Definition of paleosol and geosol; memo to John Catt
Posted by Roger Morrison 10 Oct 1999 21:37:17
This is a slight revioion of the memo that I sent to John Catt a couple
weeks ago. It covers some of the issues discussed during sessions of
the Paleopedology Commission (particularly those of the Glossary Work
Group) at the INQUA Congress in Durban, as well as various comments that
I've received ia e-mail during the past several months. I'm unable to
respond personally to each e-mail comment, so this exposMEMORANDUM TO
JOHN CATT, CHAIR, WORK GROUP ON THE GLOSSARY
OF PEDOLOGY, AND TO MEMBERS OF THE INQUA PALEOPEDOLOGY COMMISSION Comments by Roger Morrison, Chair, Work Group on Pedostratigraphy
RE: DEFINITIONS OF PALEOSOL AND
GEOSOL
7 October 1999 This is a revision of my memo to John Catt of 26/27 September,
after helpful suggestions by Leon Follmer (neglecting to
follow all of them), plus some amplifications of my views. These comments are further explication of arguments that I
presented during the 1999 INQUA Congress to members of the
Work Group on Glossary of Paleopedology and other sessions of
the Paleopedology Commission. Some topics are controversial.
Let's discuss them further via E-mail, but let us paleoped-
ologists not just argue among ourselves, at the cost of hurting
our efforts to: (1) Get pedostratigraphic units admitted into the International
Stratigraphic Guide, and (2) Improve the definition and classification of pedostrati-
graphic units in the North American Stratigraphic Code
(e.g., recognition of pedocomplexes, pedofacies, etc.) PALEOSOL I urge that PALEOSOL be defined in the Glossary of Paleopedology
in as simple terms as possible, as a generic name without arti-
ficial restriction, especially as to age limitation and burial
requirement. Its definition should be entirely separate from and
broader than that of a pedostratigraphic unit (PSU) in the 1983
North American Stratigraphic Code (NASC). Some workers confuse
the two, but they are not the same. PALEOSOL should be more in-
clusive, including both buried and relict (surface) old soil
profiles without restriction as to age. The definition of PALEOSOL ought to encompass ALL old soil pro-
files, from those only a few centuries old through the entire
Phanerozoic into the Precambrian. It should include all soil
profiles developed on landscapes of the past if their pedogenic
pathways have changed, whether the soil profiles are buried or
at the present land surface. This the sense this term is under-
stood and used by the majority of workers. Definition of the term PALEOSOL is a keynote feature of the
Glossary. The Glossary definition should contain the broadest
current usage of this complex, but hopefully generic term,
avoiding attempts by special-bias factions to impose and
codify restrictions that invade the widest responsible usage. QED, PALEOSOL should be defined more broadly than a PSU is in
the NASC, and without age and burial restriction! Also, please ignore the remonstrations of: (1) A few academics who believe that PRIORITY takes precedence
over all other definitions of paleosol, especially as to the
requirement that a paleosol must be buried. For decades,
hundreds of scientific articles have described relict soil
profiles just below the present land surface of old alluvial-
fans, stream-terraces, moraines, etc. These ancient soil pro-
files are true paleosols, that should not be left out in
limbo, as does the NASC with its definition of PSU and Geosol. (2) Adherents to a "steady-state theory of pedogenesis", who
believe that ALL soil profiles exposed at the present land
surface are in equilibrium with existing environment, even
those on ancient moraines, alluvial fans, stream terraces,
etc. Thus, at present land surfaces there are no paleosols, only "modern
soils" everywhere. Geomorphologists well versed in
pedolody know how false this idea is.
At the session of the Work Group on this Glossary during the
15th INQUA Congress in Durban, South Africa, two months ago, a
proposal was made to restrict the term PALEOSOL to just those
soil profiles that are Pleistocene or older, older than the
Holocene Epoch. I strongly recommend AGAINST such a restriction
(and against ANY age restriction) for these reasons: (1) At the Pleistocene-Holocene boundary (which is controversial)
there was no significant threshold-crossing in styles and rates
of pedogenesis, beyond the effects of the interglacial-glacial
climate cycles of the Quaternary Period.
(2) The exact stratigraphic level and age of the Pleistocene-
Holocene boundary has not yet been decided by the International
Geological Congress, although it has been discussed for decades.
