Boggs S. Principles of Sedimentology and Stratigraphy
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12.5 Nomenclature and Classification of Lithostratigraphic Units
417
sheets melt. Water may also be tied up on land in lakes, reservoirs, and ground-
water aquifers. Finally, fluctuations in ocean temperature (Table 12.2) may pro-
duce small variations in sea level. Changes in volume of an ocean basin may be
brought about by a variety of causes. Sediment infill of an ocean basin, for exam-
ple, would cause sea level to rise. Changes in the volume of the mid-ocean ridge
system may be another cause. Changes in volume of mid-ocean ridges occur as a
result of variations in rates of seafloor spreading. An increase in rates of seafloor
spreading causes an increase in volume of mid-ocean ridges and a consequent rise
in sea level, and a decrease in spreading rates generates a decrease in ridge vol-
ume and a corresponding fall in worldwide sea level. Pitman (1978) suggests, for
example, that change in the rate of seafloor spreading from 2 cm/yr to 6 cm/yr in
the modern ocean could produce a rise in sea level of more than 100 m during a
period of seventy million years. Correspondingly, a decrease in spreading rate
back to 2 cm/yr for the next seventy million years would cause sea level to drop
by more than 100 m. Other possible ways of changing the volume of an ocean
basin include glacial isostatic rebound, upwarping or downwarping of the
seafloor, and continental collision (Table 12.2). Isostatic rebound after glaciation
produces gradual rise in a land surface, which has been depressed owing to
weight of the ice, after weight of the ice has been removed by melting.
Changes in sea level and the methods that stratigraphers use to determine
the magnitude of sea level changes from the stratigraphic record are further dis
cussed under "Sequence Stratigraphy" in Chapter 13. The effects of sea-level
change on the stratigraphic characteristics of sedimentary rocks are examined in
detail in that section.
12.5 NOMENCLAT URE AND CLASSIFICAT ION
OF LITHOSTRATIGRAPHIC UNITS
To bring order to strata and to understand to the fullest extent the geologic history
recorded in these strata, it is necessary to have a formal system for defining, clas
sifying, and naming geologic units. Such a stratigraphic procedure promotes sys
tematic study of the physical properties and successional relationships of
sedimentary strata and is essential for interpretation of depositional environments
and other aspects of Earth history. The need for systematic organization of strata
was recognized as early as the latter half of the 18th century by European scien
tists such as Johann Gottlob Lehman, Giovanni Arduino, and Georg Christian
Fiichsel, who made early attempts to organize strata on the basis of relative age
(Krumbein and Sloss, 1963). The gradual evolution of these efforts to organize and
classify strata continued through the 18th and 19th centuries and eventually cul
minated in formulation of the internationally used Geologic Time Scale and the
Geologic (Stratigraphic) Column (Chapters 14 and 15). This evolution is one of the
more fascinating chapters in the history of stratigraphic study. Succinct sum
maries of these early efforts at stratigraphic classification are given by We ller
(1960), Krumbein and Sloss (1963), and Dunbar and Rogers (1957).
Development of the Stratigraphic Code
Local study of rock strata requires subdivision of the Stratigraphic Column into
smaller units that are systematically arranged on the basis of inherent properties
and attributes. The purpose of stratigraphic classification is thus to promote under
standing of the geometry and successions of rock bodies. To ensure uniform usage
of stratigraphic nomenclature and classification, attempts have been underway for
several decades to adopt a code of stratigraphic nomenclature that formulates
views on stratigraphic principles and practices designed to promote standardized
418
Chapter 12 I Lithostratigraphy
classication and formal nomenclature of rock materials. In the United States,
such codes have been drafted by the Committee on Stratigraphic Nomenclature,
1933, and its successors, the American Commission on Stratigraphic Nomencla
ture, 1961, and the North American Commission on Stratigraphic Nomencla
ture, 1983. The Code of Stratigraphic Nomenclature published by the American
Commission on Stratigraphic Nomenclature in 1961, and revised slightly
1970, standardized terminology and practices used in stratigraphy in the Unit
ed States at that time and was widely accepted by North American geologists.
