Marcellus Formation

Marcellus black shale at Marcellus, N.Y. 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. 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. Pyrite is especially abundant near the base, 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, The more organic-rich black shales can be bituminous, but are too old to contain bituminous coal formed from land plants. 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. 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 above Walpack Bend, where it leaves the buried valley eroded from Marcellus Shale bedrock 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. 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. 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 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, 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. and the beds are steeply overturned, with a reverse dip angle of up to 40° south. 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. 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, 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, ==Stratigraphy==

Stratigraphically, 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 West Virginia, the Marcellus may be separated from the brown shales of the Mahantango by occasional sandstone beds and concretions, 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. In Pennsylvania, the Marcellus forms a sharp conformable contact with the Onondaga's Selinsgrove Limestone member. A thin pyrite-carbonate bed is also found at the base of the Marcellus black shale in the exposures of south central Pennsylvania, above a thin calcerous green shale bed, which lies upon the Onondaga limestone. The local disappearance of units of the Onondaga suggests that its upper contact with the Marcellus can be erosional. In Erie County in western New York, both the upper and lower contact of the Marcellus are eroded away. In eastern West Virginia the Marcellus overlies the Onesquethaw Group, consisting of the dark gray or green, calcitic, mostly nonfissil Needmore Shale, which grades westward into the Huntersville Chert. South of the Mason-Dixon line, due to the difficulty in differentiating the Millboro and Needmore shales with the limited exposures available, and initial uncertainty in correlation with the New York survey, they were mapped as the Romney Formation, a unit containing all the Middle Devonian strata, The correlations were established by 1916 through tracing the New York exposures across Pennsylvania and Maryland into West Virginia, so under the principle of scientific priority, , New York southwest across the Allegheny Plateau and then along the Ridge-and-Valley Appalachians to Tennessee. Note the Marcellus grades up to the Milboro and Chattanooga black shales. In 1843 it was described without being named by Hall, and more than 100 years passed before it was eventually named for the natural gas field in Tioga County, Pennsylvania, where it was encountered when drilling gas wells. It is a regional stratigraphic marker, and correlate laterally equivalent strata. The volcanic origin of the ash is evidenced by its distinctive mineralogy–the ash was deposited directly upon the water, so its angular quartz grains differ from the clastic sediments rounded through the erosion process that carries them to the sea. consisting of coarse crystal tuff or tuffaceous shale, The Tioga ash bed zone consists of eight ash beds labeled according to their stratigraphic order from A (oldest) to H (youngest), and another bed known as the Tioga middle coarse zone. Named members A local Purcell limestone member, of inter-bedded calcitic shale and limestone, a bioclastic packstone, The Union Springs, Cherry Valley, and Oatka Creek merge beneath Lake Erie, into the Bell Shale, Rockport Quarry Limestone, and Arkona Shale of Ontario. and thin silt bands at the bottom. To the east, it becomes the Bakoven Member, a darker, less organic shale with fewer limestone layers. where it divides into the Cardiff member lying above the Chittenango member in central New York. above the Bierne Member shale, lies the Halihan Hill Bed, a highly bioturbated bioclastic limestone. In south central Pennsylvania, the Marcellus is mapped with three members, from top to base: The Mahanoy Member (Dmm), a dark gray to grayish black silty shale and siltstone; the Turkey Ridge Member (Dmt), an olive to dark-gray fine to medium grained sandstone; and the Shamokin Member (Dms), a dark gray to grayish black fissile carbonaceous shale that is calcareous in places near the base. The Turkey Ridge is commonly mapped in the Mahantango Formation, or included in the Montebello Formation (Dmot), and only the Shamokin correlates with the Marcellus on adjacent map sheets. In extreme eastern Pennsylvania, the Broadhead Creek member, a dark gray silty shale with dark gray shaly limestone concretions, appears above the Stony Hollow and Union Springs, in a layer up to thick. (Goniatites vanuxemi) fossil from the Marcellus Formation. ==Fossils==

