Stratigraphic range: Famennian
A fossil in the Chagrin Shale
|Unit of||Ohio Shale|
|Underlies||Cleveland Shale, Cussewago Sandstone|
|Named for||Chagrin River|
|Named by||Charles S. Prosser|
The Chagrin Shale is a shale geologic 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 (already in use). 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 National Geologic Map Database.
The Chagrin Shale is a gray or greenish-gray argillaceous shale consisting of gray siltstone, silty gray shale, soft gray clay shale, and (uncommonly) 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 50 feet (15 m) 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 5,000 pounds per square inch (34,000 kPa) to 15,000 pounds per square inch (100,000 kPa). The strength of the rock is much lower near soil/rock interface (where there is stress relief), and if there is weathering.
The Chagrin Shale is found in north-central and northeastern Ohio, and in northwestern Pennsylvania. The Chagrin Shale reaches a maximum thickness of 1,200 feet (370 m) 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.
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.
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 ichnogenus Chagrinichnites.
Hydrogen sulfide and (more frequently) 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.
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