Tectonic cleavage has developed in Lower Devonian limestones of the Hudson Valley Fold-Thrust Belt in New York State. Morphology and distribution of cleavage in these limestones is controlled by the amount of clay-quartz matrix present and by strain. Only limestones with greater than 10% clay-quartz matrix developed widespread cleavage. X-ray diffraction analyses indicate that the clay matrix was altered during deformation; the width of the 001 illite peak in cleaved lime wackestone is less than that in uncleaved lime wackestone from the same stratigraphic level. A minimum of 10% clay is necessary to provide interconnectivity between sites of dissolution and the free-fluid system. Preliminary analyses of bulk chemistry indicate that calcite is removed from the local rock system during cleavage development, supporting proposals that circulation of fluid through the rock plays a major role in the development of cleavage. Textural evidence suggests that both pressure solution and free-face dissolution contribute to the removal of ions at grain boundaries. Cross-cutting relations indicate that cleavage was initiated early during the development of the fold-thrust belt, but continued to develop during the late stages of folding.
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