PETROLOGICAL AND ISOTOPIC EVIDENCE FOR DIAGENETIC EVOLUTION IN THE CHERRY VALLEY CARBONATES AND ADJACENT MUDROCKS OF THE MARCELLUS "SHALE" FROM WEST VIRGINIA, PENNSYLVANIA, AND NEW YORK
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The Cherry Valley carbonates are thin, laterally continuous limestones that separate the upper and lower organic-rich mudstones of the Marcellus “shale”. Textural and compositional heterogeneity within these carbonates provide evidence of diagenetic evolution. The Cherry Valley carbonates contrast lithologically and petrophysically with bounding organic-rich mudstones, regionally the East Berne Member of the Oatka Creek Formation (overlying) and Bakoven Member of the Union Springs Formation (underlying), and comprise carbon isotope-depleted, methanogenic nodules and pelagic limestones. Petrologic evaluation of these units indicates the presence or absence of early nodule formation controlled the effect of burial, thermal, and exhumation diagenesis on these carbonates. The isotopic compositions of these nodular carbonates, which were deposited as fossiliferous mudstones, reflect an initial depletion of 13C that was recrystallized by isotopically heavier carbon during burial diagenesis. In equivalent mudstones where nodules were not formed, isotopic compositions are comparable to that of unaltered dacryoconarid fossils which are the dominant source of calcite in these rocks. Three depositional facies, which do not reflect subsequent alteration, are related to the Cherry Valley carbonates: (1) a basal calcareous mudstone facies that are rich in algal cysts; (2) an intermediate limestone rich in pelagic fossils that classically defines the Cherry Valley carbonates; (3) an upper-bounding calcareous mudstone facies. Eight cores from West Virginia (3), Pennsylvania (2), and New York (3) are qualitatively and quantitatively described by petrologic methods, including thin section petrography and scanning electron microscopy, and geochemical analyses to integrate lithological, textural, and spatial relationships among the diagenetic framework of these rocks at a range of scales. Compositional analyses describe the distribution of mineral phases and textures, as determined by petrological observation, and include reflectance spectroscopy, X-ray diffraction, and energy dispersive X-ray spectroscopy. Stable carbon and oxygen isotope compositions of carbonate-derived CO2 gas are measured using off-axis integrated cavity output laser spectroscopy. The general diagenetic sequence for the Cherry Valley carbonates begins with (1) micritization of calcareous mudstones and bedded limestones; (2) methanogenetic formation of calcite or barite nodules; (3) prismatic calcite growth on fossils or allochemical grains (4) crystallization of organogenetic dolomite rhombohedra; (5) pore-filling cements that include sparry carbonates, barite, and pyrite; (6) catagenesis and mobilization of hydrocarbons via fractures and stylolites; (7) late stage calcite crystallization or dolomitization; (8) dedolomitization associated with exhumation.