During Eocene time, about 50 million years ago, large lakes developed in the basins between the Laramide uplifts in the Central Rocky Mountains, in what are now Wyoming, Utah, and Colorado. The adjacent mountains, including what are now the Uinta and Wind River ranges, shed sediment into the lakes. No surprises there.
The sedimentary layers in the Green River Formation that accumulated in these lakes are thin, mostly fine-grained materials, mostly silts and muds, but there were some sands and limy sediments too. The climate was sharply seasonal, with a wet growing season and an arid season, and this is reflected in the sediments that show annual deposits, very much analogous to tree rings. These thin layers are called varves, and they average about two-tenths of a millimeter in thickness. Pretty thin. But they add up to an almost continuous record of time spanning 6 million years, from about 53 to 47 million years ago.
While the lakes were accumulating sediment, there was an extensive volcanic area not too far away. The Absaroka Volcanics were erupting in what is now northwestern Wyoming. Although the Absaroka Volcanics extend into much of Yellowstone National Park, they have nothing directly to do with the Yellowstone geysers and supervolcano. 50 million years ago, when the Absaroka Volcanics were erupting, the Yellowstone Hot Spot probably did not even exist. Eruptions related to subduction went on for millions of years, adding up to a pile more than 10,000 feet thick in places. The fine ash from those volcanoes settled into the lakes where the Green River Formation was accumulating, and they give us minerals that provide accurate age dates on the sedimentation in the Eocene lakes.
|Stingray fossil photo by Didier Descouens,|
used under Creative Commons license.
Besides the fossils, the Green River Formation contains a couple other economic resources. It is one of the world’s most voluminous oil shales. At times, the lakes became anoxic – another possible cause for some of the animal deaths – and the organic matter washing into the sediment did not decompose, but became entrapped in the rock. Oil shale is solid rock – no liquid at all, in contrast to a shale oil, like the Bakken formation, where there is liquid oil, but it is trapped in tiny tiny pores that are poorly interconnected if at all. Oil shale is like the tar sands we talked about last month, but even more solid rock. It’s very energy-intensive to get oil out of oil shale. Techniques are always improving, but I think the energy return on energy invested for oil shale is something like 2 or 3 to 1 – meaning, you spend one unit of energy to get back 2 or 3. For conventional oil resources, that return is 20 to 40. So, while there might be 3 trillion barrels of oil equivalent in place in the Green River Formation, it will be very expensive to get it out. There are no current plans that I know of to exploit this resource, although there is of course research going on.
The other economic resource in the Green River Formation is trona – the world’s largest deposit of trona, by far. What is trona? Sodium carbonate. So what? You make use of trona every day – it is a critical additive to glass, which makes the melting point of silica lower and more manageable, as well as cheaper in terms of the energy cost to produce glass. Virtually all common glass today uses sodium carbonate in its manufacture. About 80% of all the trona in the world is produced from mines in southwest Wyoming. It accumulated in one of the Eocene lakes during a time of extensive evaporation when the chemical conditions were right for its crystallization.
Trona is called soda ash in the industry, and the U.S. produces about 12 million tons of it a year, more than half of which is exported. It’s one of the few mineral products for which the United States is a net exporter, and it’s a $1.8-billion-dollar business. Turkey is a distant second in terms of world production, at about 12% of the total. I have a bit more about trona in my other book, What Things Are Made Of.
—Richard I. Gibson
See also Roadside Geology of the Yellowstone Country, by William J. Fritz and Robert C. Thomas (Mountain Press, 2011)
Stingray fossil photo by Didier Descouens, used under Creative Commons license.