Geology Reference
In-Depth Information
Reconstructin
g T
errace Heights
A
B
C
D
Cover
old
?
?
???
Strath
old
H
act
1
H
act
2
bedrock
modern river
H
app
H
act
H
app
H
act
Base Level
old
1
Base Level
mod
Cover
mod
?
?
Strath
mod
Base Level
old
2
terms
old strath unknown
modern cover unknown
old strath known
modern cover unknown
old base level
unknown
Actual Height of Strath = H
act
= H
app
- Cover
old
+ Cover
mod
- Base Level
old
H
act
= actual strath height
H
app
= apparent terrace height
Fig. 7.24
Reconstruction of differential uplift from terrace heights.
Uncertainties in the height of an uplifted bedrock strath above the strath's position when it formed. A. Geomorphic
cross-section with a preserved, uplifted strath that is buried with terrace gravels. The modern river rests on a fill above the
most recently formed strath. B. Uncertainties in the actual strath height due to unknown position of the old strath (the
contact is covered) and unknown depth of fill beneath the modern river. C. If the modern river appears to be flowing on
bedrock, then the apparent and actual strath height may be almost equivalent. D. Additional uncertainty is added if the
base level of the river has changed, such that the strath initially formed above or below the modern river level.
other and contain sedimentological evidence that
former rivers flowed
across
the site of the present
range, thereby connecting these stratigraphic rem-
nants, this implied continuity would be strong evi-
dence for surface uplift (Fig. 7.23). Even if it can
only be shown that a river system on one side of a
range had a distant source area on the far side of
the range, this geometry also demonstrates growth
of the range since deposition. Calculation of the
amount of surface or tectonic uplift will depend
on how reliably the paleoelevation of the ancient
depositional system can be constrained. Corre-
lations of the Neogene stratigraphic record in
Alaska have been used to argue that, only a few
million years ago, low-altitude rivers flowed across
the present site of Mt. McKinley (Denali), the
highest peak in North America (Fitzgerald
et al.
,
1995). Although far from the present coast, these
strata provide a reference frame for assessing
surface uplift.
document differential bedrock uplift through
time, if certain assumptions are met. It must be
assumed that the longitudinal gradients of the
modern and ancient valley bottoms were approxi-
mately the same and that any differences in
former and present base levels must be small
compared to the magnitude of rock uplift. Both
aggradational and strath terraces can be used to
reconstruct former river profiles. In terms of
reconstructing relative rock uplift, however, meas-
uring between former and current strath surfaces
is preferable, because, in each case, the lowest
level of bedrock erosion is being recorded.
Although surveying a terrace's height above the
modern river is straightforward, determining the
height of a strath above the base level at which it
was formed can be considerably more complex
(Fig. 7.24). Uncertainties can arise due to the
unknown amount of fill beneath the modern river,
the unknown thickness of terrace cover (if the
strath itself is not visible), and the unknown posi-
tion of base level at the time the strath was formed.
Despite such uncertainties, deformed terraces
clearly record differential uplift. Typically, the
modern river profile is used as a reference frame
River profiles
Although commonly far removed from coastal
areas, reconstructed river profiles can be used to