New concepts and techniques, particularly the concept of global plate tectonics,
have
developed in the past few decades. These developments revolutionized
the earth sciences and necessitated revision of the 1961 Code. In order to incor
porate new concepts and techniques, the North American Commission on
Stratigraphic Nomenclature published a new North American Stratigraphic
Code in May 1983. For the convenience of readers, this Code is reproduced in
full in Appendix C.
Since publication of the 1983 Code, the North American Commission on
Stratigraphic Nomenclature (NACSN) has become aware of several places in the
Code where inconsistencies are present, clarification is needed, or updating and
revision are required. Such revision and updating are currently in progress. Are
vision of the 1983 Code wi likely be published in 2005 (Randall Odor, U.S.
Geological Survey, personal communicaon, 2004). Unfortunately, this new Code
will not be available in time to incorporate into the fourth edition of this book. It
should, however, be available on the Inteet prior to publication as an open-le
report at http://www.agiweb.org/nacsn/. Some proposed changes to the Code
have already been published as North American Commission on Stratigraphic
Nomenclature Notes 63 and 64 (see Ferrusquia-Villafranca et aL, 2001, and Lenz
et al., 2001 ).
The International Stratigraphic Guide, published by the International Sub
commission on Stratigraphic Classification in 1976 and 1994 (Hedberg, 1976; l
vador, 1994), provides a comprehensive treatment of stratigraphic classification,
terminology, and procedures from an international point of view. In this book I ac
cept and use the terminology of the North American Stratigraphic Code. Readers
should be aware, however, that some departures from the Code may appear in the
Inteational Satigraphic Guide. See also Whittaker et al., 1991, A Guide to Strati
graphical Procedure (in the U.K.).
Major pes of Stratigraphic Units
The various categories of stratigraphic units recognized by the Code are summa
rized in Ta ble 12.3. Note that some stratigraphic units (e.g., lithostratigraphic
units, biostratigraphic units) are based on observable characteristics of rocks. Such
units are identied in the field on the basis of physical or biological properties that
can be measured (e.g., grain size), sensed by instruments (e.g., magnetic polari),
or described (e.g., sedimentary structures, kinds of fossils). Others are related to
geologic ages of rocks. Stratigraphic units having time significance may be actual
units of rock (e.g. chronostratigraphic units) that formed during particular time
intervals or they may simply be divisions of me (e.g., geochronologic units) and
not actual rock units.
Categories and ranks of all stratigraphic uts as defined in the 1983 Code,
and modified slightly in the proposed 2005 Code (Ferrusquia-Villafranca et al.,
2001), are shown in Table 12.4. Procedures and requirements for defining formal
stratigraphic units are set forth in detail in the Code (Appendix C). These proc
dures clude requirements for picking a name, designating a stratotype or type
section, describing the units, specifying the boundaries between units, and pub
lishing appropriate descriptions of the units in a recoized scientific medium.
12.5 Nomenclature and Classification of Lithostratigraphic Units
419
Material categories based on content or physical limits (composition, texture, fabric, structure, color, fossil content)
Lithostratigraphic units-conform to the law of superposition and are distinguished on the basis of lithic characteris
tics and lithostratigraphic position
Lithodemic unitsonsist of predominantly intrusive, highly metamorphosed, or intensely deformed rock that gen-
erally does not conform to the law of superposition
Magnetopolarity units-bodies of rock identified by remnant magnetic polarity
Biostratigraphic units-bodies of rock defined and characterized by their fossil content
Pedostratigraphic unitsonsist of one or more pedologic (soil) horizons developed in one or more lithic units now
buried by a formally defined lithostratigraphic or allostratigraphic unit or uts
Allostratigraphic units-mappable stratiform (in the form of a layer) bodies defined and identified on the basis of
bounding discontinuities
Categories expressing or related to geologic age
Material categoes to define temporal spans (stratigraphic units that serve as standards for recognizing and iso
lating materials of a particular age)
Chnostratigraphic unit-bies of rock established to serve as the material reference for all rocks formed
during the same spans of time
Polarity-chronostratigraphic units-divisions of geologic time distinguished on the basis of the record of magne
topolarity as embodied in polarity-chronostratigraphic units
Temporal (nonmaterial) categories-( not material units but conceptual units, Le., divisions of time)
Geochronologic unitsivisions of time distinguished on the basis of the rock record as expressed by chronos
tratigraphic units
Polari-chronologie units-divisions of geologic time distinguished on the basis of the record of magnetopolarity
as embodied in polarity-chronostratigraphic units
Diachronic unitsompri the unequal spans of time represented by one or more specific diachronous rock
bodies, which are bodies with one or two bounding surfaces that are not time synchronous and thus "transgress"
time
Geochronometric units-isochronous units (units having equal me duration) that are direct divisions of geologic
time expressed in years
Source: North American Commission on Stratigraphic Nomenclature, 1983, North AmPrlcan Stratigraphic Code: Am. Assoc. Petleum Geolo
g
ists Bull., v. 67.
