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Constraints on Sedimentation Rates of the Woodford Shale from Modeling of Uranium Diffusion
Anna Cruse1, Stanley T. Paxton2, Michael (Mike) Aufill 3
1Oklahoma State University , 2Unitted States Geological Survey Oklahoma Water Science Center , 3Newfield Exploration
Uranium is a redox-sensitive element that is known to be enriched in black shales deposited in anoxic environments. The details of the reaction mechanism by which uranium is fixed in sediments, and ultimately preserved in rocks remain the subject of study today. Clearly, however, a major factor controlling the accumulation of uranium in sediments is its diffusion from an oxic water body into euxinic porewaters across the sediment-water interface. This diffusion is driven by the development of a concentration gradient as uranium is fixed in sediments. While the relative importance of other biogeochemical processes remains to be determined, box models of uranium diffusion can be used to constrain sedimentation rates in the rock record.
We have used gamma-ray spectrometry to determine uranium concentrations in several outcrops of the Woodford Shale, and collected hand samples for organic carbon analyses. The highly laminated nature of the Woodford Shale suggests that the bottom waters of the Devonian sea was anoxic during deposition. We have applied a simple two-dimensional ocean-sediment box model to investigate sedimentation rates. Assuming the slowest sedimentation rates, model results indicate that the highest observed uranium concentrations were fixed in the sediments in less than 10,000 years. Considering the preserved thickness of outcrops today, and assuming reasonable compaction values, sedimentation rates must have varied dramatically during deposition. These results may also suggest the possibility of previously unrecognized nonconformities in the rock record. Ultimately, this work will allow calculations of sedimentary organic carbon fluxes, which are critical improvements to current exploration models.Uranium is a redox-sensitive element that is known to be enriched in black shales deposited in anoxic environments. The details of the reaction mechanism by which uranium is fixed in sediments, and ultimately preserved in rocks remain the subject of study today. Clearly, however, a major factor controlling the accumulation of uranium in sediments is its diffusion from an oxic water body into euxinic porewaters across the sediment-water interface. This diffusion is driven by the development of a concentration gradient as uranium is fixed in sediments. While the relative importance of other biogeochemical processes remains to be determined, box models of uranium diffusion can be used to constrain sedimentation rates in the rock record.We have used gamma-ray spectrometry to determine uranium concentrations in several outcrops of the Woodford Shale, and collected hand samples for organic carbon analyses. The highly laminated nature of the Woodford Shale suggests that the bottom waters of the Devonian sea was anoxic during deposition. We have applied a simple two-dimensional ocean-sediment box model to investigate sedimentation rates. Assuming the slowest sedimentation rates, model results indicate that the highest observed uranium concentrations were fixed in the sediments in less than 10,000 years. Considering the preserved thickness of outcrops today, and assuming reasonable compaction values, sedimentation rates must have varied dramatically during deposition. These results may also suggest the possibility of previously unrecognized nonconformities in the rock record. Ultimately, this work will allow calculations of sedimentary organic carbon fluxes, which are critical improvements to current exploration models.AAPG Search and Discover Article #90097 © 2009 Tulsa Geological Society, Tulsa, Oklahoma
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