The Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 Martinsburg Formation (Om) is a mapped bedrock

Bedrock

In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

 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...

, 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...

, Virginia

Virginia

The Commonwealth of Virginia , is a U.S. state on the Atlantic Coast of the Southern United States. Virginia is nicknamed the "Old Dominion" and sometimes the "Mother of Presidents" after the eight U.S. presidents born there...

, and West Virginia

West Virginia

West Virginia is a state in the Appalachian and Southeastern regions of the United States, bordered by Virginia to the southeast, Kentucky to the southwest, Ohio to the northwest, Pennsylvania to the northeast and Maryland to the east...

. It is named for the town of Martinsburg, West Virginia

Martinsburg, West Virginia

Martinsburg is a city in the Eastern Panhandle region of West Virginia, United States. The city's population was 14,972 at the 2000 census; according to a 2009 Census Bureau estimate, Martinsburg's population was 17,117, making it the largest city in the Eastern Panhandle and the eighth largest...

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

Great Appalachian Valley

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

 in Pennsylvania.

Description

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

 and slate

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. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...

. Local members of impure sandstone; thin, argillaceous limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 or phyllitic

Phyllite

Phyllite is a type of foliated metamorphic rock primarily composed of quartz, sericite mica, and chlorite; the rock represents a gradation in the degree of metamorphism between slate and mica schist. Minute crystals of graphite, sericite, or chlorite impart a silky, sometimes golden sheen to the...

 shale are present in the Cumberland Valley

Cumberland Valley

The Cumberland Valley is a constituent valley of the Great Appalachian Valley and a North American agricultural region within the Atlantic Seaboard watershed in Pennsylvania and Maryland....

 and the Lehigh Valley

Lehigh Valley

The Lehigh Valley, known officially by the United States Census Bureau as the Allentown-Bethlehem-Easton, PA-NJ metropolitan area and referred to locally as The Valley and A-B-E, is a metropolitan region consisting of Lehigh, Northampton, Berks, and Carbon counties in eastern Pennsylvania and...

.

The Martinsburg has three defined members in the Lehigh Valley

Lehigh Valley

The Lehigh Valley, known officially by the United States Census Bureau as the Allentown-Bethlehem-Easton, PA-NJ metropolitan area and referred to locally as The Valley and A-B-E, is a metropolitan region consisting of Lehigh, Northampton, Berks, and Carbon counties in eastern Pennsylvania and...

: The Bushkill Member which is defined as the basal slate unit. The Ramseburg Member which is composed on alternating turbitide sandstone units with interbeds of shale and siltstone. The Pen Argyl Member, which the upermost slate unit with some minor phyllite and shale beds.

Depositional environment

The shales (and slates) of the Martinsburg were deposited

Sedimentary depositional environment

In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock...

 in a large forearc

Forearc

A forearc or forarc, also called arc-trench gap is a depression in the sea floor located between a subduction zone and an associated volcanic arc. It is typically filled with sediments from the adjacent landmass and the island arc in addition to trapped oceanic crustal material...

 basin resulting in a flysch

Flysch

Flysch is a sequence of sedimentary rocks that is deposited in a deep marine facies in the foreland basin of a developing orogen. Flysch is typically deposited during an early stage of the orogenesis. When the orogen evolves the foreland basin becomes shallower and molasse is deposited on top of...

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

Iapetus Ocean

The Iapetus Ocean was an ocean that existed in the Neoproterozoic and Paleozoic eras of the geologic timescale . The Iapetus Ocean was situated in the southern hemisphere, between the paleocontinents of Laurentia, Baltica and Avalonia...

. Turbidites 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 can be the Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

-East Timor

East Timor

The Democratic Republic of Timor-Leste, commonly known as East Timor , is a state in Southeast Asia. It comprises the eastern half of the island of Timor, the nearby islands of Atauro and Jaco, and Oecusse, an exclave on the northwestern side of the island, within Indonesian West Timor...

 tectonic setting.

Fossils

In the Pen Argyl Member, both Graptolite

Graptolite

Graptolithina is a class in the animal phylum Hemichordata, the members of which are known as Graptolites. Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous...

s and Conodont

Conodont

Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...

s are found. The 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...

 Cryptolithus and the brittle star Protasterina are common the Swatara Gap area.

Age

The Martinsburg was deposited during the Upper Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

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

Hamburg, Pennsylvania

Hamburg is a borough in Berks County, Pennsylvania, United States. The population was 4,114 at the 2000 census. The town is named after Hamburg, Germany.-Geography:Hamburg is located at ....

, an allochthonous klippe

Klippe

thumb|right|350px|Schematic overview of a thrust system. The shaded material is called a [[nappe]]. The erosional hole is called a [[window |window or fenster]]. The klippe is the isolated block of the nappe overlying autochthonous material....

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

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...

. The Shawangunk Formation

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 Formation and Clinton Group has not been conclusively...

, Tuscarora 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.-Description:...

, and the Juniata Formation

Juniata Formation

The Ordovician Juniata Formation is a mapped bedrock unit in Pennsylvania and Maryland. It is a relative slope-former occurring between the two prominent ridge-forming sandstone units: the Tuscarora Formation and the Bald Eagle Formation in the Appalachian Mountains.-Description:The Juniata is...

, all lie unconformably atop the Martinsburg. Below it, the Chambersburg Formations and Myerstown Formations lie on a possible unconformity.

Economic Uses

Slate

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. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...

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.