Research paperOrganic matter accumulation, redox, and diagenetic history of the Marcellus Formation, southwestern Pennsylvania, Appalachian basin

- Trace element and framboid size data elucidate the depositional history of the Marcellus Shale.
- We recognize 4th order sequences in the trace element profiles not obvious in core.
- Periods of bottom water anoxia were triggered by enhanced primary productivity.
- Evidence of a fluctuating chemocline in the depositional history of the Marcellus is demonstrated.
- Anaerobic oxidation of methane formed carbonate concretions and mobilized barium.

Variations in the concentration of redox sensitive elements combined with pyrite framboid size data documented from a Marcellus Formation (Middle Devonian) core recovered from southwestern Pennsylvania elucidate the redox, organic matter accumulation, and diagenetic history of these deposits in this region of the basin. Uranium and Mo enrichment and Fe/Al display sharp increases coincident with diminishing Th/U upward through the initial 3rd order trangressive systems tract (lower Union Springs Member). These data as well as abundant small (<6 μm) pyrite framboids record establishment of strongly reducing benthic conditions, perhaps related to the expansion of an oxygen minimum zone induced by increased surface productivity. Strongly anoxic, even euxinic, conditions were interrupted by episodes of dysoxia, perhaps seasonal or longer term. Overlying regressive systems tract (RST) deposits record modestly improved redox conditions, likely a reflection of a receding oxygen minimum zone as base level dropped. A subsequent 3rd order base level rise and renewed expansion of the oxygen minimum zone triggered by increased surface productivity resulted in the accumulation of the organic-rich lower Oatka Creek Member. Still, the mix of abundant small and subordinate large (>10 μm) framboids preserves the record of oxygen deficient to sulfidic bottom conditions frequently interrupted by episodes of (dys)oxia. Improving redox conditions through the overlying RST were accompanied by a two-fold increase in Al and consequent dilution of the organic matter flux and authigenic trace metal uptake at the sediment-water interface. The upper half of the Oatka Creek comprises a depositional sequence not obvious from core inspection or gamma-ray signature but revealed by Mo enrichment and Al concentration profiles. Diagenetic modification of the Marcellus includes several horizons of authigenic calcium carbonate concretions and marked Ba enrichment. Both features reflect the effects of non-steady state microbial diagenesis within a methane-rich sedimentary column.