Galatia Channel:Excello Shale Member

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  • Figure 4 Diagram showing units between the Houchin Creek and Herrin Coals, including members newly named in this report.

Excello Shale Member

The Excello Shale is the black, slaty, phosphatic shale that overlies the Houchin Creek/Mulky Coal across the Illinois and Midcontinent Basins. This member is typical among Middle and Upper Pennsylvanian black “sheety” phosphatic shale units of the Illinois and Western Interior Basins. It is hard, highly fissile, and well jointed, with a density lower than normal for shale because of a carbon content as high as 18.5%. Phosphatic laminae and bands of small lenses are common, as are large (to 3.3 ft [~1 m]) spheroidal dolomite concretions. The upper part of the Excello tends to be mottled, burrowed, and calcareous, grading into overlying limestone. Near its depositional limits on the western and northern basin margins, black sheety shale gives way to mottled gray, green, and olive mudstone that is weakly fissile (James and Baker 1972)[1].The Excello carries a highly restricted marine fauna of inarticulate brachiopods, ammonoids, bivalves, fish remains, and conodonts. Articulate brachiopods have been found in their carbonate concretions (Wanless 1957[2], 1958[3]). Burrows are rare, except near the upper contact. Like other black phosphatic shales, the Excello produces very high (typically off-scale) inflections on gamma-ray logs (Figure 5b). Thickness varies from a few centimeters to about 8.2 ft (2.5 m), with no regional trends evident. The Excello is nearly coextensive with the Houchin Creek Coal. The lower contact is sharp and shows evidence of erosion; in some cores, the coal is absent and the Excello rests directly on underclay.

  • Figure 5b Wireline log illustrating the typical response of key units. (a) Electric log of Carter Oil No. 1 Beers well in sec. 28, T8S, R4E, Williamson County, Illinois (county no. 2107). B, Brereton Limestone; H, Herrin Coal; Sp, Springfield Coal; Han, Hanover Limestone; HC, Houchin Creek Coal; SV, Survant Coal; MQ, Mecca Quarry Shale; C, Colchester Coal. (b) Gamma-ray–resistivity log of Peabody Natural Gas No. 2 Short, in sec. 14, T7S, R7E, Hamilton County (county no. 25375).

Black shale such as the Excello clearly was deposited in marine water under anoxic reducing conditions, lacking circulation or agitation by waves and currents. Without taking account of eustasy, Zangerl and Richardson (1963)[4] advocated deposition of black shale in shallow lagoons having floating mats of algae that impeded circulation. In contrast, Heckel (1977)[5] placed black shale deposition during highstand under starved-basin conditions in water deep enough (330 ft [100 m]) that wind-driven circulation did not affect bottom waters. We favor Heckel’s model, but noting that he placed peat formation during transgression, we suggest that black shale developed during transgression to early highstand. Transgression was rapid (Archer et al., 2016)[6] and often heralded by erosion or “ravinement” (Gastaldo et al. 1993)[7]. Sedimentation slowed and water circulation ceased in steadily deepening water. Peat on the sea floor consumed oxygen and contributed a great deal of carbon to bottom sediments. Rivers may have delivered additional carbon, carrying plant matter into the sea (James and Baker 1972[1]; Banerjee et al. 2010[8]; Holterhoff and Cassady 2012[9]).

Additional Reading

Excello Shale Member on ILSTRAT

Primary Source

Nelson, W.J., S.D. Elrick, W.A. DiMichele, and P.R. Ames, 2020, Evolution of a peat-contemporaneous channel: The Galatia channel, Middle Pennsylvanian, of the Illinois Basin: Illinois State Geological Survey, Circular 605, 85 p., 6 pls.

References

  1. a b James, G.W., and D.R. Baker, 1972, Organic geochemistry of a Pennsylvanian black shale (Excello) in the Midcontinent and the Illinois Basin: Kansas Geological Survey, Bulletin 204, Part 1, p. 3–10.
  2. Wanless, H.R., 1957, Geology and mineral resources of the Beardstown, Glasford, Havana, and Vermont Quadrangles: Illinois State Geological Survey, Bulletin 82, 233 p., 7 pls.
  3. Wanless, H.R., 1958, Pennsylvanian faunas of the Beardstown, Glasford, Havana, and Vermont Quadrangles: Illinois State Geological Survey, Report of Investigations 205, 59 p.
  4. Zangerl, R., and E.S. Richardson, 1963, Paleoecological history of two Pennsylvanian black shales: Fieldiana (Field Museum, Chicago), Geological Memoir 4, 352 p.
  5. Heckel, P.H., 1977, Origin of phosphatic black shale facies in Pennsylvanian cyclothems of Mid-Continent North America: American Association of Petroleum Geologists Bulletin, v. 61, no. 7, p. 1045–1068.
  6. Archer, A.W., S. Elrick, W.J. Nelson, and W.A. DiMichele, 2016, Cataclysmic burial of Pennsylvanian Period coal swamps in the Illinois Basin: Hypertidal sedimentation during Gondwanan glacial melt-water pulses, in B. Tessier and J.-Y. Reynaud, eds., Contributions to modern and ancient tidal sedimentology: Proceedings of the Tidalites 2012 Conference: International Association of Sedimentologists, Special Publication 48, p. 217–231.
  7. Gastaldo, R.A., T.M. Demko, and Y. Liu, 1993, Application of sequence and genetic stratigraphic events to Carboniferous coal-bearing strata: An example from the Black Warrior basin, USA: Geologische Rundschau, v. 82, p. 212–226.
  8. Banerjee, S., A. Raymond, and M.M. Tice, 2010, Oxygen levels and sub-Milankovitch sedimentary cycles in the Hushpuckney Shale (Swope Formation, Kasimovian, Pennsylvanian): Geological Society of America, Abstracts with Programs, v. 42, no. 5, p. 528.
  9. Holterhoff, P., and K. Cassady, 2012, Late Pennsylvanian and Early Permian black shale-limestone bed-set couplets of the Eastern Shelf, Midland Basin (Texas): Climate-driven redox cycles of the inner platform realm: American Association of Petroleum Geologists Annual Meeting, Search and Discovery Article 90142.