Colchester Coal Member
Lithostratigraphy: Kewanee Group >>Carbondale Formation >>Colchester Coal Member
Chronostratigraphy: Paleozoic Erathem >>Pennsylvanian Subsystem >>Desmoinesian Series
Allostratigraphy: Absaroka Sequence
Nelson, W.J., 2020, Pennsylvanian Subsystem in Illinois: Illinois State Geological Survey Bulletin (in press).
Edited and figures drafted by Jennifer M. Obrad.
Worthen (1868, p. 11) wrote, “Coal No. 2 is the lower seam at Murphysboro, the Colchester coal in McDonough County, the Morris coal of Grundy County. . .” Worthen (1870, p. 96–97) described Coal No. 2 at various localities in Fulton County. Until 1960, coal seams in Illinois were informal and commonly designated by numbers rather than names. Moreover, the Colchester was miscorrelated with the older Davis and Murphysboro Coals. Kosanke et al. (1960) formalized the usage as the Colchester (No. 2) Coal Member.
Callary (2009) wrote, “Several writers have suggested that the name was transferred from Colchester, England or from Chester, England, with prefixed col, generally explained as a form of ‘coal.’ However, in an 1890 letter quoted by Moon, Lewis H. Little wrote, ‘[I] laid out the original town and named it after Steven Chester of New York.’. . . The source of the prefix remains a mystery.” Because a town in Randolph County, Illinois, was already named Chester, the post office would have required a different name for the town in McDonough County.
The Colchester Coal was previously called the No. 2 coal in Illinois. Early authors miscorrelated it with the Murphysboro, Davis, and possibly other seams in southern Illinois. In Indiana, the Colchester previously was known as Coal IIIa and Velpen coal. The names Schultztown coal bed and “S” coal bed formerly were used in western Kentucky.
Wanless (1956) listed the type area as Secs. 12 and 13, T5N, R4W, McDonough County, but no specific section was designated as the type. Several detailed sections made by ISGS geologists in the type area illustrate the relationship of the Colchester Coal to enclosing strata (Figure 4-18).
Core from Indiana Geological Survey borehole SDH-306, drilled east of Winslow in sec. 2, T 2 S, R 7 W, Pike County, Indiana, has been nominated as the principal reference section for the Colchester. The Indiana Survey borehole ID number is 115871. The Colchester Coal is 1.0 ft (30 cm) thick at a depth of 140.6 to 141.6 ft (42.9 to 43.2 m) in this core (Figure 4-19).
Hasenmueller and Ault (1991).
A core description and gamma-ray and resistivity logs were published by Hasenmueller and Ault (1991). They are on file with the Indiana Geological Survey and can be downloaded from the Indiana Geological Survey website. The core resides at the Indiana Geological Survey core storage facility in Bloomington, Indiana, under call number 609.
The Colchester rests on underclay above the Coxville Sandstone, which in turn overlies (or cuts through) the Seelyville Coal. The black, phosphatic Mecca Quarry Shale directly overlies the Colchester, except in northern Illinois, where the gray Francis Creek Shale occurs between the Colchester and Mecca Quarry.
Extent and thickness
The Colchester is the most extensive coal seam in the Illinois Basin and one of the most extensive in the world. Within the basin, Greb et al. (2003) estimated an areal extent of 41,313 mi2 (107,000 km2). This is much greater than the extent of the Springfield and Herrin Coals, which are absent due to nondeposition in large areas of the basin. On the basis of solidly established correlations to the Midcontinent and Appalachian Basins, the Colchester was probably originally continuous from Oklahoma to Pennsylvania. Cecil et al. (2003) followed the cyclothem of which the Colchester is a part from Maryland to California.
Few, if any, depositional interruptions affect the Colchester in the Illinois Basin. Where the coal is absent, later erosion is nearly always the cause. The Colchester is thickest in central to north-central Illinois, where it formerly was mined extensively. The greatest known thickness is about 54 in. (137 cm) in small areas of LaSalle County (Jacobson 1985; Cady 1915). In the McLean County Coal Company mine, which was active at Bloomington during the early 20th century, in-mine seam thicknesses ranged from 34 to 50 in. (86 to 127 cm). Patches of coal thicker than 39 in. (100 cm) are scattered through the intervening area, but in most of Illinois, the thickness of the Colchester has not been mapped (Korose et al. 2003). The average thickness in the type area is about 26 in. (66 cm). Andros (1915) reported a range of 22 to 48 in. (56 to 122 cm) where the coal was mined west of the Illinois River. The coal is considerably thinner in many places where it was not mined. Without specifying localities, Shaver et al. (1986) reported a maximum thickness of about 36 in. (91 cm) in Indiana. Regionally, the coal thins toward the south and east. It seldom exceeds 1.0 ft (30 cm) in southern Illinois and western Kentucky.
Returning to northern Illinois, Jacobson’s (1985) mapping indicated that the coal tends to thin across present-day anticlines and thicken into synclines. This observation implies that these structures were active during Colchester peat formation.
