STEPHEN F. COLLINGS AND RALPH H. KNODE
Wyoming Fuel Company, Lakewood, Colorado and Crawford, Nebraska
INTRODUCTION
The Crow Butte uranium deposit is located in northwest Nebraska near the Town of Crawford (Figure 1). The deposit was discovered during the fall of 1980 by Wyoming Fuel Company, operator of the Crow Butte Joint Venture. The Crow Butte Joint Venture is owned 50 percent by Wyoming Fuel Company, a subsidiary of KN Energy, Inc.; formerly Kansas-Nebraska Natural Gas Company, Inc., 40 percent by Ferret Exploration Company, Inc., and 10 percent by First Exploration Company. A preliminary announcement made in January 1981 indicated a "probable potential" reserve in excess of 25,000, 000 pounds U308. Drilling during 1981 and 1982 confirmed these reserves and indicated the presence of more than 30,000,000 pounds U308 having an average grade in excess of 0.25 percent U308.
Wyoming Fuel Company was initially attracted to the area in 1978 by favorable regional geology for sandstone uranium deposits and weak radioactivity noted in regional oil and gas holes. Exploration drilling began in 1979 following the formation of the Crow Butte Joint Venture. Drilling has continued up to the present, at which time the Joint Venture is ready to proceed with a pilot scale solution mine. Environmental baseline data have been acquired and the permit applications for the pilot scale mine have been submitted to the Nuclear Regulatory Commission and to the applicable state agencies in Nebraska.
GEOLOGICAL SETTING
The Crow Butte deposit is in the Pine Ridge country of northwest Nebraska. The main portion of the deposit lies north of the Pine Ridge escarpment on gradual sloping terrain with local relief of less than 100 feet. The Pine Ridge escarpment surrounds the main deposit on three sides and is about 500 feet higher in elevation than the northward sloping plain (Figure 2). The Pine Ridge is covered with ponderosa pine and is formed by a major sandstone unit.
Figure 2. Pine Ridge - Crow Butte on left. View looking northeast.
Crow Butte Deposit is in the middle distance. Gering and
Monroe Creek Formations form the Pine Ridge.
The climate, typical of a semi-arid continental climate is characterized by warm summers, cold winters, light precipitation and frequent changes in the weather. The average precipitation is 15.5 inches distributed mostly during the spring and summer. Average temperature ranges from 23 degrees F in January to 74 degrees F in July, with extremes ranging from -29 degrees F to 110 degrees F
The land in the area of the deposit is largely privately owned and used for agriculture with a population density of about 12 persons per square mile. Winter wheat and hay are the principal crops and cattle are the principal livestock.
Nearby Crawford is a town of 1320 people whose economy is based on agriculture and tourism. Fort Robinson State Park is located immediately west of the town; other nearby points of interest are the Nebraska National Forest to the southeast and the Black Hills, 90 miles to the north. Chadron, with a population of 5933, the largest community in the immediate area, is 25 miles to the east. Scottsbluff, located 75 miles to the south, is the largest community and the principal trade center for northwest Nebraska and east-central Wyoming.
GENERAL STRATIGRAPHY
Sedimentary strata ranging from late Cretaceous through Tertiary age exposed throughout the project area (Figure 3). Pleistocene alluvial and colluvial material are abundant along the north slope of the Pine Ridge. Figure 4 is a generalized stratigraphic column for the area.
Pierre Shale
The Pierre Shale of Late Cretaceous age is the oldest formation encountered in WFC's test holes. The Pierre is a widespread dark gray to black marine shale, with relatively uniform composition throughout. The Pierre outcrops extensively in Dawes County north of the project area, (Figure 3). The Pierre is essentially impermeable to the degree that in areas of outcropping Pierre, water for domestic and agricultural needs is piped in from wells from other formations.
