Some U.S. opinions about pedostratigraphic units, geosols, paleosols...
Posted by Roger Morrison 04 Jul 1999 20:25:39
To members and friends of the INQUA Paleopedology Commission The attachment below was prepared in response to e-mail that I've lately
received from workers all over the wworld, commenting on what
pedostratigraphic units are like in their work areas, how they ought to
be defined, and other problems in pedostratigraphy. To provide a basis
for discussion, I start with explaining how the current North American
Stratigraphic Code defines pedostratigraphic units and geosols. These
definGEOSOL, PALEOSOL AND PEDOSTRATIGRAPHIC UNITS IN THE U.S.A.
Roger Morrison, Chair, Work Group on Pedostratigraphy, INQUA Commision
on Paleopedology
Former Senior Research Geologist (retired), U. S. Geological Survey DEFINITIONS OF GEOSOL AND PEDOSTRATIGRAPHIC UNITS IN THE NORTH AMERICAN
STRATIGRAPHIC CODE GEOSOL is recognized as a formal pedostratigraphic term only in the
latest (1983) North American Stratigraphic Code (NASC), where it is
defined and explicated thus: 1. A GEOSOL is a PEDOSTRATIGRAPHIC UNIT (PSU), which is defined in the
NASC as "a body of rock that consists of one or more pedologic horizons
developed in one or more lithostratigraphic, allostratigraphic, or
lithodemic units [and] is OVERLAIN by one or more formally defined
lithostratigraphic or allostratigraphic units. A PSU is a buried,
traceable, three-dimensional body of rock that consists of one or more
differentiated pedologic horizons" (art. 55). Note A. According to the NASC, a PSU MUST be buried at its type
locality (this is requisite for determining its stratigraphic
relations). Thus, this Code does not recognize relict paleosols as PSUs
(these are paleosols occurring on the surfaces of old landforms,
moraines, river terraces, alluvial fans, etc. without an overlying
lithostratigraphic or allostratigraphic unit). Note B. A PSU is defined and recognized in the NASC on basis of its
solum (A and B horizons only, not O nor C horizon except in rare cases
where the C horizon is integral and has a definite lower boundary).
Therefore, a PSU may be all or only a part of a buried pedologic unit (a
laterally extending sequence of soil profiles as in a catena). 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 properties. Therefore, a PSU is characterized by
the range of physical and chemical properties exhibited in the type
area, rather than in a single type section; consequently, a PSU has a
composite stratotype [akin to a chronocatena](arts. 55 & 56). 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. Name of a PSU should be
chosen from a geographic feature in the type area, not from a time-span.
Note. Many Quaternary geosols and paleosols correlate strongly with
interglacials and interstadials, leading some workers to name these PSUs
after interglacials or interstadials and to regard them as
chronostratigraphic units. This practice, though tempting, is wrong,
because PSUs are independent from chronostratigraphic units,
conceptually and physically. [However, see below.] 4. GEOSOL is the fundamental and only unit in the pedostratigraphic
classification. 5. GEOSOL is used in two ways in the Code: if the G is capitalized 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 or
"welded" soils can be distinguished, formal names of PSUs based on the
horizon boundaries can be retained. Where the horizon boundaries of the
respective merged or "welded" ["amalgamated"] soils cannot be
distinguished, formal pedostratigraphic classification is abandoned and
a combined name such as Hallettville-Jamestown geosol may be used
informally (art. 57). IS "GEOSOL" UNNECESSARY (AND PERHAPS ALL PEDOSTRATIGRAPHIC TERMS,
INCLUDING "PALEOSOL")?
