72
S�cm�high ripple that forms in 30 min is equivalent
to an instantaneous sedimentation rate of 876000
m/ka. Clearly, this number bears no relationship to
real long-term sedimentation rates, but it serves to
emphasize the extremes of sedimentation rate, to
compare with more geologically typical rates discussed later. Tidal sand waves (group 3) have similarly very high instantaneous rates. In the Bay of
Fundy, Dalrymple (1984) demonstrated that in one
tidal cycle sand waves migrate a distance about
equivalent to their average height, which is 0.8 m.
Bay of Fundy tides are semidiurnal, and so this migration is equivalent to a sedimentation rate of
584000 m/ka. Climbing-ripple stratification represents intervals of extremely rapid sedimentation.
Data collected by Bristow (1993a) from large bars in
the modern Brahmaputra River suggested that for
very short periods as much as 3.75 m of sediment
may accumulate in the space of 1 h, an instantaneous
sedimentation rate of 33 x106 m/ka!
The deposits formed by seasonal or more irregu�
lar runoff events in rivers (group 5) have extremely
variable instantaneous sedimentation rates. The
flood deposit in Bijou Creek, Colorado, described by
McKee et al. (1967) was formed by the most violent
flood in 30 years. It formed 1 to 4 m of sediment in
about 12 h, an instantaneous sedimentation rate of
730000-2920000 m/ka. Assuming no erosion, and a
repetition of such floods every 30 years, this translates into a rate of 33-133 rnfka averaged over a few
hundred years. In fact, scour depths during the flood
ranged from 1.5 to 3m, and true net preservation of
any one flood deposit over periods of hundreds or
thousands of years may be negligible. Long-term
rates measured over hundreds to thousands of years
are likely to be an order of magnitude less, in the
range of 10-1 m/ka.
The point bars (group 6) in the Wabash River are
on average about 5 m thick. The aCtive area of each
bar is about 200 m wide. If the bars migrate at a
maximum rate of 2 km in 50 years (Jackson 1976b)
they would take 5 years to migrate one point-bar
width, which is equivalent to an instantaneous sedimentation rate of 1000 m/ka. Comparable rates may
be calculated from the migration of distributary
mouth bars. A typical Mississippi distributary migrates 3 km in 100 years (Gould 1970; Fig. 3.6). The
mouth-bar deposits, from the mouth of the channel
to the toe of the distal bar, are about 4 km wide (in a
dip direction) and about 45 m thick. This lateral
migration is equivalent to an instantaneous vertical
sedimentation rate of 340 m/ka. Data from modern
Dutch tidal deposits summarized by Yang and Nio
(1989) showed that ebb-tide deltas (group 6 deposConcepts of Scale
its) accumulate at rates of 100 to 450 m/ka for periods of about 20 years, before abandonment occurs.
If we assume that a distributary will only build out
across a given area of the delta front once during the
migration of one major delta lobe, we can calculate
the sedimentation rate of the mouth bar averaged
over the life of the lobe (group 7). In the postglacial
Mississippi delta, major lobes are formed and abandoned in about 1000 years (Kolb and Van Lopik
1966; Frazier 1967; Fig. 3.8 above), giving an average
sedimentation rate for that period of 45 m/ka. The
recurrence interval of delta lobes themselves depends on subsidence rates, sediment -supply, and the
configuration of the continental shelf. In the case of
the Mississippi complex, the river is attempting to
switch discharge (and delta construction) to the
Atchafalaya River, where one of the earliest lobes
developed about 6 to 8 ka before present (Sect.
10.3.1). On this scale deposition of a 45-m-thick
mouth-bar deposit represents a sedimentation rate
of 5.6 to 7.5 m/ka, although this calculation does not
take into account the bay-fill and other facies
interbedded with the mouth bar deposit. This compares with typical values for Holocene sedimentation rates_ of 6 to 12 m/ka that are commonly quoted
for the Mississippi delta complex (e.g., Weimer
1970). Other deltas do not accumulate so rapidly. For
example the Rh6ne delta has a thickness of only 50
m, accumulated since about 8.2 ka, indicating an
average sedimentation rate of 0.6 m/ka (Oomkens
1970).
A few sedimentation rates can be calculated for
tidal-inlet and barrier deposits. The Galveston Island barrier is 12 m thick and 3.5 ka old at its base
(Bernard et al. 1962; Fig. 3.7D), indicating an average
sedimentation rate of 3.4 m/ka. A tidal inlet at Fire
Island, New York, has migrated 8 km in 115 years
(Kumar and Sanders 1974). The depositional slope
from spit crest to channel floor is about 500 m wide,
suggesting that at any one point the entire tidal-inlet
fill could fo rm by lateral accretion in about 7 years.
The sequence is 12 m thick, indicating an. instantaneous sedimentation rate of 1714 m/ka. This migration rate is unusually rapid. Tidal inlets at Sapelo
Island) Georgia, appear to have migrated only about
2.5 km since the postglacial sea-level rise (Hoyt and
Henry 1967; Fig. 3.7C), indicating a sedimentation
rate of 4.5 m/ka.
