Abstract

Midcontinent Pennsylvanian phosphate nodules (concretions), that occur in widespread black and gray shales, have not been largely affected by reworking processes that commonly enrich other phosphate deposits with respect to P. A sequence of events consisting of 1) upwelling, 2) development of a prolific pelagic biota, 3) decay of dead pelagic organisms and fecal material that released P to the interstitial waters of the black muds, and 4) cementation by apatite, pyrite, quartz, and calcite is proposed to have formed the phosphate nodules prior to compaction of the enclosing shales. Phosphate nodules are best developed at the top (and sometimes bottom) of black-shale units, indicating that nodular growth was enhanced near the boundary between anoxic and disoxic conditions. The conditions under which these phosphates formed are generally similar to those cited for Holocene pericontinental phosphorites. This study provides an example of an epicontinental phosphate deposit that fits Cook and McElhinny's (1979) spatial and temporal model for distribution of pericontinental phosphorites.

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