March 19. Ordovician Bighorn Dolomite

Much of what is now North America – especially the western part – was under a warm, shallow sea during most of the Ordovician. The continent was near the equator, and all that warm, shallow water undoubtedly contributed to the proliferation of life that we see in the Ordovician. In the west, toward the end of the Ordovician, a wide carbonate bank developed, similar in some ways to the shallow waters off the west side of Florida today. We call the rocks that lithified from those sediments the Bighorn Dolomite for prominent outcrops in the Big Horn Mountains of Wyoming. The carbonate platform where the Bighorn and equivalent rocks were deposited was much larger than the Florida coast – it extended from what is now Yukon Territory in Canada to northern Mexico. All of that area was pretty much tropical to sub-tropical during the Ordovician.

Wide, flat, shallow water zones are sensitive to subtle changes in sea level. As planet earth approached the end of the Ordovician, polar ice caps were growing, on the way to a major glacial epoch that probably contributed to the mass extinction at the end of the period. Details of cycles in the sediments of the Bighorn Dolomite coincide quite well with the sea level changes related to the initiation of this ice age.

The Bighorn Dolomite is pretty pure dolomite. Dolomite, you recall, is calcium magnesium carbonate – almost the same as calcite, calcium carbonate, which is the mineral that makes limestone. Dolomite has that added magnesium, which can be added during lithification, the process that turns soft sediment into hard stone. Adding magnesium to the molecular structure makes the rock more porous, and the Bighorn is a useful aquifer in places.

Out here in the arid west, limestones and dolomites are resistant rocks. In rainy country, like the Midwest and eastern parts of the United States, such rocks tend to dissolve in the weakly acidic rain water – and that’s not really modern acid rain, but a very weak acid, carbonic acid, created when rain falls through the atmosphere and reacts with carbon dioxide. Acid, even weak acid, dissolves carbonates eventually – and that takes a lot longer where there isn’t much rain. So here in Montana and Wyoming, carbonates make prominent ridges, and the Bighorn Dolomite is no exception. In places like the Tensleep Canyon on the west flank of the Big Horn Mountains along highway 16, the Bighorn is a near vertical cliff that adds to the scenic beauty of that drive – a drive that I recommend highly. In some places, the Bighorn is as much as 400 feet thick – but in many locations, the Bighorn Dolomite is not present – not because it was not deposited, but more likely because it was eroded away during later times when the land was uplifted above sea level by mountain-building events.

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The mineral dolomite, calcium magnesium carbonate, which comprises the rock dolomite or dolostone, was named for Déodat Gui Sylvain Tancrède Gratet de Dolomieu, who lived from 1750 to 1801. He was a Knight of Malta, Professor of Mineralogy, and geologist to Napoleon. He wasn’t quite the first to recognize the mineral dolomite, but he got the credit and the mineral was named for him during his lifetime. He found his specimens in the Dolomites, a part of the Alps also named for him. In politics, he helped engineer the surrender of the island of Malta to Napoleon, which pissed off the Grand Master of the Knights of Malta. Dolomieu was imprisoned for 21 months in solitary confinement. He was freed, but in broken health, died at the young age of 51.

—Richard I. Gibson

Further reading
http://jsedres.geoscienceworld.org/content/82/8/599.abstract