436
12.6.3 Bedforms and Cycles
jones (1977) suggested that the style of preservation
of bedforms in fluvial deposits should reflect the
discharge regime of the river in which they were
formed. He classified discharge regimes into five
broad types, as noted earlier in this chapter. Critical
parameters are the ratio of maximum to minimum
discharge and the rate at which discharge changes.
Bedforms have a characteristic ('lag" or ''relaxation
time", which is the time required for them to respond to changes in flow conditions. This time will
be much longer for large forms because of the volume of sand to be moved and, in fact, very large
dunes in major rivers, such as the Brahmaputra, are
unable to undergo continual adjustment to changed
flow conditions and tend simply to be abandoned
during the falling stage. Bedforms are rarely in
perfect equilibrium with flow because of constantly
changing conditions, but in general, the type of
bedform assemblage and the various styles of
bedform modification should provide clues as to the
discharge regime. jones (1977) proposed several
models for field testing, but his work has not been
systematically followed up by subsequent researchers. Following are some points regarding the generation and preservation of features sensitive to
discharge regime.
Is evidence of the discharge regime likely to be
preserved in the sedimentary record? Rising-stage
changes are modified by peal< flows and have low
preservation potential. Falling-stage changes are
more preservable, taking into account the effects of
bedform lag. Jones (1977) suggested that "sand
waves" (also termed ''linguoid bars,) are most likely
to preserve differences in discharge regime because
of their relatively long lag time. These correspond
to what we would now term medium- to largescale two-dimensional dunes (Ashley 1990; see Sect.
5.3.1). Most three-dimensional dunes are small
enough that they are able to respond rapidly to
changes in discharge, and differences between high
and low stage are not as likely to be. preserved.
Rivers characterized by short, sharp floods, such
as the Tana (Norway), have rapidly falling discharge, during which the larger bedforms may be
abandoned. They may become emergent, or may be
draped by smaller forms deposited by low-stage runoff flowing in small channel< eroded between the
larger forms. Flow around emergent bedforms may
erode part of some avalanche faces and deposit
cross-bed sets at a high angle to the dip of the major
foreset. The final abandonment surface becomes a
What Does Fluvial Lithofacies Reveal About Climate?
surface of reactivation during the next flow cycle.
The contrast between large, abandoned forms and
the small, low-stage modifications may survive into
the sedimentary record as evidence of this distinctive discharge regime.
In cases ofless rapid fall in discharge, a discharge
style displayed by such large rivers as the Brahmaputra and.the Mississippi, dune scale may have time
to become adjusted to reduced flow conditions, leading to the superimposition of smaller forms on the
large dunes. Small eddies in the lee of the superimposed forms are likely to erode the crest of the major
form, generating convex-up erosion surfaces that
are buried by avalanching as the superimposed
forms continue to migrate. In perennial rivers with
slow Or minor changes . in discharge, such as the
Platte and Loup rivers, c
'
ompl, ete reworking of the
bedforms c"an occur in any channel that remains
active. Channel migration and avulsion events therefore result in the preservation of dunes displaying a
wide range of scales formed under varying discharge
conditions, with no clear preserved signal of the
discharge regime.
Application of these ideas to ancient fluvial deposits has not, to this writer's knowledge, been attempted. However, the concepts would appear to be
worth testing in conjunction with the other climatic
criteria discussed in this chapter.
In channels deep enough for macroforms to be
developed, accretion and gradual shallowing of the
channel at any given depositional site lead to a progressive lowering of flow velocity and shear stress,
resulting in a decrease in grain size and a predictable
change in the style of bedform generation. This has
long been known to occur on point-bar surfaces and
has formed the basis for the depositional model of
the fm ing-upward cycle, and for computer models of
meandering-river sedimentation under perennial
flow conditions (Allen 1970b; B ridge 1975; Sect. 6.7).
Are·there patterns in the style of macroform preservation that could be interpreted in terms of discharge regime?
The original descriptions of deep, perennial,
sand-bed braided rivers, such as the Brahmaputra
(Coleman 1969) and the South Saskatchewan (Cant
and Walker 1978), did not recognize the three-dimensional nature of the large sand flats, and it is now
known that they develop by lateral, oblique, down·
stream, and even upstream accretion (Bristow 1987;
Sect. 6.6). Cosets of downstream-climbing bedforms
are common (Banks 1973) and were first observed in
ancient deposits by Allen (1983a) and Haszeldine
(1983a,b). Upward fining and an upward decrease in
12.6.3 Bedforms and Cycles
jones (1977) suggested that the style of preservation
of bedforms in fluvial deposits should reflect the
discharge regime of the river in which they were
formed. He classified discharge regimes into five
broad types, as noted earlier in this chapter. Critical
parameters are the ratio of maximum to minimum
discharge and the rate at which discharge changes.