Our immediate concern here is with subdivision and nomenclature of lithostrati
graphic units. Other types of stratigraphic units are described in subsequent parts
of the text. [Note: Only minor changes in treatment of lithostratigraphic units, as
described in the 1983 Code, are expected in the revised (2005) version of the Code
(Randall Orndorff, U.S. Geological Survey personal communication, 2004). Some
of the proposed minor changes can be viewed in North American Commission on
Stratigraphic Nomenclature Note ; see Ferrusquia-Villafranca et al., 2001, or
ht: I I www.agiweb.orgl nacsnl .]
Formal Lithostratigraphic Units
The concept of formations and other formal lithostratigraphic units is briefly in
troduced in Section 12.2. In terms of size, the hierarchy of lithostratigraphic
units
in descending order is supergroup, group, formation, member, and bed (Table
12.5). Although a foation is not the largest lithostratigraphic unit, it is nonethe
less the fundamental unit of lithostratigraphic classification. All other lithostrati
graphic units are defi ned as either assemblages or subdivisions of fo rmations. Note from
Table 12.5 that a formation is defined strictly on the basis of lithology. Formations
may be defined on the basis of a single lithic type, repetitions of two or more lith
ic types, or extreme lithic heterogeneity where such heterogeneity constitutes a
form of unity when the rock unit is compared with adjacent units. For example,
a formation might be composed entirely of shale, entirely of sandstone, or of an
420 Chapter 12 I Lithostratigraphy
b 1� •
go d oha�ap�d ts
d�
No e
Cosio Sagraphic Nomena Note 63
I. Mateal categories based on content or physical limits
Lithostratigraphic
Lithodemic
Magnetopolarity
Biosatigraphic
Pe dostratigraphic
Allostratigraphic •
Supergroup
Supersui
Group
Suite
Polarity
Allogroup
E
Superzone
c
Formation Litlwdeme
u
Polarity Zone
Biozone Geosol Alloformation
(Interval,
Assemblage or
Abundance)
Member
Polarity Subzone
Subbiozone
Allomember
(or Lens, or
!
Tongue)
I
Bed(s)
or Flow(s)
IIA. Mateal categories used to
define temporal spans
liB. Nonmaterial categories related to geologic age
C
hrono-
Po lari Chrono-
I
Polarity
!
I
stratigraphic stratig
raphic
Geochronologic
Chronologie
Diachronic Geochronoetric
Eonothem
Polarity
Eon
Polarity
Eon
Superchronozone
Superchron
Erath em
Era
Era
(Supersystem)
(Superperiod)
(Superperiod)
System
Polarity Period
Polarity Cilron
Episode
Period
(Subsystem) Chronozone
(Subperiod)
"
(Subperiod)
c
Series
Epoch
� Phase
Epoch
Stage
Polarity
Age
Polarity
0 Span
Age
!
(Substage)
Subchronozone
(Subage)
Subchron
(Subage)
Chronozone
Chron
Cline
'
Chron
I
Fundamental units are italicized,
Sou re•: Ferrusquia-Vi llafranca et at, 21.
intimate mixture of sandstone and shale beds that is distinctive because of the
mixed lithology. Boundaries of formaons, as with all lithostratigraphic units, are
placed at the position of lithic change. Boundaries between different formatio
may, therefore, occur both vertically and laterally. That is, a formation may be lo
cated above or below another formation or be positioned laterally adjacent to an
other formation where lateral facies changes occur. Illustrations of different tys
of formation boundaries are given in Figure 2, Appendix C. A formation must be of
sufficient areal extent and thickness to be mappable at the scale of mapping com
monly used the region where it occurs.
