Geology Reference
In-Depth Information
CRN age of boulders sampled from a moraine
surface can in fact decrease with the true age of
the moraine. This example highlights the need
to take into account the dynamic nature of the
geomorphic surfaces on which CRN techniques
are currently being employed. Such integration
requires the marriage of cosmogenic and numer-
ical landscape evolution modeling techniques.
studies have targeted the generation of the terrace
platforms in the first place. Here we summarize a
study that addressed the issue of both generation
and demise of these useful geomorphic features
(Anderson
et al
., 1999). To capture the evolution
of a marine terraced landscape in a numerical
model, one must include uplift of the landmass,
oscillation of sea level, and erosion of the
coastline. Anderson
et al
. (1999) treated this
system of processes while focusing on parameters
relevant to the suite of five marine terraces at
Santa Cruz, California. In a two-dimensional
shore-normal model (Fig. 11.10), they employed
a long-term sea-level curve based upon the
δ
Conclusions from modeling
at the hillslope scale
Proper use of this sort of modeling to estimate
the ages of scarps and similar topographic fea-
tures requires all of the following: (i) knowledge
of initial conditions; (ii) awareness of talus-
like processes that violate the linear diffusion
process rule in the early stages of topographic
evolution; (iii) treatment of out-of-plane pro-
cesses that might remove material from the
cross-section, thereby violating the one-dimen-
sional conservation of mass equation; (iv)
explicit treatment of the generation of transport-
able particles if cohesive materials or bedrock
are involved in the original scarp; (v) awareness
of a possible dependence of the diffusion coef-
ficient on slope aspect, through a dependence
on process efficiency and possible process type
with orientation; and (vi) estimation of the diffu-
sion coefficient by appeal to nearby or climati-
cally relevant estimates from other well-dated
sites in similar materials.
Given these complexities, it is perhaps best
not to rely on a single slope within the scarp
(e.g., mid-point slope) to infer age, but rather to
construct a forward numerical model of the
entire scarp that explicitly addresses the issues
outlined previously. Modeled scarp profiles can
then be assessed against field-collected profile
data using goodness-of-fit estimates for a range
of possible models, in which the full range of
possible weathering rates and diffusion coeffi-
cients may be explored.
O
curve (Fig. 2.3), and rock uplift prescribed to
occur uniformly with shore-normal distance. The
erosion rate was specified to occur at a rate that
was damped by a crude approximation of wave
interaction with the offshore bathymetry, i.e.,
a broad, shallow shelf would dissipate more wave
energy than a steep, narrow shelf. Combination of
these processes leads to a set of marine terraces,
the number of preserved terraces being governed
largely by the rate of rock uplift, given that the
sea-level history to which all coastlines have been
subjected has been essentially the same.
The numerical model reproduces the means
by which older terraces generated during a lower
sea-level highstand can be cut away by subse-
quent, higher sea-level maxima. This selective
preservation is analogous to the moraine sur-
vival problem beautifully explained by Gibbons
et al
. (1984), in which younger, longer glaciers
can obliterate the moraines left by earlier, shorter
glaciers, resulting in a small number of recorded
advances (see Box 2.3). Whereas the algorithm
for the erosion of sea floor is crude, the model
nonetheless raises the need for knowledge of
these submarine processes. In particular, the
shape (width, convexity, etc.) of the continental
shelf left in the wake of repeated beveling of the
margin by the sea requires greater scrutiny
(Adams
et al
., 2005). Fortunately, the increasing
availability of high-resolution imaging of the sea
floor provides a basis for tackling this problem.
One might also ask why most active tectonic
coastlines display only a small suite of terraces -
two, three or five, but rarely more. Why is this? If
the rock uplift has been ongoing for millions of
18
Marine terrace generation
Whereas several models exist of the decaying sea
cliffs that bound a set of marine terraces, far fewer
