GEOLOGIC SETTING
COAL GEOLOGY
-
The Northern Rocky Mountains and Great Plains region of Wyoming,
Montana, and North Dakota contains a vast expanse of the Fort Union Formation
and equivalent Paleocene coal-bearing rocks (fig.
IN-1).
-
These coal-bearing rocks (fig.
IN-2) exist from the surface down to a depth of about 6,000 ft in shallow
basins, such as the Powder River,Williston, and Greater Great River Basins,
and from the surface to a
depth of 12,000 ft in deep basins such as the Hanna Basin.
-
The apparent rank of Fort Union and equivalent coal
ranges from lignite to subbituminous in the shallow basins and subbituminous
to bituminous in deep basins.
-
Deep basins were influenced more extensively by Tertiary
Laramide
deformation than were the shallow basins.
-
Paleotectonic history and depositional settings (fig.
IN-3) of the swamps in which this coal accumulated contributed to the
high quality of the Fort Union and equivalent coal.
COAL STRATIGRAPHY
-
The Fort Union Formation (fig.
IN-4) is present in the Powder River, Williston, and Greater Green
River Basins.
-
Stratigraphically equivalent to the Fort Union Formation
are coal-bearing rocks in the Ferris (fig. IN-5)
and Hanna Formations in the Hanna and Carbon Basins of Wyoming.
-
Major production of Fort Union coal in the Powder River Basin
is from the Wyodak-Anderson (fig. IN-6),
Rosebud, and equivalent coal beds and zones. These coal deposits, which
range from 25 to 140 ft thick, produce from 25 mines more than 38 percent
of the total U.S. coal production.
-
The producing Fort Union coal beds and zones in the Williston
Basin include the Beulah-Zap (fig. IN-7),
Hagel, and Harmon coal beds and zones. These coal beds and zones range
in thickness from 20 to 40 ft.
-
Coal production in the Ferris and Hanna Formations in the
Hanna Basin is from the Ferris Nos. 23, 25, 31, 50, and 65 coal beds and
Hanna Nos. 77, 78, 79, and 81 coal beds, which are each as much as 36 ft
thick.
-
The producing Fort Union coal zone in the Greater Green River
Basin is the Deadman coal zone (fig. IN-8),
which has beds ranging from 2 to 33 ft thick.
BIOSTRATIGRAPHY
-
Biostratigraphy uses fossils to determine age relations and
correlations of coal deposits in the Northern Rocky Mountains and Great
Plains region.
-
The most common and most useful fossils in coal and coal-bearing
rocks are microscopic spores and pollen grains of ancient plants. Thus,
palynology (the study of plant microfossils) has been applied throughout
the region to provide the biostratigraphic framework.
-
Biostratigraphy based on fossil spores and pollen (fig.
IN-9) is called palynostratigraphy.
-
In the Northern Rocky Mountains and Great Plains, palynostratigraphic
age determinations of the uplifts, basins, and coal deposits supportinterpretations
of the evolution of the basins and the origin of Fort Union Formation and
equivalent coal resources.
-
The regional palynostratigraphic zonation divides the Paleocene
into six biozones designated, from oldest to youngest, P1 through P6 (fig.
IN-10). The biozones are defined by occurrences of species of
the related genera Momipites and Caryapollenites and other species of fossil
pollen.
-
Palynostratigraphy is used to place all coal beds and zones
in the assessment region in a stratigraphic framework (fig.
IN-11).
-
Palynostratigraphy is the basis of correlations of coal-bearing
rocks between basins in the Northern Rocky Mountains and Great Plains region
(fig. IN-12).
|
