The geology of Utah, in the western United States, includes rocks formed at the edge of the proto-North American continent during the Precambrian. A shallow marine sedimentary environment covered the region for much of the Paleozoic and Mesozoic, followed by dryland conditions, volcanism, and the formation of the basin and range terrain in the Cenozoic.
Geologic history, stratigraphy, and tectonics
The eastern Uinta Mountains near the Colorado line and the Raft River-Dove Creek Mountains contain the oldest rocks in Utah from more than two billion years ago. Rubidium-strontium dating of the Red Creek quartzite in 1965 indicated an age of 2.3 billion years. Other geologists found 2.4 billion-year-old schist and gneiss in the Albion Range Green Creek Complex. Prior to the 1960s, geologists inferred the Vishnu and Farmington Canyon gneiss and schist as Archean in age, but subsequent research indicated a formation between 1.6 and 1.5 billion years ago in the Proterozoic. From 1.75 to 1.65 billion years ago, Utah was impacted by the Mazatzal orogeny. Thick sequences of sedimentary rock from the Neoproterozoic, including tillite, underlie much of the state.
Paleozoic (541–251 million years ago) Utah has numerous rocks deposited during the Paleozoic, as multicellular life diversified for the first time. Basal Cambrian quartzite, up to thick, is sometimes difficult to distinguish from Precambrian rocks. A shallow marine environment produced mottled limestone and stromatalite beds, deepening to the west. Thin shale and siltstone units of the Whirlwind Formation likely originated from sediment sources in the east. The Pioche Shale contains some of the oldest Cambrian fossils, followed by a continuous sequence of trilobite fossils in the House Range.
In eastern Utah, which rests on the craton of the proto-North American continent Laurentia, there are no Ordovician deposits, but Early Ordovician clastic limestone, Middle Ordovician quartz sandstone, and Late Ordovician dolomite is common in the west. Geologists have found intraformational limestone from the Early Ordovician, originating as limestone forming in a tidal bed is redeposited as pebbles in situ. Brachiopod, trilobite, and echinoderm fossils are common in these rocks. Middle Ordovician quartzites form distinctive orange, brown, and pink cliffs above gray limestone slopes, set beneath black dolomite cliffs in the north and west. Chert-rich dolomite cliffs form in the most widespread Ordovician unit: the Fish Haven-Ely Springs Dolomite.
Only a single stratigraphic unit—the Laketown Dolomite—formed during the Silurian, far from sediment sources in a carbonate platform environment, with few distinct fossil assemblages. Although thickest in the west, Devonian strata are present in the east, unlike Ordovician and Silurian rocks. A local orogeny produced the Stansbury Uplift, leading to downward erosion into older rock units dating to the Precambrian and the deposition of the coarse conglomerate Stansbury Formation. Shale and quartz sandstone are often interbedded with carbonates, as in the Hanauer Formation in the Thomas-Dugway Range.
In the Mississippian, up to of sediment accumulated in the Oquirrh Basin. Marine deposition was continuous, with numerous fossils in an unbroken record, such as the crinoid stems, brachiopods, and corals in the Chainman-Manning Canyon Shale. Silicified brachiopod and coral fossils mark the Redwall, Leadville, Madison, Gardison, Joana, and Lodgepole limestones, and only the northwest lacks extensive limestone formations.
Geologists focused significant attention on the Pennsylvanian rocks of the Paradox Formation in the Paradox Basin after the discovery of oil, potash, and rock salt. The basin took shape as part of the Uncompahgre Uplift, produced by the uplift of the ancestral Rocky Mountains. Several thousand feet of salt accumulated beneath arkose shed off the mountains. Up to 13,000 feet of sedimentary rocks accumulated in the Oquirrh Basin.
A small unconformity separates these rocks from those formed in the Permian. These rocks tend to be difficult to correlate, with significant lateral variation, cross-bedded sandstones indicating wing transportation, and a lack of late-Permian rocks. A major marine transgression generated the Kaibab Limestone (the rimrock of the Grand Canyon), represented by the Park City and Phosphoria groups in Utah.