Sedimentation Rate and Its Relation to Depositional Recurrence Interval
71
growth terminated by a shift in the depositional
milieu, such as a channel-avulsion event. As noted
above, however, similarity of form does not neces�
sadly indicate similarity in origins. Fluvial point
bars and downstream-accreted macroforms are
group 6 deposits. They are bounded by fourth-order
surfaces (Miall 1988a,b), although commonly, in the
ancient record, the caps of these deposits are planed
off during subsequent erosional events, and the top
surface is not preserved. Fluvial macroforms are
typically capped by the fifth-order surfaces that defi ne the base of succeeding channel-fill units. Little is
known about the bounding surfaces that enclose
sand-wave and sand-ridge deposits in the ancient
record.
In many' outcrops, the most prominent bounding
surfaces are those between group 7 elements, such as
the fifth-order surfaces that define the base of fluvial
channels (Miall 1988a,b), the "first-order" eolian
surfaces of Brookfield (1977), and the "first-order"
surfaces in the sand-wave deposits described by
Berne et al. (1991). These commonly have planar
geometry, except for the curvature at channel
cutbanks (rarely preserved), and may represent
hundreds to thousands of years of nondeposition,
for example, between events of fluvial�channel or
draa migration. There may be little to distinguish
these, at the outcrop level, from the surfaces that
define sediment bodies of groups 8 through 10 (e.g.,
eolian supersurfaces, sequence boundaries), and
recognition and classification of such surfaces then
depend on careful mapping and regional stratigraphic interpretation.
Lithostratigraphic and allostratigraphic subdivisions (formation and member boundaries) and sequence boundaries constitute a still larger scale of
bounding surface. These include the sixth-order fluvial surfaces of Miall (1988a,b) and the eolian supersurfaces of Kocurek (1988). They are discussed
further in Sects. 4.5 and 9.6.
3.4 Sedimentation Rate and Its Relation
to Depositional Recurrence Interval
It is a common observation that measured or calculated sedimentation rates vary in inverse relation to
the length of time over which the measurement is
made (Sadler 1981, provided detailed documentation). In general, the sedimentation rate of the depositional units in each of the 11 groups described here
differ markedly from those of the next group, comrnonly by an order of magnitude. The reason for this
is that sedimentation is rarely continuous for more
than a few weeks or months, at most, at any one
location, and typically is interrupted by erosion or
nondeposition, so that net accumulation is almost
always substantially less than total sedimentation.
Areas where sedimentation might be expected to be
most nearly continuous include the deep oceans,
where very slow pelagic settling occurs, and in the
sea off the mouths of perennial rivers, especially
below the infl uence of waves. In this section, an
attempt is made to evaluate variations in sedimentation rate with respect to the ten sediment groups
defined in this chapter.
An important consideration in evaluating sedimentation rates is the concept of recurrence interval.
As noted by jackson (1975), disproportionate volumes of sediment are cqmmonly moved by infrequent dynamic events. The sedimentation (and
accumulation) rate during the event may be extremely high, but must be discounted for geological
purposes by factoring in the (typically) much longer
periods of time when sedimentation is slow, or might
even be negative, as a result of erosion. For example,
a magnetostratigraphic study of a record of fluvial
fl ash flood deposits in Argentina indicated that only
one fl ood event every 50 to 500 years is · preserved in
the rock record there (Beer 1990). Spasmodic sedimentation also occurs in areas characterized by lat¥
eral accretion or progradation. A depositional
element, such as a delta distributary mouth, or a
fluvial point bar, builds rapidly in a horizontal direc�
tion, giving rise to temporarily rapid local sedimentation rates, but then the dispersal system switches
elsewhere for an extended period of time, with resultant abandonment and, perhaps, erosion of the
new deposit.
These points can be illustrated by a discussion of
some of the sedimentation rates that have been measured for the Various types and scales of deposits that
develop in rivers, tidal estuaries, and deltas. Estimates of order-of-magnitude instantaneous sedimentation rates averaged over the time span of each
of the ten groups are given in Table 3.2. An "instantaneous sedimentation rate'' is one calculated for a
short period of time and extrapolated to longer periods for purposes of comparison. Examples of the
type of calculation that can be made for some of the
groups are given later.