Like nearly all coal seams in the Illinois Basin, the Colchester typically is composed of bright-banded to semi-bright-banded coal. In northern and western Illinois, the coal is described as having weakly developed cleat and partings or laminae of shale, pyrite, and fusain, few of which are laterally continuous. A pyritic fusain band about 14 in. (36 cm) from the top of the bed was said to be widespread in western Illinois (Andros 1915). Only one or two mines in northern Illinois had a comparable feature (Cady 1915). No persistent clastic layers occur in this seam elsewhere in the basin. Splitting is a rare phenomenon, but two examples come from the type area. At a clay pit north of Colchester, D.L. Reinertsen and R.L. Berger (unpublished field notes dated 1959, ISGS Library) observed the seam split into two nearly equal “benches” by a wedge of rooted, silty mudstone as thick as 4 ft (1.2 m). Rooting implies that sediment was carried into the swamp, temporarily interrupting peat formation (Figure 4-19, column 4). About 1.4 mi (2.2 km) north in Argyle Lake State Park, a different style of splitting was observed (Figure 4-20). Two thin layers of coal, separated by silty shale, rest on underclay. Above this is a wedge-shaped body of siltstone as thick as 20 ft (6 m), topped by the main body of coal. This feature may have developed when rising water floated or rafted the main part of the Colchester peat deposit and sediment was deposited beneath the floating peat mat.
“Rolls” were reported to be common features in the Colchester Coal at some of the old mines in northern Illinois. Rolls are bodies of gray Francis Creek Shale protruding into and replacing the upper part of the coal seam. They are elongate in map view and lens-shaped in cross section. Jacobson (1985) pictured several examples. Rolls probably formed when water currents ripped out the upper layers of peat and replaced them with mud during the early stages of Francis Creek deposition.
“Horsebacks,” or claystone dikes, were common in the McLean County mine at Bloomington, according to field notes by ISGS geologists (ISGS Library). One notation indicated a “horseback” every 25 to 30 ft (7 to 9 m). Claystone dikes also occur in the Colchester in northern Illinois (Jacobson 1985).
In several mines near Oglesby in LaSalle County, “large, calcareous, boulder-like masses lie in the coal bed, and in some places entirely eliminate the coal. The material of which the boulders are composed seems to be a mixed-up mass of calcareous and carbonaceous rock, which in some places is also somewhat sandy” (Cady 1915, p. 76). This description best fits coal balls, which are masses of peat that were mineralized rather than coalified. Similar masses of coal balls have been encountered in other Illinois coal seams, notably the Herrin Coal at numerous localities in southern Illinois. The mines where Cady described the boulder-like masses lie just west of the crest of the La Salle Anticlinorium. Damberger (1970a) illustrated small pyritic coal balls from the Colchester Coal in western Illinois.
In terms of quality, the Colchester runs about average to slightly better than average for Illinois coal. Cady (1915) reported, based on three samples from each of 11 mines, an average ash content of 8.0%, with none more than 10%, and a sulfur content averaging 2.9%, with all samples between 2.0 and 4.0%. Moisture averaged 16.2%. A few mines in northern Illinois recorded relatively low sulfur, in the range of 1.5%. The relationship of low sulfur to thick gray shale (Francis Creek) seems to hold, although data are sparse (Gluskoter and Simon 1968; Gluskoter and Hopkins 1970). A study at a single mine in Fulton and Peoria Counties showed a definite inverse relationship between the thickness of Francis Creek and the content of pyritic sulfur (Hopkins and Nance 1970).
With local exceptions, both contacts are sharp and planar to gently rolling.
Well log characteristics
Typical for coal. Where the Colchester is thin, the coal and overlying Mecca Quarry Shale produce a distinctive sharp leftward inflection on many resistivity logs (Figure 4-15).
Kosanke (1950) and Peppers (1970, 1996) discussed fossil spores of the Colchester and neighboring coal seams. These spores helped establish the correlation of the Colchester beyond the Illinois Basin. Coal balls have been encountered in western Illinois yet are not common. Some are pyritic and preserve plant anatomy well but are difficult to prepare for study. Massive calcareous Colchester coal balls from another mine were heavily recrystallized, obliterating plant anatomy (T.L. Phillips, written communication, 2017).
Age and correlation
The Colchester Coal is of middle Desmoinesian age. The Whitebreast Coal Bed in Iowa and the Croweburg Coal Bed in Missouri, Kansas, and Oklahoma are direct correlatives (Figure 4-12). Equivalence with the Lower Kittanning coal bed of Ohio and western Pennsylvania also is solidly established. Other probable correlatives are the Princess No. 6 coal bed of eastern Kentucky and the No. 6 Block coal bed of West Virginia (Peppers 1996).
Environments of deposition
The peat swamp in which the Colchester formed had a truly vast extent. Clearly, this was a time of tectonic stability. Within the Illinois Basin, only the lower reaches of the coastal plain were represented. The absence of through-going fluvial systems contemporaneous with the Colchester anywhere in the basin is remarkable. Considering the facies and distribution of the overlying Francis Creek Shale, the outlet of a large river probably lay a short distance northeast of the present crop line near Chicago.
