Environmental Engineering Reference
In-Depth Information
Glacier
Landslide
Fluvial (river)
Glacial
deposits
Lacustrine (lake)
Shoreline
Lagoonal
Rockslide
Alluvial
Deltaic
Tidal
Shallow water marine
Deep water marine
Slump
Aeolian dunes
Beach
Wetlands
Aeolian (dunes)
Upper mantle
Reef
FIGURE 2.29
(See color insert.)
Sedimentary depositional environments.
on sedimentary deposits of marine origin. Table 2.4 lists 29 urban areas in the United
States and identifies the dominant geology beneath each area by type of depositional envi-
ronment. All of the sedimentary depositional environments are represented.
2.5.1 Alluvial
Alluvial deposits are formed by fast-flowing stream water. They range in size from fine silt
to boulders, and also include debris flows and rock slides from adjacent mountain slopes.
One example of an alluvial deposit is an alluvial fan, which is a flat to gently sloping mass
of sediment and loose rock material shaped like a fan. These deposits are created at the
base of a mountain slope where fast-moving steams coming out of the mountains meet the
relatively flat surfaces of a basin floor or broad valley. At this junction, the stream gradient
is significantly decreased, which removes much of the energy available for sediment trans-
port by the flowing water (Bloom 2004). The result is the deposition of sediment in a fan
shape as depicted in Figure 2.30. Alluvial fans are common in the western United States.
2.5.2 Fluvial
Fluvial deposits are sediments deposited by a river or stream. Deposition results from
fast- and slow-moving water currents on channel beds and bars. Low-velocity deposition
occurs on floodplains during floods. Here, the deposits are typically fine-grained silts
and clays, and may extend over large areas. According to Walker and Coleman (1987), the
Atlantic and Gulf coastal plains, in large part, owe their origins to fluvial deposition over
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