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Coal deposits of the United States
International Journal of Coal Geology, 8 (1987) 355-365 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
355
Coal Deposits of the United States
W. JOHN NELSON Illinois State Geological Survey, 615 E. Peabody Dr., Champaign, IL 61820, U.S.A. (Received and accepted for publication May 21, 1987)
ABSTRACT
Nelson, W.J., 1987. Coal deposits of the United States. Int. J. Coal. Geol., 8: 355-365. The coal fields of the United States can be divided into six major provinces. The Appalachian and Interior Provinces contain dominantly bituminous coal in strata of Pennsylvanian age. The coal seams are relatively thin and are mined both by surface and underground methods. Sulfur content is low to moderate in the Appalachian Province, generally high in the Interior province. The Gulf Coastal Plain Province, in Texas and neighboring states, contains lignite of Eocene age. The seams are 3-25 ft (0.9-7.5 m) thick and are minded in large open pits. The Northern Great Plains Province has lignite and subbituminous coal of Cretaceous, Paleocene and Eocene age. The coal, largely very low in sulfur, occurs in beds up to 100 ft (30 m) thick and is strip-mined. The Rocky Mountain Province contains a great variety of coal deposits in numerous separate intermontane basins. Most of it is low-sulfur subbituminous to bituminous coal of Cretaceous and early Tertiary age. The seams range from a few feet to over 100 ft (30 m) thick. Strip-mining dominates but underground mines are important in Utah and Colorado. The Pacific Coast Province, which includes Alaska, contains enormous coal resources but has seen little mining. The coal is highly diverse in physical character and geologic setting.
INTRODUCTION
A l t h o u g h t h e M i s s i s s i p p i R i v e r is c o m m o n l y t a k e n as t h e dividing line b e t w e e n " e a s t e r n " a n d " w e s t e r n " coal in e c o n o m i c studies, t h e r i v e r is n o t a logical dividing line f r o m t h e s t a n d p o i n t o f geology, coal c h a r a c t e r , or m i n i n g c o n d i t i o n s . L a r g e coal d e p o s i t s w e s t o f t h e M i s s i s s i p p i , n a m e l y t h o s e of t h e W e s t e r n I n t e r i o r B a s i n , are t y p i c a l of " e a s t e r n " r a t h e r t h a n " w e s t e r n " coal. T h e U.S. Geological S u r v e y , u s i n g c r i t e r i a o f geology, coal quality, a n d m i n i n g p r a c t i c e , g r o u p s A m e r i c a n coal fields i n t o six m a j o r p r o v i n c e s ( Fig. 1 ). A m o n g these, t h e A p p a l a c h i a n a n d I n t e r i o r P r o v i n c e s r e p r e s e n t " e a s t e r n " coal in t h e
0166-5162/87/$03.50
© 1987 Elsevier Science Publishers B.V.
EASTERN OR APPALACHIAN INTERIOR GULF COASTAL NORTHERN GREAT PLAINS ROCKY MOUNTAINS PACIFIC COAST
COAL PROVINCES
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Narragansett Basin Pennsylvania anthracite Pocahontas and New River Fie~ds Lookout Mountain Field Triassic basins Michigan Basin Eastern Interior or Illinois Basin
Coal basins, fields or districts
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Fig. 1. Coal deposits of the U.S. ( Modified from Trumbull, 1960).
A B C D E F
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8 Western Interlol Basin 9 Arkoma Basin 10 North*central Texas 1 I Williston Basin 12 Powder River Basin 13 Hanna Basin 14 Rock Springs or Green River Field
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15 16 17 18 19 20 21
Castle Gate Crested Butte Raton Mesa Field San Juan Basin Black Mesa Field Kaiparowits Field Henry Mountains F~eld
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357 normal manner of speaking, while the Gulf Coast, Northern Great Plains, Rocky Mountain, and Pacific Coast Provinces contain the "western" coal. EASTERN OR APPALACHIANPROVINCE
Extent and geology The Eastern Province includes the coterminous coal fields of the Appalachian Mountains and plateaus, and several disconnected basins farther east. This is the oldest and most extensively developed coal province in the U.S., and it contains roughly 2/5 of the nation's bituminous coal plus practically all the anthracite (Table 1 ). The minable coal is dominantly of Pennsylvanian age.
Character of coal Rank increases from west to east in the Appalachian Province. High-volatile bituminous coal of the Allegheny and Cumberland Plateau gives way to lowvolatile coal in the folded Appalachians, anthracite in eastern Pennsylvanian, and meta-anthracite or graphite in Rhode Island. This change in rank reflects increasing original depth of burial and greater heat and pressure associated with mountain-building activities to the east and southeast. Most reserves and current mining are in high- and medium-volatile bituminous coal. This is largely used for electrical generation or industrial processes, but some of it is employed for coke making. The nation's largest reserves of coking coal, however, are in low-volatile, low-sulfur Appalachian coal in such fields as the Lookout Mountain, Pocahontas, New River, and various fields of central Pennsylvania. Anthracite was once in great demand for domestic heating because of its "smokeless" properties, but loss of market has reduced production to only 5 or 6 million tons annually. Much Appalachian coal is low enough in sulfur to comply with current environmental regulations. This unfortunately, has led to extensive burning of metallurgical-grade coal in power plants. Some Appalachian coal is higher in sulfur (1-4%), but generally not so high as most coal from the Interior Province.
