Geoscience Reference
In-Depth Information
Table 14.1
Morphometric data for the example fans illus-
trated on Figure 14.14(d).
cal style following changes in critical power relationships
(see above, Section 14.3.2; Figures 14.3(b) and 14.7(b)).
Some such changes, though probably relatively minor,
may be brought about through intrinsic thresholds op-
erating within the fan morphosedimentological system
(Schumm, Mosley and Weaver, 1987), but major 'regime
changes' are likely to be the result of extrinsically con-
trolled thresholds related to major changes in the dynamic
factors: tectonics, climate or base level.
How these factors interact, and how their signals are
preserved in the sediment or morphological record, has
been the subject of much research (Ritter
et al
., 1995). In-
terestingly, most geological work dealing with the ancient
rock record and necessarily larger bodies of sediment, of-
ten in the context of the evolution of sedimentary basins
over relatively long timescales, stresses tectonic controls
(Harvey, Mather and Stokes, 2005). Research on Quater-
nary fans, on the other hand, operating over the generally
shorter timescales related to Quaternary climatic cycles,
stresses climatic controls.
The controls themselves are not totally independent.
One of the responses to tectonic activity would be a change
in base level; base-level changes related to main valley
incision or to changes in sea level or pluvial-lake level are
either indirectly or directly related to the global climatic
changes of the Quaternary.
For Quaternary fans the overriding control appears to
be climatic change and its influence on sediment gen-
eration and supply to fan environments (Frostick and
Reid, 1989; Roberts, 1995; Harvey, 2002b, 2005; Pope
and Wilkinson 2005). In cases of local tectonic stability,
and where fans toe out to stable base levels, the sedimen-
tological and morphological sequences are wholly proxi-
mally and climatically controlled. In cases where there is
local tectonic activity or base-level change the primary cli-
matic signal is modified by tectonics or base-level change
(see below).
1
2
3
4
5
6
Honda
27.0
0.032
0.020
0.029
22-30
1.6
Tabernas, SE
Spain
Grotto Canyon
10.6
0.085
0.064
0.075
60-120
1.3
Death Valley,
California
Ceporro
a
0.4
0.081
0.043
0.063
2.0-2.5
1.9
Tabernas, SE
Spain
Corrachos
a
4.6
0.033
0.014
0.020
7-10
2.4
Carrascoy, SE
Spain
La Sierra
b
3.3
0.055
0.045
0.045
20-25
1.2
Tabernas, SE
Spain
1. Drainage area (km
2
).
2. Fan gradient (dimensionless)
G
.
3. Channel gradient (dimensionless)
S
.
4. Distal fan gradient (dimensionless).
5.
G
/
S
.
a
Fan surfaces crusted by calcrete.
b
Fan surfaces crusted by calcrete; fan subjected to tectonically induced
distal dissection (see Section 14.2.4, Figure 14.13(b) and Figure
14.14(d)).
Alternatively, if exposed to sufficient wave energy, the
distal zones may be subject to erosion, and hence pro-
file shortening and steepening (Figure 14.16). This was
the case for the Cabo de Gata east coast fans during the
last interglacial marine highstand and again during the
Holocene (Harvey
et al
., 1999), when distally induced
dissection cut back through the fans, one of the few docu-
mented cases of a base-level rise-triggering incision (see
also Scheepers and Rust, 1999).
14.4.2
Interactions between the dynamic
controls: case studies of alluvial fan response to
Late Quaternary environmental change
14.4
Alluvial fan dynamics
14.4.1
Expressions of fan dynamics
Three examples of interactions between the dynamic con-
trols are developed from some of the examples given in
the text in the previous sections (Figure 14.18).
Combinations of the passive and dynamic factors con-
trolling fan evolution are expressed in the fan sediment
sequences and particularly by the fan morphology, both
in plan view (Figure 14.17) and in profile. Within the con-
text of the passive controls (see above Section 14.3.1),
alluvial fans respond to changes in the dynamic controls
(see above, Section 14.3.2), by changes in erosional or
14.4.2.1
Southeast Spain
Throughout southeast Spain a common sequence of Qua-
ternary alluvial fan aggradation and dissection can be
