Geology Reference
In-Depth Information
modern channels to dissected remnants of
long-abandoned surfaces. Weathering of clasts
on the surface of a fan can permit the relative
ages of different segments of a fan to be
defined. Following isolation from deposition,
weathering processes, such as fracturing, rind
and varnish development, reddening of the
underside of clasts, and granular disintegration,
begin to modify the appearance and character
of the fan surface (McFadden
et al.
, 1982; Ritter
et al.
, 1993). Commonly, in arid regions, readily
visible contrasts in surface color permit
classification of the relative ages of different
fan surfaces (Bull, 1991). In more humid fans,
more time-consuming quantification of changes
in the depositional surface may be required to
divide the fan into surfaces of differing ages. In
order to use a fan surface as a reference against
which to measure deformation, only similarly
aged portions of the fan should be used to
define a reference plane.
Many arid alluvial fans are dominated by
deposition from debris flows (Whipple and
Dunne, 1992). The surface topography of
debris-flow deposits (up to several meters) and
the roughening of the fan surface that results
from them can determine the scale at which the
local surface of the fan can be considered to
have a predictable geometry and, therefore, to
be useful as a geomorphic marker. Even sur-
faces with considerable roughness, however,
can often be used successfully as markers,
because, despite its irregularities, the average
surface gradient can be defined (Avouac
et al.
,
1993), and offsets of this gradient by faulting
can be readily recognized (Fig. 2.15).
The surface topography of mass movement
deposits, such as debris flows, mudflows, and
landslides, often depends in part on their
water content and viscosity at the time of
deposition (Whipple and Dunne, 1992). The
higher the water content, the smoother the
upper surface of the deposit and the greater
the downstream travel distance are likely to
be. These mass movement deposits have the
advantage of having formed as the result of an
instantaneous event, thereby avoiding the
problem of diachrony in the surface seen, for
example, in Cajon Creek fluvial terraces (Fig.
2.14). Moreover, it is not uncommon that mass
movements overrun or contain within their
deposits organic debris that can be radiocar-
bon dated. As a consequence, a date from any-
where within them can often pinpoint the age
of the deposit closely. In contrast, a
14
C date
within the strata of an aggradational terrace
simply provides a lower limit on the time of
abandonment of the depositional surface.
Multiple dates are commonly needed on
such surfaces in order to judge the rate of
aggradation and the timing of abandonment
(see Fig. 2.14).
Erosional surfaces
It has long been recognized that, during long
intervals of tectonic quiescence, topography
can be beveled off by erosional processes and
a low-relief landscape can be produced.
Isolated erosional remnants (monadnocks or
inselbergs) may be surrounded by
pediments
:
erosional surfaces of low relief carved into
bedrock. Across broader geographic regions,
peneplains
characterized by slightly undulating
and generally featureless topography can
result from long-continued erosion and
deposition in the absence of active deformation.
When such erosional surfaces formed in the
distant geological past, they were commonly
buried by subsequent deposition, such that a
regionally extensive unconformity is preserved.
When such low-relief surfaces (pediments,
peneplains, regional unconformities) are
uplifted tectonically, they can form prominent
markers in the landscape. Typically these
Lava flows, debris flows, and landslides
Despite the meter-scale roughness of the upper
surface of most lava flows, the surfaces of
elongate flows can provide excellent geomor-
phic markers. Topographic gradients along the
flow surface can be measured directly, and
numerous surface features can be recognized for
cross-fault correlation. The highly resistant flows
have a high preservation potential, and the flow
itself is often directly datable with radiometric
methods.
