The

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

Mahantango Formation is a mapped bedrock unit in

Pennsylvania Pennsylvania (; ( Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, ...

West Virginia West Virginia is a state in the Appalachian, Mid-Atlantic and Southeastern regions of the United States.The Census Bureau and the Association of American Geographers classify West Virginia as part of the Southern United States while the Bur ...

, and

Maryland Maryland ( ) is a state in the Mid-Atlantic region of the United States. It shares borders with Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware and the Atlantic Ocean to ...

. It is named for the North branch of the

Mahantango Creek Mahantango Creek is a U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map, accessed August 8, 2011 tributary of the Susquehanna River in Dauphin County, Pennsylvania, Dauphin, Northumberland County, P ...

in Perry and Juniata counties in Pennsylvania. It is a member of the

Hamilton Group The Hamilton Group is a Devonian-age geological group in the Appalachian region of the United States. It is present in New York, Pennsylvania, Maryland, Ohio, West Virginia, northwestern Virginia and Ontario, Canada. It is mainly composed of mar ...

, along with the underlying the Marcellus Formation Shale. South of

Tuscarora Mountain Tuscarora Mountain is a mountain ridge of the Appalachian Mountains in the Ridge and Valley province in central Pennsylvania. It reaches its highest point on Big Mountain (Pennsylvania) at above sea level. The mountain is named after the Tuscaro ...

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 Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

, characterized by coarsening-upward cycles.

Stratigraphic Setting

The Mahantango Formation is typically found conformally overlying the Marcellus Formation, and underlying the Tully Limestone (where present). The Mahantango has been divided into four members: *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 Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

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 Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too sm ...

, siltstone, and very fine-grained sandstone. The Sherman Ridge and Fisher Ridge are

ridge-former A cliff-former is a geological unit of bedrock that is more resistant to erosion than overlying or underlying strata and consequently produces outcrops with high slope angles. It is more or less equivalent to ''ridge-former'', and may be contrast ...

s. 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,

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

s, and

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a l ...

ns.

Brachiopods

Athyris ''Athyris'' is a brachiopod genus with a subequally biconvex shell that is generally wider than long and a range that extends from the Silurian into the Triassic. ''Athyris'' is the type genus for the Athyrididae, which belongs to the articulate ...

Chonetes ''Chonetes'' is an extinct genus of brachiopods. It ranged from the Late Ordovician to the Middle Jurassic. Species The following species of ''Chonetes'' have been described: * ''C. (Paeckelmannia)'' * ''C. baragwanathi'' * ''C. billingsi' ...

Mucrospirifer ''Mucrospirifer'' is a genus of extinct brachiopods in the class Rhynchonellata (Articulata) and the order Spiriferida. They are sometimes known as "butterfly shells". Like other brachiopods, they were filter feeders. These fossils occur mainly ...

* Protoleptostrophia * Tropidoleptus

Crinoids

* Ancyrocrinus

Trilobites

* Basidechenella *

Bellacartwrightia ''Bellacartwrightia'' is a relatively uncommon genus of phacopid trilobite, found in the mid- Devonian Hamilton Group of New York state, mainly in strata exposed near Lake Erie Lake Erie ( "eerie") is the fourth largest lake by surface ar ...

* Dipleura *

Eldredgeops ''Eldredgeops'' is a genus of trilobites in the order Phacopida, family Phacopidae, known from the late Middle and earliest Upper Devonian of Morocco and the USA. Description Like in all sighted Phacopina, the eyes of ''Eldredgeops'' are com ...

(formerly Phacops) *

Greenops ''Greenops'' is a mid-sized Devonian trilobite of the order Phacopida, subfamily Asteropyginae. They are mainly reported from the mid-Devonian Hamilton Group of upstate New York and southwestern Ontario. A similar-looking trilobite from Morocco i ...

Bivalves

* Grammysioidea * Grammysia * Leiopteria * Modiomorpha * Orthonota

Bryozoa

Coral

Pleurodictyum ''Pleurodictyum'' is an extinct genus of tabulate corals, characterized by polygonal corallites. Colonies commonly encrust hard substrates such as rocks, shells and carbonate hardgrounds. Distribution Fossils of ''Pleurodictyum'' have been fou ...

* Trachypora

Gastropods

Bembexia † ''Bembexia'' is a genus of fossil sea snails, marine gastropod mollusks. This genus is placed in the subfamily Eotomariinae, of the family Eotomariidae. The shell characters resemble those of Balbinipleura. These snails have been interprete ...

* Crenistriella * Cyclonema * Glyptotomaria * Loxonema *

Platyceras ''Platyceras'' is a genus of extinct Paleozoic sea snails, marine gastropod mollusks in the family Platyceratidae. This genus is known from the Silurian to the Middle Triassic periods and especially abundant in the Devonian and Carboniferous.Pal ...

Cephalopods

Tornoceras ''Tornoceras'' is a strongly involute, subdiscoidal Middle and Upper Devonian goniatite with a suture that forms six to ten lobes. '' Aulatornoceras'', '' Protornoceras'', '' Epitornoceras'', '' Lobotornoceras'' are among related genera inclu ...

* Agoniatites *

Spyroceras ''Spyroceras'' is a genus of pseudorthocerids from the Devonian of North America and Europe, defined by Hyatt in 1884. Pseudorthocerids are a kind of orthocertaoid, a taxonomic group within the Nautiloidea. Specifically ''Spyroceras'' belongs t ...

Michelinoceras ''Michelinoceras'' is the oldest known genus of the Michelinocerida, more commonly known as the Orthocerida, characterized by long, slender, nearly cylindrical orthocones with a circular cross section, long camerae, very long body chambers, and a ...

* Bactrites * Striacoceras

Age

Relative age dating of the Mahantango places it in the Middle

period, being deposited between 392 and 385 (±3) million years ago. It rests

conformably Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are primarily igne ...

atop the Marcellus Formation shale. Its upper contact is also conformable to the Trimmers Rock Formation and Harrell Shale. In 2012, Read and Erikson reported the formation as Givetian in Virginia.

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 Delta commonly refers to: * Delta (letter) (Δ or δ), a letter of the Greek alphabet * River delta, at a river mouth * D (NATO phonetic alphabet: "Delta") * Delta Air Lines, US * Delta variant of SARS-CoV-2 that causes COVID-19 Delta may also re ...

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.

References

External links

{{Navboxes , title = Chronostratigraphic context of the Mahantango Formation by state , list = {{Chronostratigraphy of Maryland {{Chronostratigraphy of New York {{Chronostratigraphy of Ohio {{Chronostratigraphy of Pennsylvania, abbreviated=yes {{Chronostratigraphy of Virginia {{Chronostratigraphy of West Virginia , state=collapsed Geologic formations of Maryland Geologic formations of New Jersey Geologic formations of New York (state) Geologic formations of Pennsylvania Devonian System of North America Devonian Maryland Devonian geology of New Jersey Devonian geology of New York (state) Devonian geology of Pennsylvania Givetian Stage Shale formations of the United States Limestone formations Siltstone formations Shallow marine deposits Deltaic deposits Beach deposits Tidal deposits Cliff-formers Devonian southern paleotemperate deposits