Fourteen Coeymans-age patch reefs and bioherms have been identified along the Silurian-Devonian outcrop belt in northeastern Pennsylvania, northwestern New Jersey, and central New York. Detailed petrographic analysis of samples from five reefs has led to development of a regional diagenetic model.

Diagenetic fabrics show evidence of near-surface conditions that developed in five successive stages. The reconstruction of the diagenetic history of the reefs is as follows: (1) lithologic development of the reef facies, (2) syndepositional infilling by detrital micrite in pores, fractures, and cavities to form geopetal structures, (3) early subaerial emergence in which minor amounts of calcite cement were precipitated in a freshwater vadose environment, (4) a regional transgression that briefly resubmerged the reefs, followed by subaerial exposure, which led to a mixing of fresh and marine pore waters resulting in localized development of silica cements and dolomite precipitation, and (5) continued subaerial exposure of the reefs where large-scale freshwater phreatic conditions resu ted in total porosity occlusion by stable low-Mg calcite.

Stage 4 shows the most regional variation in fabrics and conditions. An abundance of various forms of silica has been described in the Coeymans reefs of central New York. These varied forms resulted from fluctuations in pH and salinity. Stage 5 is the most pervasive within the reefs studied. Common phreatic fabrics are syntaxial overgrowths on crinoidal grainstones, drusy calcite rims, and neomorphic spar with crystals coarsening and completely filling the central parts of all remaining pores.

The model developed in this study leads us to infer that Coeymans reefs found in the shallow subsurface would not be favorable hydrocarbon reservoirs. The possibility does exist that localized porosity development occurs in untested reefs within the deeper subsurface portions of the basin.

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