DEPOSITIONAL SETTINGS AND
THEIR INFLUENCE ON COAL RESOURCES
About 55 to 60 million years ago when the Fort Union
and equivalent coal-forming peat accumulated, the Northern Rocky Mountains
and Great Plains region was primarily continental (Brown, 1958). Marine
and coastal-plain environments were restricted to areas near the Cannonball
Sea in what is now North Dakota and South Dakota. Areas west and southwest
of the Cannonball Sea, in what is now Montana and Wyoming, were in constant
change as a result of continuing uplift of mountain ranges and formation
of basins between these uplifts. After a few million years had elapsed,
river systems and their associated floodplains, lakes, and swamps formed
a network of waterways and wetlands in the basins.
EARLY PALEOCENE
Large river systems (fig.
IN-13) in what is now Montana and Wyoming generally flowed eastward
and northeastward toward the Cannonball Sea (fig.
IN-13) in what is now North Dakota and South Dakota (Flores, 1986).
Coal-forming swamps or mires formed between river channels near the headwaters
and downstream parts of river systems. Along the coast of the Cannonball
Sea, delta, barrier, and tidal deposits (fig.
IN-13) were formed (Flores, 1998).
MIDDLE PALEOCENE
Some of the large river systems changed to short, high-gradient
streams as mountain building (fig. IN-14)
continued to reshape the landscape. Coal-forming mires along river channels
and associated lakes were widespread from the headwaters of the rivers
to the coastal plain. The Cannonball Sea retreated northeastward, followed
by an encroaching coastal plain where mires formed on abandoned delta,
barrier, and tidal deposits.
LATE PALEOCENE
As mountain building continued, short, high-gradient
drainage systems within developing basins were either ponded into lake
systems (fig. IN-15) or flowed through the
basins toward the seaway. Large coal-forming mires (fig.
IN-15) developed in the basins between river channels and along lake
margins. The extent and distribution of the swamps were controlled partly
by the size and pattern of these networks of rivers and lakes, and partly
by mountain-building activity. As a result, coal beds that formed from
these swamps are generally discontinuous and lenticular. Many of the thick
peat accumulations, which resulted in thick coal beds, developed in multiple,
stacked, domed mires (fig. IN-15) that sheltered
the peat from floods and burial by sediments. Coal beds in the Powder River
Basin in excess of 200 ft thick reflect the repeated life cycle of swamp
growth, demise, and rejuvenation over long periods of time within tectonically
subsiding basins.
INFLUENCE
OF DEPOSITIONAL ENVIRONMENTS
Depositional environments influence the thickness, shape
or geometry, and distribution of Fort Union and equivalent coal. Generally,
coal thickness depends on how long peat is permitted to accumulate within
the swamp or mire. In this region, peat accumulation was primarily influenced
by incursion of river sediments due to floods. Thus, how far the swamp
was from river channels, how low or high was the topography of the mire,
and
the chemical and biological conditions existing in the
mire determined the nature and duration of peat accumulation. Thick peat
formed in chemically highly reduced, raised mires removed from river channels,
with plant growth sustained by high rate of rainfall. The shape and distribution
of coal beds are reflected by the location of the peat-forming swamps in
the depositional environments. For example, peat-forming swamps between
river channels formed lenticular shapes. Furthermore, when these river
channels are
abandoned, these discontinuous, lenticular, peat-forming
swamps advanced and coalesced over channel deposits, forming a continuous
bed. The quality of coal (as demonstrated by sulfur, ash, and trace
elements
contents) is directly related to the depositional environment.
For example, peat that accumulated in coastal swamps that were transgressed
by brackish and marine water commonly produces coal that has a high sulfur
content. Peat that accumulates in fluvial or continental areas
far removed from marine influence produces coal that has a low sulfur content.
Peat mires that are protected from sediment-carrying floods by raised topography
produce coal that contains low amounts of ash or fine particulates. However,
peat mires that are topographically low-lying and flooded by sediments
produce coal high in ash and trace-element content.
COAL QUALITY
-
Fort Union and equivalent Paleocene coal is considered clean,
low contaminant-bearing, and compliant with the 1990 Clean Air Act Amendments.
Coal in the Powder River and Williston Basins contains less
sulfur and ash than coal produced from other regions in the conterminous
United States, as shown in table IN-1 (arithmetic
means for the Powder River Basin based on 279 samples; arithmetic means
for the Williston Basin based on 281 samples).
When sulfur values are compared on pounds of SO 2 per million
Btu basis (table IN-2), Powder River Basin
coal has the lowest mean content of SO 2 per million Btu of any coal in
the conterminous United States.
The quantity of trace elements of environmental concern named
in the 1990 Clean Air Act Amendments (antimony, arsenic, beryllium, cadmium,
chromium, cobalt, lead, manganese, mercury, nickel, selenium, and uranium)
is of increasing importance in meeting compliance standards.
Powder River Basin coal has among the lowest concentrations
of most of these elements when compared to coal from other coal-producing
regions in the conterminous United States (table
IN-3) on a whole-coal, remnant-moisture basis.
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