The Cacapon Mountain anticline of the folded Appalachian Mountains contains rocks ranging in age from Early Silurian to Early Devonian. The Middle Silurian Bloomsburg Formation approximates a thin, viscous plate embedded in a less viscous medium. The presence of scolithuslike burrows, useful as strain markers, makes the Bloomsburg Formation an excellent unit for studying the folding of a sedimentary plate.

Examination of the microscopic fabric of rock containing a so-called “fracture cleavage” reveals the presence of discontinuous, platy zones containing tectonically oriented quartz and mica fragments. It is deduced that the shape and orientation of the quartz fragments is largely due to differential solution; however, there is some evidence of mechanical rotation of the fragments. As the “fracture cleavage” does not fit the classical definition (being locally penetrative), it has been designated Sp cleavage for the purposes of this paper.

Finite strain analysis shows that the cleavage postdates lithification, that it formed perpendicular to bedding and parallel to the plane of the finite strain ellipsoid containing λ1 and λ2, and that it is a plane of flattening. The Sp cleavage is interpreted as a phenomenon marking a transition point in the material behavior of the rock. The transition point was reached after the rock underwent a maximum two-dimensional irreversible strain of ε = 0.06. This small strain is in contrast to the values of ε = 0.30 that have been suggested for slaty cleavage. During progressive deformation, the Sp cleavage became structurally passive, permitting it to be used as a strain marker in the determination of strain about fold profiles (ac surface).

Limited field evidence suggests that the preferred orientation shown by burrow cross sections may be controlled by paleocurrent directions, making the burrows a possible current indicator. Finally, it is suggested that the fabric imposed by the burrowing organisms may have localized and controlled the development of the Sp cleavage.

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