Sedim entation Rate and Its Relation to Depositional Recurrence Interval
73
China, by McKee et a!. (1983), who studied the decay
of short-lived radionucleides in the uppermost 15
em of recent deposits, representing about 100 days of
accumulation. The uppermost 200 em of section,
representing about 100 years of accumulation,
yielded a rate an order of magnitude lower, 5.4 em/
year (54 m/ka). Oomkens (1970) quoted sedimentation rates of 35 em/year (350 m/ka) for the delta front
of the modern Rhone River. The monthly rate at the
Yangtze mouth and that determined for the Rh6ne
compare with the migration rates calculated for Holocene point bars and distributary-mouth bars,
whereas the yearly rate at the Yangtze Mouth is
similar to that of a delta lobe of the Mississippi.
Of particular relevance to the subject of this book
are the sedimentation rates of fluvial deposits at the
groups 6 to 8 level, corresponding to deposits
formed by long-term geomorphic processes, including those related to geomorphic thresholds, and the
largest scale of autogenic depositional feature (as
discussed in detail in Chap. 10). Rates of sedimentation on modern alluvial fans and fluvial floodplains
have been measured using l4C dates on plant material, and tephrochronology. Available data were
summarized by Miall (1978d) and shown to encompass a wide range, from 0.08 to 50 m/ka. However,
these measurements were not correlated to specific
scales of architectural units, such as the depositional
groups defined here. Bridge and Leeder (1979) also
assembled data from modern fluvial systems in order to establish meaningful sedimentation rates for
use in their computer simulation models. They used
a range from 5 to 40 m/ka, with an average of 20 m/
ka, for their models of channel-belt deposition based
on autogenic avulsion processes (group 7).
Leeder (1975) developed a quantitative model for
the estimation of floodplain accretion rates based on
the ages of paleosols in Recent sediments1 particularly those in the arid American southwest region.
The maturity of the paleosol was assessed using various facies criteria (see Sect. 7.4.2), and this was related to the age of the deposit, using 14C dating. It was
suggested that rates of sedimentation in ancient alluvial deposits could be estimated using this model.
However, Wright (1990) indicated that the ages of
the original Recent American paleosols have been
substantially revised, and he adduced a considerable
body of additional data to indicate that the relationship between paleosol maturity and age varied by as
much as an order of magnitu d e. He showed that
Leeder's (1975) mature "stage 4" paleosols in the
Quaternary record require as little as 3 ka or as much
as 1 million years to develop. The reasons for these
variations are discussed in Sect. 7.4.2. Kraus and
Bown (1993) also employed paleosols in an estimation of sediment accumulation rates, but used a different technique that avoids problems of correlation
to Recent equivalents. In their well-studied Eocene
Willwood Formation succession in Wyoming, they
recorded numerous paleosols, and were able to construct an artifical composite section composed entirely of paleosol units, by substituting channel and
floodplain clastics with a paleosol by detailed lateral
correlation. They were able to estimate the relative
time required for each type of paleosol to form by
this same process of lateral correlation. Thus, it was
found that four vertically stacked stage 1 paleosols
are stratigraphically, and therefore temporally, equivalent to a single stage 3 paleosol. Each paleosol
could therefore be weighted according to its relative
time of development, and a time-thickness plot
developed. Given precise biostratigraphic and radiometric brackets on the top and base of the succession, they could then convert this graph to absolute
accumulation rates, and correct this to a sediment
accumulation curve by correcting for compaction.
The results indicated a variation in sedimentation
rate between 0.1 and 2 m/ka. An examination of
unsteady sedimentation processes by Friend et al.
(1989) suggested that in the Siwalik sediments of
Pakistan individual beds of a few meters in thickness, corresponding to channel-flll increments
(group 6) and complete channel-fills (group 7) were
deposited at rates of up to 1 m/ka.
Badgley and Tauxe (1990) carried out a detailed
study of a transition interval between normal and
reversed magnetic polarity within the interfingering
fluvial units previously described by Behrensmeyer
and Tauxe (1982). This interva11 estimated to last
between 4 and 10 ka, varied in thickness from 0.5 to
2.7 m, indicating a fivefold variation in net-accumui ation rate. Badgley and Tami:e (1990) were also able
to demonstrate diachroneity of a major paleosol hOrizon and a channel-fill sandstone unit, both of
which crossed this transitional polarity interval.
