Chapter 13
Seque11ce Stratigraphy
13.1 Introduction
The currently accepted definition of «sequence" is
that proposed by Vail et al. (1977, p. 53). It is a
"stratigraphic unit composed of a relatively conformable succession_ of genetically related strata ...
bounded at its top and base by unconformities or
their correlative conformities". However� the term
sequence has had a checkered history. Sloss (1963)
defined stratigraphic sequences as "rock-stratigraphic units of higher rank than group, megagroup,
or supergroup, traceable over major areas of a continent and bounded by unconformities of interregional scope." This useage has not received universal
acceptance. In some of the literature "sequence" has
been used as a synonym for "succession", and it has
been employed for cyclic or unconformity-bounded
units of varying dimen' sions and for time spans ranging from thousands of years to hundreds of millions
of years.
Since the ground-breaking publication by Vail et
a!. (1977), sequence stratigraphy has evolved into
one of the central, theoretical underpinnings of the
science of stratigraphy. It comprises a set of predictions that can be made about the facies and architecture of sediments formed in a given range of
environments under specified allogenic controls. It
is the predictability of sequences that give them their
value for regional correlation and for one of the main
practical applications of such correlation - petroleum exploration. However, modern work has demonstrated that sequences are generated by a variety
of causes, including local- to continental-scale tectonism, eustatic sea-level change, and climate
change (see reviews by Miall 1990, Chap. 8; Plint et al.
1992). More than one generating mechanism may be
active at any one time, and it is not uncommon for
sequences of several types and scales to be superimposed on each other within the same succession.
Interpreting generating processes from the rock
record requires the construction of a detailed stratigraphic fr amework. Questions about the way in
whi
.
ch various allogenic controls actually work has
stimulated much numerical modeling of basin evolution, aided in recent years by graphical simulation
using computers.
Most recent work has focused on the sequence
architecture of shallow marine deposits, because one
of the major preoccupations of sequence stratigraphers has been sea-level change, and facies characteristics in the lower coastal-plain to shallow-shelf
environment are particularly sensitive to changes in
sea level. Much less is known about the sequence
stratigraphy of nonmarine deposits, although this
is now rapidly changing, as summarized in the recent reviews by Shanley and McCabe (1994) and
Posamentier and Allen (in press).
Sequence-stratigraphic analyses of fluvial deposits commenced, in effect, with the work of Allen
(1974b). As noted in Sects. 2.3.7 and 10.4.3 (Fig.
10.32), Allen developed a series of models of alluvial
stratigraphy based on simple deductive reasoning.
His model SB is an interpretation of the stratigraphic
architecture that might develop in an alluvial system
characterized by a cycle of dissection and aggradation, controlled by base-level changes over a vertical
interval greater than channel depth. This is precisely
the set of conditions under which nonmarine stratigraphic sequences develop, according to modern
work. Building on this work and on that of Bridge
and Leeder (1979), Ross (1990) made some simple
predictions regarding variations in channel density
in response to variations in aggradation rate, but he
did not formalize a sequence model. The current
state of knowledge of the effects of base-level change
on fluvial systems and of the control tectonic movements exert on rivers is discussed at length in Chap.
II (see in particular Sect. 11.2.2). It is beyond the
scope of this book to enter into the current controversy regarding the relative importance of regional
tectonism versus eustatic sea-level change in the
generation of base-level change.
The first modern sequence models1 those of Vail
et a!. (1977), were based essentially on the architecture of divergent or passive continental margins)
Seque11ce Stratigraphy
13.1 Introduction
The currently accepted definition of «sequence" is
that proposed by Vail et al. (1977, p. 53). It is a
"stratigraphic unit composed of a relatively conformable succession_ of genetically related strata ...
bounded at its top and base by unconformities or
their correlative conformities". However� the term
sequence has had a checkered history. Sloss (1963)
defined stratigraphic sequences as "rock-stratigraphic units of higher rank than group, megagroup,
or supergroup, traceable over major areas of a continent and bounded by unconformities of interregional scope." This useage has not received universal
acceptance. In some of the literature "sequence" has
been used as a synonym for "succession", and it has
been employed for cyclic or unconformity-bounded
units of varying dimen' sions and for time spans ranging from thousands of years to hundreds of millions
of years.
Since the ground-breaking publication by Vail et
a!. (1977), sequence stratigraphy has evolved into
one of the central, theoretical underpinnings of the
science of stratigraphy. It comprises a set of predictions that can be made about the facies and architecture of sediments formed in a given range of
environments under specified allogenic controls. It
is the predictability of sequences that give them their
value for regional correlation and for one of the main
practical applications of such correlation - petroleum exploration. However, modern work has demonstrated that sequences are generated by a variety
of causes, including local- to continental-scale tectonism, eustatic sea-level change, and climate
change (see reviews by Miall 1990, Chap. 8; Plint et al.
1992). More than one generating mechanism may be
active at any one time, and it is not uncommon for
sequences of several types and scales to be superimposed on each other within the same succession.
Interpreting generating processes from the rock
record requires the construction of a detailed stratigraphic fr amework. Questions about the way in
whi
.
ch various allogenic controls actually work has
stimulated much numerical modeling of basin evolution, aided in recent years by graphical simulation
using computers.
Most recent work has focused on the sequence
architecture of shallow marine deposits, because one
of the major preoccupations of sequence stratigraphers has been sea-level change, and facies characteristics in the lower coastal-plain to shallow-shelf
environment are particularly sensitive to changes in
sea level. Much less is known about the sequence
stratigraphy of nonmarine deposits, although this
is now rapidly changing, as summarized in the recent reviews by Shanley and McCabe (1994) and
Posamentier and Allen (in press).
Sequence-stratigraphic analyses of fluvial deposits commenced, in effect, with the work of Allen
(1974b). As noted in Sects. 2.3.7 and 10.4.3 (Fig.
10.32), Allen developed a series of models of alluvial
stratigraphy based on simple deductive reasoning.
His model SB is an interpretation of the stratigraphic
architecture that might develop in an alluvial system
characterized by a cycle of dissection and aggradation, controlled by base-level changes over a vertical
interval greater than channel depth. This is precisely
the set of conditions under which nonmarine stratigraphic sequences develop, according to modern
work. Building on this work and on that of Bridge
and Leeder (1979), Ross (1990) made some simple
predictions regarding variations in channel density
in response to variations in aggradation rate, but he
did not formalize a sequence model. The current
state of knowledge of the effects of base-level change
on fluvial systems and of the control tectonic movements exert on rivers is discussed at length in Chap.
II (see in particular Sect. 11.2.2). It is beyond the
scope of this book to enter into the current controversy regarding the relative importance of regional
tectonism versus eustatic sea-level change in the
generation of base-level change.
The first modern sequence models1 those of Vail
et a!. (1977), were based essentially on the architecture of divergent or passive continental margins)