STATUS OF THE COCALICO FORMATION, SOUTHEASTERN PENNSYLVANIA

The greenschist grade Cocalico Formation is a generally equivalent hinterland version of the allochthon-rich anchizone grade Martinsburg Formation in SE Pennsylvania. Stose (1946) included the Cocalico with the “Taconic Sequence in Pennsylvania”, drawing comparisons to the allochthonous rocks in the Great Valley (GV) (foreland) of Pennsylvania where he proposed the “Hamburg klippe”. The Cocalico is now recognized as complexly infolded with the Taconian nappes of the Lebanon Valley. It is composed of shale to coarser clastic rocks, including green and reddish-purple units that Stose called “tuffaceous” (unconfirmed).

The alternative to Stose's Hamburg klippe is allochthons thrust into the Martinsburg foreland basin. There, graptolites and conodonts have been used to separate autochtonous Martinsburg from Taconic allochthons of the Dauphin Formation in the GV. Using this model, the Cocalico would be a segment of the Martinsburg foreland and also a composite terrane containing autochthonous and allochthonous rocks. We consider the clastic rocks transported on the Alleghenian Yellow Breeches thrust as part of the Cocalico sequence (Cocalico North) and separated from the main body of the Cocalico sequence (Cocalico South) by the truncating Mesozoic basin.

Few fossils have been recovered from the Cocalico South. Some dubious graptolites were reported and a single conodont occurrence is identifiable only as Ordovician. Based on appearance alone some of the siliceous “red” units resemble radiolarian cherts in the Taconic allochthons of the GV. Other greywacke/shale units compare to Martinsburg lithologies. The Cocalico north has deformed graptolite-bearing phyllite with an exclusively “biserial” fauna indicating a probable early Late Ordovician age permissive for Martinsburg time. Some rocks in the Cocalico North also resemble Dauphin allochthons north of the Yellow Breeches thrust. Thus, the evidence supports the Cocalico being a metamorphosed segment of the Martinsburg foreland comparable to the allochthon-influenced area in the GV. The degree of allochthony for the Cocalico is however, unknown, and its specific relationship to the rocks in the Great Valley awaits more study.