Although the Pierre is up to 5,000 feet thick in other areas, in Dawes County deep oil tests have indicated thicknesses of 1,200 to 1,500 feet. Aerial exposure and subsequent erosion greatly reduced the vertical thickness of the Pierre prior to Oligocene sedimentation. Consequently, the top of the present day Pierre contact marks a major unconformity and exhibits a paleotopography with considerable relief (DeGraw, 1969).
As a result of the extended exposure
to atmospheric weathering, an ancient soil horizon or Paleosol was formed
on the surface of the Pierre Shale. It is known as the "Interior Paleosol
Complex" of the Pierre Shale (Shultz and Stout, 1955, p.24) and is readily
observed in certain outcrop exposures. The Paleosol is generally absent in
areas of Chadron Sandstone channels.
The Pierre Shale is the confining
bed below the Basal Chadron Sandstone member which is the host for uranium
mineralization (Figure 5). The black marine shale is an ideal confining bed
with measured permeabilities of less than 0.0001 millidarcies. The log characteristics
of the Pierre Shale are shown on Figure 6 and illustrate its impermeable nature.
White River Group
The White River Group is Oligocene
in age and consists of the Chadron and Brule Formations. The Chadron is the
oldest Tertiary Formation of record in northwest Nebraska. It lies with marked
unconformity on top of the Pierre Shale (Figure 5). Regionally, the vertical
thickness of the Chadron Formation varies greatly. This is attributed to the
extreme variability of the Basal Sand unit of this formation. The Chadron
Formation is comprised of three distinct members.
Basal Sandstone Member: The Basal
Sandstone is the depositional product
of a large, vigorous braided stream
system which occurred during early Oligocene
(approximately 36 to 36 million years
before present). Regionally, the Basal Sandstone
ranges in thickness from 0 to 350 feet.
Uranium mineralization occurs exclusively
within the Basal Chadron Sandstone Member, a coarse grained arkosic sandstone
with frequent interbedded thin clay beds and clay galls (Figures 5 and 6).
Occasionally the lower portion of the Basal Member is a very coarse, poorly
sorted conglomerate (Figure 7). Thickness of the Basal Chadron within the
ore trend is about 40 feet.
Figure 5. Pierre Shale - Basal Chadron Sand-
stone Contact, Whitehead Creek,
Section 36, T34N, R54W, Sioux County
Thin
section examination of the Basal Chadron Sandstone reveals its composition
to be 50% monocrystalline quartz, 30 to 40% undifferentiated feldspar, plagioclase
feldspar, and microcline feldspar. The remainder includes polycrystalline
quartz, chert, chalcedonic quartz, various heavy minerals and pyrite.
Core samples of the Basal Chadron
exhibit numerous clay galls up to a few inches in diameter. In addition, the
Basal Member contains frequent thin silt and clay lenses of varying thickness
and continuity. These represent flood plain, or low velocity, deposits which
normally occur during fluvial sedimentation. These lenses vary in thickness
from several inches to one or two feet. Within the ore trend, clay beds one
to two feet thick separate the Chadron Sandstone into two or more subunits.
X-ray diffraction of the Basal Sandstone has identified the following clay
minerals: illite, smectite, expandable mixed layer illite-smectite, and minor
amounts of Kaolinite.
Figure
7: Close-up Pierre Shale - Basal
Chadron Sandstone Contact. Note
pebbles and heterogenity of Chadron
Sandstone, Whitehead Creek, Sec. 36,
T34N, R54W, Sioux County.
Middle Chadron Member: The Middle
Chadron Member represents a distinct and rapid facies change from the underlying
Basal Sandstone. The lower portion of the Middle Chadron is characterized
by brick red clay (Figure 8). The brick red clay can be observed on outcrop
in northern Dawes and Sioux Counties and serves as an excellent marker bed
in drill hole cuttings. The Middle Chadron Member has been observed in virtually
all drill holes along the mineral trend. Thickness of the Middle Chadron Member
ranges from 40 to 100 feet throughout the project area.
at base of Middle Chadron is dark band overlying light
colored sandstone. Whitehead Creek, Sec. 36, T34N, R54W,
Sioux County
The Middle Chadron Member is the upper
confining bed overlying the Basal Sandstone Member. This can be observed by
the epigenetic occurrence of the uranium mineralization, which is strictly
confined to the Basal Chadron Sandstone Member. The lower part of the Middle
Member is a brick red clay with occasional interbedded gray-green clay. The
brick red clay grades upward to a light green-gray sandy claystone. The upper
part of the Middle Member is a light gray bentonitic clay.
