URANIUM DIFFUSION INTO THE PROTO-WOODFORD SHALE

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 processes such as organic complexation, microbial ecology and mineral precipitation remains to be determined, box models of uranium diffusion can be used to constrain sedimentation rates in the rock record.

The Woodford Shale of the US southern midcontinent is an Upper Devonian /Lower Mississippian black shale that spans approximately 32-million years of time, including the Frasnian-Famennian boundary. We have used gamma-ray spectrometry to determine uranium concentrations in several outcrops of the Woodford Shale, and collected hand samples for organic carbon and phosphorus analyses. Based on petrographic and lithologic observations, the highly laminated nature of the Woodford Shale suggests that the overlying Devonian sea was anoxic—at least in the bottom waters—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 preserved thickness of outcrops today, the possibility of past erosion, and assuming reasonable compaction values, sedimentation rates must have varied dramatically during deposition. The gamma-ray results can be used to tune the box model in order to determine temporal changes in sedimentation rate in the Devonian sea in response to climatic or other external forces. This, in turn, will allow quantitative calculations of organic carbon and phosphorus fluxes to these sediments, which are necessary to fully constrain hypotheses concerning paleoenvironmental reconstructions, source-rock development and extinction mechanisms.