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In the Delaware Water Gap area of Pennsylvania and New Jersey a thick sequence of Cambrian to Devonian sedimentary rocks unconformably overlies Precambrian gneiss and marble. A major angular unconformity separates Ordovician and Silurian rocks. Fracture cleavage, locally developed throughout the Paleozoic section, reflects at least two orogenies. Slaty cleavage is restricted to the Ordovician Martinsburg formation and predates overlapping Lower Silurian Tuscarora quartzite. The slaty cleavage is the only indication of metamorphism in the Paleozoic rocks.

The slaty cleavage is a near-perfect axial-plane foliation characterized by a high degree of orientation of constituent minerals, chiefly illite and quartz. Geologic evidence relates the slaty cleavage to folding of a thick, rapidly subsiding, water-bearing pelitic sequence. Rapid application of tectonic stress probably induced abnormally high pore-water pressures, approaching lithostatic pressure. Internal friction was thus drastically reduced, and rotation of mineral grains, accompanied by mass transport parallel to axial planes of developing folds, produced the high degree of preferred mineral orientation that characterizes rocks with slaty cleavage. Escape of pore water at a late stage of deformation, facilitated by parallel mineral orientation, probably aided recryst allization of clay minerals and brought about some redistribution of silica and carbonate.

In the Water Gap area, fracture cleavage has developed in relatively brittle siltstone, limestone, quartzite, dewatered shale, and slate. Two types of fracture cleavage are common–one related to inter-bed shear during folding (bed-de-limited fracture cleavage), and another, the false or slip cleavage of the slate belt, apparently related to shearing accompanying alimited differential flowage, essentially the microlithon mechanism of De Sitter.

If the mechanism proposed here is valid, slates form only at one time in an orogenic cycle, i.e., during the initial stages of folding. Furthermore slate is not necessarily an end member of the metamorphic sequence. It may be expected that shales involved in regional metamorphism would be deeply buried and dewatered priorto deformation, hence would pass directly from fracture-cleaved shale to phyllite and schist without passing through a slate stage. If involved in regional metamorphism, slates ordinarily will show a second period of deformation, characterized by development of fracture cleavage.

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