City Creek Nature Notes – Salt Lake City

September 20, 2016

September 1st

Fault

7 p.m. After the canyon opens up at mile 1.3, the first expansive view of the upper canyon occurs at milepost 1.5. On the north side of the upper canyon is unnamed peak at 8283 feet in altitude and on the south side is Black Mountain. Just up canyon of unnamed peak 8283, two fins run down the peak’s flank. In the sunset light, the closest is tan in the farthest is grey. The two fins descend down to a Depression era picnic area, upper Rotary Park, then climb Black Mountain. The grey fin is the main geologic attraction of the Upper Rotary Park picnic area at mile 5.75. There, the fin creates a vertical gully at the end of the paved road where it becomes a horizontal inclined ledge runs along the top of Black Mountain. Runners, who participate in the annual June Wasatch Steeplechase, and more energetic hikers will be familiar with this ledge. It is a jumbled rock scramble that takes considerable time to traverse. A 1987 geologic map by Van Horn and Crittenden (U.S.G.S. I-1762) reveals the scene’s geology.

On the canyon’s north side and at the western base of unnamed peak 8283, Freeze Creek , a north-south running side canyon, begins at Lower Rotary Park at mile 3.75. Freeze Creek marks a major geologic division in the upper canyon. Freeze Creek contains a normal fault. To the down canyon west side of the creek are layers of young Tertiary rock. On the up canyon east side of the creek and running up to unnamed peak 8283, the rocks consist of the some of the oldest layers of stone in the Wasatch. From there and continuing to the 9100 foot Grandview Peak, hidden behind the unnamed Peak 8283, the surface is 500 million year old Cambrian rock. At one point along the Freeze Creek trail up to Rudy’s Flat, recent lake bed sediment sits next to 500 million year older Cambrian rock. The transition occurs because a normal fault along Freeze Creek has raised the up-canyon side faster than the down-canyon side. Erosion has erased the intervening time between them. (Van Horn and Crittenden call the normal fault the Rudy’s Flat Fault.)

On the north side of the upper canyon and descending the flank of unnamed peak 8283, rocks layers are in reverse chronological order: Cambrian, Devoian and Mississippian. Titanic geologic forces have turned the stones upside down from their original deposition. The older Cambrian rocks are higher on the mountain and proceed to younger strata are at the Lower and Upper Rotary picnic areas.

On the south side of the upper canyon, the canyon wall rises to Black Mountain and the geology is even more contorted. Although hidden by a ridge from my viewpoint at mile 1.5, a thrust fault rises south south east from lower Rotary Park to Smuggler’s Gap at the eastern end of Black Mountain. There the thrust fault makes an abrupt 135 degree turn and runs horizontally southwest along the top of Black Mountain. The thrust fault creates the rock scramble ledge along the top of Black Mountain. Here again the thrust fault reverses the order in which rock layers were originally deposited. On the higher canyon side of Black Mountain’s summit is older Mississippian rock while on the cityside, visible from the Avenues neighborhood, the rock is of the younger Permian age.

Faults are also responsible for two miniature rapids in the canyon. The Freeze Creek fault creates on at the overlook in Lower Rotary Park at mile 4.5. A second fault creates another mini-rapid at the Weeping Rock Memorial Grotto campsite at the bridge into the last campsite at Upper Rotary Park near mile 5.5.

A 2009 Salt Lake County geologic hazards report shows that none of these faults have been recently active. However, the thrust faults have also shattered the rock in adjacent Red Butte and Dry Fork Canyons. The University of Utah Seismology Department has recorded about 23 small earthquakes at less than 2.0 magnitude and one earthquake between 2.0 and 4.0 magnitude at a small complex of faults in Red Butte between 1967 and 2003. They recorded two earthquakes between magnitude 2.0 and 4.0 in Dry Fork Canyon at the base of Black Mountain.

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