Among the smallest sedimentary structures are
small-scale ripples (group 2 of this work). These
typically migrate a distance equivalent to their own
wavelength in 20 to 60 min (Southard et al. 1980). A
71
growth terminated by a shift in the depositional
milieu, such as a channel-avulsion event. As noted
above, however, similarity of form does not neces�
sadly indicate similarity in origins. Fluvial point
bars and downstream-accreted macroforms are
group 6 deposits. They are bounded by fourth-order
surfaces (Miall 1988a,b), although commonly, in the
ancient record, the caps of these deposits are planed
off during subsequent erosional events, and the top
surface is not preserved. Fluvial macroforms are
typically capped by the fifth-order surfaces that defi ne the base of succeeding channel-fill units. Little is
known about the bounding surfaces that enclose
sand-wave and sand-ridge deposits in the ancient
record.
In many' outcrops, the most prominent bounding
surfaces are those between group 7 elements, such as
the fifth-order surfaces that define the base of fluvial
channels (Miall 1988a,b), the "first-order" eolian
surfaces of Brookfield (1977), and the "first-order"
surfaces in the sand-wave deposits described by
Berne et al. (1991). These commonly have planar
geometry, except for the curvature at channel
cutbanks (rarely preserved), and may represent
hundreds to thousands of years of nondeposition,
for example, between events of fluvial�channel or
draa migration. There may be little to distinguish
these, at the outcrop level, from the surfaces that
define sediment bodies of groups 8 through 10 (e.g.,
eolian supersurfaces, sequence boundaries), and
recognition and classification of such surfaces then
depend on careful mapping and regional stratigraphic interpretation.
Lithostratigraphic and allostratigraphic subdivisions (formation and member boundaries) and sequence boundaries constitute a still larger scale of
bounding surface. These include the sixth-order fluvial surfaces of Miall (1988a,b) and the eolian supersurfaces of Kocurek (1988). They are discussed
further in Sects. 4.5 and 9.6.
3.4 Sedimentation Rate and Its Relation
to Depositional Recurrence Interval
It is a common observation that measured or calculated sedimentation rates vary in inverse relation to
the length of time over which the measurement is
made (Sadler 1981, provided detailed documentation). In general, the sedimentation rate of the depositional units in each of the 11 groups described here
differ markedly from those of the next group, comrnonly by an order of magnitude. The reason for this
is that sedimentation is rarely continuous for more
than a few weeks or months, at most, at any one
location, and typically is interrupted by erosion or
nondeposition, so that net accumulation is almost
always substantially less than total sedimentation.
Areas where sedimentation might be expected to be
most nearly continuous include the deep oceans,
where very slow pelagic settling occurs, and in the
sea off the mouths of perennial rivers, especially
below the infl uence of waves. In this section, an
attempt is made to evaluate variations in sedimentation rate with respect to the ten sediment groups
defined in this chapter.
An important consideration in evaluating sedimentation rates is the concept of recurrence interval.
As noted by jackson (1975), disproportionate volumes of sediment are cqmmonly moved by infrequent dynamic events. The sedimentation (and
accumulation) rate during the event may be extremely high, but must be discounted for geological
purposes by factoring in the (typically) much longer
periods of time when sedimentation is slow, or might
even be negative, as a result of erosion. For example,
a magnetostratigraphic study of a record of fluvial
fl ash flood deposits in Argentina indicated that only
one fl ood event every 50 to 500 years is · preserved in
the rock record there (Beer 1990). Spasmodic sedimentation also occurs in areas characterized by lat¥
eral accretion or progradation. A depositional
element, such as a delta distributary mouth, or a
fluvial point bar, builds rapidly in a horizontal direc�
tion, giving rise to temporarily rapid local sedimentation rates, but then the dispersal system switches
elsewhere for an extended period of time, with resultant abandonment and, perhaps, erosion of the
new deposit.
These points can be illustrated by a discussion of
some of the sedimentation rates that have been measured for the Various types and scales of deposits that
develop in rivers, tidal estuaries, and deltas. Estimates of order-of-magnitude instantaneous sedimentation rates averaged over the time span of each
of the ten groups are given in Table 3.2. An "instantaneous sedimentation rate'' is one calculated for a
short period of time and extrapolated to longer periods for purposes of comparison. Examples of the
type of calculation that can be made for some of the
groups are given later.
Among the smallest sedimentary structures are
small-scale ripples (group 2 of this work). These
typically migrate a distance equivalent to their own
wavelength in 20 to 60 min (Southard et al. 1980). A