Mining conditions and practice Eastern coal seams range from less than 2 ft (0.6 m) to more than 12 ft (3.6 m) thick, but most beds mined are 3 to 7 or 8 ft (0.9 to 2.1 or 2.4 m). Through most of the province the coal beds are level or nearly so, but moderate folding is encountered in the mountains, and the anthracite is strongly folded. Deeper
358 TABLE 1 Demonstrated reserve base of coal in the United States, by province and state (U.S. DOE, 1982) ( millions of short tons) Province State EASTERN Alabama Georgia Kentucky Maryland N. Carolina Ohio Pennsylvania Tennessee Virginia W. Virginia Total
INTERIOR Arkansas Illinois Indiana Iowa Kansas Kentucky Michigan Missouri Oklahoma Texas a Total
Anthracite
7,092.0 125.5 7,217.5
96.4
96.4
Bituminous
Subbituminous
Total
3,916.8 3.6 12,927.5 822.4 10.7 19,056.1 23,188.8 983.7 3,345.9 39t776.2 104,031.7
3,916.8 3.6 12,927.5 822.4 10.7 19,056.1 30,280.8 983.7 3,471.4 391776.2 111,249.2
288.7 67,606.0 10,586.1 2,197.1 993.8 21,074.4 127.7 6,069.1 1,637.8 6~100 + 116,680.7 +
385.1 67,606.0 10,586.1 2,197.1 993.8 21,074.4 127.7 6,069.1 1,637.8 6~100 + 116,771.1 +
GULF COASTAL Alabama Arkansas Texas Total
NORTHERN GREAT PLAINS Montana North Dakota South Dakota Wyomingh Total
Lignite
1,083.0 25.7 121659.7 13,768.4
1,385.4
103,277.4 22,985.8
15,765.2 9,952.3 366.1
1,385.4
126,263.2
28,083.6
1,083.0 25.7 12~659.7 13,768.4
120,428.0 9,952.3 366.1 221985.8 153,732.2
359 TABLE 1 (continued) Province State
Anthracite Bituminous
ROCKY MOUNTAINS Arizona Colorado 25.5 Idaho New Mexico 2.3 Utah Wyoming Total 27.8 PACIFIC COAST Alaska Oregon Washington Total GRAND TOTAL 7,341.7
410.0c 9,086.1 4.4 1,835.7 6,476.5 4,460.5 22,273.2 697.5
Subbituminous
Lignite
3,979.9
4,189.9
2,683.4 1.1 42,477.7 49,142.1
4,189.9
303.7 1,001.2
5,443.0 17.5 1,169.4 6,629.4
245,372.9
182,035.0
14.0
Total
410.0 17,281.4 4.4 4,521.4 6,477.6 46,938.2 75,633.0
8.1 22.1
6,154.5 17.5 1,481.2 7,653.2
44,063.9
478,813.6
~FromEvans (1974) hU.S.DOE doesnot distinguishGreat Plains fromRockyMountainProvincein Wyoming.Reserves for Great Plains (Powder River Basin) from Glass, 1978. Reserves for Rocky Mountains from U.S. DOE minus Glass's estimate for Powder River Basin. CThisfigureseems highlyquestionablesince Arizona's coal is normallyclassed as subbituminous, but this is the figure given throughout U.S. DOE. mines tend to be gassy, and water influx is often a problem, especially where sandstone directly overlies coal beds. The rugged terrain and multiplicity of coal seams provide a b u n d a n t opportunities for small companies. Hundreds of small drift mines producing less t h a n 100,000 tons per year operate, especially in K e n t u c k y and West Virginia. Most larger mines are slope or shaft operations, below drainage. These are best established in the Pittsburgh Coal and other thick, extensive seams of northern West Virginia and southwestern Pennsylvania. Several shaft mines of 1,000 ft ( 300 m) deep recently have been opened near Birmingham, Alabama. Strip mining is widespread, but mostly the domain of small- to medium-sized outfits, because few large blocks of strippable reserves exist. Terrain generally dictates contour-mining a n d / o r mountain-top removal. A common practice is to contour-strip as deep as overburden allows, t h e n auger the coal or " p u n c h m i n e " it from the bench with a continuous miner. INTERIOR PROVINCE
Extent and geology The Interior Province comprises several separate basins of bituminous coal, extending from Texas to Michigan (Fig. 1 ). Like Appalachian coal, Interior
360 coal is found in rocks of Pennsylvanian age. Nearly half of America's bituminous coal reserves are found here (Table 1 ). Coal in the Interior Province was mostly deposited far from mountainous uplifts, and the peat swamps were invaded more often by the sea than was the case in Appalachian Province. These geologic facts influence the quality of the coal. The Arkoma Basin of Arkansas and eastern Oklahoma is an exception. Geologically, and in terms of coal character and mining conditions, the Arkoma Basin belongs with the Appalachian rather than Interior Province.
Character o[ coal Except in the Arkoma Basins, all of the coal in the Interior Province is highvolatile bituminous. Rank has advanced as far as semi-anthracite in the eastern tip of the Arkoma Basin. In the Illinois Basin, rank increases slightly from youngest coals to oldest, and increases toward the southeast in all seams, reflecting greater original depth of burial. Rank in the Western Interior Basin increases southward. The high sulfur content of most Interior Province coal has hurt its market position in recent years. The normal range is 3 - 5% sulfur, as mined. This sulfur is largely attributable to influx of suifate-bearing sea water into the peat swamps before the peat was buried. Low-sulfur coal is present in the Arkoma Basin and in certain localities within the Illinois Basin where coal is overlain by non-marine rock. Some of the low-sulfur coal is suitable for coke making, generally in blend with Appalachian low-volatile bituminous.
