THE NATURE OF THE SETTERS FORMATION IN THE PENNSYLVANIA PIEDMONT

Knopf and Jonas (1929) and Plank and Schenck (1997) describe the Setters Formation near Baltimore, MD and in Delaware as: lower member feldspathic quartz-mica schist; middle member quartzite with thin partings of microcline or muscovite; upper member pelitic schist or gneiss. The Setters is modeled as transgressive clastic sediments deposited on basement gneiss and succeeded by Cockeysville Marble and Wissahickon schist.

Rocks identified with the Setters Formation in Chester County, PA include three lithologies that correspond to members in MD-DE: microcline gneiss – 50-80% microcline with quartz, biotite, and locally abundant post-kinematic muscovite; quartzite – quartz with minor microcline and biotite; pelitic schist – quartz, biotite, muscovite, plagioclase, microcline, sillimanite, garnet and tourmaline. However, the lithologies of the Setters Formation in Pennsylvania do not share the same internal stratigraphy as in MD-DE. The primary Setters lithology is microcline gneiss, which commonly constitutes the entire section between basement gneiss and marble. The microcline gneiss has some trace element and REE characteristics similar to alkaline igneous rocks, and may be of volcanic or volcaniclastic origin. A single body of quartzite, located north of the Avondale Massif, includes layers of microcline gneiss from cm to 2 m thick. Pelitic schist forms a 100 m thick layer within microcline gneiss on the south side of the massif. Furthermore, the stratigraphic relationships are not everywhere a straightforward succession of basement gneiss-Setters-Cockeysville Marble-Wissahickon Formation. A marble layer approximately 500 m thick lies between the quartzite and microcline gneiss of the Setters north of the Avondale Massif. Setters-like metaclastic rocks occur as layers within marble in three quarries. Marble lenses lie within Wissahickon pelitic schist in several locations and within Setters microcline gneiss in another. A lens of rock identical to pelitic Setters lies within Wissahickon pelitic gneiss 5 miles southwest of Avondale.

Assuming these relations reflect original stratigraphy, they suggest that the depositional setting was less stable than transgression onto a passive margin. The complex interlayering also calls into question the definition of these rock units in Pennsylvania.