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
Perry, also known as pear cider, is an alcoholic beverage made from fermented pears, traditionally the perry pear. It has been common for centuries in England, particularly in Gloucestershire, Herefordshire, and Worcestershire. It is also made ...
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
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, in the United States,
it extends throughou ...
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
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, in the United States,
it extends throughou ...
, 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 brachiopod
Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...
s, crinoid
Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...
s, 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 ...
s, 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
* 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
* 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
''Dipleura'' is a genus of trilobites in the order Phacopida. It was described by Green in 1832, and the type species is ''Dipleura dekayi''. The type locality was in the Hamilton Group in New York.
Distribution
Fossils of this genus have b ...
* 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
* 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 Carbonifero ...
Cephalopods
* Tornoceras
* 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
* Bactrites
* Striacoceras
Age
Relative age dating of the Mahantango places it in the Middle 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 ...
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
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, in the United States,
it extends throughou ...
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
Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...
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
The Geology of Pennsylvania consists of six distinct physiographic provinces, three of which are subdivided into different sections. Each province has its own economic advantages and geologic hazards and plays an important role in shaping everyd ...
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