Colorado Group Explained

Colorado Group
Type:	Geological formation
Period:	Cretaceous
Prilithology:	Shale, Chalk, Sandstone
Otherlithology:	Siltstone, Conglomerate, Limestone, concretionary beds
Namedfor:	Colorado, specifically for the hogbacks and plains facing the Front Range of that state
Namedby:	F.V. Hayden^[1]
Region:	,,,,
Country:	United States, Canada
Subunits:	Colorado (type) and Kansas: Graneros, Greenhorn, Carlile, Niobrara Canada: Phillips Sandstone, Second White Speckled Shale, Bowdoin Sandstone, Cardium Sandstone, Martin Sandy Zone, Medicine Hat Sandstone, First White Speckled Shale
Underlies:	Montana Group, Belly River Group, Lea Park Formation, Milk River Formation
Overlies:	Dakota Group, Blairmore Group, Mannville Group, Swan River Group
Thickness:	more than 1000m (3,000feet)^[2]

Colorado is a geologic name applied to certain rocks of Cretaceous age in the North America, particularly in the western Great Plains. This name was originally applied to classify a group of specific marine formations of shale and chalk known for their importance in Eastern Colorado. The surface outcrop of this group produces distinctive landforms bordering the Great Plains and it is a significant feature of the subsurface of the Denver Basin and the Western Canadian Sedimentary Basin. These formations record important sequences of the Western Interior Seaway. As the geology of this seaway was studied, this name came to be used in states beyond Colorado but later was replaced in several of these states with more localized names.

The USGS convention has been to use Colorado Group where the rocks are further divided into formations, Colorado Formation where no beds are developed enough to be mapped as formations, and Colorado Shale where the unit is composed of little more than shale with no distinctive structures (such as in north-central Montana).

History of exploration

• In 1862, F.B. Meek and F.V. Hayden described their “Upper Missouri River” series; Dakota, Benton, Pierre, Niobrara, and Fox Hills.^[3]
• In 1871, Hayden crossed Kansas and Colorado on the recently completed Kansas Pacific Railway, and between Abilene and Limon confirmed the whole series within the two states.^[4]
• Hayden proposed term "Colorado Group" in 1876 to embrace the Benton, Niobrara, and Pierre units for their collective exposures in the dramatic hogbacks and incised plateaus facing the Rocky Mountain front ranges of Colorado.^[5] The group was described by A. Hague and S.E. Emmons in 1877.^[6]
• However, by 1878, C. A. White restricted the Colorado Group to the Benton and Niobrara, which are the formations found within the flatirons and secondary hogbacks on the east flank of the Dakota Hogback.^{[7] [8] [9]}
• During the last decade of the 19th Century, Cretaceous rocks in Colorado and western Kansas were a focus of considerable study. The Benton and Niobrara were particularly associated in the Smoky Hills of Kansas, the Arkansas River valley across southeastern Colorado, and the Colorado Front Range. G. K. Gilbert observed that of Meek and Hayden's five Cretaceous units, only the Benton and Niobrara (incidentally the Colorado Group) were chalky; moreover, the chalkiness was expressed as curiously rhythmic bedding. A well-known glaciologist, Gilbert correctly theorized that the reason for these peculiar rhythmites was periodic astronomical solar forcing, and that these rhythms were absent in the Pierre only because there was no source of carbonate in the Pierre environment. His theory was confirmed with recognitions that Milankovitch cycles can be expressed in conditions of total global absence of glaciers.^{[10] [11] [12]}  Gilbert subsequently replaced the Benton with five formations based on the changes he observed in the chalkiness; non-chalky Graneros, chalky Greenhorn, non-chalky Carlile, massively chalky Timpas (later abandoned in favor of Fort Hays), and chalky Apishapa (later abandoned in favor of Smoky Hill).^[13]   
• The Colorado Classification was initially adopted throughout the extent of the Western Interior Seaway, including Canada. However, in the 21st Century, some states, such as Wyoming and Montana, are abandoning the term.^[14] Donald E. Hattin advised that the Colorado Group should not be used in Kansas because he considered its units to be "too lithologically diverse";^[15] yet, Colorado Group remains listed in the geologic succession in Kansas.

