An integrated assessment of a type euxinic deposit: Evidence for multiple controls on black shale deposition in the Middle Devonian Oatka Creek Formation

An integrated lithologic, paleontologic, and multi-proxy geochemical study of the Middle Devonian Oatka Creek Formation, a black shale in the northern Appalachian Basin, indicates that a number of different factors contributed to organic carbon-rich black shale deposition. Conditions leading to this organic-rich sedimentary deposit were ultimately controlled by a relative sealevel rise, dominantly eustatic but with possible contributions from local tectonics, which cut off the supply of carbonate to the basin. Geochemical proxy evidence - such as Mo/Ti, Fe/Ti, C-org, S-py, and delta(34)S(py) - suggests that as sealevel continued to rise after the carbonate supply was cut off, a threshold was crossed at which point conditions in the basin shifted from dominantly anoxic to dominantly euxinic (anoxic-sulfidic bottom waters). Concurrent with the shift to dominantly euxinic conditions, the supply of siliciclastic sediments was cut off, resulting in a condensed horizon, as evidenced by the elemental ratios of Si/Al and K/(Fe + Mg) and the relative concentration of eolian silt as determined petrographically and from scanning electron microscopy. Sediment starvation in the basin appears to have facilitated the biogeochemical (re)cycling of C, N, and P. Specifically, the elemental ratios of C, N, and P and the stable carbon isotope composition of organic matter suggest that the preferential regeneration of P under anoxic conditions (and of N during the oxic phase of oxic/anoxic oscillation) led to enhanced primary production in surface waters, thereby maintaining euxinic conditions in the bottom waters through respiration of settling organic matter. Finally, it is observed that, though conditions in the basin seem to have remained consistently anoxic-sulfidic for some time after the initial shift to euxinic conditions, a progressive increase in siliciclastic sedimentation led to a corresponding decrease in the enrichment of various redox-related elements, illustrating the overriding control that sedimentation can have on geochemical proxy records.