Marcellus Formation

Marcellus Formation Stratigraphic range: Middle Devonian PreЄ Є O S D C P T J K Pg N
Marcellus shale exposure above Marcellus, N.Y. The vertical joints create sheer cliff faces.
Type	Geological formation
Unit of	Hamilton Group
Sub-units	See: Named members
Underlies	Mahantango Formation
Overlies	Onondaga Formation
Thickness	up to 900 feet (270 m)
Lithology
Primary	Shale
Other	Slate, Limestone, Sandstone
Location
Region	Appalachian Basin of eastern North America
Extent	600 miles (970 km)
Type section
Named for	Marcellus, New York
Named by	James Hall, 1839

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 Middle Devonian age 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 and export.

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 paleontological 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.