Mahantango Formation
Encyclopedia
The Devonian
Mahantango Formation is a mapped bedrock "unit" in Pennsylvania
, New Jersey
, New York
and Maryland
. 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.
and shale
, characterized by coarsening-upward cycles. It is typically found conformally overlying the Marcellus Formation
, and underlying the Tully Limestone (where present).
The Mahantango has been divided into four members:
The Sherman Ridge and Fisher Ridge are ridge-formers.
In south-central Pennsylvania, the Mahantango includes the Clearville, Frame, Chaneysville, and Gander Run Members.
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.
, Crinoid
, Trilobite
, Bivalve, and Bryozoan.
period, being deposited between 392 to 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.
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
Mahantango Formation is a mapped bedrock "unit" in Pennsylvania
Pennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...
, New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...
, New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
and Maryland
Maryland
Maryland is a U.S. state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware to its east...
. It is named for the North branch of the Mahantango Creek
Mahantango Creek
Mahantango Creek is a tributary of the Susquehanna River in Dauphin, Northumberland, and Schuylkill counties, in Pennsylvania in the United States....
in Perry
Perry County, Pennsylvania
As of the census of 2000, there were 43,602 people, 16,695 households, and 12,320 families residing in the county. The population density was 79 people per square mile . There were 18,941 housing units at an average density of 34 per square mile...
and Juniata counties in Pennsylvania. It is a member of the Hamilton Group
Hamilton Group
The Devonian Hamilton Group is a mapped bedrock unit in New York, Pennsylvania, Maryland, and West Virginia. In Virginia, it is known as the laterally equivalent Millboro Shale.The group is named for the village of Hamilton, New York...
, along with the underlying the Marcellus Formation
Marcellus Formation
The Marcellus Formation is a unit of marine sedimentary rock found in eastern North America...
Shale. South of Tuscarora Mountain
Tuscarora Mountain
Tuscoarora 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 at 2,458 feet above sea level....
in south central Pennsylvania, the lower members of this unit were also mapped as the Montebello Formation.
Description
The Mahantango is defined as a gray, brown, and olive siltstoneSiltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
and shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
, characterized by coarsening-upward cycles. It is typically found conformally overlying the Marcellus Formation
Marcellus Formation
The Marcellus Formation is a unit of marine sedimentary rock found in eastern North America...
, and underlying the Tully Limestone (where present).
The Mahantango has been divided into four members:
- Tully Limestone: a fossiliferous calcaerous 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 sandstones coarsening upward. sandstoneSandstoneSandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
. - 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): a Silty ClaystoneClaystoneClaystone is a geological term used to describe a clastic sedimentary rock that is composed primarily of clay-sized particles ....
, 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.
Depositional environment
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 deltaRiver delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...
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
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
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.
Fossils
There are numerous marine fossils found in the Mahantango including BrachiopodBrachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
, Crinoid
Crinoid
Crinoids are marine animals that make up the class Crinoidea of the echinoderms . Crinoidea comes from the Greek word krinon, "a lily", and eidos, "form". They live both in shallow water and in depths as great as 6,000 meters. Sea lilies refer to the crinoids which, in their adult form, are...
, Trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
, Bivalve, and Bryozoan.
Age
Relative age dating of the Mahantango places it in the Middle DevonianDevonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
period, being deposited between 392 to 385 (±3) million years ago. It rests conformably
Unconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
atop the Marcellus Formation
Marcellus Formation
The Marcellus Formation is a unit of marine sedimentary rock found in eastern North America...
shale. Its upper contact is also conformable to the Trimmers Rock Formation and Harrell Shale.