Boggs S. Principles of Sedimentology and Stratigraphy
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PARTV
Stratigraphy and
Basin Analysis
Well-stratified Paleozoic
sedimentary rocks
exposed in the North Rim
of the Grand Canyon,
Arizona.
397
398
E
mphasis in preceding parts of this book has been on sedimentary processes,
the environments in which these processes take place, and the properties of
sedimentary
rocks generated in these environments. In this final
part, we
focus on a different aspect of sedimentary rocks. Our concern here is not so much
sedimentary processes and detailed rock properties but rather the larger scale ver
tical and lateral relationships between units of sedimentary rock that are defined
on the basis of lithologic or physical properties, paleontological characteristics,
geophysical properties, age relationships, and geographic position and distribu
tion. It is the study of these characteristics of layered rocks that encompasses the
discipline of stratigraphy. Understanding the principles and terminology of
stratigraphy is essential to geologic study of sedimentary rocks because stratigra
phy provides the framework within which sediments can be studied systematical
ly. It allows the geologist to bring together the details of sediment composition,
texture, structure, and other features into an environmental and temporal synthe
sis from which we can interpret the broader aspects of Earth history.
Prior to the 1960s, the discipline of stratigraphy was conceed particularly
with stratigraphic nomenclature; the more classical concepts of lithostratigraphic,
biostratigraphic, and chronostratigraphic successions in given areas; and correlation
of
these successions between areas. Lithostratigraphy deals with the lithology or
physical properties of strata and their organization into units on e basis of litho
logic character. Biostratiaphy is the study of rock units on the basis of the fossils
they contain. Chronostratigraphy ( chrono = time) deals with the ages of strata
and their time relations. These established principles are still the backbone of
stratigraphy; however, today's students must go beyond these basic principles.
They must also acquire a thorough understanding of depositional systems and be
able to apply stratigraphic and sedimentological principles to interptation of stra
ta within the context of global plate tectonics. is means, among other thgs, be
coming familiar with comparatively new branches of stratigraphy that have
developed since the early 1960s as new concepts and methods of studying sedimen
tary rocks and other rocks by remote-sensing techniques have unfolded. For exam
ple, the concept of depositional sequences, which are packages of strata bounded by
unconformities, has gained particular promence since the late 1970s. This concept
has now become so important that we refer to study of sequences as sequence
stratigraphy. Two new oshoots of stratigraphy that have made particularly impor
tant contributions to our understanding of the physical stratigraphic relationships,
ages, and environmental significance of subsurface strata and oceanic sediments are
seismic stratigraphy, which is the study of stratigraphic and depositional facies as
terpreted from seismic data, and magnetostratigraphy, which deals with
strati
graphic relationships on the basis of magnetic properties of sedimentary rocks and
layered volcanic rocks. We also recognize sub-branches of stratigraphy such as event
stratigraphy (correlation of sedimentary units on the basis of marker beds or event
horizons), cyclostratigraphy (study of short-period, high-frequency, sedimentary
cycles the stratigraphic record, particularly by use of oxygen-isotope data), and
chemostratigraphy (correlation on the basis of stable isotopes such as oxygen, car
bon, and strontium). Application of these new concepts and techniques makes pos
sible subdivision of stratigraphic successions to relatively small units, e.g.,
< 1000-3000 years for Quaternary strata and 225,000-2,000,000 years for Triassic
strata (Kidd and Hailwood, 1993). Such fe-scale stratigraphic resolution is now
commonly referred to as high-resolution stratigraphy.
We explore all of these stratigraphic concepts in the next few chapters. In the
final chapter of the book, we take a look at basin analysis. Basin analysis is a kind
of umbrella under which we integrate and apply all of the sedimentologic and
stratigraphic principles presented in this book. Together with fundamental tecton
ic concepts, basin analysis allows us to develop an understanding of the rocks that
fill sedimentary basins in order to interpret their geologic history and evaluate
their economic significance .
