58
There also exists a hierarchy of physical scales,
which the same two examples illustrate - the cross�
bed fo reset at one extreme to the basin-fill at the
other extreme (Fig. 3.1). At least 14 orders of magnitude are represented, from the few square centimeters in area of the smallest scale of ripple fo reset, to
the tens of thousands of square kilometers of a major
sedimentary basin. This chapter, which is based on
an earlier review by Miall (1991a), is a systematic
exploration of these wide ranges in scales.
The ways by which earth scientists study sedimentary processes and the resultant depositional
products vary according to the scale of interest.
Bedforms in flumes are studied during experimental
runs of, at most, few days duration. Nonmarine and
marginal-marine sediments and processes have
been much analyzed in modern environments, using
studies of surface processes, and by sampling the
sediments themselves in trenches and shallow cores.
The use of old maps and aerial photographs extends
the record as far back as about 100 years, and 14C
dates may enable stratigraphic records of the last few
tens of thousands of years to be calibrated. Many
sedimentological studies draw on geomorphological
work on landforms and Recent sediments. However,
such work is hampered by the specific, and possibly
nongeneralizable nature of the Recent record, such
as the Holocene deglaciation, climatic change, and
rapid rise of global sea levels. Stratigraphic studies
typically deal with much longer time periods, as
represented by the deposits of basin fi lls, which may
have taken hundreds of thousands to millions of
years to accumulate. Intermediate scales, represented by such major depositional elements as large
channels and bars, delta lobes, draas, coastal barriers
and shelf sand ridges, which may represent thousands to tens of thousands of years of accumulation,
are particularly difficult to document in the ancient
record and to analyze in modern environments. The
time scales of the relevant sedimentary processes are
diffi cult to resolve, and the physical scale of the
deposits falls between the normal size of large outcrops and the well spacing or the scale of geophysical
resolution in the subsurface. Yet, it is this scale of
deposit that is of particular interest to economic
geologists, representing as it does the scale of many
stratigraphic petroleum reservoirs and their internal
heterogeneities.
Geomorphologists have devoted considerable attention to the problem of time scales and their effects
on analysis and prediction (Cullingford et al. 1980;
Hickin 1983; Schumm 1985a). As Hickin (1983, p. 61)
has stated, "time-scale selection largely determines
Concepts of Scale
the questions that we can ask." Schumm (198Sa)
showed that the significance of an event diminishes
as the time-scale increases. Thus, an individual
volcanic eruption, a spectacular geological event at
the time of its occurrence (a "megaevent", to use
Schumm's term), diminishes in geological importance as the millenia go by and other eruptions take
place, until eventually, after perhaps millions of
years, all evidence of the eruption is lost (it becomes
a "nonevent") as a result of erosion or burial of
the rocks and landforms formed by the eruption.
Events that seem random in the short term (such
as turbidity-current events) may assume a regular
episodicity, or even cylicity, with definable recurrence intervals, if studied over a long enough time
scale. Many events occur only when some critical
threshold has been passed, such as the buildup of
deposists on a depositional slope leading to gravitational instability and failure. In several essays,
Schumm (1977, 1979, 1985a, 1988; Schumm and
Brakenridge 1987) has discussed the concept of
((geomorphic thresholds" and their impact on sedimentary processes. Such thresholds reflect both
autogenic and allogenic processes, and are characterized by a wide range of time scales (Fig. 3.2) and
scales of cyclicity (Fig. 3.3). Schumm (1985a) also
provided a table which illustrates the magnitude of
various geological events as related to the time scale
over which they are considered (Table 3.1).
The incorporation of hierarchical scale concepts
into fluvial studies requires an architectural approach. Earlier approaches to the architectural study
of fluvial deposits are described in Chap. 2, notably
the work of ).R.L. Allen and A. Ramos and his colleagues. The main classification used in this book is
described in Chap. 4. The current explosion of interest in sequence stratigraphy represents an increasing
interest in large-scale stratigraphic architecture, and
its dependence on such allogenic controls as tectonics and sea-level change. This work is referred to
briefly here, but the main discussion is presented in
Chap. 13. The applications of the concepts to the
study of reservoir heterogeneities are discussed in
Chap. 14.