Many underground mines operated in the Colchester Coal during the late 19th and early 20th century in north-central Illinois. The proximity to market in Chicago was their prime advantage. Through time, mines in the Colchester lost market to operations in thicker seams in central and southern Illinois. The last underground mines in the Colchester closed around the time of World War II, although surface mining continued until recently. Resources of roughly 17.4 billion tons (15.8 billion metric tons) remain in the ground (Keystone 2010).
- Andros, S.O., 1915, Coal mining practice in District III: Illinois State Geological Survey, Coal Mining Investigations Bulletin 9, 30 p.
- Cady, G.H., 1915, Coal resources of District I (longwall): Illinois State Geological Survey, Coal Mining Investigations Bulletin 10, 149 p.
- Callary, E., 2009, Place names of Illinois: Urbana, University of Illinois Press, 425 p.
- Cecil, C.B., F.T. Dulong, R.R. West, R. Stamm, B. Wardlaw, and N.T. Edgar, 2003, Climate controls on the stratigraphy of a Middle Pennsylvanian cyclothem in North America: SEPM (Society for Sedimentary Geology) Special Publication 77, p. 151–180.
- Damberger, H.H., 1970a, Petrographic character of the Colchester (No. 2) Coal Member at the Banner Mine, Peoria and Fulton Counties, Illinois: Illinois State Geological Survey, Guidebook Series No. 8, p. 99–105.
- Gluskoter, H.J., and M.E. Hopkins, 1970, Distribution of sulfur in Illinois coal: Illinois State Geological Survey, Guidebook Series No. 8, p. 89–98.
- Gluskoter, H.J., and J.A. Simon, 1968, Sulfur in Illinois coals: Illinois State Geological Survey, Circular 432, 28 p.
- Greb, S.F., W.M. Andrews, C.F. Eble, W.A. DiMichele, C.B. Cecil, and J.C. Hower, 2003, Desmoinesian coal beds of the Eastern Interior and surrounding basins: The largest tropical peat mires in Earth history: Geological Society of America, Special Paper 370, p. 127–150.
- Hasenmueller, W.A., and C.H. Ault, 1991, Reference core and correlation of key beds in the Petersburg and Linton Formations (Pennsylvanian) in Indiana: Indiana Geological Survey, Occasional Paper No. 57, 8 p.
- Hopkins, M.E., and R.B. Nance, 1970, Sulfur content of the Colchester (No. 2) Coal Member at the Banner Mine, Peoria and Fulton Counties, Illinois: Illinois State Geological Survey, Guidebook Series 8, p. 96–98.
- Jacobson, R.J., 1985, Coal resources of Grundy, La Salle, and Livingston Counties, Illinois: Illinois State Geological Survey, Circular 536, 58 p., 6 pls.
- Keystone, 2010, Keystone Coal Industry manual: Jacksonville, Florida, Mining Media International, 631 p.
- Korose, C.P., S.D. Elrick, and R.J. Jacobson, 2003, Availability of the Colchester Coal for mining in northern and western Illinois: Illinois State Geological Survey, Illinois Minerals 127, 21 p.
- Kosanke, R.M., 1950, Pennsylvanian spores of Illinois and their use in correlation: Illinois State Geological Survey, Bulletin 74, 128 p., 2 pls.
- Kosanke, R.M., J.A. Simon, H.R. Wanless, and H.B. Willman, 1960, Classification of the Pennsylvanian strata of Illinois: Illinois State Geological Survey, Report of Investigations 214, 84 p. and 1 pl.
- Peppers, R.A., 1970, Correlation and palynology of coals in the Carbondale and Spoon Formations (Pennsylvanian) of the northeastern part of the Illinois Basin: Illinois State Geological Survey, Bulletin 93, 173 p.
- Peppers, R.A., 1996, Palynological correlation of major Pennsylvanian (Middle and Upper Carboniferous) chronostratigraphic boundaries in the Illinois and other coal basins: Geological Society of America, Memoir 188, 111 p. and correlation chart.
- Shaver, R.H., C. H. Ault, A.M. Burger, D.D. Carr, J.B. Droste, D.L. Eggert, H.H. Gray, D. Harper, N.R. Hasenmueller, W.A. Hasenmueller, A.S. Horowitz, H.C. Hutchison, B. Keith, S.J. Keller, J.B. Patton, C.B. Rexroad, and C.E. Wier, 1986, Compendium of Paleozoic rock-unit stratigraphy in Indiana—A revision: Indiana Geological Survey, Bulletin 59, 203 p.
- Wanless, H.R., 1956, Classification of the Pennsylvanian rocks of Illinois as of 1956: Illinois State Geological Survey, Circular 217, 14 p.
- Worthen, A.H., 1868, Geology and palaeontology: Geological Survey of Illinois, v. 3, 574 p., 20 pls.
- Worthen, A.H., 1870, Geology and palaeontology: Geological Survey of Illinois, v. 4, 508 p., 31 pls.
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