Stratigraphy of Goblin Valley State Park

 

During the Jurassic Period, about 145-170 million years ago, Utah was located at the equator as North America and Eurasia was pulling away from South America and Africa.  At that time, Goblin Valley State Park was covered by shallow seas.  Just to the northwest of the park there were granitic highlands composed of sand, silts, and clays.  Through the process of erosion, the sediments were deposited into the seas, shorelines, river channels and in playas from the highlands.  There are four rock formations that are exposed at Goblin Valley State Park: Entrada Sandstone, Curtis, Summerville, and Morrison (Figure 11).  The Entrada, Curtis, and Summerville Formations are part of the San Rafael Group.  In this section, each of these formations will be described along with the type of environment it was at the time of deposition. 

 

Entrada Sandstone

Goblin Valley State Park mostly consists of alternating horizontal layers of sandstones and horizontal layers of siltstone and shale.  These sediments are the make up of the Entrada Sandstone.  It was named from the Entrada Point in the northern part of the San Rafael Swell.  At the time of deposition, 170 million years ago, Utah was located near the equator and shallow seas covered what is now Goblin Valley State Park.  These sands, silts, and clays were the result of erosion of higher ground and deposited in the shallow seas. Goblin Valley State Park was a tidal flat whereas the oscillations of tides gave rise to the deposition of interbedded sandstone, siltstone, and shale (Figure 12). 

Figure 11: Four formations exposed in Goblin Valley, as seen from Wild Horse Butte.  Picture taken by S. Bennett

There are many factors that contributed to the sculpting of these goblins.  Erosion by water had a large effect.  Goblin Valley lies on a plateau and therefore is surrounded by higher relief, where water is transported downhill to this area.  Sandstone is more resistant to erosion and weathering than siltstone and shale, therefore this factor contributed to the molding of these figures. 

Another factor that played a role is the jointing and fracturing within Goblin Valley State Park, which creates a zone of weakness.  The sharp edges of an unweathered joint are more susceptible to weathering, which explains the spherical-shaped goblins in the sandstone beds.  Therefore, with the siltstone, shale, and the unweathered joints being less resistant to erosion and weathering, Goblin Valley has given rise to these unique features (Figures 13 and 14).

 

 

Figure 12: Interbedded Sandstone and Siltstone with Shale.  Picture taken by Dr. M. Fischer

 

Figure 13: Mushroom-like Goblins. Picture taken by S. Bennett

 

 

 

 

            

                                    

Figure 14: Goblin Valley near campsite.        Picture taken by S. Bennett

 

 

Curtis Formation

The Curtis Formation, the greenish-gray layer that overlies the Entrada, was deposited at a time of an ancient sea in Late Jurassic.  This name was given from the Curtis Point of the San Rafael Swell.  Evidence that there was an existence of a sea includes: (1) the presence of glauconite (Figure 16), a mineral that forms in oceans, and (2) ripple marks, which occur when waves oscillate back and forth, and therefore, this area is suggested to have once been a beach (Figure 17).

 

Figure 15: Entrada and Curtis Formations within Goblin Valley State Park                  Picture taken by Dr. M. Fischer

                                                     

 

Figure 16: Glauconite. Taken from http://www.britannica.com/eb/art/print?id=5693&articleTypeId=1

 

 

 

Summerville Formation

The ocean waters from the time the Curtis Formation was deposited retreated northward, which left behind a tidal flat once again where Goblin Valley State Park is now located.  Here, the deposition of the Summerville Formation, consisting of shale, siltstone, and sandstone, sited above the Curtis Formation, occurred.   This formation, with beds of dark brown to light colored sediments, was named after the Summerville Point of the San Rafael Swell.  You can see this layer on the Wild Horse Butte at the north end of the park (see Figure 11).  Gypsum can be found within this layer which indicates a dry climate where the tidal flat evaporated. 

 

Figure 17: Ripple Marks Picture taken by S. Bennett

 

 

Morrison Formation

The last rock layer exposed in Goblin Valley State Park, the youngest strata found in Goblin Valley State Park, is the Morrison Formation.  During this time, there was uplift and erosion and the Colorado Plateau was majority comprised of continental conditions.  The floodplains, playas, and local streams deposited mudstones, sandstones, and conglomerates.  These sediments are the make up of the Morrison Formation.  The Morrison Formation could only be viewed on the top of Wild Horse Butte within the park (see Figure 11). 

 

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