60
Concepts of Scale
Table 3.1. Perceived importance of geologic events as related to size and age. (After Schumm 1985a)
Relative
1 day
1 year
10 yr
102 yr
103 yr
105 yr
106 yr
108yr
magnitude
of event
Megaevent
Local soil
Gully
Meander Volcanic
Terrace
Continental Major
Mountain
slip or
cutoff
eruption
formation glaciation
fo lding,
building
flow
faulting
Mesoevent
Rill
Local soil
Gully
Meander Volcanic
Terrace
Continental Major
slip or
cutoff
eruption
fo rmation
glaciation
folding,
flow
faulting
Microevent Sand
Rill
Local soil Gully
Meander
Volcanic
Terrace
Continental
grain
slip or
cutoff
eruption
fo rmation
glaciation
movement
flow
Nonevent
Sand
Rill
Local soil Gully
Meander
Volcanic
Terrace
grain
slip or
movement
flow
of regional climatic and tectonic controls on eolian
deposits. The eventual availability of a common hierarchy of architectural units based on time scale
and physical scale would facilitate more of this type
of correlation of events and cross-fertilization of
ideas between contrasting environments. This is especially important in an era of compartmentalization of knowledge, where sedimentary geologists
studying different aspects of a basin fill might specialize in such diverse areas as (for example) fluid
hydraulics, facies analysis, geochemistry, paleoclimatology, regional tectonics, or geophysical subsidence models, yet have very little common language
to facilitate exchange of ideas and concepts. The
classification and grouping discussed in this paper
are offered as a first step toward a <(grand unified
theory'' of architectural classification.
Architectural scale concepts are also important in
the petroleum industry. Reservoir heterogeneities
are present at at least four scales (Fig. 3.4) that can be
related to the scale hierarchy described here. The
analysis of each scale of heterogeneity requires different techniques, as described throughout the rest
of this book.
3.2 The Grouping of Architectural Units
in Clastic Rocks
According to Depositional Time Scale
A tentative classification and grouping of architectural units into ten classes spanning at least 12 orders
of magnitude of time scale is shown in Table 3.2.
Deposits in most clastic environments can be ascutoff
eruption
formation
signed a grouping based primarily on the time scale
represented by the deposit, or by the total time
elapsed during the formation of the deposit and its
bounding surfaces (which may be considerably
greater). The grouping here is based primarily on the
latter criterion - the recurrence interval of the depositional process. In some cases, such as sedimentgravity flows, there is a marked contrast between the
duration of a depositional event and its recurrence
interval. The latter may vary by more than an order
of magnitude within a basin or between basins, but
quantifying such variations may be difficult or impossible using present techniques. Because of such
variation, the grouping of some types of deposit in
the discussion that follows is somewhat arbitrary.
In a recent attempt to systematize our knowledge
of large-scale subaqueous bedforms, Ashley (1990)
suggested that the scale of a bedform may simply be
a reflection of the physical space, volume of sediment, and elapsed time available for its formation
Within a · qowing system. Dunes, sand waves, and
other two- and three-dimensional bedforms, which
have gone by various names in the past, may all be
part of a genetically related continuum. Whereas this
argues against any attempt to impose an arbitrary
hierarchical size classification on hydrodynamic
sedimentary structures, the fact remains that the
timing of depositional and erosional events within
trains of bedforms is strongly controlled by rhythmic or episodic events on various time scales (diurnal, seasonal, etc.). Packages of cross-bedded strata
and their enclosing bounding surfaces are, therefore,
amenable to a hierarchical classification based on
depositional recurrence interval, as discussed in this
book.
Concepts of Scale
Table 3.1. Perceived importance of geologic events as related to size and age. (After Schumm 1985a)
Relative
1 day
1 year
10 yr
102 yr
103 yr
105 yr
106 yr
108yr
magnitude
of event
Megaevent
Local soil
Gully
Meander Volcanic
Terrace
Continental Major
Mountain
slip or
cutoff
eruption
formation glaciation
fo lding,
building
flow
faulting
Mesoevent
Rill
Local soil
Gully
Meander Volcanic
Terrace
Continental Major
slip or
cutoff
eruption
fo rmation
glaciation
folding,
flow
faulting
Microevent Sand
Rill
Local soil Gully
Meander
Volcanic
Terrace
Continental
grain
slip or
cutoff
eruption
fo rmation
glaciation
movement
flow
Nonevent
Sand
Rill
Local soil Gully
Meander
Volcanic
Terrace
grain
slip or
movement
flow
of regional climatic and tectonic controls on eolian
deposits. The eventual availability of a common hierarchy of architectural units based on time scale
and physical scale would facilitate more of this type
of correlation of events and cross-fertilization of
ideas between contrasting environments. This is especially important in an era of compartmentalization of knowledge, where sedimentary geologists
studying different aspects of a basin fill might specialize in such diverse areas as (for example) fluid
hydraulics, facies analysis, geochemistry, paleoclimatology, regional tectonics, or geophysical subsidence models, yet have very little common language
to facilitate exchange of ideas and concepts. The
classification and grouping discussed in this paper
are offered as a first step toward a <(grand unified
theory'' of architectural classification.
Architectural scale concepts are also important in
the petroleum industry. Reservoir heterogeneities
are present at at least four scales (Fig. 3.4) that can be
related to the scale hierarchy described here. The
analysis of each scale of heterogeneity requires different techniques, as described throughout the rest
of this book.
3.2 The Grouping of Architectural Units
in Clastic Rocks
According to Depositional Time Scale
A tentative classification and grouping of architectural units into ten classes spanning at least 12 orders
of magnitude of time scale is shown in Table 3.2.
Deposits in most clastic environments can be ascutoff
eruption
formation
signed a grouping based primarily on the time scale
represented by the deposit, or by the total time
elapsed during the formation of the deposit and its
bounding surfaces (which may be considerably
greater). The grouping here is based primarily on the
latter criterion - the recurrence interval of the depositional process. In some cases, such as sedimentgravity flows, there is a marked contrast between the
duration of a depositional event and its recurrence
interval. The latter may vary by more than an order
of magnitude within a basin or between basins, but
quantifying such variations may be difficult or impossible using present techniques. Because of such
variation, the grouping of some types of deposit in
the discussion that follows is somewhat arbitrary.
In a recent attempt to systematize our knowledge
of large-scale subaqueous bedforms, Ashley (1990)
suggested that the scale of a bedform may simply be
a reflection of the physical space, volume of sediment, and elapsed time available for its formation
Within a · qowing system. Dunes, sand waves, and
other two- and three-dimensional bedforms, which
have gone by various names in the past, may all be
part of a genetically related continuum. Whereas this
argues against any attempt to impose an arbitrary
hierarchical size classification on hydrodynamic
sedimentary structures, the fact remains that the
timing of depositional and erosional events within
trains of bedforms is strongly controlled by rhythmic or episodic events on various time scales (diurnal, seasonal, etc.). Packages of cross-bedded strata
and their enclosing bounding surfaces are, therefore,
amenable to a hierarchical classification based on
depositional recurrence interval, as discussed in this
book.