The Pierre Shale (/pɪər ʃeɪl/), the Upper Cretaceous rock division in the United States (the Cretaceous Period lasted from about 146 million to 65.5 million years ago). Shale makes up half to three-fourths of all sedimentary rock and accounts for more of the surface of the Earth than any other rock. Furthermore, the artificial origin of shale gives it a particular geochemical meaning. The Pierre Shale is an Upper Cretaceous geological formation or sequence that occurs in the Great Plains east of the Rocky Mountains, from the Pembina Valley in Canada to New Mexico.
After the shale is metamorphosed, metals concentrated in shale by sedimentation and diagenesis can be source materials for ore deposits, and understanding the way these metals are distributed in a large shale body may lead to better searching for such ore deposits. The Pierre Shale was described in 1862 in the Proceedings of the Academy of Sciences by Meek and Hayden (Philadelphia). They referred to it as dark-gray shale, fossiliferous, with gypsum veins and seams, and iron oxide concretions. Depending on the amount of metals in the shale and the extent of the weathering involved, shale metals can be released into the atmosphere during weathering and erosion, with beneficial or detrimental effects on the soil and the chemical content of the water.
Collections of Pierre Shale
The Pierre Shale at the type locality is about 700 feet (210m) high. It overlies the division of Niobrara and underlies the beds of the Fox Hills. It was named after an incident in South Dakota near Fort Pierre on the Missouri River. It happens in Montana, Colorado, Minnesota, New Mexico, Wyoming, and Nebraska in South Dakota. The Pierre consists of a dark gray shale of about 600 m (about 2,000 feet), some sandstone, and several bentonite layers (altered volcanic-ash falls that look and feel much like soapy clays). In 1956, the Geological Survey began examining the mineralogical and chemical composition of the Pierre Shale and equivalent rocks by comparing time-equivalent marine and nonmarine rocks to determine their genesis. Furthermore, many physical properties of the shale were determined and interpreted by compositional factors controlling them.
The Pierre Shale was deposited in the Western Interior Seaway and is of marine origin. It is related to other marine shales, such as the Bearpaw Shale, the Mancos Shale, and the Lewis Shale, that occur farther west. It coincides with the creation of Lea Park in central Alberta. The Pierre is overlain by the Fox Hills Formation’s marginal marine deposits. The Pierre Shale can be as little as 200 m (about 650 feet) thick in some regions. In the Campanian Period of the Late Cretaceous, much of the formation was deposited. However, the discovery in the uppermost beds of the Pierre Shale in the Raton, New Mexico region of the fossils of Baculites baculus Meek and Hayden indicates that deposition persisted here through the early Maastrichtian.
Many sedimentary Pierre-equivalent rocks are rich in volcanoes. Volcanic material was the predecessor of a great deal of detrital material accumulated in the basin, but only where volcanic particles are found in the field rocks is the word “volcanic” used. In the fields of Florence and Canon City in Fremont County, Colorado, and the Boulder Oil Field in Boulder County, Colorado, the Pierre Shale is the host structure for commercial oil deposits. More recently, in the Raton Basin in southern Colorado, natural gas has been produced. Volcanic material is known mainly as sand-size clay-aggregate grains that are composed of strongly smectitic clay obviously altered from volcanic glass, away from the volcanic source areas. Abundant coarse biotite flakes and euhedral feldspar grains are typically followed by such material.
The formation of the Pierre Shale is typically too impermeable for the extraction of hydrocarbons, but it is created in areas where it is naturally fractured or artificially fractured. In interpreting differences in mineral and chemical composition, other rock forms, such as marl, bentonite, and shale directly above bentonite, and rocks classified by their grain size, such as siltstone or sandstone, were also considered. Such rocks are later discussed as appropriate. The mineralogical composition is fairly straightforward for most of the Pierre Shale and equivalent rocks, and most of the major components are distributed uniformly.
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