Surficial Geologic map of the Greater Omaha Area, Nebraska and Iowa

This map provides geologic information for land-use decisions.

Geologic mapping, in support of the USGS Omaha-Kansas City Geologic Mapping
Project, shows the spatial distribution of artificial-fill, alluvial, eolian,
and glacial deposits and bedrock in and near Omaha, Nebraska. Artificial fill
deposits are mapped chiefly beneath commercial structures, segments of
interstate highways and other major highways, railroad tracks, airport runways,
and military facilities, and in landfills and earth fills. Alluvial deposits
are mapped beneath flood plains, in stream terraces, and on hill slopes. They
include flood-plain and stream-channel alluvium, sheetwash alluvium, and
undivided sheetwash alluvium and stream alluvium. Wind-deposited loess forms
sheets that mantle inter-stream areas and late Wisconsin terrace alluvium.
Peoria Loess is younger of the two loess sheets and covers much of the
inter-stream area in the map area. Loveland Loess is older and is exposed in a
few small areas in the eastern part of the map area. Glacial deposits are
chiefly heterogeneous, ice-deposited, clayey material (till) and minor
interstratified stream-deposited sand and gravel. Except for small outcrops,
glacial deposits are covered by eolian and alluvial deposits throughout most of
the map area. Bedrock is locally exposed in natural exposures along the major
streams and in quarries. It consists of Dakota Sandstone and chiefly limestone
and shale of the Lansing and Kansas City Groups. Sand and gravel in flood plain
and stream-channel alluvium in the Platte River valley are used mainly for
concrete aggregate. Limestone of the Lansing and Kansas City Groups is used for
road-surfacing material, rip rap, and fill material.

Surficial deposits are mapped where they are estimated to be at least 1 m
thick. Most of the surficial deposits are poorly exposed. The surficial
deposits were locally observed in foundation excavations, road cuts, and other
exposures in 1995 and 1996. Thin, discontinuous sheetwash alluvium (Qsw) and
small artificial-fill deposits (af) were not mapped. Small outcrops of
Loveland Loess, till, Dakota Sandstone, and limestone outcrops of the Lansing
and Kansas City Groups that are too small to map are shown as symbols. Mapping
in and near Omaha is modified from that of Miller (1964, plates 2 and 3 at
1:24,000 scale). Mapping in the rest of the map area is based primarily on
interpretation of 1:15,840- and 1:20,000-scale county soil maps (Bartlett,
1975; Borchers and others, 1984; Branham, 1989; Nixon, 1982) and interpretation
of aerial photography, supplemented by examination of artificial and natural
exposures. The aerial photography includes: 1:40,000-scale, black-and-white,
aerial photographs taken in April and May 1990 and in March and April 1993, and
also 1:60,000-scale, color infrared, aerial photographs taken in May 1981 and
April 1982. Age assignments for surficial deposits are based in part on soil
development. Soil-horizon designations and textural terms, such as silt loam
and clay loam, are those of the Soil Survey Staff (1975). Grain-size ranges
given for surficial deposits are estimates that are based on the modified
Wentworth scale (American Geological Institute, 1982). Dry matrix colors of the
surficial deposits were determined by comparison with Munsell Soil Color Charts
(Munsell Color, 1973). In general, colors of the surficial deposits are
similar to those of the bedrock and other materials from which the deposits
were derived. Unweathered to slightly weathered surficial deposits observed in
the map area commonly range from gray (5Y 6/1) to very pale brown (10YR 8/4).
In this report, the terms "alluvium" and "alluvial" refer to surficial
material transported by running water confined to channels (stream alluvium) as
well as material transported by running water not confined to channels
(sheetwash alluvium). The term "colluvial" here refers to surficial material
transported down slopes chiefly by mass-movement (gravity-driven) processes,
such as creep. All of the radiocarbon and thermoluminescence ages in this
report are uncalibrated. The thickness and grain-size ranges of surficial map
units are based chiefly on interpretation of approximately 300 unpublished
drill-hole logs and approximately 30 water-well logs on file with the
Conservation and Survey Division, University of Nebraska-Lincoln, and test-hole
data summarized by Burchett (1965) and Burchett and Smith (1989a, 1989b).
Metric units are used in this report. (See text for complete bibliographic
references.)

Coverages included in this geospatial database:
>ompnt100 coverage: Contains geologic points for sites classified as bedrock,
Loveland Loess, or Till.

>ombdy100 coverage: Contains lines defining state and county boundaries
derived from USGS DLG data.

>omtrn100 coverage: Contains lines defining linear transportation
features (highways and roads) derived from USGS DLG data.

>omhyp100 coverage: Contains lines defining hypsographic features derived from
USGS DLG data.

>omhyd100 coverage: Contains polygons and lines defining hydrographic features
derived from USGS DLG data.

>ommea100 coverage: Contains lines defining scarps bordering former channels
of the Missouri River.