The Chagrin Shale is a shalegeologic formation in the eastern United States that is approximately 365 million years old. The Chagrin Shale is a grayish shale that begins thin and deep underground in north-central Ohio. As it proceeds east, the formation thickens, rises to the surface, and contains greater amounts of siltstone.
Identification and name
The Chagrin Shale was identified in 1873 and named for the Chagrin River in 1903. John Strong Newberry, director of the Ohio State Geological Survey, first identified the formation in 1873. He called it the Erie Shale, but it was discovered that the name "Erie Shale" was preoccupied. Ohio State University professor of geology Charles S. Prosser further described the formation in 1903, and proposed the name "Chagrin Shale" because the shale presented such excellent outcroppings near the Chagrin River. Dr. Prosser's suggested nomenclature was adopted. Details of the type locality and of stratigraphic nomenclature of the Chagrin Shale, as used by the U.S. Geological Survey, are available on-line at the NationalGeologic Map Database.
Lithology
The Chagrin Shale is a gray or greenish-gray argillaceous shale consisting of gray siltstone, silty gray shale, soft gray clay shale, and grayish-black shale. The primary minerals in the shale are chlorite, illite, kaolinite, and quartz. Thin to massive beds of siltstone and sandstone are common. The amount of siltstone increases from west to east, at times forming beds up to thick. Thin layers of ironstone and marcasite, as well as concentrations of marcasite, occur throughout the shale. The Chagrin Shale is classified as a weak to medium-strong rock, with a compressive strength anywhere from to. The strength of the rock is much lower near soil/rock interface, and if there is weathering.
Distribution
The Chagrin Shale is found in north-central and northeastern Ohio, and in northwestern Pennsylvania. The Chagrin Shale reaches a maximum thickness of in eastern Ohio. In Ohio, the Chagrin Shale is thin in the west, underlying the Cleveland Shale and overlying the Huron Shale. It is deep underground here, and thickens as it proceeds east. Studies show that the Chagrin Shale in Ohio grades into and between the Cleveland and Huron shales. The Chagrin Shale also extends south into West Virginia. The unit is also present in Kentucky, where it is mapped as the Chagrin Shale tongue of the Ohio Shale.
Stratigraphy
The Chagrin Shale is Famennianstage rock approximately 365 million years old. It is a member of the Ohio Shale. The Chagrin Shale is thin in the west, underlying the Cleveland Shale and overlying the Huron Shale. It is deep underground here, and thickens as it proceeds east. Studies show that the Chagrin Shale grades into and between the Cleveland and Huron shales.
Paleobiology
The Chagrin Shale preserves fossils dating to the Late Devonian period. Marine fossils found in the formation include the coelacanth fish Chagrinia, plants, and trace fossils including the ichnogenusChagrinichnites.
Geochemistry
and methane gas are found in the Chagrin Shale. On some occasions, these pockets of gas have proven quite large, and when reached by drills have vented for several weeks. Water infiltration of the formation on a sustained or large scale is rarely seen.
