Sidescan-sonar imagery has enabled detailed mapping of the surficial geology of the lake
floor. The sidescan-sonar imagery is presented such that a strong acoustic
signal (backscatter) is white and a weak backscatter signal is black. As
a generality, the post-impoundment sediment has a lower-backscatter and
more uniform signature than the pre-impoundment surface. The use of sidescan-sonar
imagery has allowed mapping the extent of the post-impoundment sediment
throughout the lake (Fig. 3).
Here we briefly describe some of the findings from the eastern, central
and western portions of the lake.
The post-impoundment sediment in Iceberg Canyon has a moderate backscatter
signature in some areas and a low-backscatter signature in others. The northern
half of the canyon shows numerous pits in the sediment surface, which appear
to be gas-escape structures (Fig.
4). The seismic data suggest a high gas content in the sediment
of this part of the lake (see sediment distribution
and thickness section). The steep walls of Iceberg Canyon show as uniform
high-backscatter bands to either side of the post-impoundment sediment.
In the eastern portion of the lake, the post-impoundment sediment surface
has more varied backscatter strength than in the central and western parts
of the lake.
Gregg Basin has a broader floor than Iceberg Canyon, and the post-impoundment
sediment fill is flanked primarily by alluvial fans along the western side
of the basin and rock ledges along the eastern side. The post-impoundment
sediment fill is 1 km wide in the northern part of the basin and 2-3 km
wide in the southern part (Fig.
3). This sediment mostly
has a moderate backscatter signature except in the southern half of the
basin where the central part is moderate backscatter and the edges are low
backscatter (Fig. 5). Sediment
cores indicate that near-surface sediment in Gregg Basin contains numerous
fine and very-fine sand beds that are separated by silt and clay beds (Twichell
and others, 2003). The sidescan-sonar imagery also shows slightly sinuous
features on the surface of the post-impoundment sediment that can be traced
for 3.5-4 km along the southern part of the basin (Fig.
5). These features are 30-50 m wide, have floors that tend to be
moderate backscatter, and are flanked by narrow bands of high-backscatter.
Along the outside of the bends the high-backscatter areas are commonly broader.
These features are interpreted to be channels although they have no bathymetric
expression on the seismic profiles that cross them (Twichell and others,
2002). The presence of these channels on the surface of the post-impoundment
sediment indicates that they are modern channels forming by subaqueous processes.
In the central part of Lake Mead (from Virgin Canyon to Boulder Canyon)
the post-impoundment sediment surface has a low-backscatter signature except
in local areas where recent landslide deposits are still exposed on the
lake floor. The transition to post-impoundment sediment having a low-backscatter
surface occurs at the southern end of Gregg Basin where it enters Virgin
Canyon. The post-impoundment sediment cover in Temple Basin
is mostly less than 1 km wide and is broader in Virgin Basin where it reaches
widths of 2.5 km. In Overton Arm this sediment cover is broader north of
the islands in its center, and only a narrow thread of post-impoundment
sediment cover can be traced south of the islands to Virgin Basin. The surface
of the post-impoundment sediment in the central part of the lake shows no
evidence of channels. Landslide deposits do cover small parts of the sediment
surface. One landslide in the eastern part of Temple Basin (Fig.
6) occurred in 1988 when the lake was at its highest level (W. Burke,
2002, personal communication). The fact that it is still exposed on the
lake floor indicates that not much sediment has accumulated in this part
of the lake since that time.
One other feature of note in the central part of the lake is the town
of St. Thomas that now is submerged in the northern part of Overton Arm.
The streets and some foundations are still preserved on the lake floor
Boulder Basin comprises the western part of Lake Mead The post-impoundment
sediment within the basin has a
low-backscatter signature in contrast to Gregg Basin. These sediments are
surrounded by Quaternary aged alluvial fan deposits and outcrops of older
strata (Longwell, 1936; Twichell and others, 1999). The channel of the pre-impoundment
Colorado River is mimicked on the present lake floor where two moderate
backscatter bands correspond to the channel banks (Fig.
8). The preservation of the channel shape on the lake floor is probably
due to dewatering and compaction of the very fine-grained sediment deposited
in the western part of the lake.
The sidescan-sonar imagery also shows a narrow band of sediment has been
deposited on the floor of the axial valley in Las Vegas Bay since the
lake filled (Twichell and others, 2001).
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