Some geoscientists argue that GEOSOL is an unnecessary term and refuse
to use it; a few have the same opinion of PALEOSOL, preferring just the
term SOIL. SOIL was tried and then discarded, as the term for the basic
pedostratigraphic unit. SOIL was adopted in this sense in the 1961
Stratigraphic Code (American Commission on Stratigraphic Nomenclature,
1961). It was discarded because its meaning was highly ambiguous, very
different to the geologist, the soil scientist, the engineer, farmer, or
laundress, and usually has no stratigraphic significance. [It is barely
a step higher than "dirt".] It was so ambiguous that it was replaced by
GEOSOL in the 1983 Stratigraphic Code (NASC, 1983). Long live
enlightened semantics in stratigraphic nomenclature! PALEOSOL has been used for more than half a century by geomorphologists
and geologists for any soil of the past (in context of soil profiles and
catenas). It may be buried, relict at the present land surface, or
exhumed. Nonetheless, some geoscientists object to PALEOSOL and refuse
to use this term, and some refuse to use any pedostratigraphic term at
all. These Luddites in stratigraphic nomenclature are free to do as
they wish (the Stratigraphic Code is merely advisory, not compulsory),
but they will be judged appropriately by a more progressive scientific
community. ARE ALL GEOSOLS MULTISTORY PEDOCOMPLEXES?
By no means! This idea is a misreading of the NASC as to geosol/PSU
attributes. Some geosols (e.g., the Sangamon Geosol in Illinois and its
correlative the Churchill Geosol in Nevada) are known to be more than a
single soil profile at rare sites that underwent exceptionally strong
sedimentation during a long mostly pedogenic episode, such as an
interglacial. Such sites exist only in a few percent of the sites where
the geosol is exposed; at most sites these geosols occur as a single
soil profile. The NASC ought to be augmented to provide for formal
recognition of such pedocomplexes and their component paleosols (as
pedomembers?), modeled after lithostratigraphic classification (Group,
Formation, Member, Bed). Viva la Codiga Stratigraphico! MUST A GEOSOL OR PALEOSOL BE EVERYWHERE BURIED?
Not entirely. To be recognized as a pedostratigraphic unit, the NASC
requires that a PSU's stratigraphic relations be defined in explicit
terms, with a stratotype where these relations can be ascertained. Here
the PSU must be buried between deposits that bracket its stratigraphic
position. This requirement disqualifies all paleosols that occur only on
old geomorphic surfaces. Also, a PSU must be traceable over a
reasonably wide and varied terrain, akin to a chronocatena. This
requires a type area, not a single type locality. The formal name
(geosol) is retained at sites where a PSU can be demonstrated to be
exhumed. IS GEOSOL AKIN TO A CHRONOSTRATIGRAPHIC UNIT?
No. In the North American Stratigraphic Code (NASC), GEOSOLS (and
informal pedostratigraphic units such as paleosols) are defined as
independent from chronostratigraphic units, both conceptually and in
physical/material terms. Lower and upper boundaries of a PSU typically
are time-transgressive (diachronous) if traced over a large region, due
to change in altitude, latitude, and local climate. [Thus, they are
akin to diachronic units in the NASC (basic unit is Diachron).] In
contrast, the upper and lower boundaries of a chronostratigraphic unit
are isochronous (time-parallel) wherever the unit is present. In
summary, concepts of time play no part in defining a PSU, nor should
they in its name. Nonetheless, it is fundamentally established that key paleosols in
Austria, Czech Republic, Slovakia, Russia, Hungary, Germany, China,
India, Argentina, U.S., and other parts of the world correlate in time
with Quaternary interglacials and interstadials. Thus, although the
lower and upper boundaries of the paleosols are diachronous, individual
soil-forming episodes peaked lock-step throughout the world. The
soil-forming episodes were shorter in some places than in others, e.g.,
at high altitudes and latitudes, yet their peaks were globaly coeval. Consequently, there are proposals to have pedocomplexes and their
individual paleosols recognized as pseudo-chronostratigraphic
units---units that are essentially coeval but have diachronic upper
and/or lower boundaries. Another possibility is to expand the
Diachronic classification in the NASC (Diachron, episode, phase, span
are the official terms). However, as an ex-warrior in such causes, I
believe that adoption of such proposals will be a long time a-coming. EPISODIC PEDOGENIC EPISODES DURING THE QUATERNARY PERIOD
It is well established that the stronger paleosols of Quaternary age
throughout the world correlate with interglacials and interstadials.
These episodes were substantially warmer (and generally wetter) than the
intervening glacial episodes. They also were relatively short.
