Facies and Depositional Environments of the Energy Shale Member (Pennsylvanian) and their Relationship to Low-sulfur Coal Deposits in Southern Illinois

Abstract

ABSTRACT Thick deposits of Energy Shale (Carbondale Formation, Desmoinesian) are associated with low-sulfur coal deposits in the underlying Herrin (No. 6) Coal Member in southern Illinois. The Energy Shale consists of wedges up to at least 26 m thick, which thin away from the Walshville channel (interpreted as a major channel deposit), and local pods or lenses which occur farther from the channel. Facies recognition and interpretation of depositional environments are based on paleoecology, lithology, sedimentary structures, vertical and lateral relationships, and geometry. Supplementary evidence was obtained from total sulfur content and boron content. Six facies are recognized in the areas studied. The thickest and coarsest facies is adjacent to the Walshville channel and is characterized by numerous fining-upward channel-fill sandstones, siltstones, and silty shales. This facies is interpreted as a series of crevasse-distributary channel-fill, levee, and interchannel deposits formed in the proximal portions of the splays. The proximal splay facies grades laterally into a distal splay facies, which consists of shale with abundant plant remains, thin coal beds (splits from the Herrin Coal), rare siderite concretions, and siltstone lenses. The distal splay facies grades laterally into or overlies shale with laterally persistent siderite layers, interpreted as a nonmarine bay facies. The nonmarine bay facies grades vertically and late ally into shale, which is bioturbated and contains euryhaline bivalves and is interpreted as a brackish to marine bay facies. The brackish to marine bay facies grades vertically into shale, containing a moderately diverse marine fauna with some small, possibly stunted, individuals. This facies is interpreted as a marginal marine deposit. Local lenses of organic-rich, dark gray to black shale underlie isolated pods of the marginal marine facies and probably were deposited in freshwater ponds developed on the Herrin peat swamp. The Walshville channel, the proximal and distal crevasse-splay facies of the Energy Shale, and possibly the black shale pond facies of the Energy Shale were partly contemporaneous with Herrin peat deposition. The sulfur content of the Herrin Coal is highest beneath the proximal splay and marginal marine facies of the Energy Shale and is lowest (less than 1.5%) beneath the distal splay facies and the nonmarine bay facies. The sulfur content of the Herrin Coal is related to the thickness, permeability, and environment of deposition of the facies of the overlying Energy Shale.