A relict sulfate-methane transition zone in the Mid-Devonian Marcellus shale

A Marcellus Shale (Middle Devonian Oatka Creek Formation) drill core from Beaver Meadows, NY, shows ellipsoid-shaped grains with compositions that range from pure barite to intermediate barite-pyrite to pure pyrite. The composition of these grains varies over a depth of a mm to cm in the sample. Mineralogical characterization correlated with in-situ S isotope analysis was performed to better understand the diagenetic conditions under which these grains formed. The average diameter of the barite grains is 95 mu, which is much larger than that of marine barite (∼5 mu) and suggests a diagenetic origin. The mean delta34S of barite and pyrite in grains containing both minerals is 63.61 +/- 0.46 and 1.63 +/- 0.34 permil, respectively.;Results from mass balance calculations indicate that reducing sulfate from barite alone cannot be the sole source of S in the replaced grains: only ∼23% of S in pyrite comes from the dissolution of barite while the remainder derives from an additional source with delta34S = --17.6 +/- 0.6 permil. The source of lighter sulfur likely comes from bacterial sulfate reduction or anaerobic oxidation of methane. Based on our results, we suggest the following model for the formation of the barite-pyrite grains in Marcellus Shale: 1) dissolution of marine barite below the sulfate-methane transition zone (SMTZ) due to the depletion of pore water sulfate, 2) Ba2+ diffusion and re-precipitation as authigenic barite at the SMTZ, 3) burial and partial dissolution of authigenic barite below the SMTZ, 4) movement of the SMTZ (possibly due to variable methane flux) below the depth of the partially dissolved barite, and 5) precipitation of pyrite in the pore volume created by barite dissolution. In this model, the preservation of `relict' authigenic barite fronts indicates the presence of a SMTZ within the sediment. Partial to complete replacement of authigenic barite by pyrite reflects fluctuations in the SMTZ (i.e., redox) during early diagenesis. Our result is consistent with previously inferred redox conditions for the black shale present at the base of the Oatka Creek Formation. We suggest that similar barite-pyrite mineral assemblages can be used to reconstruct paleoredox conditions in similar sedimentary facies.