Boggs S. Principles of Sedimentology and Stratigraphy
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a AUTOCYCLIC MECHANISMS
SWITCHING DELTA LOBE
DUE REPEAT ED PRO
GRADATION AND RETREAT
7� -------
CONSTANT
SEA LEVEL
12.4 Ve rtical and Lateral Successions of Strata
407
STORM WAVES
MUD SLUMP
b ALLOCYCLIC MECHANISMS
CLIMATIC CHANGE AND TECTONICS
GLOBAL
SEA LEVEL
FLUCTUATIONS
�_
IN SOURCE AREA
VA RIATIONS IN
SEDIMENT INPUT
INTO DIFFERENT BASINS
CHANGING
CYCLIC
LAKE SEDIMENTS
�
VARIATIONS IN
ORGANIC
PRODUCTIVITY
WAT ER CHEMISTRY
Figure 12.7
Autocyclic (a) and allocyclic (b) mechanisms that
generate rhythmic and cyclic successions. [From Ein
sele, G., W. Rieken, and A. Seilacher, 1991, Cycles
and events in stratigraphy-Basic concepts and
terms, in Einsele, G., W. Rieken, and A. Seilacher
(eds.), Cycles and events in stratigraphy. Fig. 4, p. 9,
reproduced by permission of Springer-Verlag, Berlin.]
the basis of the mechanisms that form cyclic deposits, two kinds of cyclic
successions are recognized: autocyclic and allocyclic (Fig. 12.7). Autocyclic suc
cessions are controlled by processes that take place within the basin itself, and
their beds show only limited stratigraphic continuity. Examples include nonperi
odic storm beds and turbidites. Allocyclic successions are caused mainly by vari
aons external to the depositional basin. Fundamentally, these variations are
caused by changes in climate and tectonic movements. For example, climate can
influence sea level, waxing and waning of continental glaciers, and sedimentary
processes such as deposition of evaporites. Tectonic movements affect sea level
d water depth. Allocyclic successions may extend over long distances and per
haps even from one basin to another (Einsele, Rieken, and Seilacher, 1991a).
Several scales of allocyclic successions have been postulated, all of which are
lated in one way or another to changes in climate and sea level and tectonism
(Fig. 12.8). Ve ry long term eustatic cycles (rise and fall of global sea level) that ap
pear to take 200-500 million years (Ma) were referred to by Vail, Mitchum, and
Thompson (1977b) as first-order cycles (Table 12.1}. [Note: Miall (1997, p. 50) rec
oends against using the hierarchal classification of cycles as first-order, second
order, etc.; however, this practice is now thoroughly enenched in the literature and
is widely followed.] cycles are too large to see in normal outcrops and must be
deduced from study of large sets of field exposures or from subsurface information
408
Chapter 12 I Lithostratigraphy
Fire 12.8
Schematic representation of the
scale of cyclic sedimentation in the
stratigraphic record. Ma = million
years, a years; parasequences
are discussed in Chapter 1 3. [After
Einsele, G., W. Rieken, and A.
Seilacher, 1991 , Cycles and events
in stratigraphy-Basic concepts
and terms, in Einsele, G., W. Rick
en, and A. Seilacher (eds.), Cycles
and events in stratigraphy,
Springer-Verlag, Berlin, Fig. 2, p. 3,
reproduced by permission.]
VARVE-5CALE
MINATIONS
NON-ANNUAL
MINATIONS
(SMALL EVENTS)
ANNUAL
VARVES
(E.G. SALTS.
CARBONATES)
IN HAND SPECIMENS
AND UNDER THE
MICROSCOPE
BED-SCALE
RHYTHMS AND
CYCLES
NORMAL FIELD·SCALE
SEDIMENTARY CYCLES
(4th and 3rd order sequ.}
MILANKOVITCH
PARASEQUENCES
RHYTHMS AND BUNDLES.
