--> --> Diagenetic Evolution of the Cherry Valley Member of the Oatka Creek Formation, Marcellus Subgroup, New York

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Diagenetic Evolution of the Cherry Valley Member of the Oatka Creek Formation, Marcellus Subgroup, New York

Abstract

Textural and compositional heterogeneity within the Cherry Valley Member (CV) of the Oatka Creek Formation (Middle Devonian Marcellus subgroup) reveal a complex diagenetic history of the Appalachian Basin in New York. The CV represents laterally extensive, nodular offshore carbonates composed of pelagic fauna (e.g., goniatites), and it contrasts both lithologically and petrophysically with its bounding mudstones, regionally the East Berne Member of the Oatka Creek Formation (overlying) and Bakoven Member of the Union Springs Formation (underlying). The highly contrasting interfaces between carbonate and mudstone produce porosity- and permeability-controlled fluid flow that has influenced diagenesis. The CV is compositionally dominated by carbonates, all of which are diagenetic. Early diagenesis includes calcareous nodule formation prior to lithification, followed during burial by distinct generations inclusive of calcite, ferroan calcite, ankerite, siderite, barite, and other minerals likely representing distinct phases. The mudstones above and below have mixed matrices of illitic clays and a variable amount of calcite cement, with higher amounts coinciding with microfossil-rich laminae, and exhibit a divergent diagenetic history from the CV. Organic material is largely restricted to these mudstones and is characterized as highly dispersed, kerigenous residue that coats matrix components including pore walls hosted within clay crystallites and cements. Though the Marcellus subgroup is one of the most studied units in the United States, little work has been done to document the changes in lithology, composition, and texture from one portion of the basin to another as a function of diagenesis. The evolution of diagenesis has largely been studied within discrete zones of the basin, identifying compositional trends without specifying the diagenetic stage to which any mineral or chemical product corresponds. The CV provides a good opportunity to examine diagenetic changes to a basin-wide, contemporaneous unit that was subjected after deposition to a range of burial regimes during the Alleghanian orogeny. Qualitative petrographic descriptions and quantitative compositional analyses, including thin section petrography and scanning electron microscopy, are combined to describe the textural and compositional framework of the rocks at a range of scales. Compositional analyses include X-ray diffraction and energy dispersive X-ray spectroscopy.