298
Chapter 7 Sequences, Minor CycIes, and Event Stratigraphy
cepted unifying concept is not yet established. T~e
main reason for this deficiency is the enormous vanability inherent in the principal factors and their interplay controlling the generation of sequences and
their smaller building stones in various depositional
environments. Sequence stratigraphy is not an easy
topic; it may become fully understood o?ly .by using
sophisticated computer models. The bnef mtroduction into sequence stratigraphy of this section ~emonstrates the basic principles and gives some hmts for
the application of these principles to different
depositional environments.
.
Sequence stratigraphy is actually an old tOPlC. Geologists and stratigraphers studying sections in the
field always observed vertical and lateral facies
changes (cf. facies models in Chaps. 2 and 3). Unconformities, depositional and non-depositional
events, as weIl as repetitions in vertical successions
have been observed at many places long ago. One of
the first authors developing a more general concept
was L.L. Sloss (1963). He described thick sequences
("Sloss sequences" or supersequences, now 2nd order sequences of mostly 10 to 20 Ma in duration)
from intracratonic basins of North America, bounded
by unconformities.
Modern sequence stratigraphy started with "seismic stratigraphy" and weIl logs on passive continental margins. Seismic records reveal the large-scale
architecture of basin fills and the geometry of individual sediment bodies in a two- and three-dimensional way. They show unconformities and breaks in
the sediment buildup over long distances which can
hardly be observed in field exposures. It was recognized, mainly by a working group of EXXON Production Research Company under P. Vail, that relative sea-Ievel changes during the Earth's history affected worldwide not only coastal areas and shallow
seas, but also had significant consequences for the
sediments of deeper basins. This method led to a
better understanding of the formation of source and
reservoir rocks and allowed the identification of
stratigraphic traps (in addition to structural traps) for
oil and gas. Predictions and planning became more
safe and reduced the costs of hydrocarbon exploration.
Based on this work, the general concept of sequence stratigraphy was developed. This concept is
particularly suitable for continental margin settings
with a shelf break, slope and deeper basin, but it can
also be applied to basins with a ramp rnargin and to
other basin types.
"Classical" references include Vail et al. in Payton (1977),
Schlee (1984), Vail et al. (1984), Vail (1987, Pitman and
Golovchenko 1988; Posamentier and Vail (1988), Van
Wagoner et al. (1990). More recent work on sequence stratigraphy dealt with examples from all sorts of paleo-environrnents through geological time, from the Proterozoic up
to the modern icehouse state of the world. Part of these
results and numerous references have been summarized in
special volurnes (e.g. Posamentier et al. 1993; Williams
and Dorb 1993; Weimer and Posamentier 1994; Steel et al.
1995; Emery and Myers 1996; Gaupp and Van de Weerd
1996; Howell and Aitken 1996). Points of criticism have
been discussed by Miall (1997).
The new approach to sequence stratigraphy was first
developed for so-called second and third order s~quences which can be well identified, for example, m
deeply penetrating seisrnic records of shelf-slope-basin settings on continental margins. The thicknesses
of these sequences normally range from some tens to
hundreds of meters and they can be traced in seisrnic
cross sections over hundreds of kilometers. Exposures of this scale are rarely available on land, which
was one of the difficulties in combining and reconciling data from seisrnic records with observations on
land. Today, high-resolution seisrnic records can resolve thinner packages of strata and bridge this gap.
The present-day concept of sequence stratigr.aphy
also comprises sea- or base-level cycles of hlgher
orders (i.e. sequences or subunits of sequences reflecting shorter time periods than third order cycles)
and their superposition in all sorts of environments,
siliciclastic and carbonate depositional systems. In
addition, long-term first order cycles have been identified which had a profound influence on the evolution of the global climates and the nature and distribution of sediments on the continents and in the
ocean basins.
7.2.2 Specific Definitions and Terms
For the description and understanding of stratigraphic sequences it is necessary to define some ge.neral processes and to explain a number of specml
terms (Tables 7.1 and 7.2).
Some of these terms were introduced by members of the
EXXON group (e.g. Posamentier and Vail 1988; Van
Wagoner et al. 1990); additions or modifications in these
lists came from various other sources. Several of the
keywords in Table 7.2, e.g. the different systems tracts, the
base-level concept for continental depositional systems,
and additional terms will be discussed later. Some processes are further explained as folIows:
Base level, BL. The meaning of this term is clear for
depositional systems controlled by sea level acting as
the ultimate base level of subaerial erosion. However even in this case one has to consider that erosion' and redistribution of sediment occurs below sea
level. In continental systems which are not directly
affected by sea-Ievel change, the base level must be
defined in a different way.
