Benton Shale

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Benton Shale
Stratigraphic range: Cretaceous
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Shale bluffs at Cow Island, Montana in the Missouri River Breaks adjoining Fort Benton, the type location of the Benton Shale. As seen in this image, the overlying Niobrara Formation is not as distinctive from the Benton Shale as in the lower plains and these shales were later reclassified as Colorado Shale. The Benton name was abandoned here in favor of subdivision as Belle Fourche Shale, Greenhorn Limestone, and Carlile Shale.
Type Formation
Underlies Niobrara Formation
Overlies Dakota Sandstone
Lithology
PrimaryShale, chalky shale, chalk beds
Othermany bentonite seams,
septarians,
selenite,
occasional sandstone
Location
Region Montana, Wyoming, North Dakota, South Dakota, Colorado, Nebraska, Kansas
CountryUnited States
Type section
Named for Fort Benton, Montana
Named by Meek, F.B. and Hayden, F.V.
Year defined1862

The Benton Shale (also Benton Formation or Benton Group) is a geologic formation name historically used in Montana, Wyoming, North Dakota, South Dakota, Colorado, Kansas, and Nebraska. [1] In the "mile high" plains in the center of the continent, the named layers preserve marine fossils from the Late Cretaceous Period. The term Benton Limestone has also been used to refer to the chalky portions of the strata, especially the beds of the strata presently classified as Greenhorn Limestone, particularly the Fencepost limestone.

Contents

Naming and status

The name was applied by Meek, F.B. and Hayden, F.V. in 1862 to the gray marine shales, often chalky in the middle layers, lying above the terrestrial Dakota Sandstone and usually below the massive limestones at the base of the Niobrara Chalk. The name was taken from the type outcrop at Fort Benton, today a small city in Montana on the Upper Missouri River. [2]

Today, the Benton classification is obsolete in some regions, having been replaced by the ascending sequence Graneros Shale/Belle Fourche Shale, Greenhorn Limestone, and Carlile Shale. [3] [4] However, many old publications used the name. And the use of the Benton Group name continues in the Front Range where the Graneros Shale, Greenhorn Limestone, Carlile Shale, and Codell Sandstone may be recognized as member units. [5]

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.

See also

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The Graneros Shale is a geologic formation in the United States identified in the Great Plains as well as New Mexico that dates to the Cenomanian Age of the Cretaceous Period. It is defined as the finely sandy argillaceous or clayey near-shore/marginal-marine shale that lies above the older, non-marine Dakota sand and mud, but below the younger, chalky open-marine shale of the Greenhorn. This definition was made in Colorado by G. K. Gilbert and has been adopted in other states that use Gilbert's division of the Benton's shales into Carlile, Greenhorn, and Graneros. These states include Kansas, Texas, Oklahoma, Nebraska, and New Mexico as well as corners of Minnesota and Iowa. North Dakota, South Dakota, Wyoming, and Montana have somewhat different usages — in particular, north and west of the Black Hills, the same rock and fossil layer is named Belle Fourche Shale.

<span class="mw-page-title-main">Greenhorn Limestone</span> Geologic formation in the United States

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Juana Lopez refers to both the uppermost member of the Carlile Shale formation and to the environment that caused it to form. The Juana Lopez Member is calcareous sandstone dated to the Turonian age of the Upper Cretaceous and is exposed in the southern and western Colorado, northern and central New Mexico, and northeastern Utah. The unit has been described as "the most enigmatic" member of the Carlile Shale.

<span class="mw-page-title-main">Fencepost limestone</span>

Fencepost limestone, Post Rock limestone, or Stone Post is a stone bed in the Great Plains notable for its historic use as fencing and construction material in north-central Kansas resulting in unique cultural expression. The source of this stone is the topmost layer of the Greenhorn Limestone formation. It is a regional marker bed as well as a valued construction material of the late 19th and early 20th centuries in Kansas. This stone was very suitable for early construction in treeless settlements and it adds a notable rust orange tint to the region's many historic stone buildings. But the most famous use is seen in the countless miles of stone posts lining country roads and highways. This status gives rise to such regional appellations as Stone Post Country, Post Rock Scenic Byway, and The Post Rock Capital of Kansas. This rustic quality finds Fencepost limestone still used in Kansas landscaping today.

References

  1. National Geologic Map Database - Geolex, USGS
  2. 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.
  3. Donald E. Hattin (1962). "Stratigraphy of the Carlile Shale (Upper Cretaceous) in Kansas". State Geological Survey of Kansas. Kansas Geological Survey, University of Kansas, Lawrence, Kansas (Bulletin 156). Retrieved 2018-08-04.
  4. Donald E. Hattin (1962). "Stratigraphy and Depositional Environment of Greenhorn Limestone (Upper Cretaceous) of Kansas". State Geological Survey of Kansas. Kansas Geological Survey, University of Kansas, Lawrence, Kansas (Bulletin 209). Retrieved 2018-08-04.
  5. Marcus R. Ross; William A. Hoesch; Steven A. Austin; John H. Whitmore; Timothy L. Clarey (2010). "Garden of the Gods at Colorado Springs: Paleozoic and Mesozoic sedimentation and tectonics". Field Guide. The Geological Society of America (18): 77–93. Retrieved 2022-06-06. (Benton Group is in current use in this location.)
  6. 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
  7. F. V. Hayden, United States Geologist (1871). "IX. Sketch of the geological formations along the route of the Union Pacific Railway, Eastern Division". Final Reports of the United States Geological Survey of Nebraska and Portions of the Adjacent Territories. House Documents, otherwise Publ. as Executive Documents United States. Congress. House. Government Printing Office. pp. 66–69. Retrieved 2018-10-04. At Hays City the massive rocky layers of No. 3 are sawed into blocks, and employed in the construction of buildings. ... About eight miles west of Hays City there are about 60 feet exposed, of the dark clays of No. 2, of the Fort Benton Group.