Conodont and brachiopod biostratigraphic zones are used to divide the Phosphoria Formation and related rocks into 7 discrete time intervals of the middle and Late Permian to examine the general sedimentation patterns of the Phosphoria basin. Three depocenters existed in the middle Permian: one in southwestern Montana, one in northeastern Nevada, and one in central-eastern Nevada and adjacent Utah. The sedimentation occurred in two transgressive phases followed by sea level stillstands. A major regression occurred in late Guadalupian time. The conodont and brachiopod faunas indicate that cooler water existed in the area of maximum phosphorite deposition, generally in eastern Idaho and southwestern Montana.

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