It may be given by the regional (inclined) water table
within a river catchment, including lake levels, or it
Chapter 7 Sequences, Minor CycIes, and Event Stratigraphy
cepted unifying concept is not yet established. T~e
main reason for this deficiency is the enormous vanability inherent in the principal factors and their interplay controlling the generation of sequences and
their smaller building stones in various depositional
environments. Sequence stratigraphy is not an easy
topic; it may become fully understood o?ly .by using
sophisticated computer models. The bnef mtroduction into sequence stratigraphy of this section ~emonstrates the basic principles and gives some hmts for
the application of these principles to different
depositional environments.
.
Sequence stratigraphy is actually an old tOPlC. Geologists and stratigraphers studying sections in the
field always observed vertical and lateral facies
changes (cf. facies models in Chaps. 2 and 3). Unconformities, depositional and non-depositional
events, as weIl as repetitions in vertical successions
have been observed at many places long ago. One of
the first authors developing a more general concept
was L.L. Sloss (1963). He described thick sequences
("Sloss sequences" or supersequences, now 2nd order sequences of mostly 10 to 20 Ma in duration)
from intracratonic basins of North America, bounded
by unconformities.
Modern sequence stratigraphy started with "seismic stratigraphy" and weIl logs on passive continental margins. Seismic records reveal the large-scale
architecture of basin fills and the geometry of individual sediment bodies in a two- and three-dimensional way. They show unconformities and breaks in
the sediment buildup over long distances which can
hardly be observed in field exposures. It was recognized, mainly by a working group of EXXON Production Research Company under P. Vail, that relative sea-Ievel changes during the Earth's history affected worldwide not only coastal areas and shallow
seas, but also had significant consequences for the
sediments of deeper basins. This method led to a
better understanding of the formation of source and
reservoir rocks and allowed the identification of
stratigraphic traps (in addition to structural traps) for
oil and gas. Predictions and planning became more
safe and reduced the costs of hydrocarbon exploration.
Based on this work, the general concept of sequence stratigraphy was developed. This concept is
particularly suitable for continental margin settings
with a shelf break, slope and deeper basin, but it can
also be applied to basins with a ramp rnargin and to
other basin types.
"Classical" references include Vail et al. in Payton (1977),
Schlee (1984), Vail et al. (1984), Vail (1987, Pitman and
Golovchenko 1988; Posamentier and Vail (1988), Van
Wagoner et al. (1990). More recent work on sequence stratigraphy dealt with examples from all sorts of paleo-environrnents through geological time, from the Proterozoic up
to the modern icehouse state of the world. Part of these
results and numerous references have been summarized in
special volurnes (e.g. Posamentier et al. 1993; Williams
and Dorb 1993; Weimer and Posamentier 1994; Steel et al.
1995; Emery and Myers 1996; Gaupp and Van de Weerd
1996; Howell and Aitken 1996). Points of criticism have
been discussed by Miall (1997).
The new approach to sequence stratigraphy was first
developed for so-called second and third order s~quences which can be well identified, for example, m
deeply penetrating seisrnic records of shelf-slope-basin settings on continental margins. The thicknesses
of these sequences normally range from some tens to
hundreds of meters and they can be traced in seisrnic
cross sections over hundreds of kilometers. Exposures of this scale are rarely available on land, which
was one of the difficulties in combining and reconciling data from seisrnic records with observations on
land. Today, high-resolution seisrnic records can resolve thinner packages of strata and bridge this gap.
The present-day concept of sequence stratigr.aphy
also comprises sea- or base-level cycles of hlgher
orders (i.e. sequences or subunits of sequences reflecting shorter time periods than third order cycles)
and their superposition in all sorts of environments,
siliciclastic and carbonate depositional systems. In
addition, long-term first order cycles have been identified which had a profound influence on the evolution of the global climates and the nature and distribution of sediments on the continents and in the
ocean basins.
7.2.2 Specific Definitions and Terms
For the description and understanding of stratigraphic sequences it is necessary to define some ge.neral processes and to explain a number of specml
terms (Tables 7.1 and 7.2).
Some of these terms were introduced by members of the
EXXON group (e.g. Posamentier and Vail 1988; Van
Wagoner et al. 1990); additions or modifications in these
lists came from various other sources. Several of the
keywords in Table 7.2, e.g. the different systems tracts, the
base-level concept for continental depositional systems,
and additional terms will be discussed later. Some processes are further explained as folIows:
Base level, BL. The meaning of this term is clear for
depositional systems controlled by sea level acting as
the ultimate base level of subaerial erosion. However even in this case one has to consider that erosion' and redistribution of sediment occurs below sea
level. In continental systems which are not directly
affected by sea-Ievel change, the base level must be
defined in a different way.
It may be given by the regional (inclined) water table
within a river catchment, including lake levels, or it