It is instructive to compare these rates with those
obtained by Oomkens (1970) and McKee et al. (1983)
as a result of their studies of modern sedimentation
rates on the continental shelf off river mouths. A
short-term rate of 4.4 em/month (528 m/ka) was
determined close to the mouth of the Yangtze River )
S�cm�high ripple that forms in 30 min is equivalent
to an instantaneous sedimentation rate of 876000
m/ka. Clearly, this number bears no relationship to
real long-term sedimentation rates, but it serves to
emphasize the extremes of sedimentation rate, to
compare with more geologically typical rates discussed later. Tidal sand waves (group 3) have similarly very high instantaneous rates. In the Bay of
Fundy, Dalrymple (1984) demonstrated that in one
tidal cycle sand waves migrate a distance about
equivalent to their average height, which is 0.8 m.
Bay of Fundy tides are semidiurnal, and so this migration is equivalent to a sedimentation rate of
584000 m/ka. Climbing-ripple stratification represents intervals of extremely rapid sedimentation.
Data collected by Bristow (1993a) from large bars in
the modern Brahmaputra River suggested that for
very short periods as much as 3.75 m of sediment
may accumulate in the space of 1 h, an instantaneous
sedimentation rate of 33 x106 m/ka!
The deposits formed by seasonal or more irregu�
lar runoff events in rivers (group 5) have extremely
variable instantaneous sedimentation rates. The
flood deposit in Bijou Creek, Colorado, described by
McKee et al. (1967) was formed by the most violent
flood in 30 years. It formed 1 to 4 m of sediment in
about 12 h, an instantaneous sedimentation rate of
730000-2920000 m/ka. Assuming no erosion, and a
repetition of such floods every 30 years, this translates into a rate of 33-133 rnfka averaged over a few
hundred years. In fact, scour depths during the flood
ranged from 1.5 to 3m, and true net preservation of
any one flood deposit over periods of hundreds or
thousands of years may be negligible. Long-term
rates measured over hundreds to thousands of years
are likely to be an order of magnitude less, in the
range of 10-1 m/ka.
The point bars (group 6) in the Wabash River are
on average about 5 m thick. The aCtive area of each
bar is about 200 m wide. If the bars migrate at a
maximum rate of 2 km in 50 years (Jackson 1976b)
they would take 5 years to migrate one point-bar
width, which is equivalent to an instantaneous sedimentation rate of 1000 m/ka. Comparable rates may
be calculated from the migration of distributary
mouth bars. A typical Mississippi distributary migrates 3 km in 100 years (Gould 1970; Fig. 3.6). The
mouth-bar deposits, from the mouth of the channel
to the toe of the distal bar, are about 4 km wide (in a
dip direction) and about 45 m thick. This lateral
migration is equivalent to an instantaneous vertical
sedimentation rate of 340 m/ka. Data from modern
Dutch tidal deposits summarized by Yang and Nio
(1989) showed that ebb-tide deltas (group 6 deposConcepts of Scale
its) accumulate at rates of 100 to 450 m/ka for periods of about 20 years, before abandonment occurs.
If we assume that a distributary will only build out
across a given area of the delta front once during the
migration of one major delta lobe, we can calculate
the sedimentation rate of the mouth bar averaged
over the life of the lobe (group 7). In the postglacial
Mississippi delta, major lobes are formed and abandoned in about 1000 years (Kolb and Van Lopik
1966; Frazier 1967; Fig. 3.8 above), giving an average
sedimentation rate for that period of 45 m/ka. The
recurrence interval of delta lobes themselves depends on subsidence rates, sediment -supply, and the
configuration of the continental shelf. In the case of
the Mississippi complex, the river is attempting to
switch discharge (and delta construction) to the
Atchafalaya River, where one of the earliest lobes
developed about 6 to 8 ka before present (Sect.
10.3.1). On this scale deposition of a 45-m-thick
mouth-bar deposit represents a sedimentation rate
of 5.6 to 7.5 m/ka, although this calculation does not
take into account the bay-fill and other facies
interbedded with the mouth bar deposit. This compares with typical values for Holocene sedimentation rates_ of 6 to 12 m/ka that are commonly quoted
for the Mississippi delta complex (e.g., Weimer
1970). Other deltas do not accumulate so rapidly. For
example the Rh6ne delta has a thickness of only 50
m, accumulated since about 8.2 ka, indicating an
average sedimentation rate of 0.6 m/ka (Oomkens
1970).
A few sedimentation rates can be calculated for
tidal-inlet and barrier deposits. The Galveston Island barrier is 12 m thick and 3.5 ka old at its base
(Bernard et al. 1962; Fig. 3.7D), indicating an average
sedimentation rate of 3.4 m/ka. A tidal inlet at Fire
Island, New York, has migrated 8 km in 115 years
(Kumar and Sanders 1974). The depositional slope
from spit crest to channel floor is about 500 m wide,
suggesting that at any one point the entire tidal-inlet
fill could fo rm by lateral accretion in about 7 years.
The sequence is 12 m thick, indicating an. instantaneous sedimentation rate of 1714 m/ka. This migration rate is unusually rapid. Tidal inlets at Sapelo
Island) Georgia, appear to have migrated only about
2.5 km since the postglacial sea-level rise (Hoyt and
Henry 1967; Fig. 3.7C), indicating a sedimentation
rate of 4.5 m/ka.
It is instructive to compare these rates with those
obtained by Oomkens (1970) and McKee et al. (1983)
as a result of their studies of modern sedimentation
rates on the continental shelf off river mouths. A
short-term rate of 4.4 em/month (528 m/ka) was
determined close to the mouth of the Yangtze River )