Upper Chadron Member: The Upper Chadron
consists of massive claystones and siltstones (Figure 9). These range in color
from a dark bluegreen to greenish-brown. The sequence of green siltstones
and mudstones are generally considered fluvial channel and flood plain deposits,
with limited lacustrine and eolian material present (Vondra, 1958, p.41).
Well developed sand channels in the Upper Chadron are rarely encountered in
test holes, and of very limited lateral extent when observed. The Upper Chadron
Member averages 100 feet thick throughout the project area.
Figure 9. Upper Chadron Member.
Claystones
and Siltstones. Sugar Loaf Butte
Sec. 27, T34N, R53W, Sioux County.
Brule Formation
The Brule Formation lies conformably
on top of the Chadron Formation and combined with the Chadron comprises
the White River Group. The Brule outcrops throughout the main ore trend.
It is made up almost entirely of siltstones with minor sand channels (Figure
10). The contact between the Upper Chadron Member and the overlying Brule
Formation is a gradational one. In drill cuttings and geophysical logs the
formation boundary can only be approximated. The Brule Formation can generally
be identified by its buff to medium brown color in contrast to the greens
of the underlying Chadron.
The Brule has been subdivided into
two separate members (Shultz and stout, 1938) the Orella and the Whitney.
Differentiation of the two members in drill hole cuttings or with geophysical
logs is very difficult.
Figure 10. Brule Formation, Siltstones,
Toadstool Park, Sec. 5, T33N,
R53W, Sioux County.
The Orella lies directly on the Chadron
Formation and an approximate Brule-Chadron contact can generally be estimated
with drill cuttings but usually not on geophysical logs. The Orella is composed
of buff to brown siltstones, with persistent spotty green nodules as it grades
into the green clays of the Chadron.
The Whitney Member of the Brule is
comprised of fairly massive buff to brown siltstones, in part probably eolian
in origin (Vondra, 1958, p.19). Several volcanic ash horizons have been reported
in outcrops. They are rarely distinguishable in drill hole cuttings, but are
occasionally identified on geophysical logs. The Whitney Member frequently
becomes coarser grained upward near the Gering Formation contact. This is
marked by an increase in grain size which is difficult to detect in drill
hole cuttings but usually can be observed on geophysical logs. Some moderate
to well defined channel sands can be observed in both drill holes and on outcrops.
These upper Brule channels are limited in lateral extent and continuity but
may occasionally be water saturated in the otherwise generally impermeable
Brule.
Within the pilot mine area occasional
sand units are encountered in the upper 250 feet of the drill hole. These
represent small Brule channel sands known to occur intermittently in the Whitney
Member. The small sand units have very limited lateral continuity and although
water bearing, little water can be produced. Thus, these units do not meet
a strict definition of an aquifer. This has been demonstrated in WFC drill
holes and can be observed in cross sections throughout the R and D permit
area.
Arikaree Group
The Arikaree Group includes three
sandstone Formations which are present locally and regionally but in the main
ore trend are absent due to
erosion. The Gering Formation (Figure
2) is the oldest formation of the
Arikaree Group. The Gering Formation is Oligocene in age (Souders, 1981)
and lies unconformably on the Brule Formation.
The Gering is predominantly buff
to brown, fine grained sandstones
and siltstones. These repre sent
channel and flood plain deposits of higher
velocity than the underlying Brule. The Gering
Formation also includes some eolian material.
Thickness of the Gering Formation ranges from 100
to 200 feet (Witzel, 1974, p.50).