(I once was a member of the INQUA Holocene Commission and wrote
several articles about this boundary.) Opinions about the correct
age for this boundary range from 15 to 10 ka, a 5 kyr "bone of
contention". (3) I know of hundreds of sites in the U. S., Europe, South Amer-
ica, & Asia that have well-developed paleosols of Holocene age,
some with marked Bt horizons and some comparable in development
with Pleistocene interglacial paleosols. There is no valid
scientific basis for restricting the term PALEOSOL to just those
soil profiles that predate the Holocene. (4) I, myself, have measured and described many Holocene paleo-
sols and have formally named a few, including the Harmon School
Soil and Toyeh Soil (now both Geosols) in the pluvial Lake Lahon-
tan sequence, Nevada (U. S. Geological Survey Professional Paper
401, 1964) and the Graniteville and Midvale Soils/Geosols in the
Lake Bonneville sequence, Utah (USGS Prof. Paper 474, 1965).
These paleosols have been established pedostratigraphic units for
decades. GEOSOL GEOSOL was adopted as the term for the basic pedostratigraphic
unit in the revised North American Stratigraphic Code (NASC)
(North American Commission on Stratigraphic Nomenclature, 1983),
replacing the term SOIL in this context in the previous (1981)
American Stratigraphic Code because of the latter term's varied
and ambiguous meanings. The NASC is the only international stratigraphic code that
recognizes pedostratigraphic units. These units are not yet
recognized in the International Stratigraphic Guide (1994), but
I'm working on this issue. At the end of this memorandum is a
summary of NASC's requirements for definition of a PSU and Geosol. As far as the Glossary of Paleopedology is concerned, the
definition of GEOSOL is out of the hands and concerns of the
Glossary Work Group. It already is a fait accompli, already
codified in the NASC (see below). As with all human achievements,
this definition remains imperfect (see my recent communications
suggesting recognition of pedocomplexes and their subdivisions,
etc.), but for now this formal definition remains as the
definitive one. DEFINITIONS OF PEDOSTRATIGRAPHIC UNITS AND GEOSOL IN THE NORTH
AMERICAN STRATIGRAPHIC CODE GEOSOL was first formally adopted as a pedostratigraphic term in
the 1983 North American Stratigraphic Code (NASC), where it is
defined and explicated as the basic pedostratigraphic unit (PSU),
in these terms: 1. A PEDOSTRATIGRAPHIC UNIT (PSU) is a body of rock consisting
of one or more pedologic horizons developed in one or more litho-
stratigraphic, allostratigraphic, or lithodemic units [and]
is OVERLAIN by one or more formally defined lithostratigraphic or
allostratigraphic units. Thus, a PSU is a buried, traceable,
three-dimensional body of rock that consists of one or more
differentiated pedologic horizons. (Article 55.) RBM NOTE A. According to the NASC, a PSU MUST be buried at its
type locality (this is requisite for determining its strati-
graphic relations). Thus, this Code does not recognize relict
paleosols as PSUs. These are paleosols on old moraines, river
terraces, alluvial fans, etc that remain at the present land
surface without an overlying lithostratigraphic or allostrati-
graphic unit. On the other hand, some people misinterpret the NASC's definit-
ion of GEOSOL to mean that this term cannot be applied in cases
where a PSU is exposed at the present land surface. This is
somewhat incorrect. The NASC allows the name GEOSOL to be
retained where the unit is exhumed by erosion. RBM NOTE B. A few workers still use fuzzy semantics and mix
rock, time, event, and soil names. For example, they may use the
same name for a lithostratigraphic unit (e.g., Bull Lake Till),
and/or for the event it represents (e.g., Bull Lake Glaciation),
and/or for the time span of this event (Bull Lake Episode/Stage),
and/or for a subsequent pedogenic episode (Bull Lake soil/paleo-
sol). Of course, in the latter case the paleosol can't qualify
as a PSU or Geosol because it typically is a relict/surface
paleosol. A more correct informal name might be "Post-Bull Lake
paleosol." RBM NOTE C. According to the NASC, a PSU is defined on basis of
its solum. The solum comprises the A and B horizons only, not O
or C horizons. Therefore, a PSU may be all or only a part of a
buried pedologic unit (a laterally extending array of soil
profiles or pedons, as in a chronocatina.) Thus, a buried
pedologic unit may be somewhat more inclusive than a PSU.
2. The stratigraphic position of a PSU is determined by its
relation to overlying and underlying stratigraphic units. It
must have demonstrated traceability. It commonly will vary,
laterally and vertically, in its physical and chemical proper-
ties. Therefore, a PSU is characterized by the range of phys-
ical and chemical properties exhibited in the type area, rather
than in a type section, thus having a composite stratotype
area. (Articles 55 and 56.)
