Geology of Ohio

The Cambrian at the start of the Paleozoic marked the proliferation of multi-cellular life. By the Late Cambrian, Ohio was covered in shallow seas, creating marine and delta deposits. Sandstone, mudstone, shale, limestone and dolomite deposited atop the faulted Precambrian basement rocks. Periodic retreats of sea level during the Paleozoic eroded these rock units, leading to some unconformities in the sedimentary sequence, through the Early Permian. During some of these periods of sea level retreat, beginning in the Ordovician, the region experienced terrestrial erosion in the Pennsylvanian and Permian. The drop in sea levels was related to the uplift of the region during the Taconic orogeny to the east.

In the Middle Ordovician, a renewed marine transgression flooded the region, depositing limestone reefs. These rocks are only known from underground drilling and do not outcrop at the surface.

The Point Pleasant Formation is the oldest rock unit exposed at the surface in Ohio and formed during the Late Ordovician, outcropping along the Ohio River near Cincinnati. The formation records a transition from shallow-water limestone reef formation to a deeper water environment, marked by overlying shale and limestone, which form the Cincinnatian Series. Clay shed from the rising mountains, associated with the Taconic orogeny formed the shale units. The warm sea water created perfect conditions for abundant biological activity and the Cincinnatian Series is highly fossil-bearing, outcropping across much of southwest Ohio in river beds, hillsides and road cuts. Ohio's State Invertebrate Fossil, is a trilobite found in the formation.

The Southern Hemisphere where Ohio was located at the end of the Ordovician experienced a widespread glaciation, around 438 million years ago. Sea level dropped due to the glaciation, accompanied by a subsidence of the land. Ordovician deposition ended, marked by a paraconformable break in the Drakes Formation.

By the beginning of the Silurian, sea levels advanced again. The shallow sea in the Silurian led to the formation of subtropical carbonates, which now outcrop to the east and west of the Cincinnati Arch and at the crest of the Findlay Arch. Some of the carbonates were deposited in reefs, such as the Early Silurian Lockport Dolomite. Although they do contain some fossils, most of the Silurian rock units poorly preserve evidence of life. The units are separated by layers of evaporite, including anhydrite, gypsum and halite. Thick salt beds formed in connection with drops in sea level and land subsidence, generating the thick salt beds of the Salina Group in eastern Ohio.

A new plate collision launched the Acadian orogeny in the Devonian and led to sediment deposition in the Appalachian Basin of eastern Ohio. The Early Devonian Helderberg Limestone and Oriskany Sandstone, which formed 408 million years ago, are known in the subsurface. Some Early Devonian rocks, of the Holland Quarry Shale, were briefly exposed at the quarry in Lucas County, before it was filled in and reclaimed. Most Devonian rocks date to the Middle Devonian and outcrop between southern Ohio and Lake Erie. A large unconformity separates Silurian and Middle Devonian dolomite and limestone. The overlying Devonian rocks include the Columbus Limestone to the east of the Findlay Arch and the Detroit River Group carbonates to the west. The angular unconformity between lower Silurian rocks is marked in some places by phosphate beds, that preserve bones and teeth.

A cast of a Dunkleosteus fossil found near Cleveland, Ohio

The Columbus Limestone is especially fossil bearing and outcrops in a band between Columbus and Sandusky County, as well as on Kelleys Island in Lake Erie. The best fossil exposures are found at limestone quarries along the belt, which supply local concrete demand. Toward the end of the Middle Devonian, the marine environment shifted toward black marine shale deposition, of the Olentangy Shale and Ohio Shale. For the most part, these shale units are not fossiliferous. However, in some places, there are occurrences of brachiopod, arthropod, arthrodire and shark fossils, as well as Dunkleosteus armor plates.

At the start of the Mississippian, in the Carboniferous, 340 million years ago, river and delta sediments covered over the shales, producing the Bedford Shale, Berea Sandstone, Cuyahoga Formation and Logan Formation. The deposition of these units was briefly interrupted by a return to a stagnant marine environment, forming the Sunbury Shale. Outcrops of Mississippian rocks run parallel to Devonian rocks across central Ohio and paralleling the Lake Erie shore. In fact, Mississippian outcrops form a band of hills five to 10 miles south of the lake. In general the Mississippian rocks are moderately fossiliferous, with good crinoid preservation in Cuyahoga Formation. Pennsylvanian rocks deposited in stream channels carved into older Mississippian rocks.

The Middle Pennsylvanian Sharon sandstone and Pottsville Group preserve evidence of uplift in the east, and subsidence in the west of the Appalachian Basin. The rocks that formed in the Pennsylvanian and Permian were mainly terrestrial in origin, including non-marine shale, sandstone, coal, ironstone and limestone, along with some marine rocks, such as flint and marine shale. River deltas in the Pennsylvanian, were covered in thick, swampy vegetation, helping to produce coal deposits.

Sedimentary rocks and fossil evidence suggests that Ohio has been a terrestrial environment since the Permian. For instance, the main subdivisions of the Pennsylvanian: the Pottsville Group, Alleghany Group, Conemaugh Group and Monongahela Group, along with the Early Permian Dunkard Group, are each more terrestrial than the last. The region was situated inland, experiencing terrestrial erosion as the Proto-North American continent of Laurasia collided with Gondwana to form the supercontinent Pangaea. Rocks from the end of Paleozoic are best exposed in road cuts along I-77, between Canton and Marietta, I-77 from Cambridge to Bridgeport and cliffs on the Ohio River north of Marietta to East Liverpool. [3]

Glacial grooves on Kelleys Island.

For 245 million years, spanning the entirety of the Mesozoic in the Paleogene, of the early Cenozoic, Ohio experienced long-running uplift and weathering. Prior to the Pleistocene glaciations, the region was covered in large stream valleys, which are believed to have removed several hundred meters of rock, erasing most evidence of the Mesozoic and Cenozoic. Some bones and plant fossils, including mastodon remains, have been found in recent sediments formed since the Pleistocene. [4]