The

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

Martinsburg Formation (Om) is a mapped

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

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, ...

New Jersey New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delaware ...

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 ...

Virginia Virginia, officially the Commonwealth of Virginia, is a state in the Mid-Atlantic and Southeastern regions of the United States, between the Atlantic Coast and the Appalachian Mountains. The geography and climate of the Commonwealth ar ...

, and

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 ...

. It is named for the town of

Martinsburg, West Virginia Martinsburg is a city in and the seat of Berkeley County, West Virginia, in the tip of the state's Eastern Panhandle region in the lower Shenandoah Valley. Its population was 18,835 in the 2021 census estimate, making it the largest city in the E ...

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—a chain of valley lowlands—and the central feature of the Appalachian M ...

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 (), known colloquially as The Valley, is a geographic region formed by the Lehigh River in Lehigh County and Northampton County in eastern Pennsylvania. It is a component valley of the Great Appalachian Valley bound to the no ...

and

Lebanon Valley The Lebanon Valley is a geographic region that lies between South Mountain and the Ridge and Valley Province of eastern Pennsylvania. The valley lies almost entirely within Lebanon and Berks counties in Pennsylvania. Portions of the valley lie ...

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 minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

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 rock. ...

. 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 minerals ...

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

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 to th ...

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 foliated metamorphic rock created from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation.Stephen Marshak ''Essentials of Geology'', 3rd ed. It is primarily compo ...

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 Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the vol ...

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 (; ) was an ocean that existed in the late Neoproterozoic and early Paleozoic eras of the geologic timescale (between 600 and 400 million years ago). The Iapetus Ocean was situated in the southern hemisphere, between the paleoco ...

Turbidites A turbidite is the geologic 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 Turbidites were ...

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 Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

East Timor East Timor (), also known as Timor-Leste (), officially the Democratic Republic of Timor-Leste, is an island country in Southeast Asia. It comprises the eastern half of the island of Timor, the exclave of Oecusse on the island's north-weste ...

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 Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and ...

s and

conodont Conodonts (Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, which ...

s are found. The

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 ...

Cryptolithus ''Cryptolithus'' is a genus of extinct 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, the Czech Republic, Morocco and Tu ...

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 locomo ...

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 Sw ...

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_(geology).html"_;"title="nappe._The_erosional_hole_is_called_a__window_(geology)">window_or_fenster._The_klippe_is_the_isolated_block_of_the_nappe_overlying_aut ...

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 erosional or non-depositional 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 o ...

. The

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

, 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, USA. Description The Tuscarora is a thin- to thick-bedded fine-gr ...

, and the

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 ...

, all lie unconformably atop the Martinsburg. Below it, the

Chambersburg Formation The Chambersburg Formation is a geologic formation in Virginia. It preserves fossils dating back to the Ordovician period. See also * List of fossiliferous stratigraphic units in Virginia * Paleontology in Virginia Paleontology in Virginia refe ...

s 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 rock. ...

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