Cleveland Shale

Cleveland Shale
Stratigraphic range: Famennian
Cleveland Shale (below) and Berea Sandstone of the Bedford Formation at the Great Falls of Tinkers Creek near Bedford, Ohio
Type Formation
Unit of Ohio Shale
Underlies Bedford Formation
Overlies Chagrin Shale
Location
Region  Ohio
Country  United States
Type section
Named for Cleveland, Ohio, U.S.
Named by John Strong Newberry

The Cleveland Shale, also referred to as the Cleveland Member, is a shale geologic formation in the states of Kentucky and Ohio in the United States. Approximately 360 to 358.9 million years old, the shale is likely the regional expression of the Dasberg event, a major extinction event that occurred near the end of the Devonian period. Identified in 1870 and named for the city of Cleveland, Ohio, the black Cleveland Shale underlies much of northeast Ohio in beds of varying thickness. Exceptional marine animal fossils are found in the formation.

Identification and name

John Strong Newberry, director of the Ohio State Geological Survey, first identified the formation in 1870.[1] He called it the "Cleveland Shale" because its type locality was found at Doan Brook[2] near Cleveland, Ohio.[1] It is the regional equivalent of the Hangenberg Sandstone.[3]

Formation

The Cleveland Shale is classified as Famennian stage rock[4] roughly 360 to 358.9 million years old.[5] It is typical of sedimentary rock created in an anaerobic environment.[6] Evidence exists to suggest that the Cleveland Shale was laid down during the Dasberg event, an Upper Famennian extinction event that devastated land-based flora and marine-based fauna. This led to a significant drop in marine oxygen (an anoxic event) and atmospheric carbon dioxide, and then a brief glaciation. The global environment recovered, only to suffer another extinction, the Hangenberg event, close to the Devonian-Carboniferous boundary.[7] While the Cleveland Shale was being deposited, extensive organic matter from the land was swept into the sea then lying over Ohio.[8] Although there is dispute over how deep this sea was, the Dasberg event meant that oceans could support few to no bottom-dwelling animals. This explains why the Cleveland Shale largely lacks fossils of benthic organisms[9] and has a high carbon content that colors the shale very dark gray to black.[10][11]

Physical description

The Cleveland Shale (or Cleveland Member) is a sub-unit of the Ohio Shale Formation.[12][13] The Chagrin Shale underlies the Cleveland Shale.[14] The Bedford Shale generally overlies the Cleveland Shale, with a sharp distinction between the two. In west-central Ohio, more than 150 feet (46 m) of Bedford Shale may lie above the Cleveland Shale. In places, red and grey shale may intertongue (interlock) with the Cleveland Shale extensively. In far eastern Ohio, the Bedford Shale thins by more than 125 feet (38 m). Where the Cussewago Shale is also present, the Bedford Shale is usually less than 25 feet (7.6 m) and may be locally absent. In some areas, the Cleveland Shale is described as overstepped[12] or unconformably overlaid gradationally by Berea Siltstone and sharply by Berea Sandstone.[15]

The primary minerals in the Cleveland Shale are chlorite, illite, pyrite, and quartz.[16][lower-alpha 1] Underground, the Cleveland Shale is black,[10][6][12][17] dull grayish-black,[18] bluish-black, or brownish-black[16] in color. In exposed outcrops, it weathers to red,[18] reddish-brown,[2] or medium brown.[16] Highly weathered rock turns gray.[2][16] It is fairly fissile,[6][10][12] breaking into thin, irregularly shaped sheets[15] or flakes[16] that occasionally display crystals of pickeringite.[2] Relieved of stress once exposed, the Cleveland Shale is nonplastic[16] and can appear as if fragmented into blocks due to jointing.[10]

Pyrite basal boundary

There is a sharp and clear distinction between the Cleveland Shale and underlying Chagrin Shale.[2][15] At the very bottom of the Cleveland Shale there is a thin, discontinuous layer of pyrite.[10][lower-alpha 2] 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.[12] Portions of the pyrite layer, known as Skinner's Run Bed,[12] contain fragments of petrified wood and fossilized fish bones worn smooth by the action of water.[10] Above the pyrite layer, a limestone layer is found in west-central (but not eastern) Ohio.[18]

