Excello Shale Member
Nelson, W.J., P.H. Heckel and J.M. Obrad, 2022, Pennsylvanian Subsystem in Illinois: Illinois State Geological Survey, Bulletin (in press).
Searight et al. (1953) showed the “Excello formation” on a chart. Searight (1955) described the unit at its type locality in Macon County, Missouri, where it is approximately 4 ft (1.2 m) thick and consists of black, phosphatic shale that grades to light gray shale at the top. In the Midcontinent, the Excello underlies the Blackjack Creek Limestone and overlies the Mulky Coal. Reduced in rank to a member, the Excello Shale has been extended from Missouri into Kansas and Iowa. Nance (1970, p. 77) further extended the Excello into Illinois as the black, fissile shale overlying the Summum (now Houchin Creek) Coal in western Illinois.
The town of Excello is 11 mi (18 km) south of Macon and nearly 50 mi (80 km) north of Columbia in north-central Missouri. How the town got its name is not known.
Highwall of the coal strip pit north and south of County Highway T, 2.6 mi (4.2 km) west of U.S. Highway 63, NW¼ sec. 30, T 56 N, R 14 W, Macon County, Missouri.
Searight (1955, p. 9, 31–32, 35).
According to Gentile and Thompson (2004), the exposures described by Searight have been flooded or backfilled and the shale is no longer exposed.
The principal reference section for the Excello Shale and underlying Mulky Coal Bed is in the west cut bank of Morris Creek, NW¼ NE¼ NW¼ NW¼ sec. 33, T 44 N, R 27 W, Henry County, Missouri.
Gentile and Thompson (2004).
A reference section for Illinois is hereby proposed in core from the Freeman United Coal Mining Company’s borehole C-1-74 (Crown II Mine production shaft), located sec. 23, T 12 N, R 6 W, Macoupin County (ISGS county no. 21495). The Excello Shale Member is 6.7 ft (2.0 m) thick and lies in the depth range of 352.7 to 359.4 ft (107.5 to 109.5 m; Figure 4-44). This core also serves as a reference section for the Houchin Creek Coal.
Core is available at the ISGS Geological Samples Library (storage number C-10215). The core description is on file at the ISGS Geological Records Unit and is available on the ISGS website.
Indiana Geological Survey test hole SDH-306 has been proposed as a reference section for the Excello Shale Member in Indiana. The hole was drilled 1300 ft from north line, 2510 ft from west line of sec. 2, T 2 S, R 7 W in Pike County, Indiana. The Excello Shale is 5.3 ft (1.6 m) thick and lies in the depth interval from 21.5 to 26.8 ft.
Mastalerz et al. (2019).
Logs of SDH-306 are on file at the Indiana Geological Survey and can be accessed online via the Petroleum Database Management System under well ID #115871. Core is archived at the IGS under call number 609.
Indiana Geological Survey test hole SDH-366 has been proposed as a reference section for the Excello Shale Member in Indiana. The hole was drilled 800 ft from north line, 400 ft from west line of sec. 27, T 5 S, R 10 W in Vanderburgh County, Indiana. The Excello is 5.1 ft (1.6 m) thick and resides in the depth interval from 439.6 to 444.7 ft. Records include continuous core described by Nancy R. Hasenmueller together with resistivity, gamma-ray, neutron, and density logs.
Mastalerz et al. (2019).
Logs of SDH-366 are on file at the Indiana Geological Survey and can be accessed online via the Petroleum Database Management System under well ID #150359. Core is archived at the IGS under call number 780.
The Excello Shale overlies the Houchin Creek Coal and underlies the Hanover Limestone. Locally, an unnamed gray shale occurs between the coal and black shale. More widely, the coal is absent and the Excello rests on truncated paleosol at the top of the Lagonda Member.
