UNDERSTANDING ONE HUNDRED YEARS OF DEBATE ON THE GEOLOGY OF THE WOODVILLE DOME AND THE DOE RUN THRUST, COATESVILLE QUADRANGLE, SE PENNSYLVANIA PIEDMONT

Nearly 100 years ago, Bliss and Jonas (1916) presented a map of the Woodville dome in the SE PA piedmont that described the contact between the Wissahickon schist and the underlying Precambrian gneiss and its quartzite-marble cover as the Doe Run thrust. In the late 1920’s they reinterpreted the same contact to be conformable, placing a major regional thrust at the Martic line about 10 miles to the north. This latter interpretation formed the basis for the famous paper by Bailey and Mackin (1937) on using the down-the-plunge view to interpret regional structure. In the early 1960’s, McKinstry (1961) and Mackin (1962) debated the accuracy of the Bailey-Mackin model of the Woodville dome, relying on different data sets based on the northeastern and southwestern margins of the dome for their respective models. Neither considered the possibility of a significant discontinuity between the Wissahickon schist and the underlying rock. In the 1990’s Alcock (1994) reinvestigated the area and concluded that the structural relationships observed around the Woodville dome are most consistent with the schists having been thrust across previously folded rocks and so resurrected the Doe Run thrust of Bliss and Jonas. In 2004 Blackmer excluded the Doe Run thrust from a new geologic map of the Woodville dome that was published by the PA Geologic Survey.

Despite 100 years of investigation, easy accessibility and relatively abundant outcrop, the Woodville structure remains controversial. The author proposes that the problem results from two critical aspects of the structure. First, the Doe Run thrust and the structures of the underlying gneiss and cover are not conformable, and second, the foliation of the schist manifests the strain ellipse during Doe Run thrusting and is not parallel to formational boundaries. Thus, it runs against geologists’ strong bias to visualize planar structure as paralleling contacts. Recognition of these controls on the dome’s structure allows one to draw consistent cross sections without the special considerations that are required by models inferring a conformable contact and to explain metamorphic discontinuities that occur at the base of the Wissahickon schist elsewhere in the PA-DE piedmont.