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 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.

Description

The Marcellus consists predominantly of black shale and a few limestone beds and concentrations of iron pyrite and siderite .
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 crystals from the reaction between pyrite and limestone particles.
Fresh exposures of the pyriteiferous shale may develop the secondary mineralization of orange limonite , 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 makes it a source rock for radioactive radon gas .
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.

Geographic extent

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.

Outcrops in New York

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 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.
Close proximity to the surface of Marcellus bedrock south of the New York outcrops makes an east–west band running through the city of Syracuse a high-risk area for radon as an indoor air pollutant. From the surface exposures along the northern and eastern margins, the formation descends to depths of over below the surface in southern Pennsylvania.

Geomorphological expression

Upturned beds are exposed in sections of the folded Ridge-and-Valley Appalachians, including exposures on the flanks and axis of the Broad Top Synclinorium in south central Pennsylvania. Exposed beds are nearly horizontal on the Allegheny Plateau, but upturned to form slightly overturned beds found along the Allegheny Front.
From Wind Gap, Pennsylvania heading south, the dip of the beds steepens, becoming vertical at Bowmanstown on the Lehigh River. Nearby, in the Lehigh Gap area of Pennsylvania, the Marcellus is extensively faulted,
and the beds are steeply overturned, with a reverse dip angle of up to 40° south.
The Marcellus Shale and the fine-grained shales near the middle of the Mahantango Formation are classified by geologists as slope-formers.
Marcellus and Mahantango shale beds dipping at 60° to 75° to the west form the west facing slopes of Tonoloway Ridge on the west flank of the Cacapon Mountain anticline in the Eastern Panhandle of West Virginia.
On the eastern limb of this anticline, beds of these shales dipping to the east at a shallower angle also form the steep slopes on the east side of Warm Springs Ridge.
The Marcellus is easily eroded, and is also found underlying low areas between some Appalachian ridges, forming linear valleys of moderate relief. These bedrock surfaces are typically covered with colluvium from erosion of stratigraphically higher and more erosion-resistant strata that form the surrounding higher ground. The soils formed from the Marcellus and the overlying Hamilton shales are deep, free of stones, and well suited for agriculture.
Sampling of soil formed on the Marcellus bedrock showed the dominant mineralogy consisted of quartz, illite, montmorillonite, muscovite, and biotite, with phases of todorokite and trona appearing at depths closer to the bedrock.
Upturned beds of the soft shale also capture streams and rivers with relatively straight segments in strike valleys such as the Aquashicola Creek and McMichael Creek at the foot of The Poconos,
and the long, straight section of the Lost River in West Virginia.
Below Port Jervis, New York, the Walpack Ridge deflects the Delaware River into the Minisink Valley, where it follows the southwest strike of the eroded Marcellus beds along the Pennsylvania – New Jersey state line for to the end of the ridge at Walpack Bend in the Delaware Water Gap National Recreation Area.
The Minisink is a buried valley where the Delaware flows in a bed of glacial till that buried the eroded Marcellus bedrock during the last glacial period. This buried valley continues along the strike of the Marcellus southwest from the bend through Stroudsburg, Pennsylvania, and northeast from Port Jervis toward the Hudson River,
along the route of the Delaware and Hudson Canal.
File:Eastern North American Paleogeography Middle Devonian.gif|alt=Map of north-eastern North America showing the Illinois Basin, the Michigan Basin and to the east, the Appalachian Basin.|thumb|right|Paleogeographic reconstruction of the Appalachian Basin area during deposition of the Marcellus

Stratigraphy

, the Marcellus is the lowest unit of the Devonian age Hamilton Group, and is divided into several sub-units.
In the first Pennsylvania Geological Survey, begun in 1836, Henry Darwin Rogers classified the Marcellus as the "Cadent Lower Black Slate" which he numbered "No. VIII b." In the first New York State Geological Survey, also begun that year, James Hall established the term "Marcellus Shale" in his 1839 report titled "Marcellus Shales in Seneca County." Professor Hall also argued in 1839 against formulating geological names based on observed characteristics that may vary from place to place or need revision in the future, and in favor of location-based nomenclature where "the rock or group will receive its name from the place where it is best developed." His arguments proved persuasive, and the location-based name for this, and many of the other group names he published based on exposures in New York, were adopted in the second Pennsylvania survey, and are now widely accepted.

Overlying units

In the first New York survey, the Marcellus Shale was placed below the Hamilton Group at the base of the Erie division of the New York system, but this taxonomy is obsolete.
In current practice, the Marcellus Shale is classified as the basal unit of the Hamilton Group , lying beneath the Mahantango Formation member of this group in Pennsylvania and Maryland. In New York, the Mahantango, also of Middle Devonian age, is further divided. There the Marcellus is separated from the overlying Skaneateles Formation, a more clastic and fossiliferous dark shale, by the thin Stafford or Mottville Limestone bed.
In West Virginia, the Marcellus may be separated from the brown shales of the Mahantango by occasional sandstone beds and concretions,
or it may lie directly below the younger Late Devonian Harrel Formation because of a disconformity, which represents a gap in the geological record due to a period of erosion or non-deposition.
In eastern Ohio the Hamilton Group also lies disconformably beneath the Rhinestreet Shale Member of the West Falls Formation, another transgressive black shale tongue with similar characteristics to the Marcellus.