PLEISTOCENE MARINE
CYCLES
SHALLOW-AND
DEEP· MARINE
TRANSGRESSION
REGRESSION
CYCLES (PRE·
PLEISTOCENE)
BEDDING PHENOMENA
� AND CYCLIC SEQUENCES IN
NORMAL FIELD EXPOSURES
MACRO-SCALE
CYCUC SEQUENCES
( and 1st order)
SUPERCYCLE
to so
1,000 m
SUPER- AND MEGA·
CYCLES MAY BE CAUSED
BY REGIONAL TECTONICS
IN SETS OF LARGE
Fl ELD EXPOSURES,
LOGS OF DEEP WELLS
aap� �y d eir ptated c
s
Ty pe
First-order
Second-order
Third-order
Fourth-order
Fiith-order
Other terms
Super cycle (Vail, Mitchum, and
Thompson, 197); sequence
(Sloss, 1963)
Mesothem (Ramsbottom, 1979);
mega-cyclothem (Heckel, 1986)
Cyclothem (Wanless and Weller,
1932); major cycle (Heckel, 1986)
Minor cycle (Heckel, 1986)
Duration, m.y.
200-400
10-100
1-10
0.2-0.5
0.01-0.2
Probable cause
Major eustatic cycles caused by formation
and breakup of supercontinents
Eustatic cycles induced by volume changes
in global mid-ocean spreading ridge system
Possibly produced by ridge changes and
continental ice growth and decay
Milankovitch glacioeustatic cycles, astronomical
forcing
Milankovitch glacioeustatic cycles, astronomical
forcing
Source; Va il, P R, R. M. Mitchum, jr, and S. Thomp,on, III (l977b); Miall (1, p. 47).
derived from deep wells or seismic data (Chapter 13). They are not identified di
rectly from lithologic data but rather by application of the techniques of sea-level
analysis described in Chapter 13. They are postulated to develop by assembly of
supercontinents (e.g., Pangea) by seafloor spreading and their subsequent
breakup and dispersal. Sea level falls during times when supercontinents are as
sembled and rises during times of rapid seaoor spreading when continents are
rifting apart. Two first-order, or supercontinent, cycles appear to have taken place
during Phanerozoic time (Fig. 12.9).
12.4 Vertical and lateral Successions of Strata
Climate
0
'
__
_
-
�
jas
o
:
"'
100
Cretaceous
Zuni
0
"'
:
"'
iO
Jurassic
"'
1 § 200
Triassic
��
I� 300
Absaroka
"'
Permian
0
u
·
0
0
Kaskaskia
"'
"'
400
0
Tippecanoe
:
500
Sauk
'
o
:
gure 12.9
Illustration of first-order and second-order global sea-level cycles. First-order cycles reflect
variations
in production of new oceanic crust (in
km
2
/yr) related to formation and
breakup of continents, which cause major, long-term changes in sea level. Second-order
cles are related to changes in volume of oceanic spreading centers. [Modified from Plint
et al., 1992, Control of sea-level changes, in Walker, R. G., and N. P. james (eds.), Facies
models-Response to sea level change: Geol. Assoc. Canada, Fig. 3, p. 18, reproduced by
permission. Climate states are after Fischer (1981); the second-order cycles (Tejas, Zuni,
etc. are named for Sloss' (1 963) sequences.]
Fischer (1984) refers to climate states on Earth during cool periods as icehouse
states and those during warm periods, when greenhouse gases such as C02 were
abundant, as greenhouse states. Fischer (1984) and Frakes, Francis, and Syktus
(1992) suggest that three icehouse states and two greenhouse states existed on
Earth during late Precambrian and Phanerozoic time (Fig. 12.10). Note from
Figure 12.10 that uctuations in ancient sea levels tend to track variations in an
cient
temperatures. Temperatures also affect moisture conditions. The wettest pe
riods seem to have been during cool periods of the Carboniferous to early Permian
and the early Tertiary; the driest conditions apparently occurred during the Tr ias
sic-Jurassic and Devonian warm periods. Evaluating the exact causes of shif in
Earth's climate from warm to cool periods and back again is fraught with difficul
ty; however, these changes are related in a complex way to changes in Earth's or
bital parameters, changes in sea levels, increase in carbon dioxide levels in the
atmosphere owing to volcanic emissions, and removal of carbon dioxide from the
atmosphere by weathering of silicate rocks. For example, during times of rapid
seafloor spreading and accretion of crustal rocks (Chapter 13), sea level rises
and volcanic emissions of C02 increases dramatically (Fig. 12.9). As mentioned,
an increase in C02 levels brings on a greenhouse state. Thus, there appears to be a
reasonably good, but not perfect, correlation between higher sea levels and
warmer climates (Fig. 12.10).
