August 10. Phosphoria Formation

The Phosphoria Formation of western United States is a Permian package of rocks that’s a real mixed bag in terms of rock types. There are sandstones, limestones, black oil shale, and bedded chert. In some places there’s also dolomite, green and gray shale, and even salt in some Phosphoria equivalents. Chert, you recall, is silica, SiO2, in a very fine-grained form. Chert can form in sediments in various ways, including as a residual deposit made up of the siliceous shells of diatoms and radiolarians, microscopic plants and animals, as well as sponge spicules, the siliceous structures that sponges developed to help support their soft bodies. The Phosphoria formation contains some of the thickest bedded chert in the world, with individual beds only 6 to 10 inches thick, but adding up to many hundreds of feet of chert altogether.

The Permian in the western United States was a time of alternating transgression and regression of the sea, perhaps related to the variations in ice sheets in the southern continent of Gondwana. This gives rise to couplets of near-shore sandy and silty rocks with alternating shallow-water carbonates. In parts of the Bighorn Basin of Wyoming, these rocks serve as important oil and gas reservoirs.

Phosphoria ooids (grains about 1.5 mm across).
Photo by Richard Gibson.

The Phosphoria gets its name from an unusual rock – phosphorite, or phosphate rock. Phosphorus in the Phosphoria is sometimes in shales and mudstones, but it also occurs as pellets and pebbles of rounded calcium phosphate, more or less the mineral apatite, which also makes up your bones and teeth.

Phosphorus is a vital element for life, and phosphorus compounds are among the main components of fertilizer. How it accumulated into extensive, in fact world-class reserves during Permian time is a subject of some debate. Deep oceanic abyssal plains or shallower but low-oxygen zones on the continental shelf sometimes contain phosphorus, the result of dying animals and plants accumulating there, but the large round grains of phosphatic material in the Phosphoria must have been rolled around by waves to get the geometry and structures we see, including oolites and oncolites, round to oval clasts that can be anywhere from coarse sand size up to a couple inches or more long.

One idea for how the phosphorus got into shallow water is through upwelling oceanic water that could have brought deep-sea phosphorus into shallower water where it precipitated into nodules or as cement in sand. It could also have replaced some of the carbonate if that was the primary deposit that was forming in a particular area. Some phosphate is scattered as grains within chert. All of this may have been made possible by the fact that much of interior North America, which in Permian time was part of the supercontinent of Pangaea, was an arid climate. Prevailing winds blowing offshore could have carried nutrients from the land to support abundant offshore life that fell to the sea floor when it died, providing the source for phosphorus that was brought up to shallow zones by those upwelling currents. There’s a situation similar to this today offshore Namibia in southwest Africa, where nutrients from the onshore desert get into the ocean to support extensive life there.

There are other sources for phosphorus, including bird guano deposits, but the phosphate rock of western United States was once the primary source of phosphorus for fertilizer. Mining began in Idaho in 1906, and by the 1970s, phosphate mines in Idaho, Wyoming, Montana, and Utah produced more than 5 million tons a year. There is still some mining of phosphate rock in Idaho and Utah, but today about 85% of US phosphate comes from much younger deposits in Florida and the Carolinas. The US mines around 30 million tons of phosphate rock a year, but that’s not quite enough to satisfy demand, and we import some from Morocco – which is the world’s third largest producer, with three-quarters of all the known reserves. 

In the US phosphate rock is about a $3 billion business, putting it into the top 10 mineral businesses by value. I have quite a bit more about phosphate rock in my other book, What Things Are Made Of.

—Richard I. Gibson

References:
Bighorn Basin oil and gas  

A summary of the stratigraphy and depositional setting of Paleozoic rocks in the Dillon area, by R.C. Thomas and S. Roberts, 2007, Tobacco Root Geological Society Guidebook.

Photo by Richard Gibson