Back on April 7 when I first mentioned coral reefs, I also mentioned that you could find them around the edges of the Michigan and Illinois Basins. Today, let’s get into that a bit more.
I’ve also mentioned that North America lay along the equator during part of the Silurian. By mid- to late Silurian time, it had moved south of the equator to southern latitudes, the tropical zone where deserts like the Sahara form. So it was hot.
The Michigan Basin is that bull’s-eye geologic depression centered on the lower peninsula of Michigan. It was a bowl-like depression in the Silurian too, and around the shallow rim of the basin, on the lip of the deeper central part, coral reefs grew. The shallow platform behind the reefs was a fairly typical Paleozoic shallow water sea for some time, with the standard cast of characters such as crinoids and brachiopods.
With gentle uplift, the shallow platforms may have become dry land. The deeper sea, in the middle of the basin, was largely encircled by the edges of the platforms, and even more restricted by the reefs growing there. The result was a nearly circular sea – really a large lake – that was cut off from oceanic circulation. So combined with a really hot climate to evaporate more water than came in from rivers, the sea became saltier and saltier. It must have been much like the Caspian Sea today, but with at least some breaks in the encircling reef that periodically allowed more sea water into the basin where it could evaporate.
What do you get when you evaporate sea water? You get salt. The mineral halite, sodium chloride. In many separate beds, reflecting that episodic influx of sea water and subsequent evaporation, there’s something like 2,000 feet of Silurian salt under the lower peninsula of Michigan. One bed alone is 500 feet thick. It’s an important economic resource, and in the same strata just across Lake Huron at Goderich, Ontario, we have one of the largest underground salt mines in the world. The mine is actually about 1,500 feet down and lies beneath Lake Huron. I went there on one of my first geologic field trips, when I was at Flint Junior College in Michigan. Apart from the coolness of going into an underground salt mine – most geologists crave going underground – the thing I recall the most from that 1967 excursion was the 1500-foot descent in the mine cage. For reasons I don’t recall there were 6 or 8 nuns with us on the trip. In full habit. We all had to wear hard hats to go into the mine… and so did the nuns. I’ll never forget being crowded into a mine elevator with a clutch of nuns in regalia and hard hats… and praying loudly and fervently all the way to the bottom.
Sea water has more than salt dissolved in it, of course. So in addition to halite, you can get a whole suite of other minerals, called evaporites because they crystallize from evaporating water. Gypsum, calcium sulfate with water, is a common one – it’s the stuff most building wallboard or drywall is made from. Anhydrite, which means without water, is another common evaporite mineral. It’s gypsum, calcium sulfate, minus the water in the crystal structure. All this stuff precipitates out because the chemicals – sodium, calcium, sulfur – become unnaturally concentrated by the evaporation of water in a restricted sea, where fresh influxes don’t keep the water at a typical concentration of those elements. It’s because the evaporites are so thick that we know there was some influx – bringing in more water to evaporate to deposit more and more salt and gypsum and anhydrite.
How fast does this happen? It’s possible for halite to build up at a rate of a half a foot a year under proper conditions of chemistry and evaporation. It’s not clear how much time was involved between periods when virtually all of the water might have evaporated and the next influx of water came in, but it’s possible that the entire accumulation was deposited over a period as short as a few thousand or a few tens of thousands of years – almost instantaneously, geologically speaking.
The United States consumes about 50 million tons of salt every year. Close to half of it goes to make chemicals – sodium and chlorine are in all sorts of things. The chlorine in polyvinyl chloride (PVC) for example, comes mostly from salt that is mined or extracted from salty brines. Highway deicing is the second largest consumer of salt. All the salt on home and restaurant tables and all the food additive salt in the United States only adds up to about 5% of the total. Even with all the salt that there is in Michigan and elsewhere, the U.S. still imports 22% of the salt we need, mostly from Canada and Mexico. Salt isn’t valuable enough to ship it great distances… but the price of salt has almost doubled from 1991 to 2013, from $19 per ton to $37 per ton. In the same time frame, salt imports doubled, from 11% to 22%.
—Richard I. Gibson
Map based on Briggs (1958)
Good Reference: The Geology of Michigan, by Dorr and Eschman (U. of Michigan Press, 1970)