Global
Tectonic
Cycle
Dispersal of
Continents
Start of riing
Final coalescence
of
Pangea
Breakup of late
Progerozoic
supercontinent
409
410
Chapter 12 I Lithostratigraphy
Figure 12.10
Age
(Ma)
�
low high
Estimated mean global temperature curve for Phanerozoic time and
corresponding climate modes (Frakes et al., 1992, p.194), sea-level
curve (Vail et al., 1977b), greenhouse-icehouse climate states (Fisch
er, 1984), and times of major glaciation (Eyles, 1993). Ages from
GSA 1999 Geologic Time Scale (see Fig 15.3).
Second-order cycles have durations on the order of tens to hundreds of mil
lion years. Like first-order cycles, these macro-scale cycles are too large to study in
normal field exposures. The major cause of these cycles is attributed to volume
changes in oceanic spreading centers; that is, the volume of spreading ridges in
creases (owg to increased heating) and sea level rises during rapid spreading,
and the volume decreases, with concomitant sea-level fall, during slow spreading.
Broad, regional cycles of basement elevation (crustal flexing) that occur in respon
to convergent, divergent, and transcurrent plate motions may also contribute to
second-order cycles (Miall, 1997, p. 53). Sloss (1963) defined six major stratigraphic
cycles on the North American continent which he called sequences and to which
he gave Indian names. These sequences, rangg in age from Ter tiary to late P
cambrian, are second-order cycles that can be recognized and correlated with simi
lar cycles on oer contents (e.g., Sloss, 1979). These cycles, generated as a result
of global sea-level change, represent shorter-duration pulses superimposed on
longer-duration first-order cycles, also caused by global sea-level change.
Third-order cycles have episodicities on the order of one to ten million
years. These cycles can be cognized in normal field exposures, as well as in sub
surface well records and seismic reection profiles. They have been attributed to
fluctuations in eustatic sea level (see Chapter 13) owing to changes in spreading
ridges and/or continental ice growth and decay; however, their origin has not
been fully established and is still controversial. Also, it has not yet been definitely
proven that these cycles can be correlated on a global basis. If they are strictly
gional rather than global in scope, their origin might be related more to tectonic
mechanisms than to eustatic sea-level changes.
OBLIQUITY: the tilt of Eah's
axis changes in a 41 Ka cycle
12.4 Vertical and Lateral Successions of Strata
411
PRECESSION: wobble of Earth's
axis has a 19 to 23 Ka cycle
ECCENTRICITY: Earth's orbit
changes shape in the plane of the
ecliptic in �100 Ka and �400 Ka cycles
Figu 12.1 1
Diagram of the Earth-Moon-Sun system,
illustrating the causes of oscillations that
produce changes in the amount of solar
radiation reaching Earth. These oscilla
tions may, in turn, lead to orbitally forced
changes in Earth's climate and thus the
sedimentary record (e.g., cycles). [Modi
fied from House, M. R., 1995, Orbital
forcing timescales: An introduction, in
House, M. R., and A. S. Gale (eds.), Or
bital forcing timescales and cyclostratig
raphy: Geological Society Special
Publication 85, Fig. 9, p. 10, reproduced
by permission.]
Many cycles of smaller scale than third-order cycles have now been identi
fied. These cycles, often referred to informally as bed-scale or meter-scale cycles,
have durations less than one million years. Cycles with durations ranging from
0.2 to 0.5 million years are called fourth-order cycles, and those with durations
from 0.01 to 0.2 million years are called fifth-order cycles. It now appears that
most of these cycles are related to changes in Earth's orbital parameters (Fig.
