Geoscience Reference
In-Depth Information
amphitheatre-headed canyon tributaries to the Escalante, San Juan and Colorado Rivers. These developed at
the lithologic contact between the permeable aeolian Navajo Sandstone and the impermeable mudstones and
sandstones of the fluvial Kayenta Formation (Laity, in Baker, 1990), both part of the Triassic-Jurassic Glen
Canyon Group. Numerous minor seeps with associated alcove development also occur in parts of the Navajo
Sandstone above the main seepage zone, where thin shales interbedded with the aeolian sandstone act as aquicludes
(Laity, 1988). The regional geological structure of the Colorado Plateau exerts an important control upon valley
morphology and drainage patterns, with both pattern and form determined by the direction of groundwater flow
(see Figure 16.2). Network length, tributary length and tributary asymmetry all vary in response to geological
structure, with symmetrical systems developed in association with synclines and more asymmetric patterns found
in areas of laterally dipping strata. The orientation of zones of secondary permeability, such as faults and fractures,
also control the spacing and alignment of valleys by acting as both zones of preferential groundwater flow and
foci for groundwater emergence where they intersect trunk valley walls. The fine-grained Navajo Sandstone is
prone to granular disintegration driven by dissolution of the iron oxide, clay and carbonate cements surrounding
aeolian quartz grains, as well as intergranular pressures exerted by salt and biological weathering. The fine-grained
sediments released by weathering can be readily transported by infrequent surface water flows.
dryland canyons in the Colorado Plateau. SEM analyses
of sandstone within the 20 to 25 m thick zone of ground-
water emergence identify macropore development (Fig-
ure 16.3(a)), together with algal growth and the precipita-
tion of calcite and salt efflorescences within pore spaces,
as agents of rock weakening (Figures 16.1(c) and 16.3(b))
(Laity, 1983; Laity in Baker, 1990). In dryland environ-
ments, the presence of salts within pores may contribute to
spalling through pressures exerted by expansion and con-
traction of minerals due to thermal expansion and rehydra-
tion (Cooke and Smalley, 1968). Other processes leading
to rock wasting within Colorado Plateau sapping valleys
are the dissolution of cements by exfiltrating groundwater
and the weathering of fine shale layers within the sand-
stone formations.
The process of seepage erosion in bedrock involves at
least some intergranular flow, with weathering proceeding
by the slow release of grains within the zone of ground-
water emergence, leading to spalling and mass-wasting in
the form of rockfalls around the seepage zone. Seepage
erosion is usually focused into a narrow zone where the
discharge of groundwater is concentrated (Howard and
Selby, 2009). Experimental studies show that the main
method of drainage network development is by head-
ward erosion, which proceeds rapidly by headwall col-
lapse during the early stages of valley formation (Kochel,
Howard and McLane, 1985; Baker, 1990; Gomez and
Mullen, 1992). Theoretical studies and field observations
indicate that the velocity at which valley heads advance
is proportional to the flux of groundwater towards the
heads (Abrams
et al.
, 2009). Tributary growth occurs as
a result of permeability variations (Howard, Kochel and
Holt, 1988) and disturbances in subsurface flow (Dunne,
1980), and may also be influenced by joints and geologi-
cal structures (Laity, Pieri and Malin, 1980; Pieri, Malin
and Laity, 1980; Laity in Baker, 1990). Headward erosion
has been found to occur most effectively in gently dip-
ping lithologies with an overall regional dip of 1
◦
to 4
◦
,
with erosion of the valley head proceeding in an up-dip
direction (Howard, Kochel and Holt, 1988). In cohesion-
less sediment, seepage forces at the site of emergence
of subsurface flow are the most important controls on
headward erosion (Howard and McLane, 1988), whereas
in cohesive bedrock, mechanical and chemical weather-
ing are likely to be the dominant displacive processes
(Laity, Pieri and Malin, 1980). Mechanical weathering
16.2.3
Characteristics of drainage networks
developed by groundwater seepage erosion
Valleys developed predominantly by seepage erosion have
a number of distinctive morphological features that, to
a certain extent, may be diagnostic of the operation
of groundwater processes in their formation (Howard,
Kochel and Holt, 1988; Baker, 1990; Luo, 2000). These
are best illustrated by consideration of the characteristics
of the most intensively investigated of the dryland seepage
erosion valley systems, those of the Colorado Plateau (Fig-
ure 16.4). The most prominent morphometric properties
of these (and other nondryland) systems are summarised
in Table 16.2. They include the abrupt initiation of valleys