There are relatively sparse inclusions of fossilized marine fauna found in the Marcellus, It is also where goniatites, an extinct shelled swimmer similar to a squid, make their first appearance in the fossil record. External molds of crinoids, plant-like animals related to starfish also known as "sea lilies," are found in the formation, with the molds partially filled with limonite; brachiopod and bivalve (clam) molds have also been found in the shale. Small conical tentaculitids are commonly found in the Chittenango Member. The fossil record in this member shows the base was dominated by deposit feeders, while the upper layers were dominated by filter feeders. A diverse, eel-like conodont fauna occurs in the limestone of the Cherry Valley Member, which is also known for its rich nautiloid and goniatite cephalopod fauna. This stratigraphic interval also provides an excellent example of incursion epiboles, which are sudden appearances and disappearances of fossil taxa in relatively thin sections of the rock unit. The Cherry Valley and Union Springs also contain well-preserved anarcestida. ==Age==

On the geological timescale, the Marcellus occurs in the Middle Devonian epoch, of the Devonian period, in the Paleozoic era, of the Phanerozoic eon. Radiometric dating of a Marcellus sample from Pennsylvania placed its age at 384 million years old, and a sample from the bentonite at the top of the Onondaga at 390 ± 0.5 million years old. Relative age dating of the Marcellus places its formation in the Cazenovia subdivision of the Givetian faunal stage, or 391.9 to 383.7 million years ago (Ma). The Union Springs member, at the base of the Marcellus in New York, has been dated to the end of the Eifelian, the stage which immediately preceded the Givetian. Anoxic dark shales in the formation mark the Kačák Event, a late-Eifelian-stage marine anoxic event also associated with an extinction event. In 2012, Read and Erikson also depicted the formation as Eifelian. nomenclature for the Middle Devonian strata in the Appalachian Basin. ==Interpretation of depositional environment==

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 . 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. Early in the Acadian orogeny, as the Acadian Mountains were rising up, the black and gray shales of the Hamilton Group began accumulating as erosion of the mountains deposited terrigenous sediments from the land into the sea. The Marcellus Shale was formed from the very first deposits in a relatively deep, sediment- and oxygen-starved (anoxic), trough that formed parallel to the mountain chain. These clastic fragments of rock were carried in braided streams to the ancient Catskill Delta, a river delta probably similar to the present day Niger Delta of Africa. Smaller particles remained suspended longer in this epeiric sea, flowing offshore as turbidites in a slow but persistent underwater avalanche. They finally came to rest at the bottom of the Acadian foredeep in the Appalachian Basin, Alternatively, the basin may have been as shallow as or less, if the warm water was sufficiently stratified so that oxygen rich surface water did not mix with the anoxic bottom water. The Marcellus deposition produced a transgressive black shale, magnafacies across western Pennsylvania and eastern Ohio. Organic matter, probably dominated by plankton, also settled to the bottom, but the normal aerobic decay process was inhibited in the anaerobic environment thereby preserving the organic carbon. Uranium was also incorporated in these organic muds syndepositionally, The organic matter scavenged trace elements from the seawater, including the redox-sensitive elements uranium, rhenium, molybdenum, osmium, chromium, and selenium. The Marcellus was deposited during the development of land plants, when atmospheric oxygen was increasing, resulting in a reduction of carbon dioxide in the atmosphere, and the seawater where it was deposited. with a general coarsening upward cycle that continues into the base of the overlying Mahantango Formation. Later deep water depositional sequences formed the overlying Brallier Formation and Harrell Formation. ==Economic resources==

