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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). 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, 1958). 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.
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) advocated deposition of black shale in shallow lagoons having floating mats of algae that impeded circulation. In contrast, Heckel (1977) placed black shale deposition during highstand under starved-basin conditions in water deep enough (328.1 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) and often heralded by erosion or “ravinement” (Gastaldo et al. 1993). 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; Banerjee et al. 2010; Holterhoff and Cassady 2012).
W. John Nelson, Scott D. Elrick, William A. DiMichele, and Philip R. Ames xxxx, Evolution of a Peat-Contemporaneous Channel: The Galatia Channel, Middle Pennsylvanian, of the Illinois Basin FINISH CITATION