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Preliminary Analysis of Phosphate Nodules in the Woodford Shale, Late Devonian - Early Mississippian, Southern Oklahoma

Boardman, Darwin Rice, III
The Woodford Shale is an organic-rich, siliceous marine shale with cherty beds and phosphate nodules. Phosphate nodules are a well-known feature of the Woodford Shale that deserves further scrutiny to explain their source, growth and abundance. Geochemical and petrographical analysis of the interior of phosphate nodules as well as the encasing beds was conducted to provide insight into their genesis. Because phosphate nodules grow very slowly, they inherit a chemical signature that reflects the changing chemistry of the surrounding water during their formation. Nodules from two localities were collected and analyzed using thin section microscopy, x-ray fluorescence, x-ray diffraction and coulometric titration. Nodules were chosen based on their external morphology and internal structure. In addition, host shale was analyzed to compare the composition of nodules with the encasing beds. Highly structured nodules that are laminar or circular have a higher concentration of metals with abundance increasing in darker bands. Distribution of metals is symmetrical in highly ordered nodules and predictable. Metals distribution is more random in unstructured nodules that lack symmetrical banding. The decrease of certain metals and loss of structure in nodules is interpreted as representing less favorable conditions for phosphate growth. TOC is reduced in phosphate-bearing shale compared to beds without nodules lower in the section. Radiolarians are especially well preserved in structured phosphate nodules. The opaline biogenic silica of radiolarian tests was relatively unstable, dissolved and re-precipitated as silica minerals including chalcedony and chert.