RBM NOTE A. Designation of a PSU on basis of a type area is
similar to that required for defining a chronocatina. RBM NOTE B. I propose that the NASC allow the chief lateral
variants of a PSU in its type area to be formally recognized
as PEDOFACIES of its pedostratigraphic unit.
3. INDEPENDENCE FROM TIME CONCEPTS. Boundaries of a PSU are
time-transgressive (diachronous). Concepts of time-boundaries
or time-spans play no part in defining a PSU. The name of a PSU
should be chosen from a geographic feature in the type area, not
from a time-span. (Article 55).
RBM NOTE A. Many Quaternary paleosols/geosols correlate strongly
with interglacials and interstadials, leading some workers to
name key paleosols after the correlative interglacials or inter-
stadials and to regard them as chronostratigraphic markers.
This practice is incorrect under the NASC, because by the Code a
PSU must be defined strictly on basis of its physical strati-
graphic relations and material (physical & chemical) character-
istics, independent of time or event concepts. RBM NOTE B. According to the semantics of the NASC, PSU's cannot
qualify as chronostratigraphic units because their lower and
upper boundaries typically are time-transgressive, depending on
the altitude and latitude of various sites, etc. Nonetheless,
the maxima of the time-spans when the stronger paleosols/geosols
formed correlate lock-step worldwide with Quaternary interglacial
and interstadial maxima. This fact means that their peak times of
pedogenesis were essentially synchronous, although their begin-
nings and endings were diachronous.
This means that strong geosols have strong isochronous character-
istics. Thus, many workers regard Quaternary geosols as quasi-
chronotratigraphic marker units. Unfortunately, neither the NASC
nor any other stratigraphic code accomodates the possibility of
geosols being quasi-chronostratigraphic units. Perhaps they will
in the future. A few people suggest that PSU's are akin to diachronic units.
This is not so. A Diachronic Unit (NASC, articles 91-95) must
be represented by a specific lithostratigraphic, allostraigraph-
ic, biostratigraphic, or pedostratigraphic units or an assemblage
of such units that have comparable timespans. However, Diachronic
units (Diachrons, etc.) by the NASC should be of equal duration
at different places, despite difference in times and beginnings
at different places (this is why they are called "diachronic').
A diachronous unit is "of equal duration at different places
despite differences in the time at which it began and ended at
those places." So, diachrons lack isochronous characteristics,
which geosols do have.
4. GEOSOL is the fundamental and only unit in the pedostrati-
graphic classification.
RBM NOTE. There is need for not one but two or more levels in
the hierarchy of pedostratigraphic units. This would recognize
the common occurrence of pedocomplexes. These are composites of
more than a single paleosol, closely associated and usually seen
as a single composite soil profile but locally (in places with
rapid sedimentation during the pedocomplex episode) is divisible
into its component paleosols. Examples are many occurrences of
the Sangamon Geosol in the Midwestern U.S. and "Eemian" paleosols
in Europe. I propose that the locally identifiable subdivisions of such
compound geosols be designated as PEDOMEMBERS of the typical
geosol, similar to the division of a Formation into Members in
the lithostratigraphic classification. 5. GEOSOL is used in two ways in the NASC: If the G is capital-
ized it designates a formal PSU, but where the g is lower case
it means an informal PSU (= buried paleosol). 6. COMPOSITE GEOSOLS. Where the horizons of two or more merged
("welded" or "amalgamated") soil profiles can be distinguished,
formal names of PSUs based on the horizon boundaries can be
retained. Where the horizon boundaries cannot be distinguished,
formal pedostratigraphic c lassification is abandoned and a
combined name such as Hallettville-Jamestown geosol may be used
informally. (Article 57.)
REFERENCES
American Commission on Stratigraphic Nomenclature, 1961, Code of
Stratigraphic Nomenclature: American Association of Petroleum
Geologists Bull., v. 45, no. 5, p. 645-665. International Subcommission on Stratigraphic Classification in
International Commission on Stratigraphy in International Union
of Geological Sciences, 1994, International Stratigraphic Guide,
2d edition, A. Salvador, ed.: Boulder, Colorado, USA, Geological
Society of America and International Union of Geological
Sciences, 214 p. Morrison, R. B., 1978, Quaternary soil stratigraphy---concepts,
methods, and problems, p. 77-108 in W. C. Mahaney, ed.,
Quaternary Soils: Norwich, UK, GeoAbstracts. North American Commission on Stratigraphic Nomenclature, 1983,
North American Stratigraphic Code: American Assoc. Petroleum
Geologists Bull., v. 67, no. 5, p. 841-875.
ition will have to do for now. Roger Morrison
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