By Kit Clemons (WEST Fellow 2006-8)

The geology in Little Cottonwood Canyon is dominated by an igneous rock called quartz monzonite (very similar to granite).  This rock type makes up a large body of igneous rock known as a pluton. This pluton, called the Little Cottonwood stock, intruded the earth’s crust approximately 32 million years ago. It is the largest intrusion in the Wasatch Range.  The intrusion was emplaced approximately 6 miles deep and is now exposed by upward displacement along the Wasatch Fault. Other rocks found near the top of the canyon include: the Alta Stock, a smaller igneous intrusion similar to the Little Cottonwood Stock; the Tintic Quartzite; the Ophir Shale; and the Deseret, Gardison, and Maxfield Limestones. The sedimentary overlying the igneous rocks have been metamorphosed and bleached by the heat and fluids associated with the intrusions.

All of the rocks in the canyon have been deformed by faulting and folding which has lead to the development of multiple sets of fractures which facilitate the storage and movement of fluids. The main fluid flow paths through these rocks are through fractures which include joints and faults. Different types of fractures form in different orientations relative to the stresses on the rocks that prevailed at the time of fracturing. Fracture is a general term for any type of brittle failure. The term joint is used to describe a fracture with no offset and the term fault is used to describe a fracture with offset. Different types of fractures have different fluid-flow properties. Unmineralized joints and faults are quite permeable and provide the main flow path for water. The joints in the rocks found in the alpine zone provide the main pathway for water to infiltrate the ground.

Figure 1 – Little Cottonwood Quartz Monzonite displaying unloading joints.  The density and orientation of these joints vary with changes in the rock texture and fabric. (Joint orientations illustrated in line drawing)

Figure 2 – Little Cottonwood Quartz Monzonite displaying cooling joints.  These joints were originally vertical and have been tilted eastward by motion along the Wasatch fault.  Joints that form during cooling should are only be slightly younger than the pluton. (Joint orientations illustrated in line drawing)

Figure 3 – Missippian/Cambrian limestones east of Snowbird exhibiting tectonic joints related to folding. Tectonic joints form in response to an increase in the pore fluid pressure associated with compression during folding within the rocks. This increase in pore pressure weakens the rock making them more susceptible to tensile (pull apart) failure within the rock. Tectonic joints may form in a variety of orientations depending on the stress conditions associated with the deformation. (Joint orientations illustrated in line drawing)

References

Bergbauer, Stephan and Martal, Stephen,  1999, Formation of joints in cooling plutons, Journal of     Structural Geology Vol. 21, No. 7, pp. 821-835

Engelder, Terry, 1987, Joints and shear fractures in rock, Acad. Press, London, United Kingdom (GBR),     136 p.

Granger, A. E.; Calkins, F. C.; Crittenden, M. D., Jr; Sharp, B. J., 1952. Geology of the     Wasatch     Mountains east of Salt Lake City Utah Geol. Soc., Guidebook to the geology of Utah, no. 8.

Hintze, L.F., 1988, Geologic history of Utah: Brigham Young University Geology Studies Special     Publication 7, 202 p.

Vogel, T. A.; Cambray, F. W.; Feher, L.; Constenius, K. N. 1997. Petrochemistry and emplacement     history of the Wasatch igneous belt, central Wasatch Mountains, Utah Geological Society of     America, 1997 annual meetingAbstracts with Programs - Geological Society of America, vol.29,     no.6, pp.282,

Singhal, B. B. and Gupta, R.P, 1999. Applied Hydrogeology of Fractured Rocks. Kluwer     Academic     Publishers, The Netherlands, 395 pp.