Formal lithostratigraphic units are assigned names that consist of a geo
graphic name combined with the appropriate rank (formation, member, etc.) or an
appropriate lithic term, such as limestone, or both. Formation names thus consist
of a geographic name followed by either the word Formation or a lithic designa
tion. For example, a particular formation might be called the Otter Point Forma
tion (geographic name only) or the Eureka Quartzite (geographic name plus lithic
designation). The names of members include a geographic name and the word
12.6 Correlation of thostratigraphic Units
421
Supergroup-a formal assemblage of related or superposed groups or of groups and formations.
Grouponsists of assemblages of formations, but groups need not be composed entirely of named formaons.
Formation-a body of rock, identified by lithic characteristics and stratigraphic position, that is prevailingly but
not necessarily tabular and is mappable at Earth's surface and traceable in the subsurface. Must be of sufficient
areal extent to be mappable at the scale of mapping commonly used in the region where it occurs. The funda
mental lithostratigraphic unit-formations are grouped to form higher-rank lithostratigraphic units and are
divided to form lower-rank
Member-the formal lithostratigraphic unit next in rank below a formation and always part of some formaon.
A formation need not be divided entirely into members. A member may extend laterally from one formation to
another.
Lens (or lentil)-a geographically restricted member that terminates on all sides within a formation.
To ngue-a wedge-shaped member that extends beyond the main boundary of a formation or that wedges or
pinches out within another formation.
Bed-distinctive subdivisions of a member; the smallest formal lithostratigraphic unit of sedimentary rock.
Members commonly are not divided entirely into beds.
Flow-the smallest formal lithostratigraphic unit of volcanic rock.
urce: North American Commission on Stratigraphic Nomenclatu, 1983, North American Stratigraphic Code: ArtL Asst'. Pt'trolewrl Geologits Bul!., v. 67.
Member, or the name may have an intervening lithic designation such as Eau
Claire Sandstone Member. A group name combines a geographic name with the
word Group,
as in Arbuckle Group. e first letters of all words used in formal
names of lithostratigraphic units are capitalized.
The North American Stratigraphic Code of 1983 recoizes that some lithos
tratigraphic bodies are bounded top and bottom by discontinuities (unconformi
ties or diastems). The code introduces the name allostratigraphic unit for such
mappable stratiform bodies of sedimentary rock that are defined on the basis of
bounding, laterally traceable discontinuities rather than on the basis of lithologic
change.
The Inteational Stratigraphic Guide (Salvador, 1994) refers to an uncon
formity-bounded unit as a synthem.
Informal names may be used for lithostratigraphic units when there is in
sufficient need, insufficient information, or an inappropriate basis to justify
designation as a formal unit (Hedberg, 1976). Informal names may be ap
plied to such units as oil sands, coal beds, mineralized zones, quarry beds,
and key or marker beds. Informal names are not capitalized. Examples of in
formally designated names are "shaley zone," "coal-bearing zone," "pebbly
beds," and "siliceous-shale member."
12.6 CORRELAT ION OF LITHOSTRATIGRAPHIC UNITS
Introduction
In
the splest sense, stratigraphic corlation is the demonstration of e
q
uivalency
of stratiaphic units. Correlation is a fundamental part of stratigraphy, and
much of the effort by stratigraphers that has gone into creating formal strati
graphic units has been aimed at finding practical and reliable methods of correlat
ing these units from one area to another. Without correlation, treatment of
stratigraphy on anything but a purely local level would be impossible.
TI1e concept of correlation goes back to the very roots of stratigraphy. The
fundamental principles of correlation have been presented in numerous early
422
Chapter 12 I Lithostratigraphy
textbooks on geology and stratigraphy; especially interesting reviews of these
general principles are given in Dunbar and Rodgers (1957), Weller (1960), and
Krumbein and Sloss (1963). The continued strong interest in correlation is demon
strated by more recent publication of several books and articles dealing with cor
relation, particularly statistical methods of correlation (e.g., Agterberg, 1990; Cubitt
and Reyment, 1982; Mann, 1981; Merriam, 1981).