12.11). Earth's axis of rotation precesses (the position of the rotational pole wob
bles) in two predominant periods averaging 19,000 and 23,000 years. The axis also
changes its inclination, called obliquity, from 21S to 24.4° in a cycle of about
41,000 years. In addition, its orbit changes from almost circular to almost elliptical
(eccenicity) in two main cycles, one with a cycle of 106,000 years, the other with
a period of 410,000 years. These orbital variations produce cyclic variations in the
intensity and seasonal distribution of incoming solar radiation. Because of such
variations, incoming solar radiation may at times be reduced sufficiently to pre
vent complete summer melt of winter snowpack, leading eventually to snowpack
buildup and subsequent development of continental glaciers with resulting re
moval of large amounts of water from the ocean (lowered sea
level).
These variations in Earth's orbital behavior produce periodic changes of cli
mate, called Milankovitch cycles, which, in tum, influence sea level and deposi
tional patterns and facies (e.g., de Boer and Smith, 1994; Gale, 1998; Schwarzacher,
1993). Milankovitch was a Serbian mathematician who calculated orbital varia
tions accurately for the first time and showed how these variations affected the
amount of solar radiation reaching Earth. He suggested that these orbital cycles
caused climatic changes that led to the ice ages, thus affecting sea levels. This pos
tulated link between orbital cycles, climate, and sea level is sometimes referred to
as orbital forcing (e.g., de Boer, 1991; House and Gale, 1995). Cycles of Mi
lankovitch frequency are particularly well developed in Quaternary strata, and a
high-resolution orbital time scale graduated in precession units of 21,000 years has
been constructed for strata extending back to the base of the Miocene. Mi
lankovitch cycles may also be features of older sedimentary succession; however,
their frequencies are more difficult to identify (Gale, 1998). Milankovitch cycles
have been recognized in a variety of rock types including limestone-marl (clayey
carbonate) successions, limestone-shale successions, limestone-shale-coal succes
sions (cyclothems), chert-shale successions, evaporite deposits, and muds and
shales consisting of alternating light and dark (organic-rich) layers. e study of
412
Chapter 12 I Lithostratigraphy
Figure 12.12
Simplified, schematic repre
sentation of lithofacies. Note
that one facies may change
into another laterally or verti
cally; see also Figure 12.1 .
Such relationships are rarely
seen
in a single outcrop (e .g.,
Fig. 12.4).
short-period, high-frequency cycles (e.g., Milankovich cycles) is commonly re·
ferred to as cyclostratigraphy (e.g., Gale, 1998; House and Gale, 1995).
Note from Figure 12.8 that aual varves constitute cycles of smaller scale
than Milankovitch cycles. Stratigraphic cycles are discussed further under "Sea
Level Analysis" in Chapter 13. See also Anderson and Goodwin (1990); de Boer
and Smith (1994); Einsele, Rieken, and Seilacher (1991b); Gale (1998); nd Hse
and Gale (1995).
Sedimenta Facies
Discussion in preceding chapters dealing with depositional environments referred to
many examples of sediments of one type that grade laterally into sediments of a dif
ferent type, deposited in latera lly contiguous parts of a given depositional setting.
For example, sandy sediments of the beach shoreface may grade seaward to muddy
sediments of the shallow mer-shel£; delta-front sands and silts coonly grade
seaward to prodelta muds; and shelf-edge skeletal or oolitic carbonate sands grade
toward the open shelf to pelleted carbonate muds. I have already referred to such lat
erally equivalent bodies of sediment with distirKtive characteristics as facies. us, a
deposit may be characterized by shale facies, sandstone facies, limestone fa cies, and
so forth. The concept of facies is so important ir1 stratigraphy that a more detailed ex
planation of the meaning and significance of facies is necessary at this point.
