The

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

Martinsburg Formation (Om) is a mapped

bedrock In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of bed ...

unit in

Pennsylvania Pennsylvania, officially the Commonwealth of Pennsylvania, is a U.S. state, state spanning the Mid-Atlantic (United States), Mid-Atlantic, Northeastern United States, Northeastern, Appalachian, and Great Lakes region, Great Lakes regions o ...

New Jersey New Jersey is a U.S. state, state located in both the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. Located at the geographic hub of the urban area, heavily urbanized Northeas ...

Maryland Maryland ( ) is a U.S. state, state in the Mid-Atlantic (United States), Mid-Atlantic region of the United States. It borders the states of Virginia to its south, West Virginia to its west, Pennsylvania to its north, and Delaware to its east ...

Virginia Virginia, officially the Commonwealth of Virginia, is a U.S. state, state in the Southeastern United States, Southeastern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States between the East Coast of the United States ...

, and

West Virginia West Virginia is a mountainous U.S. state, state in the Southern United States, Southern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States.The United States Census Bureau, Census Bureau and the Association of American ...

. It is named for the town of

Martinsburg, West Virginia Martinsburg is a city in Berkeley County, West Virginia, United States, and its county seat. The population was 18,773 at the 2020 United States census, 2020 census, making Martinsburg the largest city in the Eastern Panhandle of West Virginia a ...

for which it was first described. It is the dominant rock formation of the

Great Appalachian Valley The Great Appalachian Valley, also called The Great Valley or Great Valley Region, is one of the major landform features of eastern North America. It is a gigantic trough, including a chain of valley lowlands, and the central feature of the Appal ...

in New Jersey (where it is called

Kittatinny Valley The Kittatinny Valley is a section of the Great Appalachian Valley in Sussex and Warren counties in northwestern New Jersey that is bounded on the northwest by Kittatinny Mountain, and in the southeast by the New Jersey Highlands region. The val ...

) and Pennsylvania (in the

Lehigh Valley The Lehigh Valley () is a geography, geographic and urban area, metropolitan region formed by the Lehigh River in Lehigh County, Pennsylvania, Lehigh and Northampton County, Pennsylvania, Northampton counties in eastern Pennsylvania. It is a co ...

and Lebanon Valley).

Description

The Martinsburg is defined as a gray to dark gray, and infrequently tan and purple

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

and

slate Slate is a fine-grained, foliated, homogeneous, metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade, regional metamorphism. It is the finest-grained foliated metamorphic ro ...

. Local members of impure sandstone; thin,

argillaceous Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay mineral ...

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

or phyllitic shale are present in the

Cumberland Valley The Cumberland Valley is a northern constituent valley of the Great Appalachian Valley, within the Atlantic Seaboard watershed in Pennsylvania and Maryland. The Appalachian Trail crosses through the valley. Geography The valley is bound t ...

and the

.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. The Martinsburg has three defined members in the

: The Bushkill Member which is the basal slate unit. The Ramseyburg Member which is composed of alternating turbidite sandstone units with interbeds of shale and siltstone. And the Pen Argyl Member which is the uppermost slate unit with some minor

phyllite Phyllite ( ) is a type of foliation (geology), foliated metamorphic rock formed from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation.Stephen Marshak ''Essentials of Geology'', 3rd ed. I ...

and shale beds.Epstein, J.B., Sevon, W.D., Glaeser, J.D. (1974). Geology and the Mineral Resources of the Lehighton and Palmerton Quadrangles, Carbon and Northampton Counties, Pennsylvania. The Pennsylvania Geologic Survey, Harrisburg, PA. A195c&d.

Depositional environment

The shales (and slates) of the Martinsburg were deposited in a large

forearc A forearc is a region in a subduction zone between an oceanic trench and the associated volcanic arc. Forearc regions are present along convergent margins and eponymously form 'in front of' the volcanic arcs that are characteristic of convergen ...

basin resulting in a

flysch Flysch () is a sequence of sedimentary rock layers that progress from deep-water and turbidity flow deposits to shallow-water shales and sandstones. It is deposited when a deep basin forms rapidly on the continental side of a mountain building ep ...

deposit. This basin was the result of a deepening sea due to the closing of the

Iapetus Ocean The Iapetus Ocean (; ) existed in the late Neoproterozoic and early Paleozoic eras of the geologic timescale (between 600 and 400 million years ago). It was in the southern hemisphere, between the paleocontinents of Laurentia, Baltica and Avalon ...

