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The AAPG/Datapages Combined Publications Database

Utah Geological Association


Mining Districts of Utah, 2006
Pages 604-643

Salt and Gypsum of the Arapien Shale - the Central Utah Evaporite Mineral Industry

Grant C. Willis


Production of the evaporite minerals gypsum and salt (halite) has been a major factor in the economy of the Sevier and Juab Valley area of central Utah for 150 years. Almost all commercial evaporite deposits are within the Middle Jurassic Arapien Shale, which crops out in a north-south linear belt extending from Spanish Fork Canyon to Sigurd, Utah.

The salt was used by Native Americans long before any written records were kept, and was exploited for home and farm use by early pioneers. Settlers noticed the salt and salty water within days of arrival, and soon began extracting salt from brine and quarrying block salt. Independent operators developed several small scattered salt operations in the 1850s to 1870s, but eventually, most salt production shifted to the largest and purest deposit in the Redmond Hills. For several decades, the Poulson and Bosshardt families operated side-by-side in the Redmond Hills. The Bosshardt family eventually purchased and consolidated the mines into a single operation. The mined salt is nearly pure halite, with minor calcite, trace minerals, and red clay and silt that give it a reddish tint. No accurate early production records are available. Records indicate that recent annual production has increased from 67,000 tons in 1994, to 175,000 tons in 1998, to nearly 250,000 tons in 2004. Part of this increase is due to increased production of road salt, but an increasingly important part is due to the creation of a new product that is marketed as a better flavored and healthier “natural” table salt.

Early pioneers may have used gypsum for plaster and mortar, but the first commercial plaster operation started about 1885 near Nephi and about 1907 in the Sigurd area. The mills were bought, sold, and expanded several times. The gypsum industry expanded significantly after sheetrock wallboard became a popular building material in the mid 1900s. Currently (2005), one plaster and wallboard plant operates near Sigurd. Gypsum continues to be produced from quarries in the Arapien Shale near Sigurd, Nephi, and Levan, though the primary source for the plant is presently switching to the Carmel Formation in the San Rafael Swell (not treated in detail in this paper). Most minable beds are 90 to 99 percent earthy gypsum, with minor anhydrite, carbonate, clay, and halite minerals. Early production records are not available. During the 1990s to 2004, annual production averaged about 150,000 tons in the Sigurd area, and has ranged from 100,000 to 200,000 tons for operations near Levan and Nephi. During the past few years, production from the Arapien Shale in central Utah has declined while production from the Carmel Formation on the west flank of the San Rafael Swell has increased. Most of the gypsum is used for wallboard, but some is used for plaster, concrete additive, soil conditioner, and even medicine.

The Arapien Shale is Middle Jurassic (late Bathonian to early Callovian) in age, and consists mostly of interbedded mudstone, silty sandstone, micritic limestone, anhydrite, gypsum, and halite. This highly incompetent unit was severely deformed by Cretaceous to early Tertiary Sevier orogenic thrusting and folding, and later basin-and-range normal faulting. Minor diapirism and isostatic uplift also deformed the strata. These deformational events were important in repeating outcrops at the surface, thickening and concentrating gypsum and salt beds, and fracturing and aiding alteration of anhydrite to gypsum, thereby greatly increasing production and profitability.

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