An International Consensus on Calcareous Paleosol Classification

John C. Dixon, Department of Geography, University of Arkansas, Fayetteville, AR 72701

Calcareous paleosols are widely distributed throughout the arid and semi arid regions of the world. These paleosols have been described from virtually every continent of the world, yet there is no internationally agreed upon nomenclature for the description of the horizons present within them. As a result of this lack of consistency, it is extremely difficult to reconcile soil descriptions of these soil materials from different continents. It is also consequently difficult to make significant progress toward understanding the genesis of these materials.

This abstract proposes the adoption of a universally agreed-upon system of horizon designation and gross morphological description of calcareous paleosols. I propose the adoption of the system of calcareous soil description presently used by the United States Soil Conservation Service (Soil Survey Staff, 1981; Guthrie and Witty, 1982).

All calcareous paleosols should be described in the context of their soil stratigraphic setting. That is they should not be described in isolation of the pedogenic horizons above or below. They should first be designated by the appropriate Master Soil Horizon. In many, if not most, cases this will be either a B or C horizon. Where the calcic soils are strongly indurated and calcretes have developed I would propose the adoption of the master horizon K as originally proposed by Gile et al. (1966) for a pedogenic horizon that has been engulfed and cemented by predominantly calcium carbonate.

Subordinate horizon designations should be used as appropriate. The first of these would logically be the subordinate designation of "k" to indicate the accumulation of alkaline earths, in this case calcium carbonate. Where these carbonate rich horizons are cemented or indurated it is appropriate to designate them as "m." The essential difference between "m" and "K" is the degree of induration. K horizons are completely indurated with complex morphologies and are commonly partly cemented by silica. The Ogalalla Caprock of the High Plains of the United States is a good example of a "K" horizon. Most parts of it are thick massive calcrete horizons. The substantially less strongly indurated calcareous horizons below these materials are appropriately designated "m." The third subordinate designation appropriate for calcareous paleosols is "b" indicating the buried nature of these horizons and indicating that they were horizons formed at the landscape surface before burial. Further designation of calcareous paleosols may include "t." Accumulations of illuviated clay with accompanying development of clay skins are not uncommon in calcareous paleosols. Where such accumulations occur the appropriate horizon designation is "t" to indicate a textural horizon. The ordering of subordinate letter designations for buried horizons should follow that of the SCS, with "t" always being first and "b" indicating a buried horizon being last. Successive buried horizons are designated by Arabic numbers.

Using the above SCS nomenclature calcic paleosols might be designated Btkb1 indicating a B horizon with accumulated clay and calcium carbonate which is buried. A deeper massive indurated calcrete might be designated Kb2. Calcic paleosol horizons once identified, with appropriate horizon designations attached, then would be described in accordance with the standard field descriptions of color, texture and structure.

I would further propose the adoption of a standard descriptive terminology for carbonate morphology. The one I would propose is that developed by Gile et al. (1966) and subsequently modified by Bachman and Machette (1977) and Machette (1985). This system permits more than adequate description of Stages of carbonate development. This system uses Roman numerals to designate the progressive accumulation of carbonate and accompanying development of distinctive morphologies. Stage I carbonate being the least abundant and morphologically simplest form and Stage VI carbonate being the most abundant and morphologically complex carbonate. Stages I-III are described from calcic soils while stages IV-VI are described from pedogenic calcretes. These stages are described for both fine grained and coarse grained parent materials. Recognition of these stages is easily accomplished in the field. Descriptions of carbonate morphology based on the Machette system would appear in the "remarks" column of standard soil description sheets and tables.

I believe that the adoption of a uniform system of horizon descriptions and carbonate morphology description such as that proposed above would greatly enhance the effectiveness of communication regarding the nature of calcareous paleosols from different locations. Such uniformity will also contribute to enhanced understanding of the origin of these materials.

References Cited

Bachman, G.O. and Machette, M.N., 1977. Calcic soils and calcretes in the southwestern United States. U.S. Geological Survey Open File Report 77-794, 163pp.

Gile, L.H., Peterson, F.F. and Grossman R.B., 1966. Morphological and genetic sequences of carbonate accumulation in desert soils. Soil Science, 101, 347-360.

Guthrie, R.L. and Witty, J.E., 1982. New designations for soil horizons and layers and the new soil survey manual. Soil Science Society of America Journal, 46, 443-444.

Machette, M.N, 1985. Calcic soils of the southwestern United States. In: D.L. Weide (ed.) Soils and Quaternary geology of the southwestern United States. Geological Society of America Special Paper 203, pp. 1-21.

Soil Survey Staff, 1975. Soil Taxonomy. U.S. Department of Agriculture Handbook 436, 753p.