The Benton Shale (also Benton Formation or Benton Group) is a
geologic formation name historically used in
Montana
Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columbi ...
,
Wyoming
Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...
,
North Dakota
North Dakota () is a U.S. state in the Upper Midwest, named after the Native Americans in the United States, indigenous Dakota people, Dakota Sioux. North Dakota is bordered by the Canadian provinces of Saskatchewan and Manitoba to the north a ...
,
South Dakota
South Dakota (; Sioux language, Sioux: , ) is a U.S. state in the West North Central states, North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Lakota people, Lakota and Dakota peo ...
,
Colorado
Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...
,
Kansas
Kansas () is a state in the Midwestern United States. Its capital is Topeka, and its largest city is Wichita. Kansas is a landlocked state bordered by Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to the ...
, and
Nebraska
Nebraska () is a state in the Midwestern region of the United States. It is bordered by South Dakota to the north; Iowa to the east and Missouri to the southeast, both across the Missouri River; Kansas to the south; Colorado to the southwe ...
. In the "mile high" plains in the center of the continent, the named layers preserve ''marine''
fossils
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
from the
Late Cretaceous
The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...
Period
Period may refer to:
Common uses
* Era, a length or span of time
* Full stop (or period), a punctuation mark
Arts, entertainment, and media
* Period (music), a concept in musical composition
* Periodic sentence (or rhetorical period), a concept ...
. 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
The Greenhorn Limestone or Greenhorn Formation is a geologic formation in the Great Plains Region of the United States, dating to the Cenomanian and Turonian ages of the Late Cretaceous period. The formation gives its name to the Greenhorn cycle ...
, particularly the
Fencepost limestone
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 sto ...
.
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
The Dakota is a sedimentary rock, sedimentary geologic unit name of Formation (stratigraphy), formation and Group (stratigraphy), group rank in Midwestern North America. The Dakota units are generally composed of sandstones, mudstones, clays, and ...
and usually below the massive limestones at the base of the
Niobrara Chalk
The Niobrara Formation , also called the Niobrara Chalk, is a geologic formation in North America that was deposited between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian stages of the Late Cretaceous. It is compose ...
. The name was taken from the type outcrop at
Fort Benton, today a small city in Montana on the Upper Missouri River.
Today, the Benton classification is obsolete in some regions, having been replaced by the ascending sequence
Graneros Shale
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/ ...
/
Belle Fourche Shale, Greenhorn Limestone, and
Carlile Shale
The Carlile Shale is a Turonian age Upper/Late Cretaceous series shale geologic formation in the central-western United States, including in the Great Plains region of Colorado, Kansas, Nebraska, New Mexico, North Dakota, South Dakota, and Wyom ...
. However, many old publications used the name. And the use of the Benton Group name continues in the
Front Range
The Front Range is a mountain range of the Southern Rocky Mountains of North America located in the central portion of the U.S. State of Colorado, and southeastern portion of the U.S. State of Wyoming. It is the first mountain range encountere ...
where the Graneros Shale, Greenhorn Limestone, Carlile Shale, and
Codell Sandstone
Codell is an unincorporated community in Rooks County, Kansas, United States. As of the 2020 census, the population of the community and nearby areas was 49.
History
Codell was established as a Union Pacific Railroad depot in 1887. Union Paci ...
may be recognized as member units.
[ (Benton Group is in current use in this location.)]
In the lower Missouri River, west of
Yankton, South Dakota
Yankton is a city in and the county seat of Yankton County, South Dakota, United States.
The population was 15,411 at the 2020 census, and it is the principal city of the Yankton Micropolitan Statistical Area, which includes the entirety of Y ...
, 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
The Niobrara River (; oma, Ní Ubthátha khe, , literally "water spread-out horizontal-the" or "The Wide-Spreading Water") is a tributary of the Missouri River, approximately long,U.S. Geological Survey. Many early settlers, such as Mari Sando ...
. On the shores of
Lewis and Clark Lake
Lewis and Clark Lake is a 31,400 acre (130 km²) reservoir located on the border of the U.S. states of Nebraska and South Dakota on the Missouri River. The lake is approximately in length with over of shoreline and a maximum water depth ...