The term facies was introduced into the geological literature by Nicolas
Steno in 1669 (Teichert, 1958); however, modern scientific use of the term is credit
ed to the Swiss geologist Amanz Gressly, who used the erm in 1838 in his de
scription of Upper Jurassic strata in the region of Solohurn 1n the Jura Mountams
to record marked changes in lithology and paleontology of these strata. [See Cross
and Homewood (1997) for a discussion of Gressly' s contributions to the science of
stratigraphy.] Krumbein and Sloss (1963) maintain that Gressly intended to con
fine usage of the term to lateral changes within a stratigraphic unit, such as those
illustrated in Figure 12.12. Other workers have interpreted Gressly's usage to m
clude vertical changes in the character of rock units as well (Teichert, 1958). Subse
quently, the term has been used with numerous meanings, many of which bear little
resemblance to Gressly's original meaning. These various meanings have been sum
marized and discussed by Moore (1949), Te ichert (1958), Weller (1958), Marke�ich
(1960), Walker (1992), Pirrie (1998), and many others. The extended meanings of fa
cies have included referring to all strata of a particular type as a certain facies, such
12.4 Vertical and Lateral Successions of Strata
413
as
referring to all redbeds as the "redbed facies," and even such nonstratigraphical
usage as "metamorphic facies," "igneous facies," and "tectonic facies." Because of
these rather loose and inconsistent usages of the term, the meaning of facies has
become considerably clouded.
Moore (1949) described facies as "any areally restricted part of a designated
stratigraphical unit which exhibits characters significantly different from those of
other parts of the unit." Facies comprise "one or any two or more different sorts of
deposits which are partly or wholly equivalent in age and which occur side by
side or in somewhat close neighborhood." According to Moore's definition, facies
a restricted in areal extent, but the same facies could be found at different levels
within the same stratigraphic unit. A different usage of facies that is closer to that
of Gressly usage-and to that of many European geologists-is to consider facies
simply as stratigraphic units distinguished by lithological, structural, and organic
aspects detectable in the field. The areal distribution of facies thus designated may
not be well known (Blatt, Middleton, and Murray, 1980), in contrast to the restrict
ed areal distribution required by Moore's definition. See also the discussion by
Walker (1992). Regardless of the exact definition followed in defining facies, it is
now common practice to designate facies identified on the basis of lithologic char
actestics as lithofacies, and facies distinguished by paleontologic characteristics
(fossil content) without regard to lithologic character as biofacies. A facies may be
divided into subfacies. For example, a thick cross-bedded sandstone facies might
be divisible into trough cross-bedded and tabular cross-bedded subfacies. Very
small-scale facies that can be recognized within microscope thin sections or pol
ished sections of rock are referred to as microfacies (Flgel, 1982).
An important objective of facies studies is to ultimately make environmental
interpretations from the facies. Thus, some geologists designate facies on the basis
of assumed depositional environment and speak of "continental facies," "fluvial
facies," "delta facies," and so on. Such generic usage involves subjective judg
ments that may not always be justied. It is better to make the usage of facies
purely descriptive and objective and then make subjective interpretations of envi
ronment on the basis of these descriptive facies (Hallam, 1981).
Walthes Law of Succession of Facies
Relationship of Lateral and Ve cal Facies
is implicit in the concept of facies that different facies represent different deposi
tional conditions and environments. As laterally contiguous environments in a
given region shift with time in response to shifting shorelines or other geologic
conditions, facies boundaries also shift so that eventually the deposits of one envi
ronment may lie above those of another environment. This deceptively simple
idea embodies one of the single most important concepts in stratigraphy-the
concept that a direct environmental relationship exists between lateral facies and
vertically stacked or supemposed successions of strata. This concept was first
formally stated by Johannes Walther in 1894 and is now called the law of the cor
lation (or succession) of facies, or simply Walther's Law. This law has often
been misstated as "the same facies sequences are seen laterally as vertically." The
correct statement of the law as translated by Middleton (1973, p. 979) is
The various deposits of the same facies-area and similarly the sum of the rocks
of different facies-areas are formed beside each other in space, though in a
cross-section we see them lying on top of each other ... it is a basic statement
of far-reaching significance that only those facies and facies-areas can be super
imposed primarily which can be observed beside each other at the present
time. (Walther, 1894).