Natural gas 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. The Marcellus natural gas trend, which encompasses 104,000 square miles and stretches across Pennsylvania and West Virginia, and into southeast Ohio and upstate New York, is the largest source of natural gas in the United States, and production was still growing rapidly in 2013. The Marcellus is an example of shale gas, natural gas trapped in low-permeability shale, and requires the well completion method of hydraulic fracturing to allow the gas to flow to the well bore. The surge in drilling activity in the Marcellus Shale since 2008 has generated both economic benefits and environmental concerns—and thus, considerable controversy. Iron 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. At the base of the Marcellus, in the pyrite-carbonate bed between the carbonaceous black shale and a green calcareous shale bed, As far as the ground water necessary for the conversion could penetrate, the pyrite-carbonate was converted to a usable brown hematite iron ore along the outcrops and near the bedrock surface. The Marcellus iron ore was actively mined in south Central Pennsylvania from its discovery in the late 18th century, until it was supplanted by the rich ore beds of the Iron Range of Minnesota in the early 20th century. The ore was easily located and worked from shallow pits and shafts, but once the usable upper deposits were removed, or if a mine shaft entered the bed too far below the surface, only unusable unconverted pyritic deposits were found. but the cold blast stone furnaces typically employed were inefficient, and consumed significant amounts of timber from the nearby hardwood forests, which ultimately led to their demise. A typical furnace used of hematite ore and of charcoal to produce of pig iron, The ore from the Marcellus varied in thickness, becoming unworkably thin, and even disappearing altogether in places between the workable beds. Ore found interbedded in the black slaty shale contained a relatively high proportion of carbon which was burned in the furnace, and sulfur, which produced a usable but "red-short" iron. Red-short iron has the undesirable properties of oxidizing more easily, and a tendency to crack, especially when heated to a red-hot state. In some locations in Pennsylvania the quality of the ore was quite good, with relatively deep veins containing 45% iron, and very low sulfur. Iron pigments Drainage that reacted with the pyrite inclusions also deposited a form of bog iron near several outcrops of the Marcellus. In the 19th century, iron ore from these deposits was used as a mineral paint pigment. After being heated in a kiln and finely ground, it was mixed with linseed oil, and used to paint exterior wood on barns, covered bridges, and railroad cars. became a "resort for invalids". The iron-rich waters were prescribed for anemia and related complications. Other uses The Marcellus has also been used locally for shale aggregate and common fill, In the 19th century, this shale was used for walkways and roadways, and was considered superior "road metal" because the fine grained fragments packed together tightly, yet drained well after a rain. The dark slaty shales may have the necessary cleavage and hardness to be worked, and were quarried for low grade roofing slate in eastern Pennsylvania during the 19th century. The slates from the Marcellus were inferior to the Martinsburg Formation slate quarried further south, and most quarries were abandoned, with the last significant operation in Lancaster County. The Marcellus black slate was also quarried in Monroe County, Pennsylvania, for school slates used by students in 19th-century rural schools. Carbonaceous shales, such as the Marcellus, are a possible target for carbon capture and storage for mitigation of global warming. Because carbon adsorbs carbon dioxide (CO2) at a greater rate than methane (CH4), carbon dioxide injected into the formation for geological sequestration could also be used to recover additional natural gas in a process analogous to enhanced coal bed methane recovery, but the practical value of this theoretical technique is not yet known. Scientists believe that adsorption would allow sequestration at shallower depths than absorption in deep saline formations, which must be at least below the surface to maintain liquid CO2 in a supercritical state. ==Engineering issues==

The fissile shales are also easily eroded, presenting additional civil and environmental engineering challenges. . Exposures from cut and fill road construction in Virginia and Pennsylvania have resulted in localized acid rock drainage due to oxidation of the pyrite inclusions. The newly exposed shale on the cut face weathers rapidly, allowing air and water into the unexcavated rock, resulting in acidic surface runoff after precipitation events. Acidic runoff disrupts aquatic ecosystems, and highly acidic soil contaminated by this runoff will not support vegetation, which is unsightly, and can lead to problems with soil erosion. Other sulfate minerals that can be produced by reactions with pyrite include anhydrite, melanterite, rozenite, jarosite, and alunite. The reactions have generated a heave pressure on the order of 500 kPa (10,000 pounds per square foot), but may be able to generate four times this pressure   enough to heave foundations in a 5-story building. Limestone, which is used to neutralize the acid drainage, can actually exacerbate the expansion problem by promoting sulfate–sulfate reactions that form the minerals thaumasite and ettringite, which have even higher molar volumes. Drilling boreholes through the Hamilton Group shales in the subsurface can be problematic. The Marcellus has a relatively low density, and these shales may not be chemically compatible with some drilling fluids. The shale is relatively fragile, and may fracture under pressure, causing a problem in circulating the drilling fluid back up through the borehole known as lost circulation. The formation may also be under-pressurized, further complicating the drilling process. ==See also==