Summary of Colorado Group Classifications

Meek &<br/>Hayden 1862	Hayden 1871	White 1878	Colorado Kansas (current)	Canada (current)
				Belly River
	Colorado	Pierre		Colorado
		Colorado	Colorado
Benton
Dakota

Lithology

The Colorado Group consists primarily of chalky and non-chalky shale, and incorporates conglomerate, sandstone and siltstone, rhythmite beds of chalk, chalky limestone, coquinas, phosphorite, and concretionary beds including calcite, siderite, and pyrite.^[16]

The Canadian distribution presents with more sandstone. The lower part includes the following sandstone members: Phillips Sandstone (below the Second White Speckled Shale), Bowdoin Sandstone and Cardium Sandstone in the non-calcareous shale unit. The upper part includes the Martin Sandy Zone and Medicine Hat Sandstone.^[2]

Oil/gas production

Gas is produced from the sandstone members in southern Alberta, southern Saskatchewan and in Montana, such as in the Bowdoin gas field.

Distribution

Shales of middle Albian to Santonian age are distributed throughout much of the former extent of the Western Interior Seaway, including broadly from Arizona, to Iowa and Alberta. The Greenhorn-Carlile contact represents the maximum extent of the seaway of that sequence, perhaps of the entire time of the seaway; and, so, coupled with the Graneros Shale, the "old Benton" shales are the widest durable remnant of the Western Interior Seaway.^[17] Older literature may use the term Colorado Group in this extent, but several states outside of Colorado no longer use the term in current publications; nevertheless, the evidence of correlated seaway sequences and fossil patterns remains, regardless of current names.

From Iowa to Arizona, the lithology is remarkably consistent and the bentonites and rhythmic chalk beds of the upper Greenhorn especially are geologic events that can be traced over that distance.^[18] Beyond the historic western extent of the Colorado Group usage into the Mancos Shale, the chalky beds of the group can be identified and are named accordingly, e.g, Smoky Hill, Fort Hays, Bridge Creek, Greenhorn.^[19] However, northwest of the Transcontinental Arch where western sediment sources are more dominant, altering the lithology, these names have less current use.

The Canadian Colorado Group occurs in the sub-surface throughout southern and central Alberta, western and central Saskatchewan. It is found in outcrops along the south-western edge of the Canadian Shield. The sediments of the Colorado group exceed 1000m (3,000feet) in thickness in central Alberta. In central Saskatchewan, it thins to 150m (490feet).^[2]

Relationship to other units

The rocks of Colorado Group age are overlain by the Montana Group and underlain by the Dakota Group in the Denver Basin, Powder River Basin, and Williston Basin of the western Great Plains. In Western Canada, they are unconformably overlain by the Lea Park Formation shale and unconformably underlain by the Blairmore, Mannville or Swan River Group.

The lower part is equivalent with the Ashville Formation in eastern Saskatchewan and southern Manitoba, with the upper part corresponding to the Vermillion River Formation and Favel Formation. It is equivalent to the sum of Crowsnest Formation, Blackstone Formation, Cardium Formation, and the lower Wapiabi Formation of the Alberta Group in the Canadian Rockies foothills. It correlates with the upper Smoky Group, Dunvegan Formation, Shaftesbury Formation, Paddy Member and Labiche Formation in northern Alberta. The Canadian Colorado Group was previously named Lloydminster Shale in the Lloydminster region, but the term is now obsolete.

Sub-units

United States sub-unit classifications have generally derived from the related Colorado and Kansas classifications, from top to bottom:

Colorado and Kansas classification of regional Colorado Group sub-units^[20]

			Lithology
Niobrara (calcareous)			repeating beds of buff chalk and chalky shale
			massive chalky limestone
Carlile (carbonaceous)
	Codell Sandstone		very fine-grained sandstone and siltstone, shale
	Blue Hill Shale		olive-black, carbonaceous shale; repeating thin bentonites; septarians
	Fairport Shale		olive-black, carbonaceous shale to chalky shale with repeating thin chalks; repeating thin bentonites
Greenhorn (calcareous)	Bridge Creek/ Pfeifer Shale		bench-forming Fencepost limestone marker bed olive-black, chalky shale with repeating, thin chalky limestone beds; repeating thin bentonites
	Bridge Creek/ Jetmore Chalk		bench-forming Shellrock marker bed chalky shale with very rhythmic, thin limestone beds
	Hartland Shale		chalky shale almost devoid of limestone
	Lincoln Limestone		shaly chalk with many scattered, thin skeletal limestone beds; repeating thin bentonites X-bentonite marker bed (Colorado)
Graneros (carbonaceous)	(Thatcher Limestone)		X-bentonite marker bed (Kansas) finely sandy, clayey, gray near-shore/marginal-marine shale; septarians

The Colorado Group in Canada, which retains the Santonian-time Pierre Shale correlations, is divided into an upper part which is calcareous, and a lower part, which is non-calcareous. The sub-units are defined at the base of two regional markers, called First and Second White Speckled Shale characterized by coccolithic debris.

