CLEAVAGE ASSOCIATED WITH CYLINDRICAL AND CONICAL FOLDS IN THE ORDOVICIAN WINDSOR TOWNSHIP FORMATION NEAR KUTZTOWN, PA

Meter- to decimeter-scale folds in an outcrop of interbedded sandstone and slaty shale near Kutztown, PA are associated with multiple cleavage orientations in three structurally distinct domains. Although all of the domains experienced the same deformation history, the sets of cleavage display different relationships to bedding and fold geometry from one domain to the next. We explore the significance of the cleavages with respect to progressive deformation and finite strain in the outcrop.

Cleavage in Domains 1 and 3 is associated with the limbs and hinge zones of both conical and cylindrical, reclined, SW-plunging anticlines and synclines in slaty shale. The limbs of conical folds display bedding-cleavage intersection lineations that differ by approximately 30° from limb to limb, so that the lineations form a V-shaped pattern pointing toward the apex of the cone. The intersection lineation is absent in the hinge zone of conical folds. Bedding-cleavage intersection lineation associated with cylindrical folds in Domains 1 and 3 also occupies different orientations on the fold limbs, but the intersection lineations meet at the fold hinge and overlap to form a V-pattern with a 30° apical angle. A third cleavage, approximately perpendicular to fold hinges, is poorly developed in both Domains 1 and 3. Domain 2, which lies between Domains 1 and 3, contains west-dipping sandstone layers that are not folded. Three distinct, planar cleavage orientations are prevalent throughout Domain 2, however, their relative timing relationships are not evident based on field observations.

Several hypotheses may account for the multiple and variable expressions of cleavage in the outcrop. For example, the V-shaped cleavage on the limbs of conical folds could be an expression of local-scale stress orientations during conical fold tightening. Alternatively, cleavage may develop in different orientations on previously-folded surfaces during superimposed folding events. It is also possible that a later-formed cleavage can overprint an earlier cleavage during subsequent deformation events, particularly in different lithologies. Future work will include examination of microscopic characteristics of the multiple cleavage orientations in each domain to help determine the specific mechanisms of cleavage development.