The Monroe Creek Formation is Miocene
in age and overlies the Gering Formation (Figure 2). The Monroe Creek is lithologically
similar to the Gering with buff to brown, fine grained sandstone. The unique
characteristics of the Monroe Creek is the presence of large "pipy" concretions.
These concretions consist of fine grained sand similar to the rest of the
formation with calcium carbonate cement and are extremely hard and resistent
to weathering.
The reported thickness of the Monroe
Creek Formation is 280 to 360 feet (Lugan, 1938 from Witzel, 1974, p.53).
The Harrison Formation is the youngest
member of the Arikaree Group. To date, this formation has rarely been penetrated
in WFC drill holes, thus little first hand information is available. It is
described as lithologically similar to the Gering and Monroe Creek Formations,
with fine grained unconsolidated buff to light gray sands. The Harrison Formation
is also noted for its abundance of fossil remains (Witzel, 1974, p.55)
Quarternary Alluvium
Quarternary alluvial and colluvial
material are present in the permit area ranging in depth from 0 to 40 feet.
The material consists of OligoceneMiocene rock fragments, silt, sand and gravel.
REGIONAL STRUCTURE
The most prominent structural expression
in northwest Nebraska is the Chadron Arch (Figure 11). This anticlinal feature
strikes roughly northwestsoutheast along the northeastern boundary of Dawes
County. The only surficial expression of the Chadron Arch is in the northeastern
corner of Dawes County, as well as small portions of Sheridan County and Shannon
County, South Dakota.
The Black Hills lie north of Sioux
and Dawes Counties in southwestern South Dakota (Figure 11). Together with
the Chadron Arch, the Black Hills Uplift has produced many of the prominent
structural features presently observed in the area. As a result of the uplift,
formations underlying the project area dip gently to the south. The Tertiary
deposits dip slightly less than the older Mesozoic and Paleozoic Formation
(Witzel, 1974, p.18).
The Crow Butte ore body lies in what
has been named the Crawford Basin (DeGraw, 1969). DeGraw made detailed studies
of the preTertiary subsurface in western Nebraska using primarily deep oil
well test information. DeGraw substantiated known structural features and
proposed several structures not earlier recognized. The Crawford Basin was
defined by DeGraw as being a triangular, asymmetrical basin bounded by the
Toadstool Park Fault on the northwest, the Chadron Arch and Bordeaux Fault
to the west, the Cochran Arch and Pine Ridge Fault to the south (Figure 11).
The Toadstool Park Fault, the Bordeaux
Fault and other faults occur outside WFC's project area and are assumed to
exist as described by DeGraw and others. The Pine Ridge Fault has also been
inferred from subsurface data and proposed by DeGraw (1969, p.36). This fault
trends east-west across Sioux and Dawes Counties. This fault is subparallel
to the Cochran Arch and has a reported displacement of about 300 feet with
the south side upthrown.
The Cochran Arch was also proposed
by DeGraw (1969, p.36) on the basis of subsurface data. The Cochran Arch trends
east-west through Sioux and Dawes Counties, parallel to the aforementioned
Pine Ridge Fault. Structural features subparallel to the Cochran Arch have
also been observed in drill hole data. The existence of the Cochran Arch alone
is probably enough to explain the structural high south of Crawford.
The synclinal axis of the Crawford
Basin trends roughly eastwest and plunges west into what is informally referred
to as the Inner Crawford Basin by WFC. The Inner Basin is characterized by
a rather sharp paleotopographic change in the Pierre Shale with dramatic increase
in the thickness of the Basal Chadron Sandstone.
The single most prominent structural
feature within the Crawford Basin is the previously unnamed White River Fault
(Figure 11). It is located directly north of Crawford, and strikes northeastsouthwest
with the upthrown side to the south. The total vertical displacement is 200
to 400 feet; no strike-slip movement has been detected. The disturbance of
the Chadron and Brule Formations date the fault as post-Oligocene.
