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Ione Tertiary Oxisol Soil ACEC

General Information

An Oxisol is an intensely weathered mineral soil typically associated with tropical environments. The Ione Formation represents the only occurrence of an Oxisol in the continental United States. It is direct evidence of a tropical weathering regime in California during the early Tertiary Period.

The Ione Formation occurs as a 200-mile-long series of isolated exposures along the western foothills of the Sierra Nevada, California, between Oroville in Butte County southward to Friant in Fresno County. It is comprised of fluvial, estuarine, and shallow marine deposits of Eocene age. The Ione Formation was formed between 65 and 45 million years ago when much of the present land surface of California's Central Valley was inundated by a shallow sea.

The Ione Formation is one of the most important sources of nonmetallic minerals in the western United States and has supported an active mining industry throughout most of this century. The most productive segment of the formation is near the community of Ione in west central Amador County. Within a belt 20 miles long and from 1 to 4 miles wide are over 50 clay deposits or workings, several large sand deposits, and three extensive coal basins.

Clay production is the most important of the various mining activities presently occurring in the Ione area. The Ione Formation is the only large source of super duty refractory clay west of the Mississippi River. A significant tonnage of Ione clays are used in the production of heat-resistant brick which is essential to industries employing high-temperature furnaces, such as the steel, glass, and ore smelting industries.

Nearly a century of mining activity and other land developments have irreversibly altered much of the Ione Formation. Consequently, the last undisturbed areas within the Ione Formation could disappear within the foreseeable future.

While its mineralogical and industrial properties have received considerable attention, the details of its origin and development have not. Research of the Ione Formation could yield important clues to central California's geological history as well as the history of Oxisols worldwide. However, areas with intact soil horizons must be preserved to ensure the availability of research opportunities into the future.

Because of the need to protect these unique soils, especially those with intact soil horizons, the Bureau designated a 90-acre site in southern Amador County, California, in 1986 as an Area of Critical Environmental Concern (ACEC). This site contains portions of the Ione Formation with a good representation of these soils.

For additional information on the Ione Formation, see the BLM Ione Manzanita ACEC.