Bedforms have a characteristic ('lag" or ''relaxation
time", which is the time required for them to respond to changes in flow conditions. This time will
be much longer for large forms because of the volume of sand to be moved and, in fact, very large
dunes in major rivers, such as the Brahmaputra, are
unable to undergo continual adjustment to changed
flow conditions and tend simply to be abandoned
during the falling stage. Bedforms are rarely in
perfect equilibrium with flow because of constantly
changing conditions, but in general, the type of
bedform assemblage and the various styles of
bedform modification should provide clues as to the
discharge regime. jones (1977) proposed several
models for field testing, but his work has not been
systematically followed up by subsequent researchers. Following are some points regarding the generation and preservation of features sensitive to
discharge regime.
Is evidence of the discharge regime likely to be
preserved in the sedimentary record? Rising-stage
changes are modified by peal< flows and have low
preservation potential. Falling-stage changes are
more preservable, taking into account the effects of
bedform lag. Jones (1977) suggested that "sand
waves" (also termed ''linguoid bars,) are most likely
to preserve differences in discharge regime because
of their relatively long lag time. These correspond
to what we would now term medium- to largescale two-dimensional dunes (Ashley 1990; see Sect.
5.3.1). Most three-dimensional dunes are small
enough that they are able to respond rapidly to
changes in discharge, and differences between high
and low stage are not as likely to be. preserved.
Rivers characterized by short, sharp floods, such
as the Tana (Norway), have rapidly falling discharge, during which the larger bedforms may be
abandoned. They may become emergent, or may be
draped by smaller forms deposited by low-stage runoff flowing in small channel< eroded between the
larger forms. Flow around emergent bedforms may
erode part of some avalanche faces and deposit
cross-bed sets at a high angle to the dip of the major
foreset. The final abandonment surface becomes a
What Does Fluvial Lithofacies Reveal About Climate?
surface of reactivation during the next flow cycle.
The contrast between large, abandoned forms and
the small, low-stage modifications may survive into
the sedimentary record as evidence of this distinctive discharge regime.
In cases ofless rapid fall in discharge, a discharge
style displayed by such large rivers as the Brahmaputra and.the Mississippi, dune scale may have time
to become adjusted to reduced flow conditions, leading to the superimposition of smaller forms on the
large dunes. Small eddies in the lee of the superimposed forms are likely to erode the crest of the major
form, generating convex-up erosion surfaces that
are buried by avalanching as the superimposed
forms continue to migrate. In perennial rivers with
slow Or minor changes . in discharge, such as the
Platte and Loup rivers, c
'
ompl, ete reworking of the
bedforms c"an occur in any channel that remains
active. Channel migration and avulsion events therefore result in the preservation of dunes displaying a
wide range of scales formed under varying discharge
conditions, with no clear preserved signal of the
discharge regime.
Application of these ideas to ancient fluvial deposits has not, to this writer's knowledge, been attempted. However, the concepts would appear to be
worth testing in conjunction with the other climatic
criteria discussed in this chapter.
In channels deep enough for macroforms to be
developed, accretion and gradual shallowing of the
channel at any given depositional site lead to a progressive lowering of flow velocity and shear stress,
resulting in a decrease in grain size and a predictable
change in the style of bedform generation. This has
long been known to occur on point-bar surfaces and
has formed the basis for the depositional model of
the fm ing-upward cycle, and for computer models of
meandering-river sedimentation under perennial
flow conditions (Allen 1970b; B ridge 1975; Sect. 6.7).
Are·there patterns in the style of macroform preservation that could be interpreted in terms of discharge regime?
The original descriptions of deep, perennial,
sand-bed braided rivers, such as the Brahmaputra
(Coleman 1969) and the South Saskatchewan (Cant
and Walker 1978), did not recognize the three-dimensional nature of the large sand flats, and it is now
known that they develop by lateral, oblique, down·
stream, and even upstream accretion (Bristow 1987;
Sect. 6.6). Cosets of downstream-climbing bedforms
are common (Banks 1973) and were first observed in
ancient deposits by Allen (1983a) and Haszeldine
(1983a,b). Upward fining and an upward decrease in