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Thick deposits of Energy Shale (Carbondale Formation, Desmoinesian) are associated with low-sulfur coal deposits in the underlying Herrin No. 6 coal in southern Illinois. The Energy Shale consists of wedges, up to 85 ft (25.9 m) thick, which thin away from the Walshville Channel (interpreted as a major distributary channel deposit in the study area).
Facies recognition and interpretation of depositional environments are based upon lithology, sedimentary structures, vertical and lateral relations, geometry, and paleoecology as well as organic matter content and total sulfur content.
Four facies are recognized in the surface mines studies. The thickest and coarsest facies is adjacent to the Walshville Channel and is characterized by numerous fining-upward channel-fill sequences of sandstone, siltstone, and silty shale. It is interpreted as a series of crevasse distributary channel-fills in the proximal parts of splays. This proximal splay facies grades laterally into shale with abundant plant remains, thin coal beds (splits from the Herrin No. 6 coal) and some siderite concretions. It is interpreted as the distal deposits of crevasse splays. The distal splay facies grades laterally into what is interpreted as an interdistributary bay-fill facies consisting of shale with laterally persistent siderite layers. Sixteen miles (25.8 km) from the Walshville Channel, a zo e at the top of this facies is extensively burrowed and contains pectinoid bivalves, indicating some marine influence. The bay-fill facies grades laterally and vertically into shale, containing a moderately diverse marine fauna composed mostly of stunted individuals. It is interpreted as a marginal marine facies.
The Walshville Channel and the crevasse splay facies of the Energy were partly contemporaneous with Herrin peat deposition. The sulfur content of the Herrin No. 6 coal is highest beneath the marginal marine facies (the thinnest) and the proximal splay facies (the thickest) of the Energy Shale.
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