Interglacials lasted ~15 to ~20 kyr and interstadials ~4 to ~8 kyr
within the ~100 kyr span of an average glacial-interglacial cycle during
the last 400 kyr. This means that the rate of pedogenesis was low to
almost nil during the glacial episodes but abruptly became much higher
during the warm interglacials and interstadials, causing pedogenesis to
cross various thresholds to produce well-developed paleosols. This is
the basic argument for "intermittency of Quaternary soil-forming
episodes" that I first proposed in 1964. (Many geoscientists wedded to
steady-state bias strongly opposed the idea that global temperature (not
precipitation) change was the chief control of soil development during
the Quaternary Period). Some U.S. geoscientists remain oblivious to the fact that most
Quaternary paleosols are established correlatives with specific
interglacials or interstadials (e.g., Sangamon Geosol with isotope
stage 5). These were occasional fairly short warm episodes during each
glacial-interglacial climatic cycle and the times of most active
pedogenesis. Nonetheless, these pedostratigraphic Luddites remain
guided (blinded?) by a "steady state" theory of soil genesis and
strongly resist any opposition. They commonly also reject all
pedostratigraphic concepts, saying that PALEOSOL and GEOSOL are nonsense
terms.
I'm not worried about how future geoscientists will judge their ideas vs
those of a wider, more enlightened paleopedologic community. Viva la
Codiga Stratigaphico! NAMING PROBLEMS
There is a long tradition of naming interglacials after the paleosols
that are presumed to represent them, and vice versa. Thus, in the U.S.,
the Sangamon Geosol is the basis for the Sangamonian Stage (=
Sangamonian Interglacial Episode = isotope stage 5). However, various
workers have different ideas about the physical content of the Sangamon
Geosol, and the time content of the Sangamonian Stage (e.g., Hallberg
and Kemmis, 1986; Fullerton, 1986). In order to avoid such confusion, the practice of giving similar names
to different categories of stratigraphic units now is specifically
disallowed in the North American Stratigraphic Code (1983). Unfortunately, many old usages remain deeply entrenched, such as
Sangamon Soil/Geosol and Sangamonian Stage, and Farmdale Soil/Geosol
and Farmdalian Substage. Should such usages be grandfathered in, or
should the key names be redefined? THE SANGAMON GEOSOL AND THE SANGAMONIAN STAGE
A century ago, Leverett (1898) identified the last interglacial episode
on basis of a strong soil profile, buried beneath till and loess of the
last (Wisconsin) glaciation, that he observed in railroad cuts and dug
wells in Sangamon County, Illinois. He named the soil Sangamon soil,
and the interglacial episode the Sangamon interglacial. Subsequent
workers throughout the Midwestern U. S. adopted the term Sangamon
Soil/Geosol for the soil, and "Sangamonian Stage" for the interglacial,
but with various definitions. Now these names are deeply entrenched,
despite that the North American Stratigraphic Code now disallows use of
the same name for different categories of stratigraphic units. WHAT TIME-SPAN DOES THE SANGAMON GEOSOL REPRESENT?
This is an important question, because Sangamon Soil/Geosol has become
internationally recognized as a North American proxy for the last
interglacial. Upper boundary. The Sangamon Geosol, in the majority of exposures, is
the lower part of a pedocomplex with cumulic organic-rich A/AB
horizon(s) as its upper part, called in Nebraska the Gilman Canyon
Formation. In places the upper zone is intercalated with enough
sediment (loess, etc.) so that individual paleosols can be
distinguished. In Illinois these are named the Chapin (lowest), Indian
Point, and Farmdale Geosols, correlative with isotope stages 5c, 5a, and
3 respectively. However, in most places these paleosols (especially the
Chapin) are amalgamated into the profile of the Sangamon Geosol.
