City Creek Nature Notes – Salt Lake City

January 7, 2017

January 7th

Filed under: Geology, Guardhouse gate, picnic site 6, picnic site 7 — canopus56 @ 10:02 pm

Volcanic Past

5:00 p.m. Temperatures in the canyon are in single digits and overnight the City fell to -2 degrees Fahrenheit. Three walkers and runners pass by mile 0.3, but then I have the entire canyon to myself. I am in the canyon looking for an example of Van Horn and Crittenden’s “Tv – Volcanic breccia (Tertiary)” described as “Primarily andesitic breccia consisting of angular clasts of purplish porphyrtic andesite in a fine-grained matrix. Locally, clasts are rounded and have undergone some fluvial transport.” In short, this means the layer is an intermediate form of not quite lava that includes embedded crystals. A Utah Geological Survey publication provides a photographic exemplar and is a guide to locating a sample, but I am unsure if I have found this type of rock. The Utah Geological Survey publication shows the layer between picnic sites 1 and 6 and then again from just below picnic site 7 up canyon to picnic site 9. None is visible below picnic site 6, but across from picnic site 7, I find a small that may be of the right material. About two-hundred feet below picnic site 7 and on the south bank of the stream, there is a large boulder that also may be made of a volcanic conglomerate. Van Horn and Crittenden date the age of this volcanic breccia layer between 35 and 37 million years old. Their 1987 geologic map shows extensive layers of this material going down to the red bridge. Another small outcrop of this breccia is shown on Van Horn’s map just north of the Little Twin Peaks. These deposits are geologically related to the tallest peak at the south end of the Salt Lake Valley, the 11,253 foot granite Lone Peak, and to the formation of Utah’s current geology.

Hintze and Kowallis of Brigham Young University, Willis of the Utah Geological Survey, and Stokes of the University of Utah describe how the Sevier Orogeny shaped the state’s modern landscape and the canyon, as originally proposed in 1991 by Richard Livaccari of the University of New Mexico. An orogeny is a mountain uplifting event that can last millions of years. The cause of the Sevier Orogeny was the Farallon continental plate. Currently, there are about fifteen principal continental plates, but there used to be a sixteenth, the Farallon Plate, between the well-known North American and Pacific Plates and their border, the San Andreas fault off of the United States west coast. According to Hintze and Kowallis, about 105 million years ago the Farallon plate began subducting under the west coast of the United States (Hintze and Kowallis, 67-77; Stokes, 144-145). The Farallon Plate takes its name from a remnant of the plate found at the Farallon Islands off of the coast of San Francisco. In 2012 using mathematical inverse image techniques to reconstruct the plate from seismic waves, Pavlis, Sigloch, Burdick, and colleagues visualized the remains of the Farallon Plate whose melting carcass is now embedded in the Earth’s mantle a thousand miles below the midwest (see Sigloch and Mihalynuk 2013). Willis likens the response of Utah’s surface from the Farallon Plate passing underneath to a boat riding a passing wave on water (Willis, 4). A more familiar visualization would be a resting surfer, seated on her board as a wave passes underneath.

As Utah and Nevada rode up the leading face of the wave between 105 and 80 million years ago, a cordillera – a vast north-south mountain range similar the the Canadian and United States Rockies – rose in Nevada and western Utah (Hintze, 67, Fig.s 98 and 103; Livaccari, 1106, Fig. 1(a)). Utah was compressed. Hintze and Kowallis conclude that Utah’s western border was about sixty miles closer to the current location of Salt Lake City during this event (Hintze, 5). Between 80 and 65 million years ago, those mountains eroded away and the crest of the Farallon Plate continued to migrate under Utah (Livaccari, 1106, Fig. 1(b), Hintze, 6). Central and eastern Utah, including the canyon, were part of inland sea, and the sandstones and the red conglomerates in the canyon, including the natural bridge at mile 0.9, were deposited from the erosion of the cordillera mountains to the west. Next, about 35 million years ago, the melting of the Farallon Plate as it passed under Utah resulted in a volcanic era, and volcanoes formed in west central Utah around present day Tintic (Hintze, 6). Where molten lava reached the surface, as at Van Horn’s “Tv” conglomerate layer between picnic site 6 and picnic site 9 at mile 1.1, breccia formed. Where molten lava remained trapped beneath the surface, the granite that formed Lone Peak was created (Ut. Geo. Survey, Pub. Info. 87).

Finally, as Utah slid down the backside of Fallaron wave, the land stretched (Hintze, 6). Utah’s western border expanded to its current location. As the land stretched east and west, north-south faults formed in the thin crust, and one of those lifted the Wasatch Front Mountain Range into the sky. The lifting was uneven between the canyon and Lone Peak. At the City Creek end, uplift exposed rock to the Tertiary era, but in the north in Farmington, uplift raised older Cambrian rock to the surface. At the Lone Peak southern end of Salt Lake Valley, the lifting was more extreme. Erosion removed the Cambrian rock entirely, and subsurface pockets of frozen, granitic lava were lifting to 11,000 feet. Thus, the lowly breccia in City Creek share the same parent as the lofty heights of Lone Peak.

In Thoreau’s “Journal” on January 7th, 1851, he enjoys how the winter sun lays a yellow tint on pine trees. On January 7th, 1852, he describes a sunset where one-half of the sky in the east is covered in white clouds and one-half of the sky is in west is blue and clear. On January 7th, 1853, he sees a sunrise where a white mist covers low lands. On January 7th, 1855, he describes an early thaw. The morning overcast sky is tinged with blue, green and red. On January 7th, 1857, he notes it is the fifth consecutive day of cold, windy weather, and he describes the renewing psychological effect of getting “a mile out town” and taking a walk in nature’s solitude. On January 7th, 1858, he again notes speckled crystals on the surface of fresh snow.

On January 7th, 1903, hunters collected County bounties on a total of twenty coyotes killed within or just outside the city (which then included the high Avenues). Hunter George McNeil stated that “City Creek canyon is full of them [coyotes,] and I killed two wildcats up there a few days ago.”

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