The fundamental concepts of stratigraphic correlation were already firmly
established by the 1950s and 1960s. These basic principles are still important
today; however, the emergence of new concepts and more advanced analytical
tools have changed our perception of correlation to some degree, as well as
adding new methods for correlation. The development of the field of magne
tostratigraphy since the late 1950s (Chapter 13), for example, has provided an ex
tremely important new tool for global time-stratigraphic correlation on the basis
of magnetic polarity events. Also, rapid advances in computer technology and
availability and the application of computer-assisted statistical methods to strati
graphic problems have added a new quantitative dimension to the field of strati
graphic correlation. This section will attempt to bring out some of these new
developments, along with discussion of the more "classical" concepts of strati
graphic correlation.
Definition of Correlation
In spite of the fact that the concept of correlation goes back to the early history of
stratigraphy, disagreement has persisted over the exact meaning of the term. His
torically, two points of view have prevailed. One view rigidly restricts the mean
ing of correlation to demonstration of time equivalency, that is, to demonstration
that two bodies of rock were deposited during the same period of time (Dunbar
and Rodgers, 1957; Rodgers, 1959). From this point of view, establishing the equiv
alence of two lithostratigraphic units on the basis of lithologic similarity does not
constitute correlation. A broader interpretation of correlation allows that equiva
lency may be expressed in lithologic, paleontologic, or chronologie terms (Krum
bein and Sloss, 1963). In other words, two bodies of rock can be correlated as
belonging to the same lithostratigraphic or biostratigraphic unit even though
these units may be of different ages. It is clear, from a pragmatic point of view, at
most geologists today accept the broader view of correlation. Petroleum geolo
gists, for example, routinely correlate subsurface formations on the basis of liol
ogy of the formations, the specific "signatures" recorded within the formations by
instrumental well logs, or the reflection characteristics on seismic records. The
1983 North American Stratigraphic Code (Appendix C) recognizes three principal
kinds of correlation:
1. Lithocorrelation, which links units of similar lithology and stratigraphic
position
2. Biocorrelation, which expresses similarity of fossil content and biostrati
graphic position
3. Chronocorrelation, which expresses correspondence in age and chronostrati
graphic position
Even though our concern in this chapter is correlation on the basis of litholo
gy, it is important to clarify the relationship between chronocorlation and litho
correlation. Chronocorrelation can be established by any meod that allows
matching of strata by age equivalence. Correlation of units defined by lithology may
also yield chronostratigraphic correlation on a local scale, but when traced regional
ly many liostratigraphic units transgress time boundaries. Stratigraphic units
12.6 Correlation of Lithostratigraphic Units
423
deposited during major transgressions and regressions are notably time-transgres-
sive. Perhaps the most famous North American example of a time-transgressive
formaon is the Cambrian Ta peats Sandstone in the Grand Canyon region. This
sandstone is all early Cambrian in age at e west end of the Canyon and all mid-
dle Cambrian in age at the easte end (Fig. 12.15). Thus, the Tapeats Sandstone,
which can be traced continuously through the Canyon region, correlates from one
end of the Canyon to the other as a lithostratigraphic unit but not as a chronos-
tratigrapc unit. The important point stressed here is that the boundaries defed
by criteria used to establish me correlation of stratigraphic units may not be the
same as those defined by criteria used to establish lithologic correlaon. Because
of this fact, different methods of correlation (lithocorrelation, biocorrelation,
chronocorrelation) may yield different results when applied to the same strati-
graphic succession.
Anoer point that requires some clarification is the difference between
matchg of stratigraphic units and correlation of these units. Matching has been
defined simply as correspondence of serial data without regard to stratigraphic
uni (Schwarzacher, 1975; Shaw, 1982). For example, two rock units identified in
stratigraphic sections at different localities as having essentially identical lithology
(e.g., two black shales) can be matched on e basis of lithology; however, these
units may have neither time equivalence nor lithosatigraphic equivalence. Phys
ical tracing of the units between the localities may show that one unit lies sati
graphically above e other. Matching by lithologic characteristics this
parcular case does not constitute demonstration of equivalence. Shaw (1982)
states that the process of correlation is the demonstration of geometric relation
ships between rocks, fossils, or successions of geologic data for interpretation and
inclusion in facies models, paleontologic reconstructions, or structural models.