Mining conditions and practice Coal seams mined in the Illinois Basin range from 1½ to 15 ft (0.45 to 4.5 m) thick, but normally are 3-8 ft (0.9-2.4 m) in thickness. Two seams, the Springfield and Herrin Coals, are 5 ft (1.5 m) or thicker over thousands of square miles and account for more than 90% of production. Dips are gentle, although faults are common in the southern part of the basin. Underground mines are only moderately gassy, and water seldom is a serious problem. The great continuity of coal beds, gentle dip, and level terrain render Illinois ideal for large-scale (over 1 million tons per year) area surface mines and large shaft or slope mines. Roughly half the annual production comes from underground mines and half from open pits. The proportions are about the same in Kentucky, although rougher terrain favors smaller mines. Indiana has many small and a few large surface mines, but only a handful of deep mines. The Western Interior Basin is characterized by laterally extensive but thin [less than 5 ft (1.5 m) ] coal beds, which today are mined exclusively by stripping. Most mines produce 50,000 to 500,000 t.p.y., but several have outputs of 1 million t.p.y, or more. Small- to medium-scale producers are strip-mining
361 coal in the Arkoma Basin. Both area- and contour-stripping are practiced, depending upon topography. Some of the coals mined are steeply pitching and faulted. Underground mining currently is dominant in the region, but is sure to resume when the market for coal improves. Coal has not been mined in Michigan for many years, although in the early 1900s as much as 2 million t.p.y, were produced. Most resources are accessible only to underground mining, but the small reserves, thin discontinuous seams, water problems, and other factors will deter renewed development (Ells, 1979). Bituminous coal output of Texas peaked at 1.25 m.t. in 1917 and ceased after World War II. Conditions are favorable for surface mining to resume in northcentral Texas (Evans, 1974). GULF COASTALPLAIN PROVINCE
Extent and geology The Gulf Coastal Plain Province includes the area underlain by Cretaceous and Tertiary sediments in Texas, Louisiana, Arkansas, Mississippi, and parts of adjacent states. The coal is all lignite. Resources are known in several states (Table 1 ), but only Texas has active mines. Lignite in Texas occurs in three geologic units of Eocene age: the Wilcox Group (oldest), the Yegua Formation of the Claiborne Group, and the Jackson Group. This coal originated as peat in largely freshwater swamps (Kaiser et al., 1980). Interbedded with lignite are poorly consolidated sandstone, siltstone, and mudstone.
Character of coal The oldest coal, originally most deeply buried, has the highest rank. Wilcox lignite has 24-32% moisture, about 27% fixed carbon, and yields 7,000 to 7,500 Btu/lb. as received. The younger Yegua lignite runs 36% moisture, 20% fixed carbon, and 6,000 Btu/lb., while the youngest Jackson coal averages 40% moisture, 16% fixed carbon, and less than 5,000 Btu/lb. Coal from the Wilcox Group also is better in terms of ash and sulfur; 10-11% ash and 0.9% sulfur compared with 15-40% ash and 1.0-2.3% sulfur in Yegua and Jackson coal (Keystone, 1982). This coal is used primarily in large mine-mouth electrical generating stations. Alcoa mines lignite to produce electricity for its aluminum-reduction plants northeast of Austin.
362
Mining conditions and practice Lignite in Texas is produced from large opencast pits. Output from two mines in 1982 exceeded 10 m.t., ranking the operations third and fourth in the nation. The seams mined are generally 3-10 ft (0.9-3.0 m) thick, but locally reach 25 ft (7.5 m). Maximum current mining depth is about 120 ft (36 m), but some companies are contemplating working to 250 ft ( 75 m) or deeper where multiple seams are present (Kaiser et al., 1980). The loosely consolidated overburden is easy to excavate, and the gently rolling land easy to reclaim after mining. These factors, together with low sulfur content, account for the phenomenal growth of Texas's coal industry. From about 2 m.t., in 1970, production rocketed to 33 m.t. in 1982, making Texas the eighth leading state in total output of coal. This trend shows little sign of reversing, and is likely to spread to Gulf Coastal coal outside of Texas. NORTHERN GREAT PLAINS PROVINCE
Extent and geology Coal deposits east of the Rocky Mountains proper in Montana, Wyoming, and the Dakotas are classed in the Northern Great Plains Province. Bituminous coal is present, but the great bulk of resources are subbituminous and lignite. The Paleocene Ft. Union Formation is the most prolific coal-bearing unit, with lesser deposits in strata of Cretaceous and Eocene age. The Ft. Union Formation consists mainly of detritus eroded from the Rocky Mountains when they were young, and is entirely a fresh-water deposit. The Ft. Union contains some of the world's thickest coal beds. Seams over 100 ft (30 m) thick are being mined, and one coal bed over 200 ft thick (60 m) has been reported ( Glass, 1978).
Character of coal Rank increases very gradually from east to west, toward the mountains. The Williston Basin contains lignite. Subbituminous coal is found in the Powder River Basin and other Tertiary basins of Montana, while Cretaceous coals in north-central Montana have advanced to bituminous rank. Ft. Union coal has very low sulfur content, as a result of its non-marine origin. Environmental regulations have produced enormous demand for coal in the Williston and Powder River Basins. The following are typical analyses: North Dakota lignite, 30-45% moisture, 5,700-7,700 Btu/lb, 4-9% ash, and 0.2-1.4% sulfur (Brant, 1953) ; Powder River Basin, Wyoming, 20-35% moisture, 7,100-9,700 Btu/lb, 4-15% ash, and 0.15-1.2 % sulfur ( Glass, 1978); Great Falls-Lewiston area, Montana, 1.7-4.0%
363 sulfur and highly variable ash, high-volatile bituminous B and C (Keystone, 1982).
