The Devonian Mahantango Formation is a mapped bedrock unit in Pennsylvania, West Virginia, and Maryland.Ryder, R.T., Swezey, C.S., Crangle, R.D., Jr., and Trippi, M.T., 2008, Geologic cross section E-E’ through the central Appalachian Basin from the Findlay Arch, Wood County, Ohio, to the Valley and Ridge Province, Pendleton County, West Virginia: U.S. Geological Survey Scientific Investigations Map SIM-2985, 2 sheets with 48-page pamphlet. https://pubs.er.usgs.gov/publication/sim2985Ryder, R.T., Crangle, R.D., Jr., Trippi, M.H., Swezey, C.S., Lentz, E.E., Rowan, E.L., and Hope, R.S., 2009, Geologic cross section D-D’ through the central Appalachian basin from the Findlay arch, Sandusky County, Ohio, to the Valley and Ridge province, Hardy County, West Virginia: U.S. Geological Survey Scientific Investigations Map SIM-3067, 2 sheets with 52-page pamphlet. https://pubs.er.usgs.gov/publication/sim3067Ryder, R.T., Trippi, M.H., Swezey, C.S., Crangle, R.D., Jr., Hope, R.S., Rowan, E.L., and Lentz, E.E., 2012, Geologic cross section C-C’ through the central Appalachian basin from near the Findlay Arch, north-central Ohio, to the Valley and Ridge Province, Bedford County, south-central Pennsylvania: U.S. Geological Survey Scientific Investigations Map SIM-3172, 2 sheets with 70-page pamphlet. https://pubs.er.usgs.gov/publication/sim3172 It is named for the North branch of the Mahantango Creek in Perry and Juniata counties in Pennsylvania. It is a member of the Hamilton Group, along with the underlying the Marcellus Formation Shale. South of Tuscarora Mountain in south central Pennsylvania, the lower members of this unit were also mapped as the Montebello Formation. Details of the type section and of stratigraphic nomenclature for this unit as used by the U.S. Geological Survey are available on-line at the National Geologic Map Database.
Description The Mahantango Formation is a gray, brown, and olive siltstone and shale,Berg, T.M., Edmunds, W.E., Geyer, A.R. and others, compilers, (1980). Geologic Map of Pennsylvania: Pennsylvania Geologic Survey, Map 1, scale 1:250,000.Milici, R.C., and Swezey, C.S., 2014, Assessment of Appalachian Basin oil and gas resources: Devonian gas shales of the Devonian Shale-Middle and Upper Paleozoic Total Petroleum System, in Ruppert, L.F., and Ryder, R.T., eds., Coal and Petroleum Resources in the Appalachian Basin: Distribution, Geologic Framework, and Geochemical Character: U.S. Geological Survey Professional Paper 1708, chapter G.9, 81p. https://pubs.usgs.gov/pp/1708/g9/pdf/pp1708_g9.pdf characterized by coarsening-upward cycles.
Stratigraphic setting
The Mahantango Formation is typically found conformably overlying the Marcellus Formation, and underlying the Tully Limestone (where present).
The Mahantango has been divided into four members:MacLachlan, D.B., Hoskins, D.M., Payne, D.F., (1995). Bedrock Geology of the Freeburg 7.5 minuet Quadrangle, Snyder County, Pennsylvania. ORF 95-04, The Pennsylvania Geologic Survey, Harrisburg, PA. Tully Limestone: a fossiliferous calcareous shale. The Tully is a conspicuous formation, which separates the Mahantango from the Hamilton Group. Sherman Ridge: Olive gray, fossiliferous, claystone and siltstone with interbedded fine sandstone coarsening upward. Montebello Sandstone: Olive gray, fossiliferous, coarse to fine-grained sandstone, interbedded with siltstone and claystone, in coarsening upward cycles. Fisher Ridge (also described as the Dalmatia and Turkey Ridge): silty claystone, siltstone, and very fine-grained sandstone.
The Sherman Ridge and Fisher Ridge are ridge-formers.
In south-central Pennsylvania, the Mahantango includes the Clearville, Frame, Chaneysville, and Gander Run Members.
Fossils
There are numerous marine fossils found in the Mahantango including brachiopods, crinoids, trilobites, bivalves, and bryozoans.
Brachiopods Athyris Chonetes Mucrospirifer Protoleptostrophia Tropidoleptus
Crinoids Ancyrocrinus
Trilobites Basidechenella Dipleura Eldredgeops (formerly assigned to Phacops) Greenops
Bivalves Actinodesma Grammysioidea Grammysia Leiopteria Modiomorpha Nuculites Orthonota
Bryozoa Atactotoechus furcatus
Coral Pleurodictyum Thamnoptychia
Gastropods Bembexia Crenistriella Dictyotomaria? Glyptotomaria Loxonema Paleozygopleura Platyceras
Cephalopods Tornoceras Agoniatites Spyroceras Michelinoceras Bactrites Striacoceras Koenenites?
Age Relative age dating of the Mahantango places it in the Middle Devonian period, being deposited between 392 and 385 (±3) million years ago. It rests conformably atop the Marcellus Formation shale. Its upper contact is also conformable to the Trimmers Rock Formation and Harrell Shale.Berg, T.M., et al., (1983). Stratigraphic Correlation Chart of Pennsylvania: G75, Pennsylvania Geologic Survey, Harrisburg, Pennsylvania. In 2012, Read and Erikson reported the formation as Givetian in Virginia.Paleozoic Sedimentary Successions of the Virginia Valley & Ridge and Plateau
Interpretation of depositional environments The Mahantango represents a terrestrial to marine transition zone that went through many transgressive-regression sequences. The fine-grained rocks represent a shallow sea environment and accounts for many of the fossils. Coarser grained sediments represent near-shore environments, beaches, or possibly delta lobes. These environments were tide-dominated and often had violent storms. The Montebello Sandstone member is an example of a storm dominated rock unit. Brachiopod fossils are scattered in massive sandstone beds throughout the rock unit, while the Sherman Ridge member is more laminated with fossilized ripple marks often indicating tidal current directions.Prave, A.R., Duke, W.L., Slattery, W. (1996). A depositional model for storm- and tide-dominated prograding siliciclastic shorelines from the Middle Devonian of the central Appalachian foreland basin, USA. Sedimentology, 43, 611-629.
See also Geology of Pennsylvania
References
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