3.0 Affected Environment - Knik Arm Bridge and Toll Authority

Knik Arm Crossing DraftFinal EIS
Affected Environment
Arm, extending to depths of about 60 to 65 feet below MLLW, is steep-sided to the west and
more gently sloping to the east. The channel deepens to about 180 feet below MLLW near
Cairn Point along a narrow steep-sided depression (Harding Lawson Associates 1983;
FHWA 2005a, Smith 2005; KABATA 2006a), a feature often referred to as the Cairn Point
Trench (see Section 2.6.3.2.3). This feature may be an erosional remnant from a stream
canyon that existed during the late Pleistocene lower sea level stand.
3.5.3.1.2 Geologic units
Poorly consolidated rocks of Tertiary age underlie most of the Anchorage area and the Cook
Inlet-Susitna basin. Bedrock is likely deeper than 600 to 1,000 feet in the vicinity of the
proposed project (Harding Lawson Associates 1983; Golder Associates 2003;
KABATA 2006j). Older bedrock outcrops surrounding the basin are being considered for use
as armor and filter rock in the proposed bridge approaches. Quarries in the Mat-Su and
Eklutna-Peters Creek areas contain granite, graywacke, and greenstone bedrock of
predominantly Jurassic age (PND 2005, Winkler 1992). Additional information on rock
quarries is provided in the Knik Arm Crossing Potential Armor and Rock Quarries Technical
Report (KABATA 2006l).
Unconsolidated Quaternary deposits of glacial origin overlie the older bedrock in the
Anchorage area (Figure 3.25). Deposits from the last two glacial events, the Knik and
Naptowne Glaciations dating from 75,000 to 11,000 years ago, form most of the Quaternary
units in the project area. These include 1) coarse-grained glaciodeltaic deposits interfingered
with silty clay that are associated with the Pleistocene Knik Glaciation; and 2) the Bootlegger
Cove Formation, the Elmendorf Moraine, and related outwash alluvium associated with the
late Pleistocene Naptowne Glaciation. A sensitive silty clay unit within the upper half of the
Bootlegger Cove Formation was responsible for major translational landsliding that occurred
in Anchorage during the 1964 earthquake (Updike et al. 1988).
Glacial and alluvial deposits
The following Quaternary glacial and alluvial deposits occur on the west and east sides of
Knik Arm:
• Mat-Su approach alternatives and west bluff. The Mat-Su alternatives would cross
surficial deposits composed chiefly of outwash alluvium, glacial till, morainal material,
and peat deposits laid down during the Naptowne Glaciation. Where present, peat deposits
typically extend up to depths of 5 feet (Mat-Su Borough 1995a; Natural Resources
Conservation Service [NRCS] 1998). The 100-foot-high west bluff contains about 40 feet
of sandy clay at the base of the bluff, overlain by 60 feet of silty gravelly sand and till-like
sandy gravelly clay, all of which are likely the stratigraphic equivalent of the Bootlegger
Cove Formation. Materials below the base of the bluff consist of clay and gravelly till of
the Knik Glaciation (Mat-Su Borough 1995a; KABATA 2006j). Sands and gravels of the
Elmendorf Moraine and Naptowne outwash deposits near Port MacKenzie and other
Mat-Su Valley sites are being considered for use as fill material for the proposed project
(Figure 3.26).
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