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{{Short description|Rock formations in North America}}
{{about|the stratigraphical unit|the company|Colorado Group Limited}}

{{Infobox Rockunit
| name = Colorado Group
| image =
| caption =
| type = [[Geological formation]]
| age = middle [[Albian]] to [[Santonian]] {{Fossil range|110|83| }}
| period = Cretaceous
| prilithology = [[Shale]], [[Chalk]], [[Sandstone]]
| otherlithology = [[Siltstone]], [[Conglomerate (geology)|Conglomerate]], [[Limestone]], concretionary beds
| namedfor = [[Colorado]], specifically for the hogbacks and plains facing the [[Front Range]] of that state<ref name= KGSB225/><ref name=lexicon/>
| namedby = [[Ferdinand Vandeveer Hayden|F.V. Hayden]]<ref name= KGSB225/><ref>[https://ngmdb.usgs.gov/Geolex/Units/Colorado_7668.html Geologic Unit: Colorado], Geolex.</ref>
| region = [[Alberta]], [[Saskatchewan]], [[Montana]], [[Wyoming]], [[Colorado]]
| country = [[United States]], [[Canada]]
| coordinates =
| unitof =
| subunits = '''Colorado (type) and Kansas:''' [[Graneros Shale|Graneros]], [[Greenhorn Limestone|Greenhorn]], [[Carlile Shale|Carlile]], [[Niobrara Formation|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 {{convert|1000|m|ft|-1}}<ref name=lexicon>{{Cite web|url=https://weblex.nrcan.gc.ca/html/003000/GSCC00053003233.html|title= Unit Name: Colorado Group|work = Weblex : Lexicon of Canadian Geologic Units |access-date= 2021-04-23}}</ref>
| extent =
| area =
| map =
| map_caption =
}}

'''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 (stratigraphy)|group]] of specific marine [[Formation (stratigraphy)|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 [[Sequence (geology)|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 [[United States Geological Survey|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).<ref name= PierceHunt />

== History of exploration ==
* In 1862, [[Fielding Bradford Meek|F.B. Meek]] and [[Ferdinand Vandeveer Hayden|F.V. Hayden]] described their “Upper [[Missouri River]]” series; [[Dakota Formation|Dakota]], [[Benton Shale|Benton]], [[Pierre Shale|Pierre]], [[Niobrara Shale|Niobrara]], and [[Fox Hills Formation|Fox Hills]].<ref name="meekandhayden1862">{{cite journal |author=[[Fielding Bradford Meek|Meek, F.B.]] |author2=[[Ferdinand Vandeveer Hayden|Hayden, F.V.]] |year= 1862 |title= 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 |journal= Academy of Natural Sciences of Philadelphia Proceedings |volume= 13 |page= 415-447|url= https://babel.hathitrust.org/cgi/pt?id=hvd.32044107306102&view=1up&seq=3 }}</ref>

* In 1871, Hayden crossed Kansas and Colorado on the recently completed [[Kansas Pacific Railway]], and between [[Abilene, Kansas|Abilene]] and [[Limon, Colorado|Limon]] confirmed the whole series within the two states.<ref>{{cite book |author= [[Ferdinand Vandeveer Hayden|F. V. Hayden]], United States Geologist |title= Final Reports of the United States Geological Survey of Nebraska and Portions of the Adjacent Territories |chapter= IX. Sketch of the geological formations along the route of the Union Pacific Railway, Eastern Division |series= House Documents, otherwise Publ. as Executive Documents United States. Congress. House. |chapter-url= https://books.google.com/books?id=wqEFAAAAQAAJ&pg=RA1-PA66 |pages = 66–69 |year= 1871 |location= Washington|publisher= Government Printing Office |quote = At Hays City the massive [[Fort Hays Limestone Member|rocky layers]] of [[Niobrara Formation|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 [[Carlile Formation|dark clays]] of No. 2, of the [[Benton Shale|Fort Benton Group]]. |access-date= 2018-10-04 }}</ref>

* Hayden proposed term "Colorado Group" in 1876 to embrace the [[Benton Shale|Benton]], [[Niobrara Shale|Niobrara]], and [[Pierre]] units for their collective exposures in the dramatic hogbacks and incised plateaus facing the [[Rocky Mountains|Rocky Mountain]] front ranges of [[Colorado]].<ref name= KGSB225>{{cite book |author= [[Donald E. Hattin]] |title= Stratigraphy and Depositional Environment of Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the Type Area, Western Kansas |publisher= [[Kansas Geological Survey]] |date= 1982 |section-url=http://www.kgs.ku.edu/Publications/Bulletins/225/03_nomen.html |section= History of Stratigraphic Nomenclature |access-date= April 23, 2021 }}</ref> The group was described by A. Hague and S.E. Emmons in 1877.<ref>Hague, A. and Emmons, S.E., 1877. Descriptive geology. U.S. geological exploration of the fortieth parallel, v.2.</ref>