Extent and thickness
The Excello is found almost everywhere the Houchin Creek Coal occurs, and numerous boreholes show the black shale where the coal is not developed. Small areas of erosion or nondeposition lie mainly on the western and northern margins of the basin. The thickness commonly ranges from 3.3 to 6.6 ft (1.0 to 2.0 m), making the Excello among the thickest Pennsylvanian black phosphatic shale members. A thickness of 12.7 ft (3.87 m) in a core (API 12-019-26511-00) drilled on the grounds of ISGS headquarters in Champaign is believed to be the maximum.
Black, phosphatic, highly fissile shale is characteristic. Like similar units, the Excello exhibits large, planar joints and spheroidal or ovoid carbonate concretions that may exceed 3.3 ft (1 m) in maximum dimension. In some cores and outcrops, the upper part (locally the entire thickness) of the member changes to shale that is less evenly fissile and mottled in gray, green, olive, and brown. Phosphatic laminae and lenses are common in both the “slaty” and mottled facies.
The lower contact typically is sharp, with evidence for erosion. Numerous cores show the Excello resting directly on underclay, and, in some cases, the underclay is truncated. Where the Hanover Limestone overlies the Excello, the contact is either sharp or rapidly gradational. Where limestone is missing, the contact between black Excello and overlying gray shale tends to be gradational.
Well log characteristics
Extremely high gamma-ray readings are diagnostic (Figure 4-36).
From concretions in western Illinois, Wanless (1957, p. 101) reported “fish spines, Orbiculoidea, and a small ammonoid, Anthracoceras wanlessi, which is especially diagnostic of this horizon.” In addition, Wanless (1958) tabulated Lingula in the shale and the brachiopods Composita subtilita and Crurythyris planoconvexa in concretions, along with a variety of pelecypods and Lystracanthus shark remains.
Age and correlation
Heckel (2013, p. 18) reported “an abundant conodont fauna dominated by Idiognathodus” along with other genera and a form of Gondolella that helps confirm the extension of the Excello Member from the Midcontinent to the Illinois Basin. This placement is consistent with that based on palynology of the coal (Peppers 1996). The Excello correlates with part of the Tuscarawas Shale, between the Lower and Middle Kittanning coals, in the Appalachian Basin (Heckel 2013).
Environments of deposition
See the general model for black phosphatic shale deposition.
- Gentile, R.J., and T.L. Thompson, 2004, Paleozoic succession in Missouri, Part 5, Pennsylvanian Subsystem, Volume A, Morrowan strata through Cherokee Group: Missouri Geological Survey, Report of Investigations 70, 241 p. and correlation chart.
- Heckel, P.H., 2013, Pennsylvanian stratigraphy of Northern Midcontinent Shelf and biostratigraphic correlation of cyclothems: Stratigraphy, v. 10, nos. 1–2, p. 3–39.
- Mastalerz, M., P.R. Ames, and A. Drobniak, 2019, The Survant Coal Member of the Linton Formation (Pennsylvanian) in Indiana: geometry, resources, and properties: Indiana Geological Survey, Indiana Journal of Earth Sciences, v. 1, DOI 10.14.14434/ijes.v1i0.26862.
- Nance, R.B., 1970, Limestones and phosphatic rocks from the Summum and Liverpool cyclothems in western Illinois: Illinois State Geological Survey, Guidebook Series 8, p. 75–83.
- 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.
- Searight, W.V., 1955, Guidebook to the field trip of the second annual meeting of the Association of Missouri Geologists: Missouri Geological Survey, Report of Investigations No. 20, 44 p.
- Searight, W.V., W.B. Howe, R.C. Moore, J.M. Jewett, G.E. Condra, M.C. Oakes, and C.C. Branson, 1953, Classification of Desmoinesian (Pennsylvanian) of northern Mid-Continent: American Association of Petroleum Geologists Bulletin, v. 37, p. 2747–2749.
- Wanless, H.R., 1957, Geology and mineral resources of the Beardstown, Glasford, Havana, and Vermont Quadrangles: Illinois State Geological Survey, Bulletin 82, 233 p.
- Wanless, H.R., 1958, Pennsylvanian faunas of the Beardstown, Glasford, Havana, and Vermont Quadrangles: Illinois State Geological Survey, Report of Investigations 205, 59 p.
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