[Editor's note. The following paper was presented at the 1993 Paleopedology Symposium and has been revised. An earlier draft appeared in PP NL 10 Part D. Background: Roger presents a view that represents over 60 years of training and experience in field geology (research and mapping) with a limited opportunity to engage with pedologists. Most of his experience has been in the western U.S., although he has worked in many other parts of the world. Soils and paleosols have been very important in his work and he raises questions in this paper about the relations paleosols have or should have in geological studies.]
Historical Background: For about a century, some U.S. geologists have recognized paleosols as important parts of the geologic-climatic record (notably Leverett, 1898a and 1898b). However, not until 1961 did the American Commission on Stratigraphic Nomenclature bestow upon certain paleosols the possibility of formal recognition as stratigraphic units. This stratigraphic code established a new category, soil-stratigraphic units, whose single rank was termed the soil. It did not attempt to explain in pedologic terms what constitutes a “soil.” Indeed, adoption of “soil” as the only formal name for soil-stratigraphic units was a serious mistake, because this term has widely diverse meanings to a farmer, laundress, pedologist, geologist, engineer, etc.
The stratigraphic code was revised in 1983 in part to address problems with the concept and application of soil-stratigraphic units. The commission changed the soil-stratigraphic category name to pedostratigraphic units (North American Commission on Stratigraphic Nomenclature, 1983). The new North American Stratigraphic Code offers the following definition and guidelines:
A pedostratigraphic unit is a buried, traceable, three-dimensional body of rock that consists of one or more differentiated pedologic horizons (Art. 55a). The upper boundary of a pedostratigraphic unit is the top of the uppermost pedologic horizon formed by pedogenesis in a buried soil profile. The lower boundary of a pedostratigraphic unit is the lowest definite physical boundary of a pedologic horizon within a buried soil profile (Art. 55c). The fundamental and only unit in pedostratigraphic classification is a geosol (Art. 56).
Composite geosols. Where the horizons of two or more merged or “welded” buried soils can be distinguished, formal names of pedostratigraphic units based on the horizon boundaries can be retained. Where the horizon boundaries of the respective merged or “welded” soils cannot be distinguished, formal pedostratigraphic classification is abandoned and a combined name such as Hallettville-Jamesville geosol may be used informally (Art. 57a).
The physical and chemical properties of a pedostratigraphic unit commonly vary vertically and laterally throughout the geologic extent of the unit. A pedostratigraphic unit is characterized by the range of physical and chemical properties in the type area, rather than by “typical” properties exhibited in a type section. Consequently, a pedostratigraphic unit is characterized on the basis of a composite stratotype (Arts. 8d and 57b).
Discussion. Having Geosol adopted in the North American Stratigraphic Code as the sole pedostratigraphic unit was an important step to retain the pedostratigraphic classification within the Code. Many conservative geologists wanted to eliminate the soil-stratigraphic classification adopted in the 1961 stratigraphic code. This faction still is active! If it wins, formal recognition of paleosols as stratigraphic units will be lost from the North American Code. Also, be aware that it is a slow process to make changes in the Code. The International Stratigraphic Code still has no place for pedostratigraphic units (International Commission on Stratigraphy, 1994).
I still support the concept of Geosol as defined in the North American Code, although some revision and amplification of its definition in stratigraphic and pedologic detail may be desirable. Having Geosol as the sole formal rank in the pedostratigraphic classification is inadequate for Quaternary (and pre-Quaternary) stratigraphers. The Code should recognize pedostratigraphic ranks subordinate to Geosol.
Terminology for compounded (multistory) paleosols. These are two or more paleosols separated by small thicknesses of not-soil sediment (C horizon), with the whole pedocomplex underlain and overlain by larger thicknesses of non-soil sediment or by unconformities. These occurrences typically are preserved at local sites that have had rapid deposition and no or little erosion during several separate, closely spaced pedogenic episodes. Traced laterally, these pedocomplexes commonly merge into a single composite/amalgamated paleosol.
The 1983 Code does not address this problem. I recommend (favoring practicality in mapping) that, if an amalgamated paleosol is the usual type of occurrence of a pedostratigraphic unit in a given area but locally is represented by a compounded pedocomplexes, that the amalgamated paleosol be defined formally as a Geosol, and that its various component paleosols (locally identifiable) be defined as “pedomembers” of this Geosol. This practice will be akin to lithostratigraphic rules in the North American Code.