* However, by 1878, C. A. White restricted the Colorado Group to the Benton and Niobrara, which are the formations found within the [[Flatiron (geomorphology)|flatirons]] and secondary hogbacks on the east flank of the [[Dakota Hogback]].<ref name= KGSB225 /><ref name="HattinGraneros" >{{cite book | author = Donald E. Hattin | author-link = Donald E. Hattin | title = Stratigraphy of the Graneros Shale (Upper Cretaceous) in Central Kansas, Kansas Geological Survey, Bulletin 178| publisher = University of Kansas Publications, State Geological Survey of Kansas| date = 1965 | url = http://www.kgs.ku.edu/Publications/Bulletins/178/03_strat.html | page = Stratigraphy) |quote= "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." |access-date= 2021-05-20 }}</ref><ref>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.</ref><ref name= PierceHunt >{{cite journal |author= W. G. Pierce and C. B. Hunt |title= Geology and Mineral resources of North-Central Chouteau, Western Hill, and Eastern Liberty Counties |via= United States Department of the Interior, Geology Survey. Contributions to Economic Geology. |issue= Bulletin 847 |pages= 246–247 |publisher= U.S. Government Printing Office |year= 1937 |journal= Geology |url= https://books.google.com/books?id=_ogeAQAAIAAJ}}</ref>

* 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]]. [[Grove Karl Gilbert|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 [[Glaciology|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.<ref>{{cite journal |url= http://www.kgs.ku.edu/Publications/Bulletins/GB3/ |title=Upper Cretaceous Stratigraphy and Depositional Environments of Western Kansas |author=Donald E. Hattin and Charles T. Siemers |journal=Kansas Geological Survey Guidebook |issue= 3|publisher=Kansas Geological Survey |year=1978 |quote= During the last decade of the 19th Century, considerable attention was focused upon the classification and description of Cretaceous rocks in western Kansas. }}</ref><ref>{{cite journal |author= [[Grove Karl Gilbert|G. K. Gilbert]] |journal= [[The Journal of Geology]] |date= February–March 1895 |title= Sedimentary Measurement of Cretaceous Time |publisher= [[University of Chicago Press]] |volume= 3 |issue= 2 |pages= 121–127 |doi= 10.1086/607150 |jstor= 30054556 |bibcode= 1895JG......3..121G |s2cid= 129629329 |doi-access= free }} </ref><ref>{{cite journal |author=Sageman, B. B. |author2=Rich, J. |author3=Birchfield, G E |author4=Arthur, M. A. |author5=Dean, W. E.|access-date= 2021-05-21 |journal=Journal of Sedimentary Research, Section A |title= Evidence for Milankovitch periodicities in Cenomanian-Turonian lithologic and geochemical cycles, Western Interior U.S.A. |volume= 67 |issue= 2 |date= 1997 |osti=460584 |url= https://www.osti.gov/biblio/460584 |quote= 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 … }}</ref> Gilbert subsequently replaced the Benton with five formations based on the changes he observed in the chalkiness; non-chalky [[Graneros Shale|Graneros]], chalky [[Greenhorn Limestone|Greenhorn]], non-chalky [[Carlile Shale|Carlile]], massively chalky [[Timpas Limestone|Timpas]] (later abandoned in favor of [[Fort Hays Limestone Member|Fort Hays]]), and chalky [[Smoky Hill Chalk|Apishapa]] (later abandoned in favor of [[Smoky Hill Chalk|Smoky Hill]]).<ref name="gilbert-1896">{{cite journal |last1=Gilbert |first1=G.K. |author1-link=Grove Karl Gilbert |year=1896 |title=The underground water of the Arkansas Valley in eastern Colorado |journal=U.S. Geological Survey Annual Report |volume=17 |number=2 |pages=551–601 |url=https://books.google.com/books?id=aKMNAAAAYAAJ |access-date=28 March 2021}}</ref>

* 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.<ref>{{cite web |website= geolex |title= Colorado |url= https://ngmdb.usgs.gov/Geolex/UnitRefs/ColoradoRefs_7668.html |access-date=21 May 2021 }}</ref> [[Donald E. Hattin]] advised that the Colorado Group should not be used in Kansas because he considered its units to be "too lithologically diverse";<ref>{{cite book |author= Alan F. Arbogast, William C. Johnson |access-date= 2021-05-21 |title= Surficial geology and stratigraphy of Russell County, Kansas|volume= 7 |date= 1996|series= Kansas Geological Survey Technical Series |url= http://www.kgs.ku.edu/Publications/Bulletins/TS7/02_intro.html|quote= 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. }}</ref> yet, Colorado Group remains listed in the geologic succession in Kansas.<ref>{{citation |title= Classification of Rocks in Kansas (Kansas Stratigraphic Chart) |year= 2018 |publisher= [[Kansas Geological Survey]] |url=https://www.kgs.ku.edu/Publications/Bulletins/189/13_chart.html |access-date= 2023-11-22 }}</ref>

