Phosphoria Formation, Permian geologic formation in western Idaho and Wyoming, United States.
The Phosphoria Formation is a Permian-era rock layer spanning several western states, composed of phosphorite, black shale, chert, and limestone. The layers extend hundreds of meters deep beneath the landscape surface.
The formation developed about 280 million years ago during the Early Permian period, when marine sediments accumulated in a basin between the continental margin and the North American shelf. This sediment buildup created the rock layers visible today.
The formation holds fossils of brachiopods and crinoids along with teeth from the prehistoric shark Helicoprion, revealing what marine life looked like in ancient oceans. These remains help visitors understand the creatures that once inhabited these waters.
The formation has no single outcrop to visit; its phosphorite beds were mined for phosphorus, mainly for fertilizer, in southeastern Idaho, northern Utah, western Wyoming and southwestern Montana. The thickest section lies in the Sublett Range of southern Idaho, where a basal phosphorite is overlain by chert and cherty sandstone. Elsewhere the units thin eastward and pinch out in the Green Mountains of Wyoming.
Most tooth whorls of the cartilaginous fish Helicoprion come from this formation, including the only known specimen with preserved cranial remains. Its Early Permian age rests mainly on conodont biostratigraphy.
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