The Canadian Geological Survey classification of the Colorado Group includes the following sub-units, from top to bottom:

Canadian classification of regional Colorado Group subunits

Subdivision			Lithology	Max Thickness	Reference
Upper (calcareous)			olive-black, chalk-speckled, calcareous and carbonaceous shale; minor shaly limestone	157-1NaN-1	^[21]
			muddy sandstone and siltstone	14-1NaN-1	^[22]
			Siltstone, fine grained sandstone and calcareous shale	60-1NaN-1	^[23]
Lower (non-calcareous)			marine sandstone	108-1NaN-1	^[24]
			very fine-grained sandstone and siltstone, shale	61-1NaN-1	^[25]
			olive-black, chalk-speckled, calcareous and carbonaceous shale; minor shaly limestone	70-1NaN-1	^[26]
			very fine grained sandstone and siltstone, dark calcareous shale	38-1NaN-1	^[27]

Further reading

• Open File Report . 534 . Karen W. Porter, Jennie Ridgley . Field Trip # 1 Marine Cretaceous Reservoirs in Central and Northern Montana: Road Log for Day 1 . 2006 . Montana Bureau of Mines and Geology . 2022-07-03 . - Correlation chart for the field trip over outcrops of the original Colorado classification.

Notes and References

1. https://ngmdb.usgs.gov/Geolex/Units/Colorado_7668.html Geologic Unit: Colorado
2. Web site: Unit Name: Colorado Group. Weblex : Lexicon of Canadian Geologic Units . 2021-04-23.
3. . . 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.
4. Book: F. V. Hayden, United States Geologist . Final Reports of the United States Geological Survey of Nebraska and Portions of the Adjacent Territories . IX. Sketch of the geological formations along the route of the Union Pacific Railway, Eastern Division . House Documents, otherwise Publ. as Executive Documents United States. Congress. House. . https://books.google.com/books?id=wqEFAAAAQAAJ&pg=RA1-PA66 . 66–69 . 1871 . Washington. Government Printing Office . 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. . 2018-10-04 .
5. Book: . Stratigraphy and Depositional Environment of Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the Type Area, Western Kansas . . 1982 . http://www.kgs.ku.edu/Publications/Bulletins/225/03_nomen.html . History of Stratigraphic Nomenclature . April 23, 2021 .
6. Hague, A. and Emmons, S.E., 1877. Descriptive geology. U.S. geological exploration of the fortieth parallel, v.2.
7. Book: Donald E. Hattin . Donald E. Hattin . Stratigraphy of the Graneros Shale (Upper Cretaceous) in Central Kansas, Kansas Geological Survey, Bulletin 178. University of Kansas Publications, State Geological Survey of Kansas. 1965 . Stratigraphy) . "Later, Hayden (1876, p. 45) coined the term "Colorado Group" for No. 2 (Fort Benton), No. 3 (Niobrara), and No.4 (Pierre) of Hall and Meek's Nebraska section. The Colorado Group was restricted by White (1878, p. 21) to units No. 2 and No. 3 and has remained thus defined to the present." . 2021-05-20 .
8. White, C. A., 1878, Report on the geology of a portion of northwestern Colorado: U.S. Geol. and Geog. Surv. Territories, Ann. Rept, 10, p. 5-60.
9. W. G. Pierce and C. B. Hunt . Geology and Mineral resources of North-Central Chouteau, Western Hill, and Eastern Liberty Counties . United States Department of the Interior, Geology Survey. Contributions to Economic Geology. . Bulletin 847 . 246–247 . U.S. Government Printing Office . 1937 . Geology .
10. Upper Cretaceous Stratigraphy and Depositional Environments of Western Kansas . Donald E. Hattin and Charles T. Siemers . Kansas Geological Survey Guidebook . 3. Kansas Geological Survey . 1978 . During the last decade of the 19th Century, considerable attention was focused upon the classification and description of Cretaceous rocks in western Kansas. .
11. . . February–March 1895 . Sedimentary Measurement of Cretaceous Time . . 3 . 2 . 121–127 . 10.1086/607150 . 30054556 . 1895JG......3..121G . 129629329 . free .
12. Sageman, B. B. . Rich, J. . Birchfield, G E . Arthur, M. A. . Dean, W. E.. 2021-05-21 . Journal of Sedimentary Research, Section A . Evidence for Milankovitch periodicities in Cenomanian-Turonian lithologic and geochemical cycles, Western Interior U.S.A. . 67 . 2 . 1997 . 460584 . The complex bedding pattern observed in the Bridge Creek Limestone [upper[[Greenhorn Limestone]]] is interpreted to result from the competing influences of different orbital cycles expressed through different pathways of the depositional system … .
