June 9. Deltas of the Midwest

With all the talk about limestone, you might get the impression that all of America was a big beautiful tropical seaway with nice white carbonate sand everywhere. While the shallow seas were extensive, there were other kinds of sediments deposited as well. In what is now the eastern Great Lakes, Ohio, Kentucky and West Virginia, during early Mississippian time some huge river deltas formed, as large as the Mississippi or Nile Delta today.

from USGS Prof. Paper 259

Remember that the Great lakes weren’t there – it would be hundreds of millions of years before they would form. The complex upland in eastern North America, the result of collisions with Baltica and other pieces of Europe, led to a huge river system in what is now southern Ontario. The Ontario River flowed south, into the eastern edge of the sea that covered most of the United States west of the Appalachians.

The river carried a lot of sand, silt, and mud into the branch of the sea called the Ohio Bay, and built a long linear delta complex, a peninsula that extended all the way from present-day Lake Erie to the border between Kentucky and West Virginai – almost three hundred miles long and something like 75 miles wide. The sediments in the delta are known today as the Bedford Shale and Berea Sandstone.

There was another peninsula to the west, in west-central and southwestern Ohio, but this one wasn’t a delta. It was all that was left of the Cincinnati Arch – the broad upland that got its start 130 million years earlier, during the Ordovician. As the Mississippian Period progressed and sea levels became higher and higher, the peninsula sometimes called Cincinnatia was reduced to a string of islands and shoals and eventually was pretty much completely submerged. West of Cincinnatia, in present-day Indiana and Illinois and points west, most of the Mississippian rocks are the shallow-water limestones with abundant fossils that are so common from this time across much of North America.

The Berea Sandstone and Bedford Shale represent oscillations in sea level that produced variations in the sedimentation pattern over time at various places. Modern deltas do this too, changing the position of the main channel and smaller distributary streams sometimes on an annual basis. So you can get coarse river channel sands at one time, but muds from floods over the banks of the channels at other times in the same place, and many variations in between. Deltaic systems can make pretty good reservoirs for oil and natural gas when coarse, porous sandstones are encased within impermeable fine-grained shales. And in fact the Berea Sandstone was historically an important oil and gas producer in Ohio.

—Richard I. Gibson

Reference: Geology of the Bedford shale and Berea sandstone in the Appalachian basin, USGS Prof. Paper 259 (1954) Also source of map above.