The Cleveland Shale, also referred to as the Cleveland Member, is a shale geologic formation in the eastern United States.
Identification and name
The Cleveland Shale was identified in 1870 and named for the city of Cleveland, Ohio. John Strong Newberry, director of the Ohio State Geological Survey, first identified the formation in 1870. He called it the "Cleveland Shale" and designated its type locality at Doan Brook near Cleveland. Details of the type locality and of stratigraphic nomenclature for this unit as used by the U.S. Geological Survey are available on-line at the National Geologic Map Database.
Description
The primary minerals in the Cleveland Shale are chlorite, illite, pyrite, and quartz. Underground, the Cleveland Shale is black, dull grayish-black, bluish-black, or brownish-black in color. In exposed outcrops, it weathers to red, reddish-brown, or medium brown. Highly weathered rock turns gray. It is fairly fissile, breaking into thin, irregularly shaped sheets or flakes that occasionally display crystals of pickeringite. Relieved of stress once exposed, the Cleveland Shale is nonplastic and can appear as if fragmented into blocks due to jointing.
Pyrite basal boundary
There is a sharp and clear distinction between the Cleveland Shale and underlying Chagrin Shale. At the very bottom of the Cleveland Shale there is a thin, discontinuous layer of pyrite. This pyrite layer is discontinuous because after this rock was laid down, it was eroded. The erosion increases as one moves south along the valley of the Cuyahoga River and east to the Grand River. Portions of the pyrite layer, known as Skinner's Run Bed, contain fragments of petrified wood and fossilized fish bones worn smooth by the action of water. Above the pyrite layer, a limestone layer is found in west-central (but not eastern) Ohio.
The remainder of the Cleveland Shale generally consists of a relatively hard, organic rich oil shale. It has both an upper and lower part.
Lower part
A clay shale, described as bluish or bluish-gray and as olive-black to brownish-black, forms the lower part. The lower part can be anywhere from a few inches to several feet in thickness. This layer is sometimes referred to as the Olmstead shale. This layer has been dated to between 362.6 and 361.0 million years old based on conodont biozones (Bispathodus aculeatus aculeatus to Bispathodus ultimus ultimus zones). Thin beds of gray or brown siltstone, lumps of pyrite, and layers of silica-heavy limestone with cone-in-cone structures are found in the lower part. In eastern Ohio, thin gray veins ("stringers") of siltstone appear. In western Ohio, the Cleveland Shale appears to interbed with the Chagrin Shale below it, erasing the clear boundary between the two rock formations.
Upper part
The upper part of the Cleveland Shale is a black to brownish black silty shale with occasional thin beds of gray shale and siltstone. The upper part is much richer in petroleum and kerogen. When broken open, fresh samples smell like crude oil. Where the upper part is thick, and particularly in northeast Ohio, the shale has a distinctive "rippled" appearance. The upper of the Cleveland Shale contains abundant nodules of phosphate, nodules and bands (extremely thin beds) of pyrite, bands of calcisiltite, and lamination. Almost no concretions are found in the upper part.
Geographic extent
The Cleveland Shale is a shale geologic formation in Ohio in the United States. The Cleveland Shale underlies much of northeast Ohio in beds of varying thickness.
In northeast Ohio, the member does not appear east of the Grand River. Measurements taken in northeast Ohio show the Cleveland Shale to be to thick. It is thickest around the Rocky River north of Berea, Ohio, and thins to the east, west, and south.
The Cleveland Shale is found in east-central Kentucky. In east-central Kentucky, the Cleveland Shale is more uniform in thickness, ranging from , and increases in thickness toward the east.
The unit is also present in West Virginia and in southwest Virginia, where it is mapped as the Cleveland Member of the Ohio Shale.