mudstones present, the Formation likely formed in a fluvial environment. Although not seen in hand sample, the Chinle Formation also contains the Shinarump Conglomerate, which contains pebbles of quartz and chert that were deposited in a fluvial environment (Huntoon et al., 2010). As there is quite a bit of variety in terms of grain size present in the Chinle, the depositional environment most likely was fluvial throughout, but changed slightly across the landscape, with slower waters allowing a mudstone to form, to the fastest waters creating the Shunarump Conglomerate.

In hand sample, the Wingate Sandstone is quartz-rich with planar beds. A fresh surface of the Wingate is a pink-tan when dry, and darkens slightly to a more peachy when wet. When a surface of the sandstone has been exposed and subjected to weathering, an FeMn oxide forms a darker coating known as desert varnish. The addition of dilute HCl to a sample of the Wingate does not cause any visible reaction, indicating the quartz grains are cemented together with a siliceous material rather than calcareous. The quartz grains themselves are fine grained, rounded, and very well sorted. The Wingate Sandstone seems to be moderately cemented, as some of the grains break apart but in general the hand sample tends to stay mostly intact. Examples of the Wingate Sandstone in hand sample are pictured in Figure 9.

The Wingate Sandstone is noticeable in the field by its large, vertical cliffs, often covered with desert varnish. These cliffs are often quite thick, measuring approximately 230 - 350 feet from the top to the bottom of the cliff (Doelling et al., 2010). With such a thick sandstone bed, a long depositional period is represented, dating to the Jurassic Period (201 - 145 Ma). The depositional environment for the Wingate Sandstone is believed to have been eolian and sabkha (coastal flat) environments, with large wind-blown sand dunes creating thick cross-beds of rounded, fine-grained sands (Doelling et al., 2010). With wind-blown sands, it is common to see frosted grains, although these were not noticeable in the hand samples present.

The Kayenta Formation is primarily known for its rose-brown sandstone. In hand sample, this sandstone has fine, well-sorted grains that are subrounded to subangular, forming planar laminations. With the addition of dilute HCl, some bubbles escape from the hand sample, although it was not determined if these bubbles were caused by an effervescing cement or simply air escaping from between pore spaces between grains. One of the hand samples, shown in Figure 10, includes rounded nodules of a darker material. These nodules are very fine grained and effervesce strongly with acid unlike the surrounding main body of the sample. This reaction could potentially indicate a calcite-cemented sandstone or possibly even fragments of a limestone.

The sediment grains that have formed the Kayenta Formation were likely not transported long distances, as they were not well rounded. However, the grains are well sorted, so the depositional environment could have been along a beach or near the coast, where waves would sort, weather, and rework the grains. The darker nodules present in hand sample could also indicate a fluvial or beach setting, where limestone could be formed, or possibly of calcite-cemented sands were dropped into the main sandstone. In general, the Kayenta Formation dates to the Jurassic Period, approximately 201 - 145 m.y.

The Navajo Sandstone is a quartz arenite, whose hand samples are white-gray when dry and a darker tan-gray when wet. With the addition of dilute HCl acid, neither the grains nor cement in the sandstone effervesce, indicating the absence of calcite. When macroscopically viewing the hands sample (Fig. 11), faint planar beds can be identified. Microscopically, the individual quartz grains can be seen. They are very well sorted, rounded, and many of the grains are frosted as well, with a slight etched, opaque look.

Dating to the Jurassic Period, around 185 m.y., the depositional environment of the Navajo Sandstone is commonly accepted as being an eolian setting. The Navajo Sandstone can be identified easily in outcrop, forming thick, white cliffs that can be over 1,000 feet thick (Doelling et al., 2010). The frosting, shape, and sorting of the quartz grains are all evidence to an environment of wind-blown sands that would have undergone significant weathering and transportation. Although planar beds are visible in hand sample, outcrops of the Navajo Sandstone can show the large cross-beds associated with wind-blown sand dunes, sometimes reaching as thick as 35 feet (Doelling et al., 2010).

In hand sample, the Carmel Formation shows a tan-gray limestone in sharp contact with large white crystals, between which lies a thin rust-orange coating. The grains of the sample are best viewed microscopically, as they are very fine grained. Well cemented together, these grains are sub-rounded, well sorted, and effervesce with dilute HCl. On the sides of the hand sample, dark manganese dendrites appear, showing surfaces that broke apart at slight fissures or cracks where Mn-rich water had once seeped. The hand samples observed for the Carmel Formation are shown in Figure 12.