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
The Mancos Shale or Mancos Group is a Late Cretaceous (Upper Cretaceous) geologic formation of the Western United States.
The Mancos Shale was first described by Cross and Purington in 1899 and was named for exposures near the town of Mancos, ...
of the
Colorado Plateau
The Colorado Plateau, also known as the Colorado Plateau Province, is a physiographic and desert region of the Intermontane Plateaus, roughly centered on the Four Corners region of the southwestern United States. This province covers an area of ...
correlates with the Colorado Shale, and the Tokay Tongue of the lower Mancos is the synonym for the Benton Shale.
Bentonite
There are many thin beds of volcanic ash in the unit that have
devitrified into mostly
montmorillonite
Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystals, known as clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite group, ...
. Taking its name from the formation, this material is called
bentonite
Bentonite () is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-mon ...
.
Iron sulfide
Iron sulfide or Iron sulphide can refer to range of chemical compounds composed of iron and sulfur.
Minerals
By increasing order of stability:
* Iron(II) sulfide, FeS
* Greigite, Fe3S4 (cubic)
* Pyrrhotite, Fe1−xS (where x = 0 to 0.2) (monoclin ...
in the bentonite seams converts to rust when exposed to air resulting in orange lines across exposures of Benton shale and chalk.
Gallery
;
Red Rock Canyon Open Space
Red Rock Canyon Open Space is a city park in Colorado Springs, Colorado. It is situated on the west side of the city, adjacent to Manitou Springs and south of U.S. Route 24. The park consists of a series of parallel ridges (called " hogbacks" ...
,
Colorado Springs, Colorado
Colorado Springs is a home rule municipality in, and the county seat of, El Paso County, Colorado, United States. It is the largest city in El Paso County, with a population of 478,961 at the 2020 United States Census, a 15.02% increase since ...
File:Gray shale (Benton Shale, Upper Cretaceous; Red Rock Canyon Open Space, Colorado Springs, Colorado, USA) 6.jpg, Gray Benton Shale
File:Gray shale (Benton Shale, Upper Cretaceous; Red Rock Canyon Open Space, Colorado Springs, Colorado, USA) 1.jpg, Gray Benton Shale
File:Concretionary cone-in-cone structures (Benton Shale, Upper Cretaceous; Red Rock Canyon Open Space, Colorado Springs, Colorado, USA) 13.jpg, Concretionary cone-in-cone structures in Benton Shale
File:Gray shale & bentonites (Benton Shale, Upper Cretaceous; Red Rock Canyon Open Space, Colorado Springs, Colorado, USA) 5.jpg, Bentonite seams in the Benton Shale
;Other locations
File:No. 51. The 'Hog Back' at Ellis, Kansas. (7008429651).jpg, Benton Group outcrop identified in Kansas by F.V. Hayden in 1871
File:Greenhorn_Limestone_on_Interstate_70_in_Kansas.png, Chalk rhythmites in "old Benton" Greenhorn
Greenhorn is a slang for an inexperienced person, or a slur against Portuguese people in New England, United States. It may also refer to:
Places
* Greenhorn, California, United States
* Greenhorn Mountain, a mountain in Colorado
* Greenhorn, O ...
member in Kansas
File:Fairport_Chalk_Member_of_the_Carlile_Formation_in_Ellis_County,_Kansas_01.png, Chalk rhythmites in "old Benton" Lower Carlile member in Kansas
See also
*
List of fossiliferous stratigraphic units in Montana
This article contains a list of fossil-bearing stratigraphic units in the state of Montana, U.S.
Sites
See also
* Paleontology in Montana
References
*
{{DEFAULTSORT:Fossiliferous stratigraphic units in Montana
Montana
Stratigraphic u ...
*
Paleontology in Montana
Paleontology in Montana refers to paleontological research occurring within or conducted by people from the U.S. state of Montana. The fossil record in Montana stretches all the way back to the Precambrian. During the Late Precambrian, western Mo ...
References
Shale formations of the United States
Cretaceous Colorado
Cretaceous Montana
Cretaceous geology of Wyoming
Cretaceous geology of North Dakota
Cretaceous geology of South Dakota
Cretaceous geology of Nebraska
{{Cretaceous-stub