The object of correlation is to establish equivalency of stratigraphic units between
geographically separated parts of a geologic unit. Implicit in this definition is the
concept that correlation is made between stratigraphic units, that is, lithostrati
aphic units, biostratigraphic units, or chronostratigraphic units. The difference
between correlation and matching is illustrated in Figure 12.16. Figure 12.16A
100 mi
mi
]0
m
1 0
re 12.15
Change in age of the basal
Cambrian Ta peats Sand
stone across the Grand
Canyon region. [From
Clark, T. H., and C. W.
Stern, 1968, Geological
evol ution of North Ameri
ca, 2nd ed., Fig. 7.10, p.
126, reprinted by permis
sion of John Wiley & Sons,
Inc. Originally from E. D.
McKee, 1954, Cambrian
histo of the Grand
Canyon region, Part 1.
Stratigraphy and ecology
of the Grand Canyon
Cambrian: Carnegie lnst.
Washington Pub. 563,
Washington, D.C.}
424
Chapter 12 i Lithostratigraphy
12.16
Illustration of the difference
between matching and cor
relation. A. Apparent corre
lation achieved by matching
of similar-appearing strata.
B. Actual lithocorrelation.
[After Shaw, A. B., 1964,
Time
in stratigraphy, Fig.
30.1, p. 214, McGraw Hill,
New York.]
Section
X
A
Section
y
Seion
X
B
Section
y
shows two stratigraphic sections that appear to be perfectly matched. e actual
lithocorrelation is shown in Figure 12.16B. The tie lines in Figure 12.16A do not
constitute correlation because they do not encompass equivalent lithostratigraphic
units.
Correlation can be regarded as either direct (formal) or indirect (informal)
(Shaw, 1982). Direct correlation can be established physically and unequivocally.
Physical tracing of continuous stratigraphic units is the only unequivocal method
of showing correspondence of a lithic unit in one locality to that in another.
Indirect correlation can be established by numerous methods, such as visual com
parison of instrumental well logs, polarity reversal records, or fossil assemblages;
however, such comparisons have different degrees of reliability and can never be
totally unequivocal.
Lithocorrelation
We turn now to the methods used for correlating strata on the basis of lithology.
Methods of biocorrelation and chronocorrelation are discussed in appropriate sec
tions of Chapters 14 and 15.
Continuous Latal Tracin
g
of Lithostrati
g
raphic Units
Direct, continuous tracing of a lithostratigraphic unit from one locality to another
is the only correlation method that can establish the equivalence of such a unit
without doubt. This correlation method can be applied only where strata are con
tinuously or nearly continuously exposed. The most straightforward way of trac
ing lithostratigraphic units laterally is by walking out the beds. A geologist who
traces a stratigraphic unit continuously from one locality to another by walking
along the top of a particular bed can be quite confident that correlation has been
established. Thus, the application of field boots and a bit of physical effort yields
the satisfaction of achieving a virtually unequivocal correlation. Another useful,
but somewhat more equivocal, method of tracing stratigraphic units laterally is to
follow the beds on aerial photographs. In areas where surface exposures are abun
dant and visibility is little hampered by soil or vegetation cover, lateral tracing of
thick, distinctive stratigraphic units on aerial photographs can be done rapidly
and effectively. This method is limited to tracing of distinctive beds that are thick
enough to show up on photographs of a suitable scale (e.g., Fig. 12.17).
Although physical tracing of beds is the only unequivocal method of corre
lation, it is not without limitations. The most serious of these is the fact that in
12.6 Correlation of Lithostratigraph'lc U1lt� 25
Figure 12.17
Excellent exposure of nearly flat-lying Permi
an-Pennsylvanian strata, allowing continuous
tracing of beds for considerable distances,
Goosenecks of the San juan River, Utah.
most areas, geologists cannot trace beds continuously for more than a very short
distance before encountering areas covered by soil or vegetation, structural com
plications (faults), or erosional terminations, as across a large valley. In fact, it is
often impossible to trace a given stratigraphic unit more than a few hLmdred me
ters before the u is lost for one of these reaso{ls. An additional problem may
arise if the beds being traced pinch out or merge with others laterally, a very com
mon occurrence in nonmarine strata. In such a case, tracing of an individual bed
or bedding plane will be impossible. Therefore, in practice, geologists commonly
trace a gross lithostratigraphic it (e.g., a member or a formation) consisting of
beds of like character, rather than trying to trace individual beds.