414
Chapter 12 /lithostratigraphy
Walther suggests that comparison with recent environments could always
provide the essential clues for interpretation of ancient facies. Middleton (1973) is
careful to point out that the law states not that vertical successions always repro
duce the horizontal succession of environments, but merely that only those facies
can be superimposed that can now be seen side by side. For example, the beach
and barrier-island environmental setting discussed in Chapter 9 may include sev
eral laterally adjacent environments such as beach, back-barrier lagoon, marsh,
tidal flat, tidal channel, and tidal delta. Depending upon the manner in which
these lateral environments shift with time, the vertical successions produced by
deposition in a particular barrier-island setting might consist only of beach sands
overlain by lagoonal muds and capped by marsh peats. The entire lateral succes
sion of deposits formed in the contiguous environments may not be preserved.
Tr ans
g
ressions and Re
g
ressions
The
principles embodied in Walther's Law are well illustrated by considering
transgressive and regressive sedimentary successions. As discussed in Chapters 9
and 10, transgression refers to movement of a shoreline in a landward direction,
also called retrogradation. A seaward movement of a shoreline called regression,
or progradation. Consider the growth of a delta as illustrated in Figure 12.13. As
the delta builds seaward, coarser grained deltaic sediments are deposited on top
of finer grained prodelta muds. The result is a coarsening upward vertical suc
cession of facies, generated by the progradation of laterally adjacent depositional
Figure 12.13
Walther's law illustrated by the
growth of a delta through time.
Note the successive outbuilding
of the delta at four different time
periods (Tl-T 4). With time, the
shoreline progrades from right to
left, so that at a single location
depicting a veical succession
(A), a gradual transition from
prodelta mud to coarser grained
delta deposits takes place, gener
ating a coarsening-upward suc
cession. [After Pirrie, D., 1998,
Interpreting the record: Facies
analysis, in Doyle, P., and M. R.
Bennett (eds.), Unlocking the
stratigraphical record: Advances
in modern stratigraphy, john
Wiley and Sons, Ltd., Chichester,
ri roduced by permission.]
T2
12.4 Vertical and Lateral Successions of Strata
415
environments. Note that the depositional surface (sediment-water interface) rep-
resents a time line, indicating that at any given time prodelta mud was deposited
at the same time as coarser grained delta sediments. It is the seaward migration of
the deltaic environment (regression) that brings deltaic sediments on top of
prodelta muds to create the coarsening-upward vertical succession.
Transgressio occur during a relative rise in sea level when influx of
ter
rigenous sedents from land sources is low enough to allow deeper water marine
sediments to encroach landward over nearshore deposits (coastal encroachment).
Tr ansgression will not occur during rising sea level if the inux of terrigenous sed
iments is so high that outbuilding of the shoreline takes place; instead, regression
curs. at regression may occur during a relative rise in sea level or during
static sea level if the influx of terrigenous clastics is high. Regression may also
cur during a relative fall in sea level. To summarize, transgression occurs only
during rising sea level. Regression can occur either during rising sea level, if inux
of
terrigenous detritus is hi, or during falling sea level.
Transgression followed by regression tends to produce a wedge of sediments
in which deeper water sediments are deposited on top of shallower water sedi
ments in the basal part of the wedge, and shallower water sediments are deposit
on top of deeper water sediments in the top part of the wedge (Fig. 12.14). Note
e marked coastal onlap illustrated in Figure 12.14. The initial depositional sur
face
at the base of a transgressive succession is commonly an unconformity. The
boundg surface at the base of a regressive succession can also be an unconfor
mi if a relative fall in sea level is accompanied by erosion.
Eects of Climate and Sea Level on Sedimentation tterns
e preceding discussion shows that both the rate of influx of terrigenous clastic
sediments and the change in relative sea level exert control on sedimentation
TERRIGENOUS INFLUX
SEA LEVEL
Nonma
r
ine
Facies
NO DIFFERENTIAL SUBSIDENCE
�
�
COASTA
L ENCROA
C
HM
T
_____
_ _______
___
::
.