Mining conditions and practice As in Texas, most mining in the Northern Great Plains takes place in large open pits. Of the 10 largest mines in the U.S. in 1982, 8 were in the Powder River Basin. Such mines have sent Wyoming soaring to third place among coal-producing states. The largest mine of all, with more than 16.8 m.t. in 1982, is the Black Thunder Mine near Gillette, Wyoming, The incredibly thick, gently dipping coal beds and slightly rolling terrain obviously lend themselves to such operations. Mining is not without problems: the coal readily catches fire spontaneously; many seams are burned to clinker far back from the outcrop. Reclamation also is a thorny issue. When 50-100 ft (15-30 m) of coal are removed, the land cannot be restored to original contour; and re-establishing plant growth after mining is difficult with the arid climate and extremely thin, alkaline topsoil. Approximately 18 m.t. of lignite were mined in the Williston Basin in 1982 (Keystone). The coal fields of north-central Montana have not seen activity for many years, and are not likely to do so in the near future. ROCKY MOUNTAINPROVINCE
Extent and geology Coal deposits of the Rocky Mountain are found in numerous intermontane basins and vary considerably in character, geology, and mining conditions. The principal deposits are in western Wyoming, Colorado, Utah, Arizona, and New Mexico. Most coal is bituminous and subbituminous of Cretaceous age, and subbituminous and lignite of Paleocene and Eocene age. The seams are thick; many over 10 ft (3 m) and some reach 60 ft (10 m) or more. Most coal beds are horizontal or gently folded but some are tightly folded, faulted, and coked by igneous intrusions.
Character of coal Rank of coal in the Rocky Mountain Province varies from subbituminous to anthracite, but most is subbituminous or high- to medium-volatile bituminous. Locally, higher rank is attributed to the heat from nearby igneous intrusions, as at Crested Butte, Colorado. Important deposits of coking coal occur in the Raton Mesa Field near Trinidad, Colorado and in the Book Cliffs at Price and Castle Gate, Utah. Most Rocky Mountain coal, however, is burned for steam generation. Generally, it is excellent for this purpose because of its low (under
364 1% ) sulfur content. The ash content varies widely from seam to seam, but most fields contain at least some coal with less than 8% ash.
Mining conditions and practice Aside from mines that produced coking coat, most early mines in the Rockies existed to supply fuel to railroad locomotives and for local domestic use. As these markets vanished after World War II, most mining areas declined seriously. They were vigorously reborn during the 1970s when environmental regulations brought intense demand for low-sulfur coal. This development has spread beyond old established fields, such as Hanna and Rock Springs in Wyoming, to previously untapped deposits, as in the Black Mesa Field of Arizona. However, some rich resources, notably those of Kaiparowits and Henry Mountains Fields in southern Utah, have never been exploited because of their remote location. Mining practice varies greatly, dependent upon geology and topography. Huge area-surface mines operate in broad, open basins such as Black Mesa and the San Juan and Green River Basins. Modified contour-stripping is practiced in moderately dipping coal beds of northwestern Colorado. Near Walden, northcentral Colorado, a nearly vertical 60-ft (100 m) -thick coal seam is being mined in an open pit 400 ft (120 m ) deep. Drift mines, which often must contend with faulted or pitching seams, work the rugged plateau and mesa country of the Book Cliffs in Utah and western Colorado. Squeezes and rock bursts are often a problem due to extreme thickness of overburden. Most mines in the Rocky Mountains are relatively new, modern operations, and medium to large in terms of production. PACIFIC COASTPROVINCE
Extent The Pacific Coast Province comprises the states west of the Rocky Mountains, together with Alaska. This region has large resources but has been far less mining than any other province. At present only two mines, one in Washington and another in Alaska, are operating.
Geology Coal of Tertiary age occurs in small and widely scattered basins in California, Nevada, Oregon, and Washington. The latter state has by far the largest reserves, most of them between Puget Sound and the Cascade Range. Coalbearing strata range in age from Paleocene to Oligocene, with most coal being
365 in Middle Eocene rock. Much of the coal has been deformed and metamorphosed by mountain-building and volcanic activity (Keystone, 1982 ). Alaska's coal deposits are huge but poorly known, and occur in highly diverse geologic settings. Estimates of resources vary enormously. U.S DOE (1982) lists a little more than 6 billion tons of demonstrated resources, but the ultimate resources may be as high as 5.5 trillion tons-one-sixth of all the coal in the world (Schaff, 1983).
Character of coal Both Alaska and Washington contain coal of all grades from lignite to anthracite, but demonstrated resources are largely subbituminous. Most coal in Washington has less than 1% sulfur, although some analyses approach 5%. Ash tends to be high; 8-20% or even more. Alaska contains coal of very low sulfur content; but generalizations cannot be made about such a diverse resource.
Mining conditions and practice The one operating mine in Washington is a strip pit that produces approximately 5 m.t. per year. Additional development is deterred by structural complications and somewhat unpredictable reserves and quality. Alaska obviously presents a challenging, if not hostile, environment for mining, but large reserves are located close to tidewater, and not far from populated areas. The prognosis is good for some of this coal to be developed for export market, especially to Japan.
REFERENCES Brant, R. A., 1953. Lignite resources of North Dakota. U.S. Geol. Surv., Circular 226, 78 pp. Ells, G. D., 1979. The Mississippian and Pennsylvanian (Carboniferous) Systems in the United States-Michigan, U.S. Geol. Surv., Prof. Pap. 1110 A-L: J1-J17. Evans, T. J., 1974. Bituminous coal in,Texas. Bur. of Econ. Geol., Handbook 4, 65 pp. Glass, G. B., 1978. W y o m i n g coal fields,1978. Geol. Surv. Wyo., Pub. Inf.Circ. 9, 91 pp. Kaiser, W.R., Ayers, W.B. and La Brie, L.W., 1980. Lignite resources in Texas. Bur. Econ. Geol. Rep. Inv. 104, 52 pp. Keystone Coal Industry Manual, 1983. McGraw-Hill, N e w York, NY, 1461 pp. Schaff, R. G., 1983. Coal resources of Alaska. Div. Geol. Geophys. Surv., Info. Circ. 17, 9 pp. Trumbull, J. 1960. Coal fieldsof the United States. U.S. Geol. Survey (map). U.S. Department of Energy, 1982. Demonstrated reserve base of coal in the United States on January 1, 1980. DOE/EIA-0280 (80), 39 pp.
