A NEW LOOK AT THE STRATIGRAPHY OF THE PENNSYLVANIA PIEDMONT

New 1:24,000 scale mapping in the Pennsylvania Piedmont (Coatesville 7.5' quadrangle) has led to important changes to stratigraphic interpretation in the area around the Doe Run massif. At the present time, these changes apply only to this area.

The existing stratigraphic sequence has felsic Grenville gneiss at the base, overlain in succession by Setters Quartzite and Cockeysville Marble. These are in turn overlain by the Wissahickon Formation on a contact that has been variably interpreted as sedimentary or tectonic. The “Wissahickon” here is a garnet-, staurolite-, kyanite-bearing schist. Because of uncertain connection to type Wissahickon, it will be referred to as the pelitic schist.

Current mapping moves much of what was previously identified as Setters into the gneiss. A planar-laminated, medium-gray quartz-microcline-plagioclase gneiss with minor muscovite and biotite is reinterpreted as mylonitized felsic gneiss based on mineralogic and textural criteria. Another “Setters” lithology, muscovite schist without porphyroblasts, has been reinterpreted as altered felsic gneiss. Much of the muscovite is post-kinematic, probably replacing feldspar and biotite in the original gneiss. The few remaining quartzite occurrences are found as lenses in the pelitic schist. Likewise, marble has reduced outcrop area on the new map and occurs as lenses within the schist.

To the north, the pelitic schist is bounded by another schist similar to it except for lack of staurolite, better compositional layering, and more retrogression. Wiswall (1990) interpreted this second schist as “low-grade Wissahickon” in the footwall of the Cream Valley Fault. I think it is better explained as pelitic schist retrograded and strained in the fault zone, and place the Cream Valley Fault at the contact between the second schist and the psammitic schist (previously referred to as Peters Creek) immediately north.

Tectonic interpretation: The schist-quartzite-marble assemblage is a fault-bounded slice of sediments deposited on Grenvillian basement during pre-Iapetan rifting. The psammitic schist, now structurally below the pelitic schist, is the uppermost part of the pre-drift sequence.