It is not immediately clear why the data of Bridge
and Leeder (1979) should suggest sedimentation
rates an order of magnitude greater than those of
Kraus and Bown (1993) and Friend et a!. (1989). In
the case of Kraus and Bown (1993), the main control
on absolute time was provided by radiometric dating
of bracketing volcanic units in an ancient succession, and in the study of Friend et al. (1989) absolute
time limits were derived by magnetostratigraphic
correlation. This contrasts to the estimates from
modern floodplains that comprise the Bridge and
Leeder (1979) data base. It is possible that despite
every effort to relate the calculations to specific beds
73
China, by McKee et a!. (1983), who studied the decay
of short-lived radionucleides in the uppermost 15
em of recent deposits, representing about 100 days of
accumulation. The uppermost 200 em of section,
representing about 100 years of accumulation,
yielded a rate an order of magnitude lower, 5.4 em/
year (54 m/ka). Oomkens (1970) quoted sedimentation rates of 35 em/year (350 m/ka) for the delta front
of the modern Rhone River. The monthly rate at the
Yangtze mouth and that determined for the Rh6ne
compare with the migration rates calculated for Holocene point bars and distributary-mouth bars,
whereas the yearly rate at the Yangtze Mouth is
similar to that of a delta lobe of the Mississippi.
Of particular relevance to the subject of this book
are the sedimentation rates of fluvial deposits at the
groups 6 to 8 level, corresponding to deposits
formed by long-term geomorphic processes, including those related to geomorphic thresholds, and the
largest scale of autogenic depositional feature (as
discussed in detail in Chap. 10). Rates of sedimentation on modern alluvial fans and fluvial floodplains
have been measured using l4C dates on plant material, and tephrochronology. Available data were
summarized by Miall (1978d) and shown to encompass a wide range, from 0.08 to 50 m/ka. However,
these measurements were not correlated to specific
scales of architectural units, such as the depositional
groups defined here. Bridge and Leeder (1979) also
assembled data from modern fluvial systems in order to establish meaningful sedimentation rates for
use in their computer simulation models. They used
a range from 5 to 40 m/ka, with an average of 20 m/
ka, for their models of channel-belt deposition based
on autogenic avulsion processes (group 7).
Leeder (1975) developed a quantitative model for
the estimation of floodplain accretion rates based on
the ages of paleosols in Recent sediments1 particularly those in the arid American southwest region.
The maturity of the paleosol was assessed using various facies criteria (see Sect. 7.4.2), and this was related to the age of the deposit, using 14C dating. It was
suggested that rates of sedimentation in ancient alluvial deposits could be estimated using this model.
However, Wright (1990) indicated that the ages of
the original Recent American paleosols have been
substantially revised, and he adduced a considerable
body of additional data to indicate that the relationship between paleosol maturity and age varied by as
much as an order of magnitu d e. He showed that
Leeder's (1975) mature "stage 4" paleosols in the
Quaternary record require as little as 3 ka or as much
as 1 million years to develop. The reasons for these
variations are discussed in Sect. 7.4.2. Kraus and
Bown (1993) also employed paleosols in an estimation of sediment accumulation rates, but used a different technique that avoids problems of correlation
to Recent equivalents. In their well-studied Eocene
Willwood Formation succession in Wyoming, they
recorded numerous paleosols, and were able to construct an artifical composite section composed entirely of paleosol units, by substituting channel and
floodplain clastics with a paleosol by detailed lateral
correlation. They were able to estimate the relative
time required for each type of paleosol to form by
this same process of lateral correlation. Thus, it was
found that four vertically stacked stage 1 paleosols
are stratigraphically, and therefore temporally, equivalent to a single stage 3 paleosol. Each paleosol
could therefore be weighted according to its relative
time of development, and a time-thickness plot
developed. Given precise biostratigraphic and radiometric brackets on the top and base of the succession, they could then convert this graph to absolute
accumulation rates, and correct this to a sediment
accumulation curve by correcting for compaction.
The results indicated a variation in sedimentation
rate between 0.1 and 2 m/ka. An examination of
unsteady sedimentation processes by Friend et al.
(1989) suggested that in the Siwalik sediments of
Pakistan individual beds of a few meters in thickness, corresponding to channel-flll increments
(group 6) and complete channel-fills (group 7) were
deposited at rates of up to 1 m/ka.
Badgley and Tauxe (1990) carried out a detailed
study of a transition interval between normal and
reversed magnetic polarity within the interfingering
fluvial units previously described by Behrensmeyer
and Tauxe (1982). This interva11 estimated to last
between 4 and 10 ka, varied in thickness from 0.5 to
2.7 m, indicating a fivefold variation in net-accumui ation rate. Badgley and Tami:e (1990) were also able
to demonstrate diachroneity of a major paleosol hOrizon and a channel-fill sandstone unit, both of
which crossed this transitional polarity interval.
It is not immediately clear why the data of Bridge
and Leeder (1979) should suggest sedimentation
rates an order of magnitude greater than those of
Kraus and Bown (1993) and Friend et a!. (1989). In
the case of Kraus and Bown (1993), the main control
on absolute time was provided by radiometric dating
of bracketing volcanic units in an ancient succession, and in the study of Friend et al. (1989) absolute
time limits were derived by magnetostratigraphic
correlation. This contrasts to the estimates from
modern floodplains that comprise the Bridge and
Leeder (1979) data base. It is possible that despite
every effort to relate the calculations to specific beds