Geoscience Reference
In-Depth Information
1982; Young 1985). Talus will accumulate as
a function of mechanical failure of the caprock
and fall impact. Typically coarse talus tends
to be associated with lithologies that break into
large chunks, i.e. lithologies composed of large
component parts, such as conglomerates, or
lithologies of greater mechanical strength, such
as limestones. It is not uncommon for poorly
cemented sandstones, such as aeolianites, to be
associated with no talus at all, the sediment being
carried away by contemporaneous aeolian pro-
cesses (Schumm & Chorley 1966).
Once the talus slopes have been created they
may generate their own runoff (Yair & Lavee
1976) in rainstorm events. This typically occurs
close (within 10 cm) to the surface. Depend-
ing on the balance between talus supply and
removal, the talus may be eroded to expose the
area previously protected by them, revealing talus
'flat irons'. This is common in some parts of
Utah, USA. Alternatively in hyper-arid settings
the talus may form a 'fossil' accumulation in the
modern landscape.
often well-cemented carbonates. The finest of
these lake sediments will be prone to dust ero-
sion (section 5.2.4). Where this occurs sediment
will be exported out of the basin system. One
such relatively recent example of this is Owens
Lake, California, which is recorded in some
detail in Reisner (1986) and Knudson (1991).
Owens Lake was a relatively small (110 km
2
)
water body that was located to the south of
Owens Valley (Case Fig. 5.2A). Early European
settlers were attracted to the area and it became
important for agriculture, with irrigation farm-
ing introduced. In the 1900s, however, pressure
on available water resources in the growing
conurbation of Los Angeles was high. This led
to the purchasing of the water rights of this area,
by members of the Los Angeles Water Company
posing as cattle ranchers. In 1913 many of the
local streams that fed the lake were diverted into
the Los Angeles Aqueduct. In 1927 the lake was
nothing more than a small pond. The exposed
dry, fine-grained playa sediments contain halite,
trona, thenardite and mirabilite and are suscept-
ible to erosion by strong summer and winter
winds. Dust derived from this 110 km
2
area
accounts for 1% of the total dust production
in the USA every year. In February 1989 a con-
centration of 1861
5.2.2.2 Exposed lake sediments
Lake-bed deposits in arid regions range in
thickness from a thin veneer to hundreds of
metres. As water bodies expand and contract
in response to changes in the water balance, so
former lake-bed sediments may become exposed
to further erosion by surface processes such as
mass wasting or gully erosion, being cannibalized
into the lower lake levels.
Lake sediments contain a range of grain sizes,
depending on the extent and depth of the lake
system, and the characteristics of the surround-
ing catchments feeding the lake with water and
sediment. Thus lake sediments may have been
sourced originally from terrigenous, allochthon-
ous sediments external to the lake water body.
These clastic sediments are typically transported
by runoff and river flow and thus can contain a
range of grain sizes from clay to cobble. Other
sources of lake sediment are autochthonous and
originate within or beneath the water column or
within the lake-bed sediments after deposition.
These sediments are typically fine-grained, but
gm
−3
was recorded, which
was 37 times higher than the health standard in
the State of California.
Where lake sediments become elevated in
respect to the main lake level as a function of
tectonics, then mechanical erosion of all grades
of lake sediment material becomes possible from
gully erosion. This is currently occurring in Lake
Burdur in central Turkey, where tectonically
elevated Pliocene lake sediments are being eroded
by gullying, and redeposited in the smaller, and
constantly shrinking, modern lake system.
μ
5.2.3 Sediment transport by water
Channels within arid areas may be one of
two main types: (i) perennial or (ii) ephemeral.
Perennial channels have flow for most of the
year and exist within arid areas where rivers
within the arid basins are sourced by external
inputs from mountainous regions. For example,