Lithostratigraphy
12.1 INTRODUCTION
P
erhaps the most fundamental kind of stratigraphic study is recognition, sub
division, and correlation (establishing equivalency) of sedimentary rocks on
the basis of their lithology, that is, lithostratigraphy. The term litholo is
used by geologists in two different but related ways. Strictly speaking, it refers to
study and description of the physical character of rocks, particularly in hand spec
imens and outcrops (Bates and Jackson, 198 0). It is used also to refer to these phys
ical characteristics: rock type, color, mineral composition, and grain size are all
lithologic characteristics. For example, we may refer to the lithology of a particu
lar stratigraphic unit as sandstone, shale, limestone, and so forth. Thus, lithostrati
graphic units are rock units defined or delineated on the basis of their physical
properties, and lithostratigraphy deals with the study of the stratigraphic relation
ships among strata that can be identified on the basis of lithology.
In this chapter, we begin study of stratigraphic principles by briefly dis
cussg the nature of lithostratigraphic units, followed by exploration of the vari
ous types of contacts that separate these units. We then consider the important
concepts of sedimentary facies and depositional sequences. The essentials of
st ratigraphic nomenclature and classification as they apply to lithostratigraphic
its are discussed next, including examination of the North American Code of
Stratigraphic Nomenclature. Finally, correlation of lithostratigraphic units is ex
pl ained and the various methods of correlation described.
12.2 TYPES OF LITHOSTRATIGRAPHIC UNITS
Lithostratigraphic units are bodies of sedimentary, extrusive igneous, metasedi
mentary, or metavolcanic rock distinguished on the basis of lithologic characteris
tics. A lithostratigraphic unit generally conforms to the law of superposition,
which states that in any succession of strata, not disturbed or overtued since de
pition, younger rocks lie above older rocks. Lithostratigraphic units are also com
mly stratified and tabular in form. They are recognized and defined on the basis
of observable rock characteristics. Boundaries between different units may be
placed at clearly identifiable or distinguished contacts or may be drawn arbitrarily
399
400
Chapter 12 I Lithostratigraphy
within a zone of gradation. Definition of lithostratigraphic units is based on a
stratotype (a designated type unit), or type section, consisting of readily accessible
rocks, where possible, in natural outcrops, excavations, mines, or bore holes.
Lithostratigraphic units are defined strictly on the basis of lithic criteria as deter
mined by descriptions of actual rock materials. They carry no connotation of age.
They cannot be defined on the basis of paleontologic criteria, and they are de
pendent of time concepts. They may be established in subsurface sections as well
as in rock units exposed at the surface, but they must be established on the basis of
lithic characteristics and not on geophysical properties or other criteria. Geophys
ical criteria, described in Chapter 13, may be used to aid in fixing boundaries of
subsurface lithostratigraphic units, but the units cannot defined exclusively on
the basis of remotely sensed physical properties.
Wheeler and Mallory (1956) introduced the term lithosome to refer to masses
of rock of essentially uniform character and having intertonguing relationships wi
adjacent masses of different liology. Thus, we speak of shale lithosomes, limestone
lithosomes, sand-shale lithosomes, and so forth. Krumbein and Sloss (1963) clarify
the meaning of lithosome by asking readers to imane the body of rock that would
emerge if it was possible to preserve a single rock type, such as sandstone, and d
solve
away all other rock types. The resulting sandstone body would thus appear as
a roughly tabular mass with intricately shaped boundaries. ese irregular bound
aries would represent its surfaces of contact with erosion surfaces and with other
rock masses of diering constitution above, below, and to the sides. Lithosomes
have no specified size limits and may range in gross shape from thin, sheetlike or
blanketlike units to thick prisms, or narrow, elongated shoestrings.
Of course, stratigraphic units of a single lithology rarely exist as isolated bod
ies. They are commonly in contact with oer rock bodies of different lithology.
important part of lithostratigraphy is identifyg and understanding the nature of
contacts between vertically supeosed or laterally adjacent bodies. Another impor
tant aspect is the identification of single lithosomes, groups of lithosomes, or subdi
visions of lithosomes that are so distinctive that they form lithostratigraphic units
that can be distinguished from other units that may lie above, below, or adjacent.
The fundamental lithostratigraphic unit of this type is the formation. A
formation is a lithologically distinctive stratigraphic unit that is large enough in
scale to be mappable at the surface or traceable in the subsurface. It may encom
pass a single lithosome, or part of an intertonguing lithosome, and thus consist of
a single lithology. Alternatively, a formation can be composed of two or mo
lithosomes and thus may include rocks of different lithology. Some formations
may
divided into smaller stratigraphic units called members, which, in tum,
may
be divided into smaller distinctive units called beds. Beds are the smallest
formal lithostratigraphic units. Formations having some kind of stratigraphic
unity can be combined to form groups, and groups can be combined to form
supergroups. All formal lithostratigraphic units are given names that are derived
from some geographic feature in the area where they are studied.
