Structural and temporal interpretations of the Wissahickon Formation: the first 100 years

Blackmer, Gale C., Bureau of Topographic and Geologic Survey, 3240 Schoolhouse Rd, Middletown, PA 17057.

Two main themes run through the history of structural interpretation of the Wissahickon Formation: the nature of its basal contact; and the model used to describe regional structures of the Wissahickon and associated basement massifs. Structural interpretation throughout is influenced by current ideas about stratigraphy, formation age, and structural models.

Bascom (1902) assigned a Silurian age to the Wissahickon, noting that it conformably overlies “limestone” correlated with the Chester Valley limestones, known by fossils to be Cambrian-Silurian. By 1909, she had decided that metamorphism and “eruptive material” within the Wissahickon required it to be Precambrian, lying above the younger rocks in overturned synclines. Bliss and Jonas (1916) changed the basal contact to an overthrust, analogous to structures in the southern Appalachians, Scotland and Scandinavia. In 1923, the same authors assigned the Wissahickon and all underlying metasediments to the Precambrian, abandoning the overthrust and interpreting all contacts as depositional. Bascom and Stose (1932) kept these ages, showing the regional structures as asymmetric, overturned folds. McKinstry (1961) and Mackin (1962) were influenced by Alpine structures in their classic interpretations of the basement massifs as refolded nappes. Using modern thermobarometric techniques, Alcock (1994) identified a metamorphic discontinuity at the base of the Wissahickon, and determined that it could not be in stratigraphic contact with the underlying metasediments. Field observations and local gravity surveys led him to reinstate the overthrust. In a regional mapping study, Blackmer (2004) found no structural discontinuity and some evidence for stratigraphic transition at the base of the Wissahickon. She removed the overthrust, and interpreted the basement massifs as sheath folds and hanging-wall antiforms.

The depositional age of the Wissahickon remains uncertain. Metamorphism dated by monazite growth gives an Ordovician upper limit. Interlayered amphibolites with continental initial-rift geochemistry suggest an Iapetan rifting association and Neoproterozoic-Cambrian age. Despite our modern isotopic techniques, we have barely improved on Bascom's 1902 conclusion that the Wissahickon “…is either pre-Cambrian or Lower Silurian.”
