Geochemical and textural evidence for early (shallow) diagenetic growth of stratigraphically confined carbonate concretions, Upper Devonian Rhinestreet black shale, western New York

Laterally persistent carbonate concretionary horizons are conspicuous to the Upper Devonian (Frasnian) Rhinestreet black shale of the western New York State Appalachian Plateau. Field observations, including randomly tilted concretions and differential compaction of host sediment laminae around concretions, are consistent with early diagenetic growth in unconsolidated sediment. Further, estimates of pre-cementation host sediment porosity based on the volume percentage of CaCO3 cement (74–93%) and, perhaps most importantly, the preservation of a cardhouse clay fabric in mudstones collected from concretion pressure shadow regions suggest that concretionary growth occurred rapidly within perhaps a meter of the seafloor. Petrographic examination (SEM) of matrix carbonate samples collected from six concretions reveals a relatively porous (∼4–7%) framework of calcite micrite and microsparite, euhedral pyrite (most abundant near concretion rims), disseminated pyrite framboids and randomly oriented detrital clay platelets and domains. C isotopic compositions of Rhinestreet concretion matrix carbonate range from −13.9‰ to +1.7‰ PDB suggesting derivation of C from a mixed source that included depleted and isotopically heavy C. Slightly to moderately depleted O isotopic compositions (−3.8‰ to−6.8‰ PDB) do not corroborate the shallow depth of concretion growth suggested by field and textural observations. The vertically confined nature of most Rhinestreet concretions, their large size, oblate ellipsoidal shape, variable C isotope compositions and absence of any evidence that concretions were sited at local concentrations of organic matter suggest an origin by anaerobic CH4 oxidation. Early pre-concretion sulfate reduction and associated pyrite framboid formation that occurred throughout the host sediment was renewed in thin, shallow zones of uncompacted sediment—the concretionary horizons—by CH4 and isotopically heavy dissolved carbonate diffusing upward from the zone of biogenic methanogenesis to the base of the sulfate reduction zone, probably little more than a meter below the seabed. Local variations in the mix of isotopically light CH4 and heavier dissolved carbonate may have been responsible for the wide range of δ13C isotopic compositions of Rhinestreet concretions. Crucial to this mechanism is a pause in deposition that would have held the zone of carbonate precipitation at a fixed distance below the ocean floor long enough for the large concretions to grow. Evidence that sedimentation slowed or even stopped during concretion growth includes (1) laminae that can be traced through concretions with no observed systematic changes in thickness and (2) minimal concretion center-to-edge variations in estimated pre-cementation porosity. A resumption of sedimentation and related burial brought concretion growth in each horizon to an end. The moderately depleted δ18O values of Rhinestreet concretions probably reflect the “resetting” of O isotopic compositions by warm fluids migrating through the permeable concretions during burial.