358
a delta will prograde seaward only a limited distance
before autogenic switching leads to the development
of a shorter, steeper route to the sea. And however far
the river mouth moves horizontally in response to
progradation, it must also move vertically in response to base-level change. Whatever the slope of
the continental shelf, a fall in sea level fo rces the river
mouth to drop. The slope of the shelf is therefore
relevant to questions of accommodation space, as
discussed above.
There is therefore no a priori reason why a fall in
sea level should be accompanied by creation of accommodation space and consequent fluvial aggradation and coastal onlap. The extension of a thin
fluvial sheet seaward over shoreline deposits seems
likely, but thick aggradation and coastal onlap does
not seem probable. For example; note that in the
thick clastic-wedge cycles described by Embry
(1990a, 1991) there is no wedge of onlapping fluvial
strata between the subaerial unconformity (se�
quence boundary) and the overlying transgressive
deposits. This supports my objections to the
Posamentier and Vail (1988) model. Additional support has been provided by the numerical models of
Rivenaes (1992).
With regard to horizontal shifts in the mouth of a
river (for example, as it "tracks'' a prograding delta:
Nummedal et at. 1993), the condition shown in
Fig. 11.11, Leopold et al. (1964, pp. 260-266) and
Leopold and Bull (1979) presented data showing
that a rise in base level or horizontal extension
of a profile, as in the filling of and progradation
across a reservoir, has a very minor effect on
the overall river profile. Aggradation takes place
for a few hundred meters above the reservoir.
Above this the longitudinal profile is characterized
by a transitional reach, which curves upstream
into the remainder of the river, whose slope and
longitudinal profile remain unaffected. Schumm
(1993) suggested that the rivers observed by
Leopold et al. (!964) and Leopold and Bull (1979)
were too small and the observation time too short
for these to be useful examples from a geological point of view. The major rivers draining into
the Gulf of Mexico show significant incision relat�
ing to glacioeustatic falls in base leveli for example,
the Mississippi valley is incised for 370 km upstream from its mouth, as far as Baton Rouge
(Schumm !993), with aggradation having taken
place during the subsequent rise. However, the
precise nature of the relationship between baselevel change and changes in equilibrium profile
in these rivers is confused by the nature of complex
Tectonic Control of Fluvial Sedimentation
response referred to at the beginning of this section
(and was also rendered more complex by climate
change, as discussed in Sect. 12.12.2).
The shift in the entire profile suggested by
Posamentier et al. (1988) has not been observed in
any of the rivers described by Leopold et al. (1964),
Leopold and Bull (1979), and Schumm (1993). Even
if such a shift could occur, it would create little
accommodation space. Given a slope ofO.OOl, typical
of moderate to large rivers such as the Nile (Table
11.1), simple trigonometry indicates that moving the
mouth of the river horizontally a distance of 100 krn
would create only 10 m of accommodation space,
which is entirely inadequate to explain the large
thicknesses of fluvial sediments present in some
stratigraphic sequences (e.g., see Galloway !989b).
The Yellow River of China illustrates this point (Qian
1990, as quoted by Schumm !993). The river aggraded significantly between the late seventeenth
century and !855 as a result of the growth ofits delta.
During this period the lower reach of the river was
extended nearly 200 km by deltaic progradation, and
the channel aggraded its bed to 10 m above the level
of the North China Plain. However, this effect oc�
curred only because of the confinement of the channel between artificial levees, which were breached by
a major flood in 1855. Without the levees the increase in accommodation space would have resulted
in channel and delta-lobe switching, probably without significant aggradation.
Recent work by Leckie (1994) on the rivers of the
Canterbury Plains, New Zealand, demonstrated that
transgression is not necessarily accompanied by the
immediate creation of accommodation space. Be�
cause of very high wave energies along the east coast
of South Island, New Zealand, much of the coastal
plain is undergoing erosional shoreface retreat, and
the rivers are incising their valleys near their mouths
as they grade down to the retreating shoreline. The
process is analogous to the development of the
ravinement surface that develops during transgres�
sion of a shallow shelf.
Another assertion by Posamentier et al. (1988, p.
!35) that should be commented on is the prediction
of an «overall upward-fining trend" in grain size in
intermediate positions of the stream profile (e.g., a
short distance downstream from A in Fig. !1.!1 ).