355
Coal Deposits of the United States
W. JOHN NELSON Illinois State Geological Survey, 615 E. Peabody Dr., Champaign, IL 61820, U.S.A. (Received and accepted for publication May 21, 1987)
ABSTRACT
Nelson, W.J., 1987. Coal deposits of the United States. Int. J. Coal. Geol., 8: 355-365. The coal fields of the United States can be divided into six major provinces. The Appalachian and Interior Provinces contain dominantly bituminous coal in strata of Pennsylvanian age. The coal seams are relatively thin and are mined both by surface and underground methods. Sulfur content is low to moderate in the Appalachian Province, generally high in the Interior province. The Gulf Coastal Plain Province, in Texas and neighboring states, contains lignite of Eocene age. The seams are 3-25 ft (0.9-7.5 m) thick and are minded in large open pits. The Northern Great Plains Province has lignite and subbituminous coal of Cretaceous, Paleocene and Eocene age. The coal, largely very low in sulfur, occurs in beds up to 100 ft (30 m) thick and is strip-mined. The Rocky Mountain Province contains a great variety of coal deposits in numerous separate intermontane basins. Most of it is low-sulfur subbituminous to bituminous coal of Cretaceous and early Tertiary age. The seams range from a few feet to over 100 ft (30 m) thick. Strip-mining dominates but underground mines are important in Utah and Colorado. The Pacific Coast Province, which includes Alaska, contains enormous coal resources but has seen little mining. The coal is highly diverse in physical character and geologic setting.
INTRODUCTION
A l t h o u g h t h e M i s s i s s i p p i R i v e r is c o m m o n l y t a k e n as t h e dividing line b e t w e e n " e a s t e r n " a n d " w e s t e r n " coal in e c o n o m i c studies, t h e r i v e r is n o t a logical dividing line f r o m t h e s t a n d p o i n t o f geology, coal c h a r a c t e r , or m i n i n g c o n d i t i o n s . L a r g e coal d e p o s i t s w e s t o f t h e M i s s i s s i p p i , n a m e l y t h o s e of t h e W e s t e r n I n t e r i o r B a s i n , are t y p i c a l of " e a s t e r n " r a t h e r t h a n " w e s t e r n " coal. T h e U.S. Geological S u r v e y , u s i n g c r i t e r i a o f geology, coal quality, a n d m i n i n g p r a c t i c e , g r o u p s A m e r i c a n coal fields i n t o six m a j o r p r o v i n c e s ( Fig. 1 ). A m o n g these, t h e A p p a l a c h i a n a n d I n t e r i o r P r o v i n c e s r e p r e s e n t " e a s t e r n " coal in t h e
0166-5162/87/$03.50
© 1987 Elsevier Science Publishers B.V.
EASTERN OR APPALACHIAN INTERIOR GULF COASTAL NORTHERN GREAT PLAINS ROCKY MOUNTAINS PACIFIC COAST
COAL PROVINCES
.o
E
/
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/
! 2 3 4 5 6 7
/
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Narragansett Basin Pennsylvania anthracite Pocahontas and New River Fie~ds Lookout Mountain Field Triassic basins Michigan Basin Eastern Interior or Illinois Basin
Coal basins, fields or districts
/
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D
Fig. 1. Coal deposits of the U.S. ( Modified from Trumbull, 1960).
A B C D E F
F
o
L
500 ml
8 Western Interlol Basin 9 Arkoma Basin 10 North*central Texas 1 I Williston Basin 12 Powder River Basin 13 Hanna Basin 14 Rock Springs or Green River Field
J
250
15 16 17 18 19 20 21
Castle Gate Crested Butte Raton Mesa Field San Juan Basin Black Mesa Field Kaiparowits Field Henry Mountains F~eld
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357 normal manner of speaking, while the Gulf Coast, Northern Great Plains, Rocky Mountain, and Pacific Coast Provinces contain the "western" coal. EASTERN OR APPALACHIANPROVINCE
Extent and geology The Eastern Province includes the coterminous coal fields of the Appalachian Mountains and plateaus, and several disconnected basins farther east. This is the oldest and most extensively developed coal province in the U.S., and it contains roughly 2/5 of the nation's bituminous coal plus practically all the anthracite (Table 1 ). The minable coal is dominantly of Pennsylvanian age.
Character of coal Rank increases from west to east in the Appalachian Province. High-volatile bituminous coal of the Allegheny and Cumberland Plateau gives way to lowvolatile coal in the folded Appalachians, anthracite in eastern Pennsylvanian, and meta-anthracite or graphite in Rhode Island. This change in rank reflects increasing original depth of burial and greater heat and pressure associated with mountain-building activities to the east and southeast. Most reserves and current mining are in high- and medium-volatile bituminous coal. This is largely used for electrical generation or industrial processes, but some of it is employed for coke making. The nation's largest reserves of coking coal, however, are in low-volatile, low-sulfur Appalachian coal in such fields as the Lookout Mountain, Pocahontas, New River, and various fields of central Pennsylvania. Anthracite was once in great demand for domestic heating because of its "smokeless" properties, but loss of market has reduced production to only 5 or 6 million tons annually. Much Appalachian coal is low enough in sulfur to comply with current environmental regulations. This unfortunately, has led to extensive burning of metallurgical-grade coal in power plants. Some Appalachian coal is higher in sulfur (1-4%), but generally not so high as most coal from the Interior Province.
