Geologic Setting

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Rocks of Late Ordovician age are exposed in a circular outcrop belt throughout the Cincinnati region, including southwestern Ohio, northern Kentucky, and southeastern Indiana. During the Late Ordovician, sea level was higher than present, and much of North America was covered by shallow seas. The sedimentary layers of the Cincinnati region were deposited in a shallow sea. The depth of the sea varied between 10 and 100 meters during the interval of shallow marine deposition. Using these variations in sea level, the Cincinnatian formations have been divided into six depositional sequences based on cycles of sea level rise and fall as shown on the right.

During the Late Ordovician, North America was part of the continent of Laurentia, and its position was different from the modern geography of North America. The Cincinnati region was located in the tropics about 20 degrees south of the Equator and tilted about 45 degrees from its current orientation. This tropical location placed the Cincinnati region within the hurricane belt, and the shallow seas frequently were disturbed by major storm events.

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Star=Cincinnati region, T=Taconic Mountains, A=Trancontinental Arch

Also during the Late Ordovician, a volcanic island arc collided with the eastern edge of North America. This collision resulted in mountain building and the uplift of the Taconic Mountains. The Taconic Mountains were located in approximately the same position as the modern Appalachian Mountains.

Within each depositional sequence, two types of sedimentary rocks occur: shale and limestone. The shale units were formed from deposition of clay and silt-sized sediment that weathered off of the Taconic Mountains. As these mountains weathered, an enormous amount of sediment was shed into the basin, some of which was resuspended and moved by frequent tropical storms or tsunami waves that rolled through the region. This mud supply was intermittent, and when mud was not being deposited into the area, muddy bottoms became firm enough for shell-bearing organisms to colonize the sea bed. Shells accumulated on the bottom until the bottom was once again smothered by mud. During this accumulation, the sea floor was still frequently disturbed by storms and tsunami waves that would suspend and mix the shells, redepositing them into large ripples.  These shell beds are preserved as fossiliferous limestones. The alternating layers of shale and limestone reflect alternating periods of heavy mud running off of the Taconics alternating with periods of clear water.

The fossils preserved within these rocks reflect the inhabitants of the shallow marine seafloor 450 million years ago. At that time, the dominant creatures were brachiopods, bryozoans, crinoids, and trilobites. Cephalopods swam above and were the dominant predators in the Late Ordovician ecosystem.  The community structure of this ecosystem was stable during the C1 to C3 sequences, but underwent a major shift during and after the Richmondian Invasion of the C4 and early C5 sequence.

Further reading:

For an excellent and detailed discussion of the stratigraphy and depositional settings of the Cincinnati region, visit this website by Dr. Steven Holland, University of Georgia.

A Sea without Fish: Life in the Ordovician Sea of the Cincinnati Region” by Drs. David Meyer and Richard Arnold Davis is an outstanding book written for the general public and paleontologists alike.  It is published by Indiana University Press and sold by Amazon.com and other retailers.