Few workers have considered the origin of lower Paleozoic phosphate-rich strata, especially in the light of recently proposed phosphorite depositional models. Selected samples of limestones and shales from the Cincinnatian Series have been examined to determine the environmental factors contributing to the accumulation of phosphatic sediments.

The conditions of phosphate accumulation have been interpreted from a detailed description of the stratigraphic sequence, sedimentary structures, textures, and fossil content of each locality. Microfacies analysis of the limestones revealed that phosphate is largely confined to intragranular pores of bioclasts (echinoderm debris, juvenile mollusks, bryozoan zooecia). The phosphatized bioclasts are concentrated as basal lag deposits above discontinuity surfaces, as starved ripples within shale beds, and as burrow infillings. The zones of phosphatic concentration directly overlie bioclastic wackestones.

The original phosphatization process probably occurred within the sediments that formed the bioclastic wackestones. The ichnofossils (and, in part, the body fossils) indicate that there was a low rate of sedimentation, high organic input, high initial water content (> 50%), normal oxygen concentrations, and pervasive bioturbation of a muddy substrate. The confining microenvironments necessary for the reducing conditions of phosphate precipitation were the intragranular pores of bioclasts, such as bryozoans, filled with organic-rich muds. Early diagenesis of phosphate took place within the pores, and these relatively denser allochems were subsequently winnowed from the unconsolidated muddy substrate by episodic high-energy events. The resultant deposits were phosphatic sands that al o underwent biogenic disturbance and further physical redeposition.

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