Mining conditions and practice Eastern coal seams range from less than 2 ft (0.6 m) to more than 12 ft (3.6 m) thick, but most beds mined are 3 to 7 or 8 ft (0.9 to 2.1 or 2.4 m). Through most of the province the coal beds are level or nearly so, but moderate folding is encountered in the mountains, and the anthracite is strongly folded. Deeper
358 TABLE 1 Demonstrated reserve base of coal in the United States, by province and state (U.S. DOE, 1982) ( millions of short tons) Province State EASTERN Alabama Georgia Kentucky Maryland N. Carolina Ohio Pennsylvania Tennessee Virginia W. Virginia Total
INTERIOR Arkansas Illinois Indiana Iowa Kansas Kentucky Michigan Missouri Oklahoma Texas a Total
Anthracite
7,092.0 125.5 7,217.5
96.4
96.4
Bituminous
Subbituminous
Total
3,916.8 3.6 12,927.5 822.4 10.7 19,056.1 23,188.8 983.7 3,345.9 39t776.2 104,031.7
3,916.8 3.6 12,927.5 822.4 10.7 19,056.1 30,280.8 983.7 3,471.4 391776.2 111,249.2
288.7 67,606.0 10,586.1 2,197.1 993.8 21,074.4 127.7 6,069.1 1,637.8 6~100 + 116,680.7 +
385.1 67,606.0 10,586.1 2,197.1 993.8 21,074.4 127.7 6,069.1 1,637.8 6~100 + 116,771.1 +
GULF COASTAL Alabama Arkansas Texas Total
NORTHERN GREAT PLAINS Montana North Dakota South Dakota Wyomingh Total
Lignite
1,083.0 25.7 121659.7 13,768.4
1,385.4
103,277.4 22,985.8
15,765.2 9,952.3 366.1
1,385.4
126,263.2
28,083.6
1,083.0 25.7 12~659.7 13,768.4
120,428.0 9,952.3 366.1 221985.8 153,732.2
359 TABLE 1 (continued) Province State
Anthracite Bituminous
ROCKY MOUNTAINS Arizona Colorado 25.5 Idaho New Mexico 2.3 Utah Wyoming Total 27.8 PACIFIC COAST Alaska Oregon Washington Total GRAND TOTAL 7,341.7
410.0c 9,086.1 4.4 1,835.7 6,476.5 4,460.5 22,273.2 697.5
Subbituminous
Lignite
3,979.9
4,189.9
2,683.4 1.1 42,477.7 49,142.1
4,189.9
303.7 1,001.2
5,443.0 17.5 1,169.4 6,629.4
245,372.9
182,035.0
14.0
Total
410.0 17,281.4 4.4 4,521.4 6,477.6 46,938.2 75,633.0
8.1 22.1
6,154.5 17.5 1,481.2 7,653.2
44,063.9
478,813.6
~FromEvans (1974) hU.S.DOE doesnot distinguishGreat Plains fromRockyMountainProvincein Wyoming.Reserves for Great Plains (Powder River Basin) from Glass, 1978. Reserves for Rocky Mountains from U.S. DOE minus Glass's estimate for Powder River Basin. CThisfigureseems highlyquestionablesince Arizona's coal is normallyclassed as subbituminous, but this is the figure given throughout U.S. DOE. mines tend to be gassy, and water influx is often a problem, especially where sandstone directly overlies coal beds. The rugged terrain and multiplicity of coal seams provide a b u n d a n t opportunities for small companies. Hundreds of small drift mines producing less t h a n 100,000 tons per year operate, especially in K e n t u c k y and West Virginia. Most larger mines are slope or shaft operations, below drainage. These are best established in the Pittsburgh Coal and other thick, extensive seams of northern West Virginia and southwestern Pennsylvania. Several shaft mines of 1,000 ft ( 300 m) deep recently have been opened near Birmingham, Alabama. Strip mining is widespread, but mostly the domain of small- to medium-sized outfits, because few large blocks of strippable reserves exist. Terrain generally dictates contour-mining a n d / o r mountain-top removal. A common practice is to contour-strip as deep as overburden allows, t h e n auger the coal or " p u n c h m i n e " it from the bench with a continuous miner. INTERIOR PROVINCE
Extent and geology The Interior Province comprises several separate basins of bituminous coal, extending from Texas to Michigan (Fig. 1 ). Like Appalachian coal, Interior
360 coal is found in rocks of Pennsylvanian age. Nearly half of America's bituminous coal reserves are found here (Table 1 ). Coal in the Interior Province was mostly deposited far from mountainous uplifts, and the peat swamps were invaded more often by the sea than was the case in Appalachian Province. These geologic facts influence the quality of the coal. The Arkoma Basin of Arkansas and eastern Oklahoma is an exception. Geologically, and in terms of coal character and mining conditions, the Arkoma Basin belongs with the Appalachian rather than Interior Province.
Character o[ coal Except in the Arkoma Basins, all of the coal in the Interior Province is highvolatile bituminous. Rank has advanced as far as semi-anthracite in the eastern tip of the Arkoma Basin. In the Illinois Basin, rank increases slightly from youngest coals to oldest, and increases toward the southeast in all seams, reflecting greater original depth of burial. Rank in the Western Interior Basin increases southward. The high sulfur content of most Interior Province coal has hurt its market position in recent years. The normal range is 3 - 5% sulfur, as mined. This sulfur is largely attributable to influx of suifate-bearing sea water into the peat swamps before the peat was buried. Low-sulfur coal is present in the Arkoma Basin and in certain localities within the Illinois Basin where coal is overlain by non-marine rock. Some of the low-sulfur coal is suitable for coke making, generally in blend with Appalachian low-volatile bituminous.
