Marcellus Formation

The Marcellus Formation or the Marcellus Shale is a Middle Devonian age unit of 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 unit name usage by the U.S. Geological Survey (USGS) includes Marcellus Shale and Marcellus Formation. The term "Marcellus Shale" is the preferred name throughout most of the Appalachian region, although the term "Marcellus Formation" is also acceptable within the State of Pennsylvania. The unit was first described and named as the "Marcellus shales" by J. Hall in 1839. The Marcellus consists predominantly of black shale and a few limestone beds and concentrations of iron pyrite (FeS2) and siderite (FeCO3). 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. Pyrite is especially abundant near the base, and the upper contacts of limestones, but framboidal microcrystals and euhedral crystals of pyrite occur throughout the organic-rich deposits. The Marcellus also contains uranium, and the radioactive decay of the uranium-238 (238U) makes it a source rock for radioactive radon gas (222Rn). Measured total organic content of the Marcellus ranges from less than 1% in eastern New York, to over 11% in the central part of the state, and the shale may contain enough carbon to support combustion. The more organic-rich black shales can be bituminous, but are too old to contain bituminous coal formed from land plants.