13. Gilbert . G.K. . Grove Karl Gilbert . 1896 . The underground water of the Arkansas Valley in eastern Colorado . U.S. Geological Survey Annual Report . 17 . 2 . 551–601 . 28 March 2021.
14. Web site: geolex . Colorado . 21 May 2021 .
15. Book: Alan F. Arbogast, William C. Johnson . 2021-05-21 . Surficial geology and stratigraphy of Russell County, Kansas. 7 . 1996. Kansas Geological Survey Technical Series . Regarding the inclusion of all Cretaceous rocks in Russell County within the Colorado Group, Hattin (personal communication) suggests that the term Colorado Group be discontinued because the units are too lithologically diverse to be included within one group. As a result, the term Colorado Group is not used in this report. .
16. Web site: Cretaceous Colorado / Alberta Group of the Western Canada Sedimentary Basin. Geological Atlas of the Western Canada Sedimentary Basin - Chapter 20. 2010-05-12. dead. https://web.archive.org/web/20101024130709/http://www.ags.gov.ab.ca/publications/wcsb_atlas/A_CH20/CH_20.html. 2010-10-24.
17. Book: Donald E. Hattin . Stratigraphy and Depositional Environment of Greenhorn Limestone (Upper Cretaceous) of Kansas, Kansas Geological Survey, Bulletin 209 . University of Kansas Publications, State Geological Survey of Kansas . 1975 . Depositional Environment and Paleoecology . Maximum transgression (Fig. 22) is represented by relatively pure pelagic carbonates of the Jetmore and Pfeifer Members of the Greenhorn; for this reason the sequence was named Greenhorn cyclothem by Hattin (1962, p. 124). .
18. Web site: Paleocirculation and foraminiferal assemblages of the Cenomanian–Turonian Bridge Creek Limestone bedding couplets: Productivity vs. dilution during OAE2 . Figure 4 . ResearchGate . Khalifa Elderbak . Mark Leckie . 2015 . 2021-05-23 .
19. Web site: Regional stratigraphic cross sections of Cretaceous rocks from east-central Arizona to the Oklahoma Panhandle . National Geologic Map Database . Molenaar, C.M. . Cobban, W.A. . Merewether, E.A. . Pillmore, C.L. . Wolfe, D.G. . Holbrook, J.M. . 2002 . . May 23, 2021 .
20. Book: Donald E. Hattin . Donald E. Hattin . Stratigraphy and Depositional Environment of Greenhorn Limestone (Upper Cretaceous) of Kansas, Kansas Geological Survey, Bulletin 209 . University of Kansas Publications, State Geological Survey of Kansas . 1975 . Stratigraphy of the Greenhorn Limestone .
21. Glass, D.J. (editor) 1997. Lexicon of Canadian Stratigraphy, vol. 4, Western Canada including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba, p. 452. Canadian Society of Petroleum Geologists, Calgary, 1423 p. on CD-ROM. .
22. Web site: Medicine Hat Sandstone. https://archive.today/20120707220931/http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:009567. dead. July 7, 2012. Lexicon of Canadian Geologic Units. 2009-03-01.
23. Web site: Martin Sandy Zone. https://archive.today/20120707045411/http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:009262. dead. July 7, 2012. Lexicon of Canadian Geologic Units. 2009-03-01.
24. Web site: Cardium Sandstone . https://archive.today/20120707181825/http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:002448 . dead . 2012-07-07 . Lexicon of Canadian Geologic Units . 2009-03-01 .
25. Web site: Bowdoin Sandstone. https://archive.today/20120716121748/http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:001739. dead. July 16, 2012. Lexicon of Canadian Geologic Units. 2009-03-01.
26. Glass, D.J. (editor) 1997. Lexicon of Canadian Stratigraphy, vol. 4, Western Canada including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba, p. 1052. Canadian Society of Petroleum Geologists, Calgary, 1423 p. on CD-ROM. .
27. Web site: Phillips Sandstone. https://archive.today/20120710100248/http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:011796. dead. July 10, 2012. Lexicon of Canadian Geologic Units. 2009-03-01.