Geology Reference
In-Depth Information
Dating of the aggradational fill underlying the
terrace, however, was facilitated by the discov-
ery of organic debris within the sediments. The
resultant radiocarbon ages indicate that both
the initiation of aggradation and the initiation
of incision are remarkably diachronous along
the terrace surface (Fig. 2.14B). Compared to
the upstream reaches, deposition commenced
~4 kyr earlier in the downstream reaches, and
incision of the aggraded surface began there
~7 kyr earlier. Thus, while upstream parts of the
terrace were just beginning to aggrade, the dis-
section of the downstream areas had already
begun! Even though the entire terrace is less
than ~15 kyr old, the age and time of abandon-
ment of the upper surface of the terrace varies
by ~7 kyr across a distance of ~10 km (Weldon,
1986).
The cause of this diachronous response is
unknown. Alluvial terraces form within
coupled, complex systems that include ero-
sion, transport, and deposition of sediments.
Such systems do not respond instantaneously
or uniformly to changes in controlling para-
meters, such as discharge, sediment supply,
rock uplift rate, or base-level lowering
(Humphrey and Heller, 1995). Instead, changes
in aggradation or degradation propagate up
stream and down through both alluvial and
bedrock systems at rates which may be related
to the diffusivity of each system and to the
velocity of kinematic waves that move along
alluvial and bedrock reaches (Humphrey and
Heller, 1995). The duration of these waves of
deposition or erosion often greatly exceeds
the duration of the perturbation that initiated
the wave. Many drainage basins have
equilibrium response times that are consider-
ably greater than 10
5
yr. Given that many
changes imposed on river systems, such as
discharge variations due to climate, occur on
much shorter times scales, it seems likely that
depositional and/or erosional systems will be
constantly integrating the responses to succes-
sive variations in controlling parameters. Thus,
equilibrium may be rarely attained in fluvial
systems. The diachrony displayed by the Cajon
terrace (Weldon, 1986) might typify many flu-
vial terraces. In the common absence of data
to delineate chronological differences along a
terrace, however, most workers assume the
upper terrace surface is essentially isochro-
nous along its length. Diachrony at the scale
of a few thousand years becomes less important
when older terraces are considered, but for
post-glacial and Holocene terraces, such
variability would significantly distort rates
that were calculated assuming an isochronous
terrace surface.
Alluvial fans
In many respects, the surfaces of alluvial fans
are similar to fluvial terraces. Slopes generally
decrease down-fan and any longitudinal sec-
tion displays a slightly concave upward profile.
Deposition on most fans is considerably more
episodic than in rivers, and, in addition to
channelized fluvial processes, various types
of mass flows and unchannelized flows com-
monly dominate deposition. Although most
fans are characterized by steady, down-fan
decreases in gradient, some fans appear to
be segmented (Bull, 1964), such that in cross-
section they comprise a suite of fairly straight
slopes that abruptly change where the linear
slopes intersect. Such segmentation can result
from shifting of the boundary between deposi-
tion and erosion up or down the fan. For exam-
ple, if deposition is focused on the fan apex
(also termed the fanhead), alluvium will accu-
mulate and steepen this area with respect to
the rest of the fan. If the fanhead is subse-
quently entrenched, remnants of the aban-
doned steeper surface will remain, but the apex
of deposition will shift down the fan. As aggra-
dation on this gentler, down-fan surface pro-
ceeds, the upper surface of aggradation may
eventually intersect and begin to overlap the
steeper, up-fan surface, such that an abrupt
slope change occurs at the intersection. In this
situation, the upper and lower parts of the fan
surface will have different ages. In such circum-
stances, before using fans as geomorphic mark-
ers, the age of each surface should be separately
determined. Even in the absence of fan seg-
mentation, most fans are composed of a mosaic
of surfaces of different ages, ranging from the