This remark is likely to lead to many simplistic deductions made from observed grain�size trends in
well logs. The argument is that in the new accommo�
dation space slopes are initially lower, leading to a
dumping of coarse sediment, followed by remobilization of this sediment as slopes increase again.
a delta will prograde seaward only a limited distance
before autogenic switching leads to the development
of a shorter, steeper route to the sea. And however far
the river mouth moves horizontally in response to
progradation, it must also move vertically in response to base-level change. Whatever the slope of
the continental shelf, a fall in sea level fo rces the river
mouth to drop. The slope of the shelf is therefore
relevant to questions of accommodation space, as
discussed above.
There is therefore no a priori reason why a fall in
sea level should be accompanied by creation of accommodation space and consequent fluvial aggradation and coastal onlap. The extension of a thin
fluvial sheet seaward over shoreline deposits seems
likely, but thick aggradation and coastal onlap does
not seem probable. For example; note that in the
thick clastic-wedge cycles described by Embry
(1990a, 1991) there is no wedge of onlapping fluvial
strata between the subaerial unconformity (se�
quence boundary) and the overlying transgressive
deposits. This supports my objections to the
Posamentier and Vail (1988) model. Additional support has been provided by the numerical models of
Rivenaes (1992).
With regard to horizontal shifts in the mouth of a
river (for example, as it "tracks'' a prograding delta:
Nummedal et at. 1993), the condition shown in
Fig. 11.11, Leopold et al. (1964, pp. 260-266) and
Leopold and Bull (1979) presented data showing
that a rise in base level or horizontal extension
of a profile, as in the filling of and progradation
across a reservoir, has a very minor effect on
the overall river profile. Aggradation takes place
for a few hundred meters above the reservoir.
Above this the longitudinal profile is characterized
by a transitional reach, which curves upstream
into the remainder of the river, whose slope and
longitudinal profile remain unaffected. Schumm
(1993) suggested that the rivers observed by
Leopold et al. (!964) and Leopold and Bull (1979)
were too small and the observation time too short
for these to be useful examples from a geological point of view. The major rivers draining into
the Gulf of Mexico show significant incision relat�
ing to glacioeustatic falls in base leveli for example,
the Mississippi valley is incised for 370 km upstream from its mouth, as far as Baton Rouge
(Schumm !993), with aggradation having taken
place during the subsequent rise. However, the
precise nature of the relationship between baselevel change and changes in equilibrium profile
in these rivers is confused by the nature of complex
Tectonic Control of Fluvial Sedimentation
response referred to at the beginning of this section
(and was also rendered more complex by climate
change, as discussed in Sect. 12.12.2).
The shift in the entire profile suggested by
Posamentier et al. (1988) has not been observed in
any of the rivers described by Leopold et al. (1964),
Leopold and Bull (1979), and Schumm (1993). Even
if such a shift could occur, it would create little
accommodation space. Given a slope ofO.OOl, typical
of moderate to large rivers such as the Nile (Table
11.1), simple trigonometry indicates that moving the
mouth of the river horizontally a distance of 100 krn
would create only 10 m of accommodation space,
which is entirely inadequate to explain the large
thicknesses of fluvial sediments present in some
stratigraphic sequences (e.g., see Galloway !989b).
The Yellow River of China illustrates this point (Qian
1990, as quoted by Schumm !993). The river aggraded significantly between the late seventeenth
century and !855 as a result of the growth ofits delta.
During this period the lower reach of the river was
extended nearly 200 km by deltaic progradation, and
the channel aggraded its bed to 10 m above the level
of the North China Plain. However, this effect oc�
curred only because of the confinement of the channel between artificial levees, which were breached by
a major flood in 1855. Without the levees the increase in accommodation space would have resulted
in channel and delta-lobe switching, probably without significant aggradation.
Recent work by Leckie (1994) on the rivers of the
Canterbury Plains, New Zealand, demonstrated that
transgression is not necessarily accompanied by the
immediate creation of accommodation space. Be�
cause of very high wave energies along the east coast
of South Island, New Zealand, much of the coastal
plain is undergoing erosional shoreface retreat, and
the rivers are incising their valleys near their mouths
as they grade down to the retreating shoreline. The
process is analogous to the development of the
ravinement surface that develops during transgres�
sion of a shallow shelf.
Another assertion by Posamentier et al. (1988, p.
!35) that should be commented on is the prediction
of an «overall upward-fining trend" in grain size in
intermediate positions of the stream profile (e.g., a
short distance downstream from A in Fig. !1.!1 ).
This remark is likely to lead to many simplistic deductions made from observed grain�size trends in
well logs. The argument is that in the new accommo�
dation space slopes are initially lower, leading to a
dumping of coarse sediment, followed by remobilization of this sediment as slopes increase again.