Colorado Group

This article is about the stratigraphical unit. For the company, see Colorado Group Limited.

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, and as the geology of this seaway was studied, this name came to be used in states beyond Colorado, but was later replaced in several of these states with more localized names.

Colorado Group
Stratigraphic range: middle Albian to Santonian PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological formation
Sub-units	Colorado (type) and Kansas: Graneros, Greenhorn, Carlile, Niobrara Elsewhere: Medicine Hat Sandstone, Martin Sandy Zone, Second White Speckled Shale, Cardium Formation, Bowdoin Sandstone, First White Speckled Shale, Phillips Sandstone
Underlies	Montana Group, Lea Park Formation, Milk River Formation
Overlies	Dakota Group, Blairmore Group, Mannville Group, Swan River Group
Thickness	more than 1,000 metres (3,280 ft)^[1]
Lithology
Primary	Shale, Chalk, Sandstone
Other	Siltstone, Conglomerate, Limestone, concretionary beds
Location
Region	Alberta,  Saskatchewan,  Montana,  Wyoming,  Colorado
Country	US,  Canada
Type section
Named for	Colorado, specifically for the hogbacks and plains facing the Front Range of that state^[2]^[1]
Named by	F.V. Hayden^[2]^[3]

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).^[4]

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.^[5]

• 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.^[6]

• 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.^[2] The group was described by A. Hague and S.E. Emmons in 1877.^[7]

• However, by 1878, C. A. White restricted the Colorado Group to the Benton and Niobrara, the formations found within the flatirons and secondary hogbacks on the east flank of the Dakota Hogback.^[2][8][9][4]

•  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 (Fort Hays), and chalky Apishapa.^[13]  

• The Colorado Classification was initially adopted throughout the extent of the Western Interior Seaway. 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]

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 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.^[1]

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 1,000 metres (3,280 ft) in thickness in central Alberta. In central Saskatchewan, it thins to 150 metres (490 ft).^[1]

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 Colorado Group was previously named Lloydminster Shale in the Lloydminster region, but the term is now obsolete.

The Colorado Group is divided in Canada 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.

Subunits

The Colorado Group includes the following sub-units, from top to bottom:

Subdivision	Sub-unit	Age	Lithology	Max Thickness	Reference
Upper (calcareous)	First White Speckled Shale	Santonian	olive-black, chalk-speckled, calcareous and carbonaceous shale; minor shaly limestone	157 m (520 ft)	^[20]
	Medicine Hat Sandstone	Santonian	muddy sandstone and siltstone	14 m (50 ft)	^[21]
	Martin Sandy Zone	Santonian	Siltstone, fine grained sandstone and calcareous shale	60 m (200 ft)	^[22]
Lower (non-calcareous)	Cardium Sandstone	Turonian to Coniacian	marine sandstone	108 m (350 ft)	^[23]
	Bowdoin Sandstone	Turonian	very fine grained sandstone and siltstone, shale	61 m (200 ft)	^[24]
	Second White Speckled Shale	Turonian	olive-black, chalk-speckled, calcareous and carbonaceous shale; minor shaly limestone	70 m (230 ft)	^[25]
	Phillips Sandstone	Turonian	very fine grained sandstone and siltstone, dark calcareous shale	38 m (120 ft)	^[26]

