The Marcellus Formation (also classified as the Marcellus Subgroup of the Hamilton Group, Marcellus Member of the Romney Formation, or simply the Marcellus Shale) is a unit of marine sedimentary rock found in eastern North America. Named for a distinctive outcrop near the village of Marcellus, New York in the United States, it extends throughout much of the Appalachian Basin. The shale contains largely untapped natural gas reserves, and its proximity to the high-demand markets along the East Coast of the United States makes it an attractive target for energy development.
Stratigraphically, the Marcellus is the lowest unit of the Devonian age Hamilton Group, and is divided into several sub-units. Although black shale is the dominant lithology, it also contains lighter shales and interbedded limestone layers due to sea level variation during its deposition almost 400 million years ago. The black shale was deposited in relatively deep water devoid of oxygen, and is only sparsely fossiliferous. Most fossils are contained in the limestone members, and the fossil record in these layers provides important paleontologal insights on faunal turnovers. The black shales also contain iron ore that was used in the early economic development of the region, and uranium and pyrite which are environmental hazards. The fissile shales are also easily eroded, presenting additional civil and environmental engineering challenges.
The Marcellus Formation is a black shale that may contain limestone beds and concentrations of iron pyrite (FeS2) and siderite (FeCO3). Its sedimentary structure, or bedding, is moderately well developed. Like most shales, it tends to split easily along the bedding plane, a property known as fissility. Lighter colored shales in the upper portion of the formation tend to split into small thin-edged fragments after exposure. These fragments may have rust stains from exposure of pyrite to air, and tiny gypsum (CaSO4·2H2O) crystals from the reaction between pyrite and limestone particles. Fresh exposures of the pyriteiferous shale may develop the secondary mineralization of orange limonite (FeO(OH)·nH2O), and the pale yellow efflorescence or bloom of sulfur, associated with acid rock drainage.