Wissahickon Formation | |
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Stratigraphic range: Ediacaran to Early Cambrian (Depositional) | |
![]() A hand sample of the type Wissahickon from the banks of the Wissahickon Creek | |
Type | Geological formation |
Sub-units | Mt. Cuba, Doe Run schist, Laurels schist, Greystone schist |
Underlies | Hardyston Quartzite |
Overlies | Chestnut Hill Formation |
Thickness | unknown |
Lithology | |
Primary | Schist |
Other | Gneiss and quartzite |
Location | |
Region | Piedmont of eastern North America |
Country | United States |
Extent | Southeastern Pennsylvania, northern Delaware, Northeastern Maryland |
Type section | |
Named for | The Wissahickon Creek |
The Wissahickon Formation is a mapped bedrock unit in Pennsylvania, New Jersey, and Delaware. It is named for the Wissahickon gorge in Fairmount Park, Philadelphia.
In Maryland formations, the term "Wissahickon" is no longer used. Rocks in this classification have since been divided into several units, such as Lower Pelitic Schist and Prettyboy Schist. [1]
The Wissahickon is described as a pelitic schist and gneiss with interlayers of quartzite. Color is highly variable as is the mineralogy. [2] A general description for the unit is a silver to brown garnet mica-schist.
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.
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.
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. [3]
The current map sought to divide the Wissahickon into three informal units. The type described here has been the restricted Wissahickon, or its easternmost 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. [2]
Wissahickon schist is quarried as a building stone and is used primarily as a decorative stone rather than a weight bearing stone. However, there are numerous old buildings in the Philadelphia area that are constructed almost entirely of this rock.
Schist is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes or plates. This texture reflects a high content of platy minerals, such as micas, talc, chlorite, or graphite. These are often interleaved with more granular minerals, such as feldspar or quartz.
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C and, often, elevated pressure of 100 megapascals (1,000 bar) or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition. The protolith may be an igneous, sedimentary, or existing metamorphic rock.
Metamorphism is the transformation of existing rock to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 °C (300 °F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface.
Amphibolite is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.
Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure. They are of particular interest to geologists because many granulites represent samples of the deep continental crust. Some granulites experienced decompression from deep in the Earth to shallower crustal levels at high temperature; others cooled while remaining at depth in the Earth.
Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are due to fine grained non-aligned crystals with platy or prismatic habits, characteristic of metamorphism at high temperature but without accompanying deformation. The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.
Blueschist, also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15–30 km (9.3–18.6 mi). The blue color of the rock comes from the presence of the predominant minerals glaucophane and lawsonite.
Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically 300–450 °C (570–840 °F) and 2–10 kilobars (29,000–145,000 psi). Greenschists commonly have an abundance of green minerals such as chlorite, serpentine, and epidote, and platy minerals such as muscovite and platy serpentine. The platiness gives the rock schistosity Other common minerals include quartz, orthoclase, talc, carbonate minerals and amphibole (actinolite).
The Narryer Gneiss Terrane is a geological complex in Western Australia that is composed of a tectonically interleaved and polydeformed mixture of granite, mafic intrusions and metasedimentary rocks in excess of 3.3 billion years old, with the majority of the Narryer Gneiss Terrane in excess of 3.6 billion years old. The rocks have experienced multiple metamorphic events at amphibolite or granulite conditions, resulting in often complete destruction of original igneous or sedimentary (protolith) textures. Importantly, it contains the oldest known samples of the Earth's crust: samples of zircon from the Jack Hills portion of the Narryer Gneiss have been radiometrically dated at 4.4 billion years old, although the majority of zircon crystals are about 3.6-3.8 billion years old.
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