Further reading

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

References

1. "Unit Name: Colorado Group". Weblex : Lexicon of Canadian Geologic Units. Retrieved 2021-04-23.
2. Donald E. Hattin (1982). "History of Stratigraphic Nomenclature". Stratigraphy and Depositional Environment of Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the Type Area, Western Kansas. Kansas Geological Survey. Retrieved April 23, 2021.
3. Geologic Unit: Colorado, Geolex.
4. W. G. Pierce and C. B. Hunt (1937). "Geology and Mineral resources of North-Central Chouteau, Western Hill, and Eastern Liberty Counties". Geology. U.S. Government Printing Office. United States Department of the Interior, Geology Survey. Contributions to Economic Geology. (Bulletin 847): 246–247.
5. 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.
6. 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. Washington: 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.
7. Hague, A. and Emmons, S.E., 1877. Descriptive geology. U.S. geological exploration of the fortieth parallel, v.2.
8. Donald E. Hattin (1965). Stratigraphy of the Graneros Shale (Upper Cretaceous) in Central Kansas, Kansas Geological Survey, Bulletin 178. University of Kansas Publications, State Geological Survey of Kansas. p. Stratigraphy). Retrieved 2021-05-20. 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.
9. 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.
10. Donald E. Hattin and Charles T. Siemers (1978). "Upper Cretaceous Stratigraphy and Depositional Environments of Western Kansas". Kansas Geological Survey Guidebook. Kansas Geological Survey (3). During the last decade of the 19th Century, considerable attention was focused upon the classification and description of Cretaceous rocks in western Kansas.
11. G. K. Gilbert (February–March 1895). "Sedimentary Measurement of Cretaceous Time". The Journal of Geology. University of Chicago Press. 3 (2): 121–127. Bibcode:1895JG......3..121G. doi:10.1086/607150. JSTOR 30054556. S2CID 129629329.
12. Sageman, B. B.; Rich, J.; Birchfield, G E; Arthur, M. A.; Dean, W. E. (1997). "Evidence for Milankovitch periodicities in Cenomanian-Turonian lithologic and geochemical cycles, Western Interior U.S.A." Journal of Sedimentary Research, Section A. 67 (2). Retrieved 2021-05-21. The complex bedding pattern observed in the Bridge Creek Limestone [upperGreenhorn 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. (1896). "The underground water of the Arkansas Valley in eastern Colorado". U.S. Geological Survey Annual Report. 17 (2): 551–601. Retrieved 28 March 2021.
14. "Colorado". geolex. Retrieved 21 May 2021.
15. Alan F. Arbogast, William C. Johnson (1996). Surficial geology and stratigraphy of Russell County, Kansas. Kansas Geological Survey Technical Series. Vol. 7. Retrieved 2021-05-21. 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. Geological Atlas of the Western Canada Sedimentary Basin - Chapter 20. . Archived from the original on 2010-10-24. Retrieved 2010-05-12.
17. Donald E. Hattin (1975). 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. p. 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. Khalifa Elderbak; Mark Leckie (2015). "Paleocirculation and foraminiferal assemblages of the Cenomanian–Turonian Bridge Creek Limestone bedding couplets: Productivity vs. dilution during OAE2". ResearchGate. Figure 4. Retrieved 2021-05-23.
19. Molenaar, C.M.; Cobban, W.A.; Merewether, E.A.; Pillmore, C.L.; Wolfe, D.G.; Holbrook, J.M. (2002). "Regional stratigraphic cross sections of Cretaceous rocks from east-central Arizona to the Oklahoma Panhandle". National Geologic Map Database. USGS. Retrieved May 23, 2021.
20. 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. ISBN 0-920230-23-7.
21. Lexicon of Canadian Geologic Units. "Medicine Hat Sandstone". Archived from the original on July 7, 2012. Retrieved 2009-03-01.
22. Lexicon of Canadian Geologic Units. "Martin Sandy Zone". Archived from the original on July 7, 2012. Retrieved 2009-03-01.
23. Lexicon of Canadian Geologic Units. "Cardium Sandstone". Archived from the original on 2012-07-07. Retrieved 2009-03-01.
24. Lexicon of Canadian Geologic Units. "Bowdoin Sandstone". Archived from the original on July 16, 2012. Retrieved 2009-03-01.
25. 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. ISBN 0-920230-23-7.
26. Lexicon of Canadian Geologic Units. "Phillips Sandstone". Archived from the original on July 10, 2012. Retrieved 2009-03-01.