Wissahickon Formation

Infobox Rockunit
name = Wissahickon Formation


type = Geological formation
prilithology = Schist
otherlithology = Gneiss and Quartzite
namedfor = The Wissahickon Creek
namedby =
region = Piedmont of eastern North America
caption = A hand sample of the type Wissahickon from the banks of the Wissahickon Creek
unitof = none
subunits = Mt. Cuba, Doe Run schist, Laurels schist, Greystone schist
thickness = unknown
extent = Southeastern Pennsylvania, northern Delaware, Northeastern Maryland
age = Ediacaran to Early Cambrian (Depositional)The Wissahickon Formation is a mapped bedrock unit in Pennsylvania, New Jersey, and Delaware. In Maryland, the term "Wissahickon" is no longer used and has since been divided into several units. [ [http://www.mgs.md.gov/esic/geo/lgepm.html Geologic Maps of Maryland: Eastern Piedmont Metasedimentary Rocks ] ] It is named for the Wissahickon gorge in Fairmount Park, Philadelphia.

Description

The Wissahickon is described as a pelitic schist and gneiss with interlayers of quartzite. Color is highly variable as is the mineralology.Blackmer, G.C., (2005). Preliminary Bedrock Geologic Map of a Portion of the Wilmington 30- by 60-Minute Quadrangle, Southeastern Pennsylvania. Pennsylvania Geologic Survey, Open-File Report OFBM-05-01.0.] A general description for the unit is described as a silver to brown garnet mica-schist. (see photo)

Metamorphic Grade

The highly variable nature of this rock type is also why the metamorphic grade is also complex. The existence of the minerals biotite, garnet, staurolite, and kyanite all imply a low-intermediate to high metamorphic grade. The metamorphic facies, which is described as lower to upper amphibolite facies, implies a moderate to high metamorphic temperature and a moderate pressure.

Age

Depositional Age

The age indicated on the most recent geologic map of southeast Pennsylvania shows the Wissahickon being Ediacaran to Cambrian in age. This age is a relative date since the sediments that created the Wissahickon are highly deformed and went through several deformation events.

Deformation Age

The sediments of the Wissahickon were altered during the Taconic orogeny and most dates do not place the deformation older than the Silurian. Although there is some evidence of Devonian-aged deformation. [Bosbyshell, H., (2001). Thermal evolution of a convergent orogen: Pressure-Temperature-Deformation-Time paths in the central Appalachian Piedmont of Pennsylvania and Delaware: [Ph.D. thesis] : Bryn Mawr College, Bryn Mawr, Pa., 233 p.]

Divisions

The current map sought to divide the Wissahickon into three informal units. The type described here has been the restricted Wissahickon, or its eastern most section that exists in the City of Philadelphia and eastern Delaware County. The Glenarm Wissahickon and Mt. Cuba Wissahickon are two units described in western Delaware and Chester Counties, Pennsylvania. The metamorphic and depositional histories of these two series are different from the Wissahickon type described here..]

Economic uses

The Wissahickon is quarried as a building stone and is used primairly as a decorative stone rather than a weight bearing stone. Although there are numerous old buildings in the Philadelphia area that are constructed almost entirely of this rock.

References

ee also

*http://www.dcnr.state.pa.us/topogeo/openfile/secompmap.pdf
*Geology of Pennsylvania
*Geology of Delaware
*Wissahickon Creek