Turbidites A turbidite is the geologic Deposition (geology), deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing ...

are common in the Martinsburg due to underwater landslides stirring up sediments and rushing down a slope. Limestones were deposited upon peripheral bulges in the deep seas. A modern analog to the depositional history of the Martinsburg might be the

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

East Timor Timor-Leste, also known as East Timor, officially the Democratic Republic of Timor-Leste, is a country in Southeast Asia. It comprises the eastern half of the island of Timor, the coastal exclave of Oecusse in the island's northwest, and ...

tectonic setting.

Fossils

In the Pen Argyl Member, both

graptolite Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian ( Miaolingian, Wuliuan) through t ...

s and

conodont Conodonts, are an extinct group of marine jawless vertebrates belonging to the class Conodonta (from Ancient Greek κῶνος (''kōnos''), meaning " cone", and ὀδούς (''odoús''), meaning "tooth"). They are primarily known from their hard ...

s are found. The

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

Cryptolithus ''Cryptolithus'' is a genus of extinct epifaunal, suspension-feeding, trinucleid trilobites that lived during the Ordovician period. They were mostly blind. They are found in the United States, Canada, Venezuela, the United Kingdom, France, th ...

and the

brittle star Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomot ...

Protasterina are common in the

Swatara Gap Swatara Gap is a water gap through Blue Mountain formed by the Swatara Creek in Lebanon County, Pennsylvania. PA Route 72 as well as Interstate 81 pass through the gap. The Appalachian Trail passes through the gap over the Waterville Bridge in S ...

area.

Age

The Martinsburg was deposited during the Upper

period between 457 and 445 (±5) million years ago. Near Hamburg PA, an

allochthonous River ecosystems are flowing waters that drain the landscape, and include the biotic (living) interactions amongst plants, animals and micro-organisms, as well as abiotic (nonliving) physical and chemical interactions of its many parts.Angelier ...

klippe 350px, Schematic overview of a thrust system. The shaded material is called a window or fenster. The klippe is the isolated block of the nappe overlying autochthonous material. A klippe ( German for cliff">German language">German for cliff or c ...

of rocks (known as the Hamburg klippe) is a tectonic slice of Middle Ordovician sediments placed within the Martinsburg. The Martinsburg lies above and below erosional

unconformities An unconformity is a buried erosion surface, erosional or non-depositional surface separating two Rock (geology), rock masses or Stratum, strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer ...

. The

Shawangunk Formation The Silurian Shawangunk Formation is a mapped bedrock unit in eastern Pennsylvania, New Jersey, and New York. It is named for the Shawangunk Ridge for which it is the dominant rock type. The division of the Shawangunk between the Tuscarora For ...

, Kittatinny Formation,

Tuscarora Formation The Silurian Tuscarora Formation — also known as Tuscarora Sandstone or Tuscarora Quartzite — is a mapped bedrock unit in Pennsylvania, Maryland, West Virginia, and Virginia, US. Description The Tuscarora is a thin- to thick-bedded fine-gra ...

, and the

Juniata Formation The Ordovician Juniata Formation is a mapped bedrock unit in Pennsylvania, West Virginia, Virginia, Tennessee, and Maryland. It is a relative slope-former occurring between the two prominent ridge-forming sandstone units: the Tuscarora Formation ...

, all lie unconformably atop the Martinsburg. Below it, the Chambersburg Formations and Myerstown Formations lie on a possible unconformity.Berg, T.M., et al., (1983). Stratigraphic Correlation Chart of Pennsylvania: G75, Pennsylvania Geologic Survey, Harrisburg, Pennsylvania.

Economic uses

Slate Slate is a fine-grained, foliated, homogeneous, metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade, regional metamorphism. It is the finest-grained foliated metamorphic ro ...

is still quarried out of the Martinsburg in the slate belt of Pennsylvania. Other quarries that exist in the shale partings use the rock as crushed stone for structural and other earth fill operations.

Description

Depositional environment

Fossils

Age

Economic uses

References

See also