title: Dikes of the Clear Creek Area, Wasatch Plateau, Utah
Creators: name: Thomas, William Dennis
Publisher: name: Utah State University
Year: 1976

description: The area, covered by this report, includes about 108 square miles in the northeastern part of the Wasatch Plateau in central Utah. Clear Creek, Utah, is near the center of the area.
Stratigraphy of the area is characterized by 18,000 to 20,000 feet of sedimentary rocks above the Precambrian basement. The oldest exposed stratigraphic unit is the 11ancos Formation of late Cretaceous age; the youngest exposed stratigraphic unit is the North Horn Formation of Cretaceous-Tertiary age. The sedimentary rocks are mainly sandstone, shale, and coal. Sandstone and shale are the most abundant.
Structure of the area is characterized by folds and normal faults. Folding began in mid-Cretaceous time. A later impulse occurred in late Cretaceous to early Tertiary. Folding was again rejuvenated in Eocene. The normal faults mostly trend north-south and east-west. The faults that trend north-south have more displacement and greater extent than those that trend east-west. North-south-trending faults have up to 2,500 feet of displacement. Maximum displacement, on the east-west-trending faults, is 90 feet. The north-south-trending faults are the main structural control of the topography. In many cases, they outline uplifts separated by graben. Faults that trend north-south and east-west are considered to have formed at the same time. The age of the faults is considered to be late Oligocene to Miocene.
The dikes are biotite orthoclase pyroxene lamprophyre. The igneous rock of the dikes resembles the lamprophyre type called minette. The presence of olivine and more augite than diopside probably places the rock between minette and kersantite. All dikes in the area are of the same mineral composition. Typical mineral composition of fresh dike rock is 33 percent biotite, 11 percent augite, 8 percent olivine, 4 percent magnetite, 38 percent groundmass, traces of apatite, and up to 5 percent secondary materials. The most common secondary materials are chlorite, calcite, quartz, and iron oxides. The groundmass is 85 to 90 percent orthoclase. The remainder of the material in the groundmass is plagioclase, probably albite, and mafic minerals including biotite and augite. A fe1dsparstaining technique was used to determine the composition of the dominant feldspar.
The dikes are localized along faults and joints parallel to the faults. All but two of the 28 mapped dikes in the area are localized in east-west-trending fractures.
Effects of the dikes on adjoining rocks are minimal. Dike temperature was not sufficiently high to melt the adjoining rocks. Effects on the coal are also minimal. Coke has been reported in the old underground mines of the area but was not recognized on the surface. Anthracite has formed locally near some dikes.
The lamprophyric magma is considered to have originated by the melting of hydrous peridotite in the upper mantle. Melting of the hydrous peridotite produced appreciable amounts of water t CO2, S02' and Cl. The volatile pressure increased as melting continued. At some point, the volatile pressure was sufficient to cause a rise of gaseous magma into pre-existing normal faults and joints. Mafic phenocrysts were carried in the melt until the volatile pressure was sufficiently reduced and the magma rapidly solidified to produce the extremely fine-grained groundmass. Emplacement of dikes by volatile pressure relates them to a nonexplosive diatremic type of intrusion. Emplacement of the magma occurred at a temperature between 600o and 1,000o C. The dikes of the Wasatch Plateau are middle to late Tertiary in age. Most evidence indicates them to be late Oligocene to Miocene.