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, 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. In petroleum geology, these black shales are an important source rock that filled conventional petroleum reservoirs in overlying formations, are an unconventional shale gas reservoir, and are an impermeable seal that traps underlying conventional natural gas reservoirs. To the west the formation may produce liquid petroleum; further north heating during deeper burial more than 240 million years ago cracked this oil into gas.

The Marcellus is found throughout the Allegheny Plateau region of the northern Appalachian Basin of North America. In the United States, the Marcellus Shale runs across the Southern Tier and Finger Lakes regions of New York, in northern and western Pennsylvania, eastern Ohio, through western Maryland, and throughout most of West Virginia extending across the state line into extreme western Virginia. The Marcellus bedrock in eastern Pennsylvania extends across the Delaware River into extreme western New Jersey. It also exists in the subsurface of a small portion of Kentucky and Tennessee. Below Lake Erie, it can be found crossing the border into Canada, where it stretches between Port Stanley and Long Point to St. Thomas in southern Ontario.

The Marcellus appears in outcrops along the northern margin of the formation in central New York. There, the two joint planes in the Marcellus are nearly at right angles, each making cracks in the formation that run perpendicular to the bedding plane, which lies almost level. These joints form smooth nearly vertical cliffs, and the intersecting joint planes form projecting corners in the rock faces. Once exposed, the weathered faces lose most of their organic carbon, turning from black or dark gray to a lighter shade of gray.

Outcrops of the Marcellus can contain very small beds that resemble coal. The New York outcrops, and others further south in Pennsylvania and New Jersey, were extensively excavated in the early 19th century, sometimes at great expense, in the false hope of finding minable coal seams. In Perry County, Pennsylvania along the Juniata River the false coal beds become up to .3 m (1 ft) thick, but they did not produce a valuable fuel, despite the considerable effort expended to mine it from the surrounding hills. Seaweed and marine plants probably formed the false coal. True coal is formed from terrestrial plants, which only began to appear in Marcellus and later fossils.