Obviously, this practice will be suitable only where the amalgamated paleosol represents a comparatively short time interval, such as within oxygen-isotope stage 5 or between stages 5 to 3. It should not apply to amalgamated paleosols that represent a very long time interval. For such paleosols, 1983 Code Article 57a, Composite geosols, should apply; such paleosols deserve only informal hyphenated designation that states the chief components, such as the Yarmouth-Sangamon paleosol, which represents about 700,000 years in Iowa.
Many paleosols are polygenetic and record more than one soil-forming episode. Morrison (1967, 1978) called these “composite paleosols/geosols” and Ruhe (1967) termed them “welded paleosols”; however, I now prefer the term “amalgamated” (proposed by L. Follmer, 1989 personal commun.) for such paleosols. The North American Code does not allow composite geosols to carry formal names unless soil horizons diagnostic of the individual merged paleosols can be distinguished, which rarely is possible.
The Code does not recognize litho-pedostratigraphic complexes that exist throughout the world. These complexes comprise stacks of many paleosols with varying degrees of soil-profile development. The paleosols are so numerous and closely spaced that they are integral to the lithologic character of the unit. The individual paleosols commonly are more or less amalgamated, but in places they are separated by small but varying thicknesses of intervening non-soil sediment. Traced laterally, the individual paleosols are discontinuous, being in places truncated or cut out by small disconformities and/or amalgamated with other paleosols. In other words, the individual paleosols cannot be reliably differentiated and traced very far; the whole complex is best treated and mapped as a lithostratigraphic or allostratigraphic unit. Some of the best examples are documented in loessial successions in Czechoslovakia, Austria, Hungary, and China. Well known examples in the U.S. include:
Pedofacies. The 1983 Code (Art. 57b) recognizes that lateral changes in soil-profile characteristics commonly occur if a Geosol is traced laterally, and recommends that a Geosol be characterized “on the basis of a composite stratotype for the type area”. Thus, the Code recognizes the chronocatena concept, although it doesn’t say so. However, few workers have followed this recommendation.
I recommend that each significant lateral variant of a geosol be designated formally as a pedofacies of this geosol, with a type locality. This practice is akin to designating lithofacies in lithostratigraphic units. In the future, formal definition of a Geosol should be accompanied by definition of each of its typical pedofacies, and each pedofacies should be represented by a parastratotype (1994 International Stratigraphic Code, p. 28), akin to defining members of a chronocatena.
Formal vs. Informal designation. Formal definition of a pedostratigraphic unit is not always necessary or desirable. In many cases, informal designation may be preferable. Surface (relict) paleosols exhibited on old land surfaces are allowed only informal stratigraphic names.
Definition of paleosol. The Glossary of Geology (1980) defines paleosol as “... a buried soil horizon of the geologic past. When uncovered, it is said to be exhumed. Syn: buried soil; fossil soil.” Some pedologists prefer this definition. Of course, the North American Code requires that a geosol be defined on the basis of a buried soil profile. However, many geomorphologists and Quaternary stratigraphers refer to the surface soil profiles on old landforms (e.g., ancient stream terraces and alluvial fans) as relict paleosols (Ruhe, 1967). I approve of this practice. Therefore, I do not agree with the notion that all soils exposed at the present land surface must be classed as “modern soils”.
Relict (surface) paleosols. A relict paleosol has remained exposed at the land surface after developing most of its diagnostic profile characteristics. Erosion has not destroyed or subsequent pedogenic activity has not totally transformed its original profile. A relict paleosol obviously has been exposed to all the surficial processes, pedogenic, biologic, and geologic, that have operated on this land surface since it was formed. Thus, a relict paleosol is a partial, over-printed, and imperfect record of the succession of surficial processes that have affected this surface.
If an original soil profile is weakly developed it will be transformed into and masked by any subsequent stronger pedogenesis. On the other hand, once a strongly developed profile has formed, it remains relatively unaffected by subsequent weaker pedogenesis. Clay accumulation is essentially irreversible and so is carbonate and silica accumulation in most semiarid and arid environments. A relict paleosol therefore can be said to have developed its diagnostic B-horizon characteristics during the strongest soil-forming episode to which it has been subjected. I believe that it should be named informally after the episode during which the prominent soil characteristics formed.