Subdivision of thick units of strata into smaller lithostratigraphic units such
as formations is essential for tracing and correlating strata bo in outcrop and in
the subsurface. We will come back to discussion of these formal stratigraphic units
near the end of this chapter. First, however, we examine the nature of contacts be
tween stratigraphic units and the lateral and vertical facies relationships that char
acterize strata.
12.3 STRATIGRAPHIC RELATIONS
Different lithologic units are separated from each other by contacts, which are pla
nar or irregular surfaces between different types of rocks. Vertically superposed
12.3 Stratigraphic Relations
401
strata are said to be either conformable or unconformable depending upon conti-
nuity of deposition. Conformable strata are characterized by unbroken deposi-
tional assemblages, generally deposited in parallel order, in which layers are
formed one above the other by more or less uninterrupted deposition. The surface
that separates conformable strata is a confoity, that is, a surface that separates
younger strata from older rocks but along which there is no physical evidence of
nondeposition. A conformable contact indicates that no siificant break or hiatus
in deposition has occurred. A hiatus a break or interruption in the continuity of
the geologic record. It represents periods of geolog time (short or long) for which
there are no sediments or strata.
Contacts between strata that do not succeed underlying rocks in immediate
order
of age, or that do not fit together with them as part of a continuous whole,
are called unconformities. Thus, an unconformity is a surface of erosion or
nondeposition, separating younger strata from older rocks, that represents a
significant hiatus. Unconformities indicate a lack of continuity in deposition and
rrespond to periods of nondeposition, weathering, or erosion, either subaerial
or subaqueous, prior to deposition of younger beds. Unconformities thus repre
sent a substantial break in the geologic record that may correspond to periods of
erosion or nondeposition lasting millions or even hundreds of millions of years.
Contacts are also present between laterally adjacent lithostratigraphic units.
These contacts are formed between rock units of equivalent age that developed
different lithologies owing to different conditions in the depositional environ
ment. Excluded from discussion here are contacts between laterally adjacent bod
ies that arise from postdepositional faulting. Contacts between laterally adjacent
bodies may be gradational, where one rock type changes gradually into another,
or they may be intertonguing, that is, pinching or wedging out within another for
mation (see Figures 12.1-12.4).
Conta between Conformable Strata
Contacts between conformable strata may be either abrupt or gradational. Abrupt
contacts directly separate beds of distinctly different lithology (Figs. 12.1, 12.2).
Most abrupt contacts coincide with primary depositional bedding planes that
formed as a result of changes local depositional conditions, as discussed in
Chapter 4; thus, the contacts are commonly very sharp. In general, bedding planes
represent minor interruptions in depositional conditions, Such minor deposition
al
breaks, involving only short hiatuses in sedimentation with little or no erosion
before deposition is resumed, are called diastems. Abrupt contacts may be caused
also by postdepositional chemical alteration of beds, producing changes in color
owing to oxidation or reduction of iron-bearing minerals, changes in grain size
owing to recrystallization or dolomitization, or changes in resistance to weather
g owing to cementation by silica or carbonate minerals.
Conformable contacts are said to be gradational if the change from one
lithology to another is less marked than abrupt contacts, reflecting gradual change
in depositional conditions with time (Fig. 12.1 ). Gradational contacts may be of ei
ther the progressive gradual type or the intercalated type. Progressive gradual
contacts occur where one lithology grades into another by progressive, more
less uniform changes in ain size, mineral composition, or other physical charac
teristics. Examples include sandstone units that become progressively finer
grained upward until they change to mudstones, or quartz-rich sandstones that
become progressively enriched upward in lithic fragments until they change to
lithic arenites. Intercalated contacts are gradational contacts that occur because of
an increasing number of thin interbeds of another lithology that appear upward in
the section (Figs. 12.1, 12.3).
402
Chapter 12 I Lithostratigraphy
Figure 12.1
Schematic representation of
the principal kinds of vertical
and lateral contacts between
lithologic units. Vertical con
tacts include abrupt, pro
gressive gradual, and
intercalated. Lithologic units
may be laterally continuous
or they may change laterally
by pinchout, intertonguing,
or lateral gradation. See
Figures 12.2-1 2.4 for exam
ples of actual contacts.