Mining conditions and practice Coal seams mined in the Illinois Basin range from 1½ to 15 ft (0.45 to 4.5 m) thick, but normally are 3-8 ft (0.9-2.4 m) in thickness. Two seams, the Springfield and Herrin Coals, are 5 ft (1.5 m) or thicker over thousands of square miles and account for more than 90% of production. Dips are gentle, although faults are common in the southern part of the basin. Underground mines are only moderately gassy, and water seldom is a serious problem. The great continuity of coal beds, gentle dip, and level terrain render Illinois ideal for large-scale (over 1 million tons per year) area surface mines and large shaft or slope mines. Roughly half the annual production comes from underground mines and half from open pits. The proportions are about the same in Kentucky, although rougher terrain favors smaller mines. Indiana has many small and a few large surface mines, but only a handful of deep mines. The Western Interior Basin is characterized by laterally extensive but thin [less than 5 ft (1.5 m) ] coal beds, which today are mined exclusively by stripping. Most mines produce 50,000 to 500,000 t.p.y., but several have outputs of 1 million t.p.y, or more. Small- to medium-scale producers are strip-mining
361 coal in the Arkoma Basin. Both area- and contour-stripping are practiced, depending upon topography. Some of the coals mined are steeply pitching and faulted. Underground mining currently is dominant in the region, but is sure to resume when the market for coal improves. Coal has not been mined in Michigan for many years, although in the early 1900s as much as 2 million t.p.y, were produced. Most resources are accessible only to underground mining, but the small reserves, thin discontinuous seams, water problems, and other factors will deter renewed development (Ells, 1979). Bituminous coal output of Texas peaked at 1.25 m.t. in 1917 and ceased after World War II. Conditions are favorable for surface mining to resume in northcentral Texas (Evans, 1974). GULF COASTALPLAIN PROVINCE
Extent and geology The Gulf Coastal Plain Province includes the area underlain by Cretaceous and Tertiary sediments in Texas, Louisiana, Arkansas, Mississippi, and parts of adjacent states. The coal is all lignite. Resources are known in several states (Table 1 ), but only Texas has active mines. Lignite in Texas occurs in three geologic units of Eocene age: the Wilcox Group (oldest), the Yegua Formation of the Claiborne Group, and the Jackson Group. This coal originated as peat in largely freshwater swamps (Kaiser et al., 1980). Interbedded with lignite are poorly consolidated sandstone, siltstone, and mudstone.
Character of coal The oldest coal, originally most deeply buried, has the highest rank. Wilcox lignite has 24-32% moisture, about 27% fixed carbon, and yields 7,000 to 7,500 Btu/lb. as received. The younger Yegua lignite runs 36% moisture, 20% fixed carbon, and 6,000 Btu/lb., while the youngest Jackson coal averages 40% moisture, 16% fixed carbon, and less than 5,000 Btu/lb. Coal from the Wilcox Group also is better in terms of ash and sulfur; 10-11% ash and 0.9% sulfur compared with 15-40% ash and 1.0-2.3% sulfur in Yegua and Jackson coal (Keystone, 1982). This coal is used primarily in large mine-mouth electrical generating stations. Alcoa mines lignite to produce electricity for its aluminum-reduction plants northeast of Austin.
362
Mining conditions and practice Lignite in Texas is produced from large opencast pits. Output from two mines in 1982 exceeded 10 m.t., ranking the operations third and fourth in the nation. The seams mined are generally 3-10 ft (0.9-3.0 m) thick, but locally reach 25 ft (7.5 m). Maximum current mining depth is about 120 ft (36 m), but some companies are contemplating working to 250 ft ( 75 m) or deeper where multiple seams are present (Kaiser et al., 1980). The loosely consolidated overburden is easy to excavate, and the gently rolling land easy to reclaim after mining. These factors, together with low sulfur content, account for the phenomenal growth of Texas's coal industry. From about 2 m.t., in 1970, production rocketed to 33 m.t. in 1982, making Texas the eighth leading state in total output of coal. This trend shows little sign of reversing, and is likely to spread to Gulf Coastal coal outside of Texas. NORTHERN GREAT PLAINS PROVINCE
Extent and geology Coal deposits east of the Rocky Mountains proper in Montana, Wyoming, and the Dakotas are classed in the Northern Great Plains Province. Bituminous coal is present, but the great bulk of resources are subbituminous and lignite. The Paleocene Ft. Union Formation is the most prolific coal-bearing unit, with lesser deposits in strata of Cretaceous and Eocene age. The Ft. Union Formation consists mainly of detritus eroded from the Rocky Mountains when they were young, and is entirely a fresh-water deposit. The Ft. Union contains some of the world's thickest coal beds. Seams over 100 ft (30 m) thick are being mined, and one coal bed over 200 ft thick (60 m) has been reported ( Glass, 1978).
Character of coal Rank increases very gradually from east to west, toward the mountains. The Williston Basin contains lignite. Subbituminous coal is found in the Powder River Basin and other Tertiary basins of Montana, while Cretaceous coals in north-central Montana have advanced to bituminous rank. Ft. Union coal has very low sulfur content, as a result of its non-marine origin. Environmental regulations have produced enormous demand for coal in the Williston and Powder River Basins. The following are typical analyses: North Dakota lignite, 30-45% moisture, 5,700-7,700 Btu/lb, 4-9% ash, and 0.2-1.4% sulfur (Brant, 1953) ; Powder River Basin, Wyoming, 20-35% moisture, 7,100-9,700 Btu/lb, 4-15% ash, and 0.15-1.2 % sulfur ( Glass, 1978); Great Falls-Lewiston area, Montana, 1.7-4.0%
363 sulfur and highly variable ash, high-volatile bituminous B and C (Keystone, 1982).
