Benton Shale
Benton Shale | |
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Location | |
Region | Montana, Wyoming, North Dakota, South Dakota, Colorado, Nebraska, Kansas |
Country | United States |
Type section | |
Named for | Fort Benton, Montana |
Named by | Meek, F.B. and Hayden, F.V. |
Year defined | 1862 |
The Benton Shale (also Benton Formation or Benton Group) is a
Naming and status
The name was applied by
Today, the Benton classification is obsolete in some regions, having been replaced by the ascending sequence
In the lower Missouri River, west of Yankton, South Dakota, the distinction between the Benton and the Niobrara is very clear. This is near Meek and Hayden's type location for the Niobrara, the Niobrara River. On the shores of Lewis and Clark Lake between Yankton and the Niobrara River, high bluffs of near white Fort Hays Limestone are perched above the top of the gray shales that Meek and Hayden named "Fort Benton". However, at their Fort Benton type location for the Benton Group, the Fort Hays Limestone layer is hardly distinct from the Benton Shale and is identifiable only by its major change in fossil species.
The Mancos Shale of the Colorado Plateau correlates with the Colorado Shale, and the Tokay Tongue of the lower Mancos is the synonym for the Benton Shale.[6]
Bentonite
There are many thin beds of volcanic ash in the unit that have devitrified into mostly montmorillonite. Taking its name from the formation, this material is called bentonite. Iron sulfide in the bentonite seams converts to rust when exposed to air resulting in orange lines across exposures of Benton shale and chalk.
Gallery
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Gray Benton Shale
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Gray Benton Shale
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Concretionary cone-in-cone structures in Benton Shale
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Bentonite seams in the Benton Shale
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Chalk rhythmites in "old Benton" Greenhorn member in Kansas
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Chalk rhythmites in "old Benton" Lower Carlile member in Kansas
See also
References
- ^ National Geologic Map Database - Geolex, USGS
- ^ Meek, F.B.; Hayden, F.V. (1862). "Descriptions of new Lower Silurian, (Primordial), Jurassic, Cretaceous, and Tertiary fossils, collected in Nebraska, by the exploring expedition under the command of Capt. Wm F. Reynolds, U.S. Top. Engineers, with some remarks on the rocks from which they were obtained". Academy of Natural Sciences of Philadelphia Proceedings. 13: 415–447.
- ^ Donald E. Hattin (1962). "Stratigraphy of the Carlile Shale (Upper Cretaceous) in Kansas". State Geological Survey of Kansas (Bulletin 156). Kansas Geological Survey, University of Kansas, Lawrence, Kansas. Retrieved 2018-08-04.
- ^ Donald E. Hattin (1962). "Stratigraphy and Depositional Environment of Greenhorn Limestone (Upper Cretaceous) of Kansas". State Geological Survey of Kansas (Bulletin 209). Kansas Geological Survey, University of Kansas, Lawrence, Kansas. Retrieved 2018-08-04.
- The Geological Society of America: 77–93. Retrieved 2022-06-06. (Benton Group is in current use in this location.)
- ^ Spencer G. Lucas; W. John Nelson; Karl Krainer; Scott D. Elrick (Spring 2019). "The Cretaceous System in central Sierra County, New Mexico" (PDF). New Mexico Geology. 41 (1): 10. Retrieved 2022-06-12.
Tokay Tongue, however, is simply a synonym of the unit that Meek and Hayden (1861) long ago named the "Fort Benton group" (more commonly called Benton Group or Benton Shale ... and that [name, Benton,] has long been abandoned in favor of a more detailed lithostratigraphic terminology
- dark claysof No. 2, of the Fort Benton Group.