Abstract

The Peach Bottom slate lies in the center of the Peach Bottom syncline of southeastern Pennsylvania and Maryland. The slate is underlain by the Cardiff conglomerate and the Peters Creek schist. Chlorite is almost as abundant as muscovite in the slate. Chloritoid is common in the Cardiff conglomerate and in many parts of the slate belt, especially in local zones where shearing was more intense. Bedding is almost entirely destroyed, and bedding-cleavage relations cannot be used to work out the structure of the slate. The rocks show well developed flow cleavage (schistosity), fracture cleavage, joints, and lineations. Flow cleavage strikes northeast and parallels the long axis of the Peach Bottom syncline. Fracture cleavage is parallel to axial planes of drag folds and crenulations in the slate. Lineations are parallel to fold axes. They plunge generally northeast to southeast at a moderate to steep angle. Steep cross joints, trending northwest to northeast, are the most abundant type. They exercise a marked control over surface drainage. Arching of fold axes, resulting in cross folds, occurred in several transverse zones at the same time that the syncline was compressed and overturned to the northwest. B-tectonites and B ∧ B′-tectonites are represented. B ∧ B′ is about 72°. B' is approximately the a direction. Where two sets of lineations occur, there has been no uniform overprinting. Lineations are largely contemporaneous. One period of deformation, consisting of several stages, is indicated. Conclusions based on field data are borne out by petrofabric analysis.

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