April 23. Pinnacle Reefs

Coral reefs developed around the margins of the Michigan Basin during the Silurian, but even in deeper water some corals thrived. They grew upward in cylindrical, column-like structures, keeping pace with the rate of rising sea level, like modern atolls. Some reached heights of a few hundred feet, and because of their shape they’re called pinnacle reefs. 

Pinnacle reefs are especially common in a northeast-southwest trending zone across northern Lower Michigan. A belt that’s about 150 miles long and 40 miles wide. The reefs probably grew on a linear break in the sea floor – there’s some evidence in geophysical data for such a break, which would have been the easiest place for corals to get started because it was somewhat shallower than the deepest part of the basin, to the south.

When the tall cylindrical reefs were eventually buried beneath later sediments including salt as the water evaporated, the porous limestone structures built by corals became excellent traps for oil and gas. The surrounding sediment is the seal, the stuff that keeps the oil and gas from escaping from the reservoir, which is that tall little reef. They are usually no more than a half-mile across, but there are more than 700 of them across northern Lower Michigan that have been found to contain oil and gas. A typical oil field there contains around a half-million barrels of oil, which is maybe 45 minutes worth of oil consumption in the whole United States today. To have a good oil reservoir you need good porosity – that’s the holes in the rock where the oil is found – and good permeability, which is how interconnected the pore spaces are. These little reefs have both – they average about 6% pore space, but some are as much as 37% - that’s really huge, more than a third of the rock. The permeability can be as much as 500 times that of a “good” oil reservoir.

Most of those little fields were discovered after 1970, which was the peak of oil production in the United States. In 1970, the U.S. produced around 10 million barrels of oil per day. Today, even with the dramatic additions in production from North Dakota, the U.S. only produces about 7.9 million barrels per day – but that’s significantly up from the 5 million that was usual through most of the 1990s and 2000s.

There are Silurian pinnacle reefs in Illinois, Indiana, Ohio, and Ontario as well, and some contain oil, but the reefs in Michigan are the richest.

* * *

Today’s birthday is William Dean Thornbury, born April 23, 1900, in English, Indiana. He became a professor of Geomorphology at Indiana University and wrote the textbook that was used in geomorphology classes across the United States for decades. Geomorphology is the study of landforms and the processes that create them.

—Richard I. Gibson

Links:
Pinnacle reefs 
Porosity and permeability 
Map from Michigan DEQ

April 17. Salina salt sea

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)

March 12. Michigan Basin

The Michigan Basin is a bull’s eye on the lower peninsula of Michigan – a nearly circular target painted on the geologic map of North America. It’s about 250 kilometers wide, and 5 kilometers deep. Basins like the Michigan Basin are important because they often contain important resources such as oil and natural gas, so understanding how they form helps us explore for such resources.

In some of the Ordovician rocks, called the Prairie du Chien Group, porosities are great enough to serve as natural gas reservoirs, and more than 5 billion cubic feet of natural gas has been produced from that part of the section. Not too shabby, but not too much in the grand scheme – and in fact the United States today consumes almost 100 billion cubic feet of natural gas per day, so that total historic production of 5 billion cubic feet from the Prairie du Chien of Michigan amounts to about 80 minutes’ worth of natural gas consumption today.  We’ll talk more about the Michigan Basin next month in connection with its mineral resources.

The problem is, we really aren’t sure how the Michigan Basin formed. It’s shaped like a big bowl, and clearly there was subsidence in the basin to allow for the 5 kilometers of sediment to fill it. And fill it they did – the layers of rock are thicker in the center than on the flanks.

One possible mechanism for formation suggests that the earth’s crust or upper mantle was weaker, or thinner, or of different composition, so that broad stretching on a crustal scale might have allowed this area to sink more than other areas, becoming the bowl in which the sediments were deposited. It’s a fact that a branch of the Mid-Continent Rift, the pull-apart zone that affected this region about 1.1 billion years ago – we talked about it on January 26 — but that zone was clearly very linear, oriented north-south. I suppose it might have controlled the subsiding, and the Michigan Basin is somewhat oval shaped, with the longer axis north-south, but honestly this seems to me to be a stretch. Possible, or possibly some degree of affect to the whole process, but hard to see as the one and only cause.

Some mechanisms call on thermal subsidence as the basis for the Michigan Basin. In this scenario, a relatively small portion of the upper mantle cools more than adjacent areas, and when it cools, it contracts, it shrinks, and that smaller volume is also a physically lower place, a basin in which sediments can be deposited. This is a reasonable theoretical idea, but I don’t know of any good solid evidence for it in Michigan. 

You can also get subsidence of the crust when you have an upwelling of the mantle down below. It pretty much stretches the crust above the upwelling hot mantle, and the stretched crust forms a neck, like when you pull silly putty apart – or partly apart. This has almost certainly happened in the Mississippi Salt Basin, near the Gulf Coast, but there, we have good geophysical evidence for that process which we don’t find in Michigan.

And because the basin is so symmetrical, so nearly circular, it’s been suggested that it represents a huge impact crater. But beyond the circularity – and it’s really oval, not circular – there’s no evidence for an impact.

Maps is from Devonian time;
Michigan Basin began to subside in
Late Cambrian and Ordovician time.

Back in 1990, geologist Paul Howell and his colleagues at the University of Michigan studied the sequences of sedimentation in the Michigan Basin in detail, and found a good correlation in time between the deposition and tectonic events along the east coast of North America. I mentioned this idea in connection with the Cincinnati Arch the other day. The really good coincidence of several different mountain building events – collisions – on the east coast with pulses of subsidence in Michigan suggests a causative relationship, and it does put the development of the Michigan Basin into a plate-tectonic context, rather than a simple, isolated basin subsiding a lot, but for no obvious reason.

I think that idea is most likely, that the basin is a reaction within the continent to big-time collisions happening a few hundred kilometers to the east. Push it down on the east coast, it bows up along the Cincinnati Arch, and sags beyond the arch in the Michigan and Illinois Basins. I don’t think you can quite take that to the bank, yet, but it’s an idea that works pretty well with what we see in the rocks. Howell paper proposing this tectonic mechanism is linked below.

—Richard I. Gibson

Images from Michigan State University and USGS. Devonian paleogeographic map by Ron Blakey, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Link to paper by Howell et al. (1990) PDF