March 10. Cincinnati Arch

The Cincinnati Arch is a broad, low geologic structure that brings Ordovician rocks to the surface near Cincinnati, Ohio, and Nashville, Tennessee, where it’s called the Nashville Dome. This was not a mountain range, nor even a land area all that often, but it did separate the low-lying basins to the east – the Appalachian Basin – from those to the west, the Michigan and Illinois Basins.

Map is from Devonian time,
but the Cincinnati Arch began
during the Ordovician.

The best way to look at the Cincinnati Arch probably isn’t the most straightforward. It seems like an anticline, an upward arching of the rocks that brings the older, Ordovician rocks to the surface. But in terms of earth history, it’s probably more of a long linear stable area that stayed where it was when the adjacent areas subsided. It might have been related to the Middle Ordovician mountain building further east – the Taconic Orogeny, which we’ll talk about later in March when it reaches its culmination. The entire crust might have buckled very gently as island arcs and small continental fragments collided with the eastern part of North America. You can think of it as their added mass pushing down the crust, and the crust further to the west buckled upwards in compensation.

The arch began to form in Middle Ordovician time, around 470 million years ago. If and when the arch broke the surface of the sea, it meant that marine sediments typical of the Ordovician in this area would not have been deposited. It would be an unconformity in the making, and that gap in deposition continued up into the Silurian and Devonian Periods. How do we know the alternative isn’t the correct story – what if those rocks were deposited, but were later eroded off? That would give us the same appearance today. To figure that out, you have to look at the little picture, the details, as well as the overall picture of what looks like a broad uplift.

When you study the rocks, it appears that it was a combination of both processes – non-deposition as well as erosion. When you think about it, that shouldn’t come as a surprise. Land areas today are subject to both of those processes at the same time, and of course there is some deposition of sediment on land as well – just not as extensive and continuous, usually, as the piles of sediment we find in ocean basins.

Over time during the Paleozoic Era, the Cincinnati Arch seems to have gone up and down several times. Or, alternatively, sea level rose and fell several times. Or, most likely, some combination of both happened. It’s all relative.

Back in the 1980s, I think it was, I did some work with the geological surveys of Kentucky, Ohio, and Indiana to try to figure out the details of the subsurface of the Cincinnati Arch. A well had encountered unexpected sedimentary rocks below the lowest Cambrian rocks, the Mount Simon Sandstone, and the question was, could those sedimentary rocks contain any oil and natural gas? Even though they would be early Cambrian or Precambrian in age, their presence high on the arch might have allowed younger, but deeper sourced, oil to migrate into them like the oil we talked about in Ohio on February 21.  

My contribution was to look at maps of the earth’s gravity field, which tells you things about the densities of rocks, and the magnetic field, which can help you say some things about the rock types. The project ultimately described an old basin there on the Cincinnati Arch, probably dating to Precambrian time and possibly related to the Grenville Front, a structure that represents a collision between continents about a billion years ago. We talked about it on January 29.

Bottom line, I’m pretty sure no oil or gas has been found in those rocks. But the bigger structure, the Cincinnati Arch itself, certainly has affected sedimentation patterns and structures, and a lot of the oil and gas in Ohio, Indiana, and Kentucky is related to it.

—Richard I. Gibson

Image by Ron Blakey, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.