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.

Juniata Formation Stratigraphic range: Late Ordovician
Outcrop on U.S. Route 522 at Blacklog Narrows southeast of Orbisonia, Pennsylvania.
Type	sedimentary
Underlies	Oswego Formation and Tuscarora Formation
Overlies	Bald Eagle Formation
Thickness	400–1,125 ft (122–343 m)
Lithology
Primary	sandstone, siltstone, shale
Location
Region	Appalachian Mountains
Extent	Maryland, Pennsylvania, Tennessee, Virginia,[1] and West Virginia
Type section
Named for	Juniata River in Pennsylvania
Named by	Darton and Taff[2]

Description

Conformable contact of overlying Tuscarora Formation (white rock, left) with underlying Juniata Formation (red rock, right) at the Narrows along rt. 30 in Bedford County, Pennsylvania.

Sample from roadcut on U.S. Route 322 near State College, Pennsylvania, showing slickensides

The Juniata is defined as a grayish-red to greenish-gray, thin- to thick-bedded siltstone, shale, and very fine to medium-grained crossbedded sandstone or subgraywacke and protoquartzite with interbedded conglomerate.[3][4] The Juniata is a lateral equivalent of the Queenston Shale in western Pennsylvania.

Depositional environment

The depositional environment of the Juniata has always been interpreted as mostly terrestrial or shallow marine deposits resulting in a molasse sequence produced by the Taconic orogeny.

Fossils

Very few fossils exist in the Juniata Formation, but different types of trace fossils such as tracks and burrows can commonly be found.

Age

Relative age dating of the Juniata places it in the Upper Ordovician period, being deposited between 488.3 and 443.7 (±10) million years ago. It rests conformably atop the Bald Eagle Formation in Pennsylvania and the Martinsburg Formation in Maryland,[4] and conformably below the Tuscarora Formation.[5]

Economic use

The Juniata is a good source of road material, riprap and building stone.[6]

References

1. Paleozoic Sedimentary Successions of the Virginia Valley & Ridge and Plateau
2. Darton, N.H., and Taff, J.A., 1896, Description of the Piedmont sheet (West Virginia-Maryland): U.S. Geological Survey Geologic Atlas of the United States, Piedmont folio, no. 28, 6 p.
3. 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.
4. "Allegheny Plateau and Valley and Ridge". Geologic Map of Maryland. Maryland Geological Survey. 1968. Retrieved 2008-01-26.
5. Berg, T.M., et al., (1983). Stratagraphic Correlation Chart of Pennsylvania: G75, Pennsylvania Geologic Survey, Harrisburg, Pennsylvania.
6. Doden, Arnold G.; Gold, David P. (2008). "Bedrock Geologic Map of The Mc Alevys Fort Quadrangle, Huntingdon, Centre, and Mifflin Counties, Pennsylvania" (pdf). Pennsylvania Geological Survey. Cite journal requires |journal= (help)

See also

• Geology of Pennsylvania