Growth of Present�Day Concepts, 1978-1988
B
A
s m g
c f c
LllL1J
Fig. 2.26. Stacking of three hypothetical poiilt-bar successions under COIJ-ditions of A high andB low subsidence rate
relative to aggradation rate. InA, complete successions are
preserved intact, together with overlying fl oodplain units.
In B, upper point-bar and floodplain units have been
removed by deep scour, and the resulting succession
is very similar to those produced in some low-sinuosity
multiple-channel rivers, particularly those of South
Saskatchewan type. (Miall !980)
excessive pigeon-holing, which obscures rather than
reveals whatever unity may exist among the variants".
There are two main kinds of problem with the
vertical profile as a precise analytic tool: (1) in some
cases, allogenic and autogenic processes can lead to
the same vertical succession; (2) similar vertical profiles can be generated in rivers of different channel
pattern by different autogenic processes. These
points were explored at length by Miall (1980) from
whom Figs. 2.24, 2.25, and 2.26 were taken.
The cycle shown in Fig. 2.24a may be of autogenic
or allogenic origin, or a combination of both. It
probably represents the fi lling and progressive abandonment of a relatively deep, braided channel system, but it is unclear whether cyclic repetition of this
type of sequence requires tectonic triggering of
41
channel-avulsion events (an allogenic process) or
whether it can be accomplished by autogenic lateral
channel migration. Information on lateral variabi�
lity of the section (not available in this case) may
supply some clues.
The cycles in Fig. 2.24b)c are unquestionably
autogenic in origin. Figure 2.24b shows autogenicity
on two nested levels. The smaller cycles may be
seasonal in origin or represent infrequent fl oods,
whereas the overall cycle represents a migrating
point bar, as does Fig. 2.24c. Figure 2.24d, which is
similar to the others in thi<:: kness and fining-upward
character, represents the deposits of shallow alluvialMfan distributaries in an environment dominated
by tectonic controls. The basin in which it formed
was a tectonically active one, with differential movement between basin and source area adeqUate to
create and maintain a set of alluvial fans at the basin
margin.
The question of scale must also be considered
when comparing the columns in Fig. 2.24. Diagrams
A, B, and C represent relatively large rivers, with
channel depths of several meters. Diagram D represents the deposits of fan 9-istributaries, which probably were much shallower. Autogenic cycles, related
to such causes as channel m�gration and bankfull
floods, would be on a correspondingly smaller scale.
Several thin fining-upward cycles, of probable autogenic origin, can be discerned in diagram D, nested
within the overall fining-upward succession . .
The coarsening-upward cycles of Fig. Z.25A,B resulted from rapid progadation following pulses of
relative uplift on the source areas of alluvial fans in
two different basins; they are therefore of tectonic
origin. They are similar to the succession of Fig.
2.25C, in terms of an upward increase in grain size
and scale of sedimentary structures. Cycle C, however, represents a climatic effect. During periods of
glacial �dvance, enviroP,.ments in fl uvioglacial rivers
are shifted progressively downstream, so that at a
given point the fluvial setting changes from distal to
proximal. The result is a coarsening-upward succession. The example illustrated is from a Pleistocene
su,cession. Other examples were quoted by Miall
(1980).
The question of preservation is also of importance in interpreting vertical profiles. Consider Fig.
2.26, which shows the effects of variation in subsidence rates in a basin undergoing deposition by highsinuosity streams. The main sedimentary process is
the formation of coarse channel-belt deposits by
lateral accretion. During periods of rapid subsidence, these will be rapidly buried and preserved,
B
A
s m g
c f c
LllL1J
Fig. 2.26. Stacking of three hypothetical poiilt-bar successions under COIJ-ditions of A high andB low subsidence rate
relative to aggradation rate. InA, complete successions are
preserved intact, together with overlying fl oodplain units.
In B, upper point-bar and floodplain units have been
removed by deep scour, and the resulting succession
is very similar to those produced in some low-sinuosity
multiple-channel rivers, particularly those of South
Saskatchewan type. (Miall !980)
excessive pigeon-holing, which obscures rather than
reveals whatever unity may exist among the variants".
There are two main kinds of problem with the
vertical profile as a precise analytic tool: (1) in some
cases, allogenic and autogenic processes can lead to
the same vertical succession; (2) similar vertical profiles can be generated in rivers of different channel
pattern by different autogenic processes. These
points were explored at length by Miall (1980) from
whom Figs. 2.24, 2.25, and 2.26 were taken.
The cycle shown in Fig. 2.24a may be of autogenic
or allogenic origin, or a combination of both. It
probably represents the fi lling and progressive abandonment of a relatively deep, braided channel system, but it is unclear whether cyclic repetition of this
type of sequence requires tectonic triggering of
41
channel-avulsion events (an allogenic process) or
whether it can be accomplished by autogenic lateral
channel migration. Information on lateral variabi�
lity of the section (not available in this case) may
supply some clues.
The cycles in Fig. 2.24b)c are unquestionably
autogenic in origin. Figure 2.24b shows autogenicity
on two nested levels. The smaller cycles may be
seasonal in origin or represent infrequent fl oods,
whereas the overall cycle represents a migrating
point bar, as does Fig. 2.24c. Figure 2.24d, which is
similar to the others in thi<:: kness and fining-upward
character, represents the deposits of shallow alluvialMfan distributaries in an environment dominated
by tectonic controls. The basin in which it formed
was a tectonically active one, with differential movement between basin and source area adeqUate to
create and maintain a set of alluvial fans at the basin
margin.
The question of scale must also be considered
when comparing the columns in Fig. 2.24. Diagrams
A, B, and C represent relatively large rivers, with
channel depths of several meters. Diagram D represents the deposits of fan 9-istributaries, which probably were much shallower. Autogenic cycles, related
to such causes as channel m�gration and bankfull
floods, would be on a correspondingly smaller scale.
Several thin fining-upward cycles, of probable autogenic origin, can be discerned in diagram D, nested
within the overall fining-upward succession . .
The coarsening-upward cycles of Fig. Z.25A,B resulted from rapid progadation following pulses of
relative uplift on the source areas of alluvial fans in
two different basins; they are therefore of tectonic
origin. They are similar to the succession of Fig.
2.25C, in terms of an upward increase in grain size
and scale of sedimentary structures. Cycle C, however, represents a climatic effect. During periods of
glacial �dvance, enviroP,.ments in fl uvioglacial rivers
are shifted progressively downstream, so that at a
given point the fluvial setting changes from distal to
proximal. The result is a coarsening-upward succession. The example illustrated is from a Pleistocene
su,cession. Other examples were quoted by Miall
(1980).
The question of preservation is also of importance in interpreting vertical profiles. Consider Fig.
2.26, which shows the effects of variation in subsidence rates in a basin undergoing deposition by highsinuosity streams. The main sedimentary process is
the formation of coarse channel-belt deposits by
lateral accretion. During periods of rapid subsidence, these will be rapidly buried and preserved,