Silver Reef, Utah, Geology Highlights
Geological Formations as viewed from the Museum at Silver Reef, Utah
By Robert F. Biek
Utah Geological Survey
The Pine Valley Mountains tower over 6000 feet above Silver Reef, reaching an elevation of just over 10,000 feet. The mountains are eroded from a 20 million-year-old granitic rock. If you want to see what these rocks look like, cast your eyes over the apron of sediment on which the town of Silver Reef is built. That apron of sediment forms what geologists call an alluvial fan – the sediment of debris flows that periodically come off the mountain during thunderstorms.
The large rounded boulders are mostly quartz monzonite porphyry, the granitic rock that makes up the bulk of the mountains. The rock was intruded into the earth’s uppermost crust as a shallow sill that quickly grew to 3000 feet thick. This type of igneous intrusion is known as a laccolith, and the Pine Valley Mountains are one of the world's largest such features.
One thing that makes the Silver Reef area so attractive is the abundance of colorful red sandstone west of town. These sedimentary rocks belong to the Kayenta Formation and Navajo Sandstone, the same strata made famous for the wonderful landscape of nearby Zion National Park. The Navajo Sandstone is part of the world’s largest coastal and inland paleodune field, larger even than today’s modern Sahara desert.
The quartz monzonite porphyry of the Pine Valley Mountains is a distinctive rock. The ”porphyry” part of its name indicates that it contains large crystals set in a finer-grained matrix, sort of like plums in a pudding. Here, these are large quartz and feldspar crystals floating in a finer matrix. The matrix is fine because the rock cooled relatively quickly near the earth’s surface, unlike a true granite that cools slowly deep within the earth so that all the crystals are of the same size. This shallow type of igneous intrusion with its distinctive porphyritic texture is known as a laccolith, and the Pine Valley Mountains are the eroded remnant of one of the world's largest exposed laccoliths.
Think of a laccolith as a boil beneath one’s skin—a zit on the surface of the earth. Molten rock from deep within the earth moved upward to within a few thousand feet of the surface, but instead of erupting, the magma intruded laterally into shallow sedimentary strata. There, it spread out into a mushroom-shaped intrusion—flat on the bottom and dome-shaped on top—with a thickness of 3000 feet and a maximum horizontal dimension of over 20 miles in a northeast-southwest direction. The magma was emplaced rapidly, bulging up overlying rock, which resulted in catastrophic landslides on oversteepened slopes. These landslides were several tens of square miles in size and in some cases slid several miles across the former landscape. Removal of this much overlying strata acted much like popping the cork of a champagne bottle, leading to the violent eruption of lava flows that now partly cover the landslide masses. Landslide deposits are preserved on the north flank of the mountains near Pine Valley, but if once present on its southern flank they have long since been removed by erosion.
The Pine Valley laccolith is not related to the much younger volcanic rocks that today dot the greater St. George landscape. The younger volcanic rocks are basaltic lava flows much like those erupting today on Hawaii – very fluid, black lava flows. The greater St. George area is home to several dozen such lava flows that erupted over the past two million years, the youngest of which lies like a frozen black cascade through the fantastic red rock landscape of Snow Canyon.
Silver Reef also has its own red rock landscape. The mountains west of town expose a detailed record of changing environmental conditions some 180 million years ago when what is now Utah was in the arid, low-latitude, trade-winds climatic belt, in a rain shadow created by mountains at the western margin of the continent. Here, near Silver Reef, the underlying, planar sandstone and siltstone beds of the underlying Kayenta Formation grade upward into the great sweeping crossbeds of the Navajo Sandstone. At the time the Kayenta sediments were deposited, the environment was that of a sabkha, a broad, flat coastal area with a high water table – much like parts of the coastal Arabian Penninsula today. As sand was blown onto the sabkha, it adhered to the wet surface. Eventually, the sabkha was overridden by the wind-blown sand, but the water table remained high. This led to the formation of planar sandstone beds in the lower Navajo as dry sand was blown away and wet sand remained behind. With time, large sand dunes eventually formed. The great, sweeping cross-beds of these sand dunes are preserved in the middle and upper parts of the Navajo Sandstone, where uncommon planar sandstone and limestone beds provide a record of widely scattered oases. Thus vast dune fields similar to the modern Sahara eventually overwhelmed the Kayenta playas, resulting in the Navajo Sandstone, part of the world’s largest coastal and inland paleodune field.
Where did all this sand come from? Recent research shows that most of the sand was eroded from the ancestral Appalachian Mountains, transported westward by a Mississippi-scale river system to the western shore of Jurassic North America, and was then blown southward and incorporated into the Navajo sand desert. Eventually, the sand accumulated to a thickness in excess of 2000 feet in southwestern Utah. Preservation of this tremendous thickness of wind-blown sand was made possible because of basin subsidence associated with Early Jurassic compressional deformation near the west margin of North America. This deformation caused the continental interior to flex downward, creating accommodation space for sediment accumulation.