Therefore, the Illinois State Geological Survey currently regards the
Sangamon Geosol (and Sangamonian Stage/Age) upper age limit as ~75 ka,
approximately equivalent to the boundary between istope stages 5a and 4;
thus, the Sangamonian Stage/Age is equivalent to isotope stage 5 sensu
lato, 5e through 5a. Lower boundary. At its type locality in Sangamon County, Illinois, the
Sangamon Geosol is developed in till of middle Illinoian age (Vandalia
Till Member, Glasford Formation). This till may be ~200 kyr old and
some workers suggest that development of the Sangamon Geosol started
right afterward. Consequently, this requires that the Sangamonian Stage
began about 200 ka, instead of at the beginning of isotope stage 5e,
about 130 ka, or in places, even earlier (e.g., Fullerton, 1986 and
Hallberg and Kemmis, 1986). However, the profile characteristics of the
Sangamon Geosol at this site closely resemble those where this paleosol
is developed above the late Illinoian Radnor Till, indicating that the
Sangamon Geosol (s.s.) is monogenetic in practical terms (Leon Follmer,
personal communication, 1999), and appears to have developed during a
single pedogenetic episode, not several. Likely this was because
episodes of widespread lateral erosion (pedimentation) occurred
repeatedly during glacial phases in the Midwestern U. S., repeatedly
wiping clean the surficial veneer from much of the pre-existing
landsurface. REFERENCES
American Commission on Stratigraphic Nomenclature, 1961, Code of
Stratigraphic Nomenclature: American Association
of Petroleum Geologists Bull., v.45, no. 5, p. 645-665. Curry, B. B., and Follmer, L. R., the last interglacial-glacial
transition in Illinois: 123-25 ka, in:Clark, P. U., and Lea, P. D., eds,
The last interglacial-glacial transition in North America: Geological
Society of America Special Paper 270, p. 71-88. Follmer, L. R., 1983, Sangamon and Wisconsin pedogenesis in the
Midwestern United States, in Late-Quaternary environments of the United
States, H.E. Wright, ed., v. 1, The late Pleistocene, S. C. Porter, ed.,
p. 138-144.
Fullerton, D. S., 1986, Stratigraphy and correlation of glacial deposits
from Indiana to New Jersey; in V. Sibrava, D. Q. Bowen, and G. M.
Richmond, Quaternary glaciations in the Northern Hemisphere: Quaternary
Science Reviews (Pergamon Press}, v. 5, p. 23-38; also in-pocket Chart
"Indiana to New York and New Jersey". Hallberg, G. R. and Kemmis, T. J., 1986, Stratigraphy and correlation of
the glacial deposits of the Des Moines and James lobes and adjacent
areas in North Dakota, South Dakota, Minnesota and Iowa, in V. Sibrava,
D. Q. Bowen, G. M. Richmond, eds, Quaternary glaciations in Northern
Hemisphere: Quaternary Science Reviews (Pergamon Press), v. 5, p. 65-68;
especially in-pocket chart "Central Plains". Leverett, F., 1898, The weathered zone (Sangamon) between the Iowan
loess and Illinoian till sheet: Journal of Geology, v. 6, p. 171-181. Morrison, R. B., 1978, Quaternary soil stratigraphy-concepts, methods,
and problems, p. 77-108 in W. C. Mahaney, ed., Quaternary Soils:
Norwich [UK], GeoAbstracts. Morrison, R. B., 1991, Quaternary stratigraphic, hydrologic, and climate
history of the Great Basin, with emphasis on Lakes Lahontan, Bonneville,
and Tecopa, in R. B. Morrison, ed., Quaternary nonglacial geology;
Conterminous U. S.: Boulder, Colorado, Geological Society of America,
The Geology of North America, v. K-2, p. 283-320.
North American Commission on Stratigraphic Nomenclature, 1983, North
American Stratigraphic Code: American Assoc. Petroleum Geologists Bull.
v. 67, no. 5, p. 841-875.
itions are by no means immutable; they aren't set in concrete; in fact,
they are constantly being revised in light of more advanced information
and judgemeent. I've tried to address a few key controversies in as
simple terms as possible, but please forgive my at times obscure
semantics. I hope that this material will be helpful during discussions
of the Paleopedology Commission in Durban, South Africa in August --
which I plan to attend.
Roger Morrison
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