Mining conditions and practice As in Texas, most mining in the Northern Great Plains takes place in large open pits. Of the 10 largest mines in the U.S. in 1982, 8 were in the Powder River Basin. Such mines have sent Wyoming soaring to third place among coal-producing states. The largest mine of all, with more than 16.8 m.t. in 1982, is the Black Thunder Mine near Gillette, Wyoming, The incredibly thick, gently dipping coal beds and slightly rolling terrain obviously lend themselves to such operations. Mining is not without problems: the coal readily catches fire spontaneously; many seams are burned to clinker far back from the outcrop. Reclamation also is a thorny issue. When 50-100 ft (15-30 m) of coal are removed, the land cannot be restored to original contour; and re-establishing plant growth after mining is difficult with the arid climate and extremely thin, alkaline topsoil. Approximately 18 m.t. of lignite were mined in the Williston Basin in 1982 (Keystone). The coal fields of north-central Montana have not seen activity for many years, and are not likely to do so in the near future. ROCKY MOUNTAINPROVINCE
Extent and geology Coal deposits of the Rocky Mountain are found in numerous intermontane basins and vary considerably in character, geology, and mining conditions. The principal deposits are in western Wyoming, Colorado, Utah, Arizona, and New Mexico. Most coal is bituminous and subbituminous of Cretaceous age, and subbituminous and lignite of Paleocene and Eocene age. The seams are thick; many over 10 ft (3 m) and some reach 60 ft (10 m) or more. Most coal beds are horizontal or gently folded but some are tightly folded, faulted, and coked by igneous intrusions.
Character of coal Rank of coal in the Rocky Mountain Province varies from subbituminous to anthracite, but most is subbituminous or high- to medium-volatile bituminous. Locally, higher rank is attributed to the heat from nearby igneous intrusions, as at Crested Butte, Colorado. Important deposits of coking coal occur in the Raton Mesa Field near Trinidad, Colorado and in the Book Cliffs at Price and Castle Gate, Utah. Most Rocky Mountain coal, however, is burned for steam generation. Generally, it is excellent for this purpose because of its low (under
364 1% ) sulfur content. The ash content varies widely from seam to seam, but most fields contain at least some coal with less than 8% ash.
Mining conditions and practice Aside from mines that produced coking coat, most early mines in the Rockies existed to supply fuel to railroad locomotives and for local domestic use. As these markets vanished after World War II, most mining areas declined seriously. They were vigorously reborn during the 1970s when environmental regulations brought intense demand for low-sulfur coal. This development has spread beyond old established fields, such as Hanna and Rock Springs in Wyoming, to previously untapped deposits, as in the Black Mesa Field of Arizona. However, some rich resources, notably those of Kaiparowits and Henry Mountains Fields in southern Utah, have never been exploited because of their remote location. Mining practice varies greatly, dependent upon geology and topography. Huge area-surface mines operate in broad, open basins such as Black Mesa and the San Juan and Green River Basins. Modified contour-stripping is practiced in moderately dipping coal beds of northwestern Colorado. Near Walden, northcentral Colorado, a nearly vertical 60-ft (100 m) -thick coal seam is being mined in an open pit 400 ft (120 m ) deep. Drift mines, which often must contend with faulted or pitching seams, work the rugged plateau and mesa country of the Book Cliffs in Utah and western Colorado. Squeezes and rock bursts are often a problem due to extreme thickness of overburden. Most mines in the Rocky Mountains are relatively new, modern operations, and medium to large in terms of production. PACIFIC COASTPROVINCE
Extent The Pacific Coast Province comprises the states west of the Rocky Mountains, together with Alaska. This region has large resources but has been far less mining than any other province. At present only two mines, one in Washington and another in Alaska, are operating.
Geology Coal of Tertiary age occurs in small and widely scattered basins in California, Nevada, Oregon, and Washington. The latter state has by far the largest reserves, most of them between Puget Sound and the Cascade Range. Coalbearing strata range in age from Paleocene to Oligocene, with most coal being
365 in Middle Eocene rock. Much of the coal has been deformed and metamorphosed by mountain-building and volcanic activity (Keystone, 1982 ). Alaska's coal deposits are huge but poorly known, and occur in highly diverse geologic settings. Estimates of resources vary enormously. U.S DOE (1982) lists a little more than 6 billion tons of demonstrated resources, but the ultimate resources may be as high as 5.5 trillion tons-one-sixth of all the coal in the world (Schaff, 1983).
Character of coal Both Alaska and Washington contain coal of all grades from lignite to anthracite, but demonstrated resources are largely subbituminous. Most coal in Washington has less than 1% sulfur, although some analyses approach 5%. Ash tends to be high; 8-20% or even more. Alaska contains coal of very low sulfur content; but generalizations cannot be made about such a diverse resource.
Mining conditions and practice The one operating mine in Washington is a strip pit that produces approximately 5 m.t. per year. Additional development is deterred by structural complications and somewhat unpredictable reserves and quality. Alaska obviously presents a challenging, if not hostile, environment for mining, but large reserves are located close to tidewater, and not far from populated areas. The prognosis is good for some of this coal to be developed for export market, especially to Japan.
REFERENCES Brant, R. A., 1953. Lignite resources of North Dakota. U.S. Geol. Surv., Circular 226, 78 pp. Ells, G. D., 1979. The Mississippian and Pennsylvanian (Carboniferous) Systems in the United States-Michigan, U.S. Geol. Surv., Prof. Pap. 1110 A-L: J1-J17. Evans, T. J., 1974. Bituminous coal in,Texas. Bur. of Econ. Geol., Handbook 4, 65 pp. Glass, G. B., 1978. W y o m i n g coal fields,1978. Geol. Surv. Wyo., Pub. Inf.Circ. 9, 91 pp. Kaiser, W.R., Ayers, W.B. and La Brie, L.W., 1980. Lignite resources in Texas. Bur. Econ. Geol. Rep. Inv. 104, 52 pp. Keystone Coal Industry Manual, 1983. McGraw-Hill, N e w York, NY, 1461 pp. Schaff, R. G., 1983. Coal resources of Alaska. Div. Geol. Geophys. Surv., Info. Circ. 17, 9 pp. Trumbull, J. 1960. Coal fieldsof the United States. U.S. Geol. Survey (map). U.S. Department of Energy, 1982. Demonstrated reserve base of coal in the United States on January 1, 1980. DOE/EIA-0280 (80), 39 pp.