Summary: 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, bu