Conclusion. It is fortunate that the North American Stratigraphic Code recognizes pedostratigraphic units, despite possible improvements that might be made in characterizing and subdividing formally named Geosols. It is unfortunate that the International Stratigraphic Commission does not recognize pedostratigraphic units. Let’s hope that members of the INQUA Paleopedology Commission can influence both stratigraphic commissions to correct these shortcomings.
Editor’s commentary. For a broader view of the subject more background information is needed. Source of name: Geosol as a name for the fundamental pedostratigraphic unit was first proposed by Morrison (1964) and further explicated by Morrison (1967 and 1978) and by Morrison and Frye (1965).
The Commission chose a new word for a name so that it could be unique and not be encumbered by multiple meanings that result from redefining an existing word. Significance of capitalization: The upper-case spelling of a formal term such as Geosol indicates its status as a formalized term, meaning that it has been described and characterized in a publication that is available from research libraries. The lower case geosol serves to distinguish its informal status and is used as an equivalent to buried paleosol.
The contentious aspects: Some people have been against geosol because it is a new word. In a sense, it is two words--geosol and Geosol. People with more focus have criticized the requirement that a geosol must be buried (where defined). Some people lose focus at this point and assume it must be everywhere buried. This idea is a false interpretation that has gained some popularity. In general, a geosol is a buried catena (a lateral soil profile continuum) that is characterized by several important or dominant soil types which are recognized and described in stratigraphic context. Paleosols that occur only on old land surfaces (terraces, moraines, fans, etc.) do not qualify as geosols in the formal sense because they lack an upper boundary in the stratigraphic sense. The surficial paleosol can be recognized informally in a context with geosols, but stratigraphic codes do not take a position on this idea. Hypothetically, classes of paleosols could be formalized based on a set of criteria for all paleosols. This has not been done by stratigraphers because it goes beyond the concerns and needs of stratigraphic classification. However, a separate classification of paleosols would create a problem of overlap; geosols can outcrop beyond the locality where they are defined and paleosols can be buried in parts of their occurrence. The main purpose and distinction for the two terms are that geosol is needed in stratigraphic work, whereas paleosol is a broader term that includes geosol.
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International Commission on Stratigraphy, 1994, International Stratigraphic guide: Boulder, Colorado, Geological Society of America, 214 p.
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, 1898b, The weathered zone (Sangamon) between the Iowan loess and Illinoian till sheet: Jour. Geology, v. 6, p. 171-181. Morrison, R. B., 1964, Soil stratigraphy; principles, applications to differentiation and correlation of Quaternary deposits and landforms, and applications to soil science [Ph.D. dissertation]: University of Nevada, Reno, 178 p.
, 1967, Principles of Quaternary soil stratigraphy; in Morrison, R. B., and Wright, H. E., Jr., eds., Quaternary Soils, Internat. Assoc. For Quaternary Research (INQUA), VII Congress 1965, Proc. V. 9: Reno NV, Center for Water Resources Research, Desert Research Inst., Univ. Nevada Reno, p. 1-69.
, 1978, Quaternary soil stratigraphy--concepts, methods, and problems; in Mahaney, W. C., ed., Quaternary Soils: Norwich [England], Geoabstracts, p. 77-108.
, 1991, Quaternary stratigraphic, hydrologic, and climatic history of the Great Basin, with emphasis on Lakes Lahontan, Bonneville, and Tecopa; in Morrison, R. B., ed., Quaternary non-glacial geology; Conterminous U.S.: Boulder, Colorado, Geological Society of America, The Geology of North America, v. K-2, p. 283-320.
Morrison, R. B. And Frye, J. C., 1965, Correlation of the middle and late Quaternary successions of the Lake Lahontan, Lake Bonneville, Rocky Mountain (Wasatch Range), Southern Great Plains, and Eastern Midwest areas: Nevada Bureau Mines Rept. 9, 45 p.
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Ruhe, R. V., 1967, Paleosol, in McGraw-Hill Encyclopedia of Science and Technology: New York, McGraw-Hill Book Co., p. 522.