The outcrop of the Martinsburg and Shawangunk Formations at Lehigh Gap, Pennsylvania, is an important locality for studying the development of slaty cleavage because a gradual transition from shale to slate exists within a distance of 50 m. Field, thin-section, and x-ray preferred-orientation studies show that in the uncleaved rock in the Martinsburg Formation near the contact with the overlying Shawangunk Formation, white mica is parallel to bedding whereas chlorite is about 20° off bedding. In the transition region farther from the contact, a bimodal orientation results from mica becoming oriented parallel to an incipient cleavage, nearly perpendicular to bedding, whereas chlorite retains its 20° off-bedding orientation. Farther from the contact, where the cleavage is best developed, the chlorite also is reoriented. The distinct difference in the locations at which these two platy minerals are reoriented argues strongly against their preferred orientations being produced by mechanical rotation of pre-existing tabular grains, whether during tectonic dewatering or at high temperatures. Petrographic observations of crystals parallel to cleavage that transect crystals parallel to bedding strengthen this argument. The preferred orientation associated with the cleavage appears to be a result of some solution and recrystallization process that is poorly understood.
This study makes it clear that caution is necessary in using models of mechanical rotation in relating strain to preferred orientations. The fact that other recent work suggests that such models are useful may mean either that slate may form in many different ways or that the fit of such models is fortuitous.