Devonian Period

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The Devonian Period began about 416 million years ago and ended about 359 million years ago. Devonian rocks crop out in a north-south band through the central part of the state and then eastward along the Lake Erie shore from Sandusky to Ashtabula. They are also present in northwestern Ohio although outcrops are few due to a thick cover of Pleistocene Ice Age sediments. In addition to being present in the subsurface of eastern Ohio, Devonian rocks crop out in an isolated area near Bellefontaine, in Logan County. This feature is known to geologists as the Bellefontaine Outlier because these Devonian rocks are about 30 miles from the nearest outcrop of rocks of similar age.

Ohio was in equatorial latitudes during the Devonian. The outcropping rock record representing earliest Devonian time is absent, with one minor exception, in Ohio as much of the state was apparently above sea level. Early Devonian rocks are present in the subsurface of eastern Ohio, indicating that the sea was nearby. The lone exception to the absent Lower Devonian rock record is a small lens of dark-gray shale that was discovered in Holland Quarry, in Lucas County west of Toledo, in the 1920’s. This single exposure contained the remains of early fishes, eurypterids, and plants and represents a brackish-water embayment of the sea that was present in the Michigan basin. This quarry was reclaimed long ago.

In Middle Devonian time the seas once again flooded Ohio and limy sediment began to accumulate. The most prominent of these rock units are the Columbus Limestone and the Delaware Limestone. These rocks crop out through the central part of the state and are quarried extensively throughout this area. The Middle Devonian seas teemed with life as the Columbus Limestone, in particular, is noted for abundant corals, brachiopods, clams, snails, cephalopods, trilobites, and other invertebrates. Remains of fishes, although not common, are found in these rocks. Whereas the Columbus Limestone is a light-colored rock with little land-derived clay, the Delaware Limestone has a bluish color due to silt and clay derived from rising land areas far to the east. This influx of terrestrially derived sediment carried far offshore marks the beginning of another mountain building cycle as North America collided with the northern European continent. The overlying Olentangy Shale and its equivalent in northwestern Ohio, the Silica Shale, show even a greater influx of clay and silt deposited offshore. The Silica Shale is world famous for its well-preserved fossils. This orogeny is called the Acadian and culminated at the end of the Devonian. In addition, volcanic ash beds are preserved in Middle Devonian rocks in Ohio as evidence of volcanoes associated with continental collision to the east.

In the Late Devonian, a major change occurred. The clear, shallow seas with abundant bottom-dwelling invertebrates that had dominated Ohio during much of the preceding Paleozoic periods changed to a comparatively deep, stagnant sea. Black mud with much organic material accumulated to great thickness in a wide area west of the Acadian Mountains. In Ohio, this thick, black shale is called the Ohio Shale.
 
The lower portions of the sea were anoxic (without oxygen) and no organisms could live on the soupy sea bottom. However, the upper waters were clear and oxygenated and teemed with a variety of fishes, including some of the earliest, well-preserved sharks, ray-fined fishes, and predatory, armored fishes known as arthrodires. Among the largest arthrodires was Dunkleosteus, which reached nearly 20 feet in length and had jaws in which the bone was modified for puncturing and slicing prey.
One hypothesis suggests that the Ohio Shale sea stagnated when the rising Acadian Mountains to the east blocked the moist westerly trade winds, thus depriving areas west of this orographic barrier of rainfall. This prevented streams from carrying large amounts of sediment into the sea to dilute the organic matter produced by plankton in the upper waters. Late in the Devonian Period and continuing into the Mississippian Period, a large wedge of sediment was eroded from the Acadian Mountains. This feature is known as the Catskill delta.

Perhaps the most unusual feature of the Ohio Shale is the presence of large, spherical masses of limestone, called concretions. They occur in zones and are often abundant. Concretions in the lower part of the Ohio Shale can be up to six feet in diameter and exhibit distinct layers when broken open. In the upper part of the Ohio Shale, in the Cleveland area, the concretions tend to be flattened. These features grew within the shale, soon after its deposition, as the bedding planes in the shale bend around the concretion. It is thought that the concretion grew around an organic mass as many of them contain bones or other remains of fishes that lived in the Ohio Shale sea. Many of the spherical concretions are used as yard ornaments.

The dark, highly organic Ohio Shale is overlain by gray Bedford Shale, indicating an abrupt change whereby fine-grained clastic sediments were being carried into the Ohio sea as offshore delta deposits. Extensive studies of the Bedford suggest that these sediments were derived from the Catskill delta to the east and from the Canadian shield to the north. Portions of the Bedford are red in color, suggesting oxidation of the iron minerals.

Overlying the Bedford is the Berea Sandstone. This unit is composed of angular quartz grains and can reach a thickness of 200 feet locally in northeastern Ohio. Traditionally, the Bedford and Berea were considered to be of Mississippian age. Although fossils are sparse in these units, therefore making age determination and correlation difficult, researchers in recent years have placed the Bedford and Berea in the Upper Devonian.

Life was abundant and diverse during the Devonian, both in the seas and for the first time on land. Ohio rocks preserve a remarkable record of this biological diversity. In particular, the Columbus and Delaware Limestones of Middle Devonian age contain many species of well-preserved invertebrate fossils including corals, brachiopods, clams, snails, cephalopods, and trilobites, among others. Although not common, remains of fishes also occur in these rocks. The Devonian Period is known as the age of fishes and that appellation is in part derived from the remarkable fauna of bony fishes, sharks, and armor-plated arthrodires, some of which reached 20 feet in length, that are well known from the Ohio Shale and particularly the upper member of this unit, the Cleveland Shale. These fishes lived in the upper, oxygenated waters of the stagnant Ohio Shale sea and their remains are often found in the center of large carbonate concretions. Perhaps the best-known arthrodire is Dunkleosteus terrelli, a large, ferocious predator whose bite force is estimated to have been 80,000 pounds per square inch, more than any fossil or living fish and among the most powerful among all animals.

Devonian rocks in Ohio have been an important source of industrial minerals throughout much of the state’s history. The Columbus and Delaware Limestones have been an important source of carbonate rock for agricultural lime, building stone, cement, road metal, and many other uses. The Bedford Shale was used for many years to make brick and sewer tile. The Berea Sandstone, informally known as the Berea Grit, was used by early settlers for grindstones and later as an important building stone. Many buildings in Ohio and indeed in the eastern United States were constructed from Berea Sandstone. It is still quarried in northern Ohio. Oil and gas are produced from Devonian rocks in eastern Ohio, particularly the Berea and Ohio Shale.

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