Across Utah, the Carmel Formation contains sandstone, siltstone, mudstone, limestone, and gypsum layers, together ranging from 200 to 1,000 feet thick (Morris et al., 2010). Two notable units of the Carmel Formation in this area include the Page Sandstone and the Dewey Bridge Member. However, the hand samples provided were fine grained, had calcite crystals, dendrites, and fizzed with HCl, all indicating the sample is a limestone. Limestones form in a shallow marine environment, and may contain fossils. Despite the variety of rock types present in the Carmel Formation, each represents a hydrated environment, whether that be a shallow sea, fluvial system, or tidal area. The Carmel Foramtion was formed during the Jurassic Period, around 166 - 168 m.y.

Hand samples of the Entrada Sandstone have round, very well sorted, very fine grains that are poorly cemented together. With the addition of dilute HCl acid, no visible reaction occurred, indicating the absence of a calcite cement. Although they have similar textures, there were three samples of the Entrada, each of which a slightly different color. The three samples, seen in Figure 13, range from red-brown, a lighter pink-brown, and a tan. The grains of the Entrada Sandstone form planar beds.

The Entrada Sandstone can reach thicknesses of 400 to 900 feet in outcrop, creating notable landforms such as arches at Arches National Park or monoliths at Capitol Reef National Park (Morris et al., 2010). Also dating to the Jurassic Period, the Entrada Sandstone was likely deposited around 158 m.y. in eolian or tidal flat environments. One of the most prominent units of the Entrada in the San Rafael Swell and Moab areas of Utah is the Slickrock Member, which is the specific unit that comprises the arches of Arches National Park.

The Curtis Formation contains two primary types of grains in hand sample. There are clear, colorless, rounded grains of quartz, and white-yellow, opaque, angular mineral grains (possibly feldspar). Overall, these grains are poorly sorted and are held together by a calcareous cement, as it reacts with dilute acid. As seen in Figure 14, the hand sample of the Curtis Formation is a light golden brown, but there are brighter white beds that protrude slightly. These veins are more resistant to weathering than the main body of the sandstone.

With the name of the Curtis Formation can arise some confusion, as the same unit has previously, and in other areas of Utah, been referred to as the Moab Tongue Member. Regardless of the name, the Curtis Formation is widely believed to have been formed in a Jurassic marine depositional environment (Morris et al., 2010). In outcrop, the Curtis Formation can also appear with a slight green tint, as the unit is glauconitic.

Above the Curtis Formation lies the Summersville Formation, the uppermost part of the San Rafael Group. This slope-forming unit dates to the Late Triassic - Early Jurassic, and is easy to identify in outcrop, as the Summersville is marked by the J-3 unconformity beneath it (Anderson et al., 2010). Alternating layers of siltstone and sandstone comprise this unit, with thin veins of evaporites like gypsum cross-cutting the planar beds, creating a red and white banded unit (Anderson et al., 2010). The Summersville Formation was likely deposited in a fluvial environment, with alternating periods of slight water level or current velocity changes.

The Morrison Formation creates thick cliffs in outcrop, bound by two unconformities - the J-5 and the K-0. These unconformities provide an age range of the unit of 155 - 148 m.y. during the Jurassic Period, and can also aid in identifying the unit in outcrop. The Morrison Formation consists of tan sandstones and red or gray-green shales that can reach thicknesses of over 700 feet (Anderson et al., 2010). Some important geologic members in this area contained in the Morrison Formation include the Brushy Basin, Salt Wash, and Tidwell Members. These members, as well as the other sandstones and shales of this unit, provide evidence of floodplain and overbank depositional environments (Anderson et al., 2010).

Atop the K-0 unconformity, which cuts into the top of the Morrison Formation, lie the Cedar Mountain Formation and Dakota Sandstone. The Cedar Mountain Formation is light to dark brown in outcrop and consists of mudstones with intervals of sandstones (Doelling, 2010). This unit can be seen in Arches National Park, Capitol Reef National Park, and the Grand Staircase National Monument. The Cedar Mountain Formation, along with the following unit of Dakota Sandstone, both date to the Cretaceous Period (145 - 66 Ma). The Dakota Sandstone contains multiple types of rock that can be as thick as 110 feet, including limestone, shale, and sandstone (Doelling, 2010). While the depositional environment of the Cedar Mountain Formation was likely a floodplain, the Dakota Sandstone was more of a coastal plain with lagoons (Doelling, 2010).