The remainder of the Cleveland Shale generally consists of a relatively hard,[18][lower-alpha 3] organic rich[20] oil shale.[16][17] It has both an upper and lower part.[18]

Lower part of the Cleveland Shale

A clay shale,[18] described as bluish or bluish-gray[18] and as olive-black to brownish-black,[21] forms the lower part. The lower part can be anywhere from a few inches to several feet in thickness. 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.[18] In western Ohio,[17] the Cleveland Shale appears to interbed with the Chagrin Shale below it, erasing the clear boundary between the two rock formations.[18]

Upper part of the Cleveland Shale

The upper part of the Cleveland Shale is a black to brownish black[21] silty shale[18] with occasional thin beds of gray shale and siltstone.[10] The upper part is much richer in petroleum[22] and kerogen.[16][lower-alpha 4] When broken open, fresh samples smell like crude oil.[16] Where the upper part is thick,[12] and particularly in northeast Ohio,[15] the shale has a distinctive "rippled" appearance.[12] The upper 10 feet (3.0 m) of the Cleveland Shale contains abundant nodules of phosphate, nodules and bands (extremely thin beds) of pyrite, bands of calcisiltite, and lamination.[21] Almost no concretions are found in the upper part.[16]

Hypotheses about the contact boundaries

The contact between the Chagrin Shale and Cleveland Shale has been described as interbedding. This is caused when two different depositional environments (in this case, the oxygenated sea which laid down the Chagrin Shale and the anaerobic sea rich in organic matter which laid down the Cleveland Shale) moved repeatedly back and forth over the same area.[18] Geologist Horace R. Collins called the boundary area intercalated,[17] but it is unclear what meaning he intended.[lower-alpha 5]

Different hypotheses have been suggested as the cause of the regional, irregular contact between the Cleveland Shale and Bedford Formation. Charles E.B. Conybeare has noted that the Cleveland Shale is siltier in the east and more calcareous in the west. He hypothesized that this indicates that silt flowed into the sea from east to west. Current eroded the Cleveland Shale and then laid down new sediment in the gullies which became the Bedford Formation.[11] Jack C. Pashin and Frank R. Ettensohn proposed a variation on this hypothesis. They note that the region containing the Cleveland Shale was undergoing uplift when the Bedford Formation was being deposited. This likely led to exposure and erosion of the Cleveland Shale, with sediment which became the Bedford Formation filling in these gullies. They also observe that there is evidence of diapirism (the intrusion of deformable Cleveland Shale upward into the more brittle Bedford Formation), as well as intertonguing.[25] Baird et al. note that the Cleveland Shale also tilts downward to the south. They suggest that this caused overstepping, rather than intertonguing.[12]

Location and thickness

The Cleveland Shale is found in northeast and central Ohio, and in east-central Kentucky.

In northeast Ohio, the member does not appear east of the Grand River.[12]

A thick sequence of the Cleveland Shale exposed on the north bank of the Rocky River in North Olmstead, Ohio. For scale, note the paleontologists just right of center at the base of the cliff.

Measurements taken in northeast Ohio show the Cleveland Shale to be 7 feet (2.1 m)[12] to 100 feet (30 m) thick.[18] It is thickest around the Rocky River north of Berea, Ohio, and thins to the east, west, and south.[18] In east-central Kentucky, the Cleveland Shale is more uniform in thickness, ranging from 41.4 to 50.1 feet (12.6 to 15.3 m), and increases in thickness toward the east.[21]

