The diagenetic origin and depositional history of the Cherry Valley Member, Middle Devonian Marcellus Formation

Abstract

We report δ13C, δ18O, δ34S records coupled with elemental, pyrite morphology, and fluid inclusion data spanning the Cherry Valley Member in the Middle Devonian Marcellus shale from two cores located across a depth gradient in the Appalachian basin. The goal of the study was to evaluate the diagenetic and redox history of this carbonate-interbedded layer within a prominent shale gas system. Three lithological units were identified in the Cherry Valley Member: a nodular clay-rich marlstone lower unit, a fossiliferous limestone middle unit, and a calcareous mudstone upper unit. Low δ13C and δ18O values of the carbonate matrix display a mixing trend among marine carbonate, authigenic carbonate, and late stage diagenetic cementation. The petrology of barite nodules and associated δ34Sbarite values suggest that a shoaling sulfate-methane transition zone created by methane advection contributed to the nodule growth and pyritization. Along with other early and late diagenetic processes, this mechanism provided the main carbonate source for the lower unit due to elevated rates of aerobic methane oxidation. The δ34Spyrite values, U and Mo contents, and pyrite framboid size distributions together constrain the redox history of the Cherry Valley Member. An anoxic-euxinic marine environment rather than a fully oxygenated environment continued through the lower unit in areas with deeper paleo-bathymetry until it was interrupted by an episode of dysoxia during the deposition of the middle unit. Basin-scale anoxia was then renewed during a subsequent 3rd order sea level rise in the upper unit. The entire Cherry Valley Member was ultimately altered by warm burial fluid interaction through vertically distributed veins. The combined body of evidence indicates that the duration and spatial distribution of oxygenated marine conditions during the deposition of Cherry Valley Member within the Marcellus Formation may have been overestimated.