Lithologic Similarity and Stratigraphic Position
Lithologic Similarity. Geologists working in areas where direct lateral tracing of
beds is not possible must depend for correlation of lithostratigraphic units upon
methods that match strata from one area to another on the basis of lithologic simi
larity and stratigraphic position. Because matching of strata does not necessarily
indicate correlation, correlation by lithologic similarity has varying degrees of re
liability. The success of such correlation depends upon the distinctiveness of the
lithologic attributes used for correlation, the nature of the stratigraphic succession,
and the presence or absence of lithologic changes from one area to another. Facies
changes that take place lithostratigraphic units between two areas under study
obviously complicate the problem of lithologic correlation.
Lithologic similarity can be established on the basis of a variety of rock prop
erties. These properties clude gross lithology (e.g., sandstone, shale, or lime
stone), color, heavy mineral assemblages or other distinctive mineral assemblages,
primary sedimentary structures such as bedding and cross-lamination, and even
thickness
and weathering characteristics. The greater the number of properties
that can Q� used to e�tablish a match between strata, the stronger the likelihood of
a reliable mtch. A single property such as color or thkkness may change laterally
within a given stratigraphie unit, but a suHe of distinctive lithologic properties is
less likdy to change. I cauHon again that matching of strata on the basis of litholo
gy is not a guarantee that correlation has been estab�ished. Strata with very simi
lar lithologie characteristic can orJU in similar depositional environments widely
separated fe or space. It may be quite possble, for example, to obtain an excel
len t lithologic match between � dea, well-sorted, cross-bedded, eoli sandstone
unit of Triassic age and a lithologicaHy vjrtually identicat sandstone of Jurassic age,
426
Chapter 12 I Lithostratigraphy
Figure 12.18
yet
these sandstones do not correlate as either lithostratigraphic or chronostrati
graphic units. Corre lation on the basis of lithologic identity is particularly difficult
between
cyclic successions, such as the Pennsylvanian cyclothems of the United
States midcontinent region. Very similar successions of units can be repeated over
and over in the strtigraphic section owg to the fact that similar environmental
conditions can reappear in a region time after time during repeated transgressive
regressive cycles of depositi.
The most reliable lithologic correlations are made when it is possible to
match not just one or two
distinctive beds or rk types but a succession of sever
al distinctive units. For eample, the Tr iassic and Jurassic formations of he Col
orado Plateau in the western United States consist of a highly distinctive
succession of largely nonmarine red to gen siltstone and mudstone units (the
Moenkopi, Chinte, Kayenta, Summerville, and Morison Formations) interstrati
fied
with red to white, cross-bedded
eolian (?) sandstones (the Wingate, Navajo,
and Entrada Formations). Some of these formations are shown in Figure 12.18.
This succession of formations is so dstinctive that it can be recognized and corre
lated lithologically with considerable confidence Ver wide areas on he Colorado
Plateau (Figure 12.19). In some cases, it may be possible to improve
the reliability
of correlation by applying statistical and computer-assisted techniques. These
quantitative methods can provide a probability measure of whethr a prod
correlation is valid or invalid (Agterberg, 1990).
Stratigraphic Position in a Succession. The preceding iustration points out the im
portance of position in a stratigraph succession when correlating units by litho
logic identity. Several of the Colorado Plateau formations are lithologically
similar, but because they occur in a succession of strata distinctive enough to be
correlated from one area to another, individual formations can be correlated also
by their position in this succession. Another way in which position in a strati
graphic succession is important has to do with estabshing correlation of strata by
relation to some highly distinctive and easily correlated unit or unrts. Such dis
tinctive beds serve as control units for correlation of other strata above and below.
For example, a thin, ash-fall unit or bentonite bed may be present and easily rec
ognized throughout a particular region. If it is the only such bed in the strati
graphic succession in the region, and thus carmot be consed with any other bed,
it can serve as a key bed, or marker bed, to which other strata are related. Strata
immediately above or below this control unit can be correlated with a reasonable
We ll-exposed, distinctive Triassic Formations
(Wingate, Chinle, Moenkopi) that can be
correlated over wide areas in Utah and
Colorado.