Fossil record and petroleum

The Cleveland Shale is generally considered to be fossil-poor, but there are exceptions. The basal pyrite layer contains petrified wood and fossilized fish bones.[10] The upper part is famous for its extensive and well-preserved fossil Chondrichthyes, Conodonts, Placodermi,[12] Cladoselache,[10] and Palaeoniscinoids.[26] The giant predatory placoderms Dunkleosteus terrelli, Gorgonichthys clarki, Gymnotrachelus hydei, Heintzichthys gouldii, and five subspecies (including the type specimen) of Titanichthys were all discovered in the Cleveland Shale.[27] The Cleveland Shale is classified as a konservatte-lagerstatten, which means it often preserves complete body fossils. Typical early shark preservation includes soft tissue outlines and impressions, fin rays, gill musculature, cartilage, and stomach contents.[28]

The high organic content of the Cleveland Shale makes it eminently suitable for the formation of fossil fuels. One 1981 study found that the Cleveland Shale can yield an average of 14 US gallons (53 l; 12 imp gal) of petroleum per 1 short ton (0.91 t) of rock.[29] The Cleveland Shale also contains cannel coal and "true" coal, although neither in great quantity.[16]

See also

References

Notes
  1. ↑ Quartz particles in the shale range from 2 to 7 micrometres (7.9×10−5 to 0.000276 in) in size.[16]
  2. ↑ Pyrite forms when organic material falls onto an ocean floor that is anaerobic, has little bottom current, and has extensive deposition of silt and sediment.[12]
  3. ↑ "Hard" is defined as having a compressive strength between 10,000 to 13,000 pounds per square inch (69,000 to 90,000 kPa).[19]
  4. ↑ In a 1981 study of Cleveland Shale samples in central-eastern Kentucky, the upper part of the shale was 11 percent carbon and 1.3 percent hydrogen.[22]
  5. ↑ Intercalation can be used as a synonym for interbedding.[23] The term may also mean the introduction of a new layer between two preexisting layers.[24]
Citations
  1. 1 2 Wilmarth 1938, p. 361.
  2. 1 2 3 4 5 Williams 1940, p. 19.
  3. ↑ Kaiser, Aretz & Becker 2016, p. 404.
  4. ↑ Traverse 2008, p. 227.
  5. ↑ Harding, Megan (Spring 1996). "Cleveland Shale and the Age of Fishes". The Explorer. p. 20.
  6. 1 2 3 Pashin & Ettensohn 1995, p. 57.
  7. ↑ Baird et al. 2009, pp. 8, 10.
  8. ↑ Kaiser, Aretz & Becker 2016, p. 415.
  9. ↑ Hannibal & Feldman 1987, p. 406.
  10. 1 2 3 4 5 6 7 8 9 Hannibal & Feldman 1987, p. 404.
  11. 1 2 Conybeare 1979, pp. 419-420.
  12. 1 2 3 4 5 6 7 8 9 10 11 12 13 Baird et al. 2009, p. 10.
  13. ↑ Rubel & Coburn 1981, p. 22.
  14. ↑ Pashin & Ettensohn 1995, p. 6.
  15. 1 2 3 4 Pashin & Ettensohn 1995, p. 51.
  16. 1 2 3 4 5 6 7 8 9 10 11 12 Johnson 1981, p. 171.
  17. 1 2 3 4 Collins 1979, p. E-10.
  18. 1 2 3 4 5 6 7 8 9 10 11 12 13 Pepper, DeWitt & Demarest 1954, p. 16.
  19. ↑ Vyas, Aho & Robl 1981, p. 390.
  20. ↑ Pashin & Ettensohn 1995, p. 50.
  21. 1 2 3 4 Pollock, Barron & Beard 1981, p. 204.
  22. 1 2 Bland, Robl & Koppenaal 1981, p. 188.
  23. ↑ Bates & Jackson 1984, p. 262.
  24. ↑ Neuendorf, Mehl & Jackson 2005, p. 330.
  25. ↑ Pashin & Ettensohn 1995, pp. 50-51.
  26. ↑ Hansen 2005, pp. 292-293.
  27. ↑ Hansen 2005, pp. 290.
  28. ↑ Various Contributors to the Paleobiology Database. "Fossilworks: Gateway to the Paleobiology Database". Archived from the original on 31 July 2014. Retrieved 22 June 2014.
  29. ↑ Reasoner et al. 1981, p. 12.

Bibliography

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