A review of the depositional processes in clastic
environments, and the ways by which various workers have systematized them in depositional hierarchies, has revealed some common themes, although
it is premature to propose a unified hierarchy of
depositional elements for use in the description and
classification of all clastic rocks. Most within-basin
(autogenic) or external (allogenic) sedimentary controls have durations of a rather constant and predict-
There also exists a hierarchy of physical scales,
which the same two examples illustrate - the cross�
bed fo reset at one extreme to the basin-fill at the
other extreme (Fig. 3.1). At least 14 orders of magnitude are represented, from the few square centimeters in area of the smallest scale of ripple fo reset, to
the tens of thousands of square kilometers of a major
sedimentary basin. This chapter, which is based on
an earlier review by Miall (1991a), is a systematic
exploration of these wide ranges in scales.
The ways by which earth scientists study sedimentary processes and the resultant depositional
products vary according to the scale of interest.
Bedforms in flumes are studied during experimental
runs of, at most, few days duration. Nonmarine and
marginal-marine sediments and processes have
been much analyzed in modern environments, using
studies of surface processes, and by sampling the
sediments themselves in trenches and shallow cores.
The use of old maps and aerial photographs extends
the record as far back as about 100 years, and 14C
dates may enable stratigraphic records of the last few
tens of thousands of years to be calibrated. Many
sedimentological studies draw on geomorphological
work on landforms and Recent sediments. However,
such work is hampered by the specific, and possibly
nongeneralizable nature of the Recent record, such
as the Holocene deglaciation, climatic change, and
rapid rise of global sea levels. Stratigraphic studies
typically deal with much longer time periods, as
represented by the deposits of basin fi lls, which may
have taken hundreds of thousands to millions of
years to accumulate. Intermediate scales, represented by such major depositional elements as large
channels and bars, delta lobes, draas, coastal barriers
and shelf sand ridges, which may represent thousands to tens of thousands of years of accumulation,
are particularly difficult to document in the ancient
record and to analyze in modern environments. The
time scales of the relevant sedimentary processes are
diffi cult to resolve, and the physical scale of the
deposits falls between the normal size of large outcrops and the well spacing or the scale of geophysical
resolution in the subsurface. Yet, it is this scale of
deposit that is of particular interest to economic
geologists, representing as it does the scale of many
stratigraphic petroleum reservoirs and their internal
heterogeneities.
Geomorphologists have devoted considerable attention to the problem of time scales and their effects
on analysis and prediction (Cullingford et al. 1980;
Hickin 1983; Schumm 1985a). As Hickin (1983, p. 61)
has stated, "time-scale selection largely determines
Concepts of Scale
the questions that we can ask." Schumm (198Sa)
showed that the significance of an event diminishes
as the time-scale increases. Thus, an individual
volcanic eruption, a spectacular geological event at
the time of its occurrence (a "megaevent", to use
Schumm's term), diminishes in geological importance as the millenia go by and other eruptions take
place, until eventually, after perhaps millions of
years, all evidence of the eruption is lost (it becomes
a "nonevent") as a result of erosion or burial of
the rocks and landforms formed by the eruption.
Events that seem random in the short term (such
as turbidity-current events) may assume a regular
episodicity, or even cylicity, with definable recurrence intervals, if studied over a long enough time
scale. Many events occur only when some critical
threshold has been passed, such as the buildup of
deposists on a depositional slope leading to gravitational instability and failure. In several essays,
Schumm (1977, 1979, 1985a, 1988; Schumm and
Brakenridge 1987) has discussed the concept of
((geomorphic thresholds" and their impact on sedimentary processes. Such thresholds reflect both
autogenic and allogenic processes, and are characterized by a wide range of time scales (Fig. 3.2) and
scales of cyclicity (Fig. 3.3). Schumm (1985a) also
provided a table which illustrates the magnitude of
various geological events as related to the time scale
over which they are considered (Table 3.1).
The incorporation of hierarchical scale concepts
into fluvial studies requires an architectural approach. Earlier approaches to the architectural study
of fluvial deposits are described in Chap. 2, notably
the work of ).R.L. Allen and A. Ramos and his colleagues. The main classification used in this book is
described in Chap. 4. The current explosion of interest in sequence stratigraphy represents an increasing
interest in large-scale stratigraphic architecture, and
its dependence on such allogenic controls as tectonics and sea-level change. This work is referred to
briefly here, but the main discussion is presented in
Chap. 13. The applications of the concepts to the
study of reservoir heterogeneities are discussed in
Chap. 14.
A review of the depositional processes in clastic
environments, and the ways by which various workers have systematized them in depositional hierarchies, has revealed some common themes, although
it is premature to propose a unified hierarchy of
depositional elements for use in the description and
classification of all clastic rocks. Most within-basin
(autogenic) or external (allogenic) sedimentary controls have durations of a rather constant and predict-