Figure 12.2
Abrupt contact (arrow) between massive
bedded sandstone below and fine-grained
conglomerate above. Miocene deposits near
Blacklock Point, Southwest Oregon Coast.
Contacts between Laterally Adjacent lithosomes
In addition to vertical boundaries deeated by contacts, stratigraphic ts also
have fite lateral boundaries. They do not extend indefinitely laterally but must
eventually terminate, either abruptly as a result of erosion or more gradually by
change to a different lithology. Some sedimentary w1its are laterally discontuous
in the sense
that lateral changes lithology may occur within single outcrops or at
least within a local area. Many nonmarine deposits, such as alluvial-fan deposits,
exhibit such lateral discontuity. Lateral changes may be accompanied by pogres
sive tg of units to extction-pinch-outs (Figs. 12.1, 12.4); lateral splitting of
a lithologic unit into many thin Lmits that pinch-out independently-intertonguing;
12.3 Stratigraphic Relations
403
Figure 12.3
Gradation from sandstone below through a
zone of intercalated thin conglomerates and
sandstones (light) to conglomerate at the
top of the section. Miocene deposi near
Blacklock Point. Southwest Oregon Coast.
Figure 12.4
Pinch-out. Note how the sandstone bed
(light)
pinches out abruptly to the right and
disappears into the conglomerate. Creta
ceous (?) deposits, near Ashland, southern
Oregon.
or progressive lateral gradation, similar to progressive vertical gradation. We
speak of sedimentary units that do not terminate ithin single outcrops or local
areas as being laterally continuous (Fig. 12.1). If traced far enough, of course such
units must also eventually terminate. Many marine strata such as shelf sandstones,
limestones, and central-basin evaporites exhibit lateral continuity.
Unconformable Contacts
As mentioned, contacts between strata that do not succeed underlying rocks verti
cally
in immediate order of age are called unconformities. Four types of uncon
formable contacts (unconformities) are recoed: (1) angular unconformity, (2)
disconformity, (3) paraconformity, and (4) nonconformity (Fig. 12.5). Unconformi
ts are oized by an angular relationship between strata (angular unconfor
mities} the presence of a marked erosional surface separating these strata (e.g.,
dionformities), marked disparity in age of rocks above and below the unconfor
ty (e.g., paraconformities) , and the nature of the rocks underlying the surface of
404
Chapter 12 I Lithostratigraphy
Figure 12.5
Schematic representation of four basic kinds of unconfor
mities. Arrows indicate the unconformity surface. For the
purpose of illustration, the youngest strata below the un
conformity surface in each diagram is shown to have a
(hypothetical) age of 1 00 million years and the oldest
strata above the unconformity surface an age of 50 mil
lion years, indicating a hiatus in each case of 50 million
years. [Modified from Dunbar, C. 0., and ]. Rodgers,
1957, Principles of stratigraphy: john Wiley & Sons, New
Yo rk, Fig. 57, p. 117, reprinted by permission.]
A. Angular Unconformity
B. Disconformity
C. Paracontormity
D. Nonconformity
unconformity. The rst three types of unconformities occur between bodies
of
sedimentary rock. Nonconformities occur between sedimentary rock and meta
morphic or igneous rock.
An
lar Unconformi
An angular unconformity is a type of unconformity in which younger sediments
rest upon the eroded surface of tilted or folded older rocks; that is, the older rocks
dip at a dierent, commonly steeper, angle than
do the younger rocks (Fig. 12.5A).
The unconformity surface may be essentially planar or markedly irregular. gular
12.3 Stratigraphic Relations
405
Figure 12.6
Major angular unconformity
(arrow) between tilted sedimentary
rocks of the Grand Canyon Group
(Precambrian) and overlying, nearly
horizontal, Tapeats Sandstone
(Cambrian). Younger Paleozoic stra
ta are exposed above the Tapeats.
View from Desert Overlook, South
east Rim, Grand Canyon, Arizona.
unconformities may be confined to limited geographic areas (local tmconformities)
or may extend for tens or even hundreds of kilometers (regional unconformities).
Some angular unconformities are visible in a single outcrop (Fig. 12.6). By con
trast, regional unconformities between stratigraphic units of very low dip may not
be apparent in a single outcrop and may require detailed mapping over a large
aa before they can be identified.
