Widespread occurrences of Eocene and later lacustrine sediments are present
throughout the western United States. Many of these deposits, particularly in eastern
Oregon, Idaho, Nevada, and Arizona, contain thick accumulations of diatomaceous
earth that were formed by the gradual accumulation of the tests (skeletons) of
diatoms. Diatoms, a type of one-celled algae, are widespread, even ubiquitous, in surface
waters of all climates and salinities. They have been reported in marine, fluvial,
and lacustrine sediments from Eocene to the Present.
Even though diatoms are widespread, special environmental conditions are
needed to form a rock composed primarily of diatom tests in sufficient thickness to be
of economic value. Two key conditions appear to be required: a mechanism to restrict
the input of clastic sediments and a source of dissolved silica. In addition, two other
conditions appear to be important: some mechanism to accommodate a sufficient
thickness of diatomaceous sediments and a chemical control that allows the deposition
and preservation of silica but not carbonates. If these conditions are met, thick and
pure deposits of diatomaceous earth will be formed that can be economically mined.
Individual deposits in Arizona, Kansas, and Oregon are being studied to provide
examples of how these environmental controls effect deposit formation.