DIVIDING THE WISSAHICKON FORMATION IN THE PENNSYLVANIA-DELAWARE PIEDMONT - A PROGRESS REPORT

The Wissahickon Formation underlies approximately 1100 sq. km. in southeastern Pennsylvania and northern Delaware. Recent mapping and analytical work support a tentative division of the formation into three subunits with distinct tectonic origins.

The easternmost subunit includes the type locality and is informally called Type Wissahickon. The rocks are characterized by pelitic and psammitic layering that probably represents original graded bedding. Monazite shows metamorphism at 432 Ma and at 377-369 Ma. Amphibolite interlayers have back-arc basin geochemistry.

The northwesternmost subunit is informally called the Glenarm Wissahickon. It is a pelitic schist in contact with Baltimore Gneiss (Laurentian basement) and its Glenarm Group metasedimentary cover rocks, and includes interlayers of continental initial-rift (CIR) amphibolite. It may have been deposited on the rifted margin of Laurentia prior to initiation of sea-floor spreading.

The origin of the third subunit, informally called Mt. Cuba Wissahickon, is more ambiguous. This subunit is psammitic and pelitic gneiss lying between the Glenarm and Type Wissahickon, and includes amphibolites with both CIR and ocean-floor geochemistry. Psammitic gneiss at Yorklyn, DE, near the center of the subunit’s map extent, yielded a detrital zircon suite (1740-735 Ma) indicating a Laurentian sediment source. The Mt. Cuba in this area is in contact with the Baltimore Gneiss and Glenarm Group, and may also have originated on the rifted margin. Ordovician forearc igneous rocks of the Wilmington Complex intrude the easternmost Mt. Cuba. It may be that the continental sediments were carried to a subduction zone in Iapetus and incorporated into the Wilmington Complex forearc accretionary complex. Alternatively, there may be a cryptic boundary within the Mt. Cuba separating the eastern, arc-related portion from the western portion with Laurentian affinity. Metamorphic ages lend credence to this hypothesis. Monazite from the eastern Mt. Cuba indicates metamorphism at 448-430 Ma and at 480-472 Ma. The older period corresponds to the age of arc magmatism in the Wilmington Complex, suggesting proximity to the active arc. In contrast, monazite from Yorklyn shows metamorphism only at 425 Ma, indicating that it was not affected by arc magmatism.