Chapter 3
Collcepts of Scale
3.1 Time Scales and Physical Scales
in Sedimentation
It has become a geological truism that many sedimentary units accumulate as a result of short intervals of rapid sedimentation separated by long
intervals of time when little or no sediment is deposited (Ager 1981, 1993). It is also now widely realized
that rates of sedimentation measured in modern
depositional environments or the ancient record
vary in proportion to the time scale over which they
are measured. Sadler (1981) documented this in detail, and showed that measured sedimentation rates
vary by 11 orders of magnitude, from 10-4 to 107
m/ka. This wide variation reflects the increasing
number and length of intervals of nondeposition or
erosion factored into the measurements as the
FLUVIAL ARCHITECTURE:
SCALE, CONTROLS AND TIME
CONTROLS
PROCESSES
length of the measured stratigraphic record increases. Breaks in the record include such events as
the nondeposition or erosion that takes place in
front of an advancing bedform (a few seconds to
minutes), the nondeposition due to drying out at ebb
tide (a few hours), up to the major regional unconformity generated by orogeny (millions of
years). The variation in sedimentation rate also reflects the variation in actual rates of continuous
accumulation, from the rapid sand flow or grain-fall
accumulation of a cross-bed foreset lamina (time
measured in seconds), and the dumping of graded
beds from a turbidity current (time measured in
hours to days), to the slow pelagic fill of an oceanic
abyssal plain (undisturbed in places for hundreds or
thousands of years, or more). There clearly exists a
wide variety of time scales of sedimentary processes
(Fig. 3.1).
TECTONICS
SUBSIDENCE STYLE
AND RATE
lateral
'"'
TECTONICS
CLIMATE
BASE LEVEL
COMPACTION
CHANNEL TYPE
BEDFORMS
HYDROGRAPH
SEDIMENT LOAD
SUBSIDENCE RATE
SLOPE CHANGES
MIGRATION/AVULSION
INCISION &
t. ..=:=-rc _- -- -�\-_j AGGRADATION
BANKFULL SCOUR
illlo-"'-' -'�-i INCISION/
AGGRADATION
MIGRATION/AVULSION
Fig. 3.1. Hierarchies of scale and time in fluvial deposits. (Leeder 1993)
Collcepts of Scale
3.1 Time Scales and Physical Scales
in Sedimentation
It has become a geological truism that many sedimentary units accumulate as a result of short intervals of rapid sedimentation separated by long
intervals of time when little or no sediment is deposited (Ager 1981, 1993). It is also now widely realized
that rates of sedimentation measured in modern
depositional environments or the ancient record
vary in proportion to the time scale over which they
are measured. Sadler (1981) documented this in detail, and showed that measured sedimentation rates
vary by 11 orders of magnitude, from 10-4 to 107
m/ka. This wide variation reflects the increasing
number and length of intervals of nondeposition or
erosion factored into the measurements as the
FLUVIAL ARCHITECTURE:
SCALE, CONTROLS AND TIME
CONTROLS
PROCESSES
length of the measured stratigraphic record increases. Breaks in the record include such events as
the nondeposition or erosion that takes place in
front of an advancing bedform (a few seconds to
minutes), the nondeposition due to drying out at ebb
tide (a few hours), up to the major regional unconformity generated by orogeny (millions of
years). The variation in sedimentation rate also reflects the variation in actual rates of continuous
accumulation, from the rapid sand flow or grain-fall
accumulation of a cross-bed foreset lamina (time
measured in seconds), and the dumping of graded
beds from a turbidity current (time measured in
hours to days), to the slow pelagic fill of an oceanic
abyssal plain (undisturbed in places for hundreds or
thousands of years, or more). There clearly exists a
wide variety of time scales of sedimentary processes
(Fig. 3.1).
TECTONICS
SUBSIDENCE STYLE
AND RATE
lateral
'"'
TECTONICS
CLIMATE
BASE LEVEL
COMPACTION
CHANNEL TYPE
BEDFORMS
HYDROGRAPH
SEDIMENT LOAD
SUBSIDENCE RATE
SLOPE CHANGES
MIGRATION/AVULSION
INCISION &
t. ..=:=-rc _- -- -�\-_j AGGRADATION
BANKFULL SCOUR
illlo-"'-' -'�-i INCISION/
AGGRADATION
MIGRATION/AVULSION
Fig. 3.1. Hierarchies of scale and time in fluvial deposits. (Leeder 1993)