Middle Ordovician carbonate rocks with a variety of mud-supported to grain-supported textures have been sampled in the central and northern Appalachians. Progressive fabric alterations similar to those in older (Cambrian-Ordovician) mud-supported carbonates of the central Appalachians also are observed in the Middle Ordovician rocks. These alterations include increase in matrix (micrite) crystal size, elongation and increased preferred orientation of matrix crystals, increased twinning in echinoderm fragments, and progressive loss of original textures. Scanning electron microscopy of the Middle Ordovician carbonate rocks has revealed other progressive changes in microfabric including increased sinuosity of grain boundaries between matrix crystals.

The progressive alteration observed in these rocks evidently is related to increasing intensity of tectonic deformation. The tectonic factors responsible for the fabric alterations also act to destroy porosity and permeability and, in part, to effect the generation, migration, and ultimate destruction of hydrocarbons. Thus, studies of the fabric changes in carbonate rocks may provide valuable information on the possible occurrence of oil or gas in a basin or in a particular part of a basin being explored, regardless of whether or not the carbonate rocks themselves are potential reservoirs. A straightforward petrographic tool of this type would be especially valuable in exploration in the deeper Appalachian basins. Carbonate fabric studies also may be useful in the discovery and explor tion of metallic deposits such as lead and zinc in the Appalachians.

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