Disconformi
An unconformity surface above and below which the bedding planes are essen
tially parallel and in which the contact between younger and older beds is
marked by a visible, irregular or uneven erosional surface is a disconformity (Fig.
12.5B). Disconformities are most easily recognized by this erosional surface,
which may be channeled and which may have relief ranging to tens of meters.
Disconformity surfaces, as well as angular unconformity surfaces, may be
marked also by "fossil" soil zones (paleosols) or may include lag-gravel deposits
at lie immediately above the unconformable surface and that contain pebbles
of the same lithology as the lithology of the underlying unit. Disconformities are
psumed to form as a result of a significant period of erosion throughout which
older
rocks remained essentially horizontal during nearly vertical uplift and sub
sequent dowt
'
warping.
Pa raconfonni
A paraconformity is an obscure unconformity characterized by beds above and
below the unconformity contact that are parallel and in which no erosional
surface
or other physical evidence of unconformity is discernible. The uncon
formity contact may even appear to be a simple bedding plane (Fig. 12.5C).
Paraconformities are not easily recognized and must be identified on the basis of
a
gap in the rock record (because of nondeposition or erosion) as determined
from paleontologic evidence such as absence of fa unal zones or abrupt faunal
changes. In other words, rocks of a particular age are missing, as determined by
fossils or other evidence.
406
Chapter 12 I Lithostratigraphy
Nonconfoity
An unconformity developed between sedimentary rock and older igneous or mas
sive metamorphic rock that has been exposed to erosion prior to being covered by
sediments is a nonconformity (Fig. 12.50). Nonconformity surfaces probably rep
resent an extended period of erosion.
Significance of Unconfoities
The presence of unconformities has considerable significance in sedimentological
studies. Many stratigraphic successions are bounded by unconformities, dicat
g that these successions are incomplete records of past sedimentation. Not only
do unconformities show that some part of the stratigraphic record is missing, but
they also indicate that an important geologic event took place during the time pe
riod (hiatus) represented by the unconformity-an episode of uplift and erosion
or, likely, an extended period of nondeposition.
12.4 VERTICAL AND LATERAL SUCCESSIONS OF STRATA
Nature of Vertical Successions
As discussed, conformities and unconformities divide sedimentary rocks into ver
tical successions of beds, each characterized by a particular lithologic aspect. Dif
ferent types of beds can succeed each other vertically in a great variety of ways,
and distinctions can be drawn between rock units characterized by lithologic uni
formity, lithologic heterogeneity, and cyclic successions. Rock units that have
complete lithologic uniformity are rare, although many beds may display a high de
gree of uniformity in color, grain size, composition, or resistance to weathering.
Beds that are most likely to be uniform are fe-grained sediments deposited
slowly under essentially uniform conditions in deeper water, or coarser sediments
that have been deposited rapidly by some type of mass sediment transport mech
anism such as grain flow. By contrast, heterogeneous bodies of sedimentary strata
are characterized by internal variations or irregularities in properties. Heteroge
neous units may include strata such as extremely poorly sorted debris-flow de
posits, as well as thick units broken inteally by thinner beds characterized by
differences in grain size or bedding features.
Cyclic Successions
Many stratigraphic successions display repetitions of strata that reflect a succes
sion of related depositional processes and conditions that are repeated in the
same order. Such repetitious events are referred to as cyclic sedimentation or
rhythmic sedimentation. Cyclic sedimentation leads to the formation of vertic
successions of sedimentary strata that display repetitive orderly arrangement of
different kinds of sediments. The term cyclic sediment has been used for a wide
variety of repetitious strata including such small-scale features as presumed an
nually
deposited varves in glacial lakes as well as large-scale sediment cycles
caused by long-period, recurring migration of depositional environments. Other
common examples of repetitive deposits clude rhythmically bedded turbidites,
laminated evaporite deposits, limestone-shale rhythmic successions, coal cy
clothems (repeated cycles involving coal deposition), black shale deposits, and
chert deposits. Cyclic successions occur on all continents in essenally every
stratigraphic system. They are produced by processes that range in geograpc
scope and duration from very local, short-term events-such as seasonal climatic
changes that generate varves-to global changes in sea level that may involve en
tire geologic periods.