Figure 12.14
Coastal on lap owing to marine transgression and regression. During relative rise in sea
level, littoral facies may be transgressive, stationa, or regressive. Neritic (shallow shel
facies may be deepening, shallowing, or compensating (maintaining a given depth). Note
the wedge of sediment formed during a cycle of transgression-regression. [From Vail, P. R.,
R. M. Mitchum, Jr., and S. Thompson, Ill, 1977, Seismic stratigraphy and global change of
a level. Part 3: Relative changes of sea level from coastal on lap
, in Payton, C. E.
(ed.),
Seismic stratigraphy-Application to hydrocarbon exploration: Am. Assoc. Petroleum Ge
ologists Mem. 26, Fig. 4, p. 67, reprinted by permission of AAPG, Tulsa, Okla.]
416 Chapter 12 I Lithostratigraphy
patterns in coastal areas and on the continental shelf. In turn, terrigenous influx is
itself influenced by tectonism and climatic conditions (e.g., Church and Coe,
2003). Te ctonism produces changes in elevation of sediment source areas and thus
affects rates of erosion, which generally increase with increase in land elevation.
Also, source areas at higher elevations and with steeper slopes tend to shed coarser
sediment than do those at lower elevations. Climate regulates sediment influx by
controlling rates of weathering and erosion, sediment transport conditions, and
sedimentation mechanisms. For example, in a given geographic area, significantly
greater terrigenous influx will occur during periods of heavy rain (e.g., during the
winter rainy season), when erosion rates are accelerated and stream transport is
increased, than during dry periods. On a shorter time scale, more sediment, and
coarser sediment, may be eroded and transported during a single unusually large,
high-velocity flood that occurs only once every hundred years than during all e
smaller floods that may have occurred during the preceding hundred years. [See
Clifton (1988) and Ager (1993a) for discussion of the sedimentologic consequences
of large, rare, convulsive geologic events.] Thus, the rates of sediment influx and
the grain sizes of sediments delivered to coastal areas from continents have varied
throughout geologic time in response to these variables of tectonism and climate.
Changes in sea level also affect sedimentation patterns in coastal areas.
Changes in sea level that are worldwide and that affect sea level on all continents
essentially simultaneously are called eustatic sea-level changes, as mentioned.
Changes of sea level that affect only local areas are referred to as relative sea-level
changes. Relative sea level changes may involve some global eustatic change but
are also affected by local tectonic uplift or downwarping of the basin floor and
sediment aggradation (buildup). Local tectonics and rates of sedimentation have
little or no effect on worldwide sea levels.
Eustatic sea level changes have been attributed to a variety of causes, all of
which can be lumped under changes in volume of water and changes in volume of
the ocean basins (Table 12.2). The most important changes in water volume are
tied to continental glaciation. Sea level drops during glacial stages when seawater
is locked up on land as ice, and it rises during interglacial stages as continental ice
ble 12.2
Poslad
mechanisms of
sea-lel change
Mechanisms
1. Ocean steric (thermohaline) volume changes
Shallow (0-500 m)
Deep (500000 m)
2. Glacial accretion and wastage
Mountain glaciers
Greenland Ice Sheet
East Antarctic Ice Sheet
West Antarctic Ice Sheet
3. Liquid water on land
Groundwater aquifers
Lakes and reservoirs
4. Crustal deformation
Lithosphere formation and subduction
Glacial isostatic rebound
Continental collision
Seafloor and continental epirogeny
Sedimentation
Source: Revelle, 1990.
me scale (yr)
0.1-100
10-10,000
10-100
100-100,000
1,000-100,000
100-10,000
100-100,000
100-100,000
100,000-10
8
100-10,000
100,000-10
8
100,000-10
8
10,000-10
8
Order of magnitude
0-1 m
0.01-10 m
0.1-1
m
0.1-10 m
10-100 m
1-10 m
0.1-10 m
0.01-0.1 m
1-lOO m
0.1-10 m
10-100 m
10-100 m
1-100 m