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

August 9. Permo-Carboniferous glaciation

Glacial development in the southern continent of Gondwana began during the Carboniferous. We attributed the cyclic nature of coal cyclothems to alternating high- and low-stands of the sea related to advance and retreat of glaciers. The glacial period lasted into the Permian, so it is usually called the Permo-Carboniferous glaciation, and it lasted close to 90 million years, the longest glacial epoch in Phanerozoic history, the past 550 million years.   

Some of the best evidence for the glaciers comes from South Africa, where rocks called tillites are many hundreds of feet thick. Till is the poorly sorted deposit of material left behind by a glacier, and is has a distinctive chaotic character. You can get till-like deposits in various ways, however, but in addition the cobbles and pebbles within till are often striated – scratched by other rocks held firmly in glacial ice. Together with broader geometry of the deposits, including striated pavements over which glaciers carrying rocks flowed, we’re sure that these sediments were laid down by glaciers.

Permian Gondwana reconstruction and inferred ice cap (blue outline) from A. du Toit, South African geologist, 1937 (Our Wandering Continents). Du Toit's work was remarkably prescient. The map above differs only slightly from modern reconstructions of Gondwana.

The tillites in South Africa have been well known for more than 100 years. South Africa lay in the heart of Gondwana, well inland and probably at a relatively high elevation, and not far from the South Pole. This made for conditions favoring snow and ice accumulation. The ultimate causes of the Permo-Carboniferous glaciation aren’t completely clear, but the presence of a large polar continent is almost certainly an important factor. Other factors include the overall geometry of seaways and precipitation patterns. As Pangaea formed and equatorial seaways closed, oceanic circulation changed. Interiors of continents, distant from the sea and especially in mid-latitudes like the Sahara today would see low precipitation, and in cold climates the precipitation that did fall would likely be snow.

Details of stratigraphy in the South African tillites suggest that there may have been at least four distinct glacial periods within the overall glacial epoch, lasting 5 to 7 million years each, with intervening interglacial periods.

Water frozen in ice obviously affects sea levels, as we’ve said, but it can also affect carbon dioxide levels in the atmosphere by locking oxygen in water into the ice. Or was it the other way around? In late Carboniferous time CO2 levels were falling dramatically, perhaps partly as a result of the vast extent of plant life around the globe. Did that reduce greenhouse conditions, cooling the planet and enhancing the growth of glaciers? The glacial maximum appears to have been in early Permian time, and the glaciation seems to have pretty much ended later in the Permian, coinciding with a rebound in CO2 levels. The interplay of all these factors is still under considerable investigation.

Rocks virtually identical to the glacial deposits of South Africa were found in South America, East Africa, Antarctica, India, and Australia, and this discovery was another nail in the coffin helping to define the supercontinent of Gondwana. Together with fossil evidence like glossopteris that we talked about yesterday, the concept of a continuous sheet of ice was important to the idea that the southern continents had once been assembled into one landmass.

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On this date, August 9, 1138 a.d., an earthquake destroyed the city of Aleppo, Syria. The generally accepted death toll, 230,000, makes it one of the deadliest earthquakes in history. Aleppo sits more or less at the end of a transform fault that is the boundary between the Arabian Plate and Africa, right where that fault impinges on the Anatolian Plate in Turkey, a small block that is more or less amalgamated to the Eurasian Plate. This tectonic activity is obviously modern, but its heritage goes back to the Permian, when Africa, as part of Gondwana, was beginning to impact Eurasia. It’s still doing that, 270 million years later.

—Richard I. Gibson

Links:
Permian glaciation 
Permo-Carboniferous glaciation
Gondwana glaciation 
Crowell, 1978

August 8. Glossopteris

Glossopteris leaves

One of the first lines of evidence leading to the acceptance of continental drift was a shrub called glossopteris. It was a seed fern that lived during Permian time (with some questionable successors in the Triassic). They were among the dominant plants of the Permian, and contributed significantly to the Permian coal beds that are important reserves today in South America, South Africa, India, Australia, and even Antarctica, where they were collected by Robert Scott’s expedition. That’s right, despite the episode titled “The End of Coal” last month, plants did continue to make coal under the right circumstances.  

Glossopteris seeds were large – too large to be distributed by the wind. That observation and the presence of glossopteris across the southern continents that are now widely separated led Austrian geologist Eduard Suess to suggest that the southern continents had once been attached, either via land bridges or in a single supercontinent, Gondwana. It wasn’t much of a leap to connect Gondwana with the other continents in Pangaea, although it was tectonic and stratigraphic evidence that led to that, rather than glossopteris. Some glossopteris-like fossils have been found in the northern continents, but I think the general consensus is that it was restricted to the southern continent of Gondwana.

Glossopteris distribution (dark green) across Gondwana

The plant was a shrub or tree, with some as tall as 100 feet. The name means “tongue-leaf” for the simple lobate shape of the leaves.

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Benjamin Silliman was born August 8, 1779, in New Stratford, Connecticut. He was a professor of chemistry and geology at Yale, where he built up the mineralogy collection that formed the core of the Yale Peabody Museum. He was one of America’s first real science professors, and he established the American Journal of Science, the oldest scientific journal in the U.S.

—Richard I. Gibson

Photo by Daderot (public domain) 

Glossopteris distribution map by Petter Bøckman (public domain, via Wikipedia)