{|class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"
|+ Summary of Colorado Group Classifications
|-
! [[Fielding Bradford Meek|Meek]] & [[Ferdinand Vandeveer Hayden|Hayden]] 1862 !! Hayden 1871 !! White 1878 !! Colorado Kansas (current) !! Canada (current)
|-
| colspan=3 | [[Fox Hills Formation|Fox Hills]] || rowspan=2 | [[Montana Group|Montana]] ||style="white-space: nowrap;"| [[Belly River Group|Belly River]]
|-
|[[Pierre Shale|Pierre]] || rowspan=3 | Colorado || Pierre || rowspan=3 | Colorado
|-
|[[Niobrara Formation|Niobrara]] || rowspan=2 | Colorado || rowspan=2 | Colorado
|-
|[[Benton Shale |Benton ]]
|-
|colspan=5 | [[Dakota Formation|Dakota]]
|-
|}

==Lithology==
The Colorado Group consists primarily of chalky and non-chalky [[shale]], and incorporates [[Conglomerate (geology)|conglomerate]], [[sandstone]] and [[siltstone]], [[rhythmite]] beds of [[chalk]], chalky [[limestone]], [[coquina]]s, [[phosphorite]], and concretionary beds including [[calcite]], [[siderite]], and [[pyrite]].<ref>{{Cite web|url=http://www.ags.gov.ab.ca/publications/wcsb_atlas/a_ch20/ch_20.html|title=Cretaceous Colorado / Alberta Group of the Western Canada Sedimentary Basin|author=Geological Atlas of the Western Canada Sedimentary Basin - Chapter 20|accessdate=2010-05-12|url-status=dead|archiveurl=https://web.archive.org/web/20101024130709/http://www.ags.gov.ab.ca/publications/wcsb_atlas/A_CH20/CH_20.html|archivedate=2010-10-24}}</ref>

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 Formation|Cardium Sandstone]] in the non-calcareous shale unit. The upper part includes the [[Martin Sandy Zone]] and [[Medicine Hat Sandstone]].<ref name=lexicon/>

===Oil/gas production===
[[Natural gas|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.<ref name="HattinPaleo" >{{cite book |author= Donald E. Hattin |title= Stratigraphy and Depositional Environment of Greenhorn Limestone (Upper Cretaceous) of Kansas, Kansas Geological Survey, Bulletin 209 |publisher= University of Kansas Publications, State Geological Survey of Kansas |date= 1975 |url= http://www.kgs.ku.edu/Publications/Bulletins/209/07_paleo.html |page= Depositional Environment and Paleoecology |quote= 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). }}</ref> 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.<ref>{{cite web |url= https://www.researchgate.net/figure/West-to-east-transect-from-Blue-Point-Arizona-to-Sioux-City-Iowa-see-Fig-1-showing_fig3_287206299 |at=Figure 4 |title=Paleocirculation and foraminiferal assemblages of the Cenomanian–Turonian Bridge Creek Limestone bedding couplets: Productivity vs. dilution during OAE2 |website=ResearchGate |author=Khalifa Elderbak |author2=Mark Leckie |date= 2015 |access-date= 2021-05-23 }}</ref> 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.<ref name= AZtoOK >{{cite web |title= Regional stratigraphic cross sections of Cretaceous rocks from east-central Arizona to the Oklahoma Panhandle |url= https://ngmdb.usgs.gov/Prodesc/proddesc_45091.htm |website= National Geologic Map Database |author=Molenaar, C.M. |author2=Cobban, W.A. |author3=Merewether, E.A. |author4=Pillmore, C.L. |author5=Wolfe, D.G. |author6=Holbrook, J.M. |year= 2002 |publisher= [[United States Geological Survey|USGS]] |access-date= May 23, 2021 }}</ref> 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 Alberta|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 {{convert|1000|m|ft|-1}} in thickness in central Alberta. In central Saskatchewan, it thins to {{convert|150|m|ft|-1}}.<ref name=lexicon/>

==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 [[unconformity|unconformably]] overlain by the [[Lea Park Formation]] shale and unconformably underlain by the [[Blairmore Group|Blairmore]], [[Mannville Group|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, Canada|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:
{|class="wikitable"
|+ Colorado and Kansas classification of regional Colorado Group sub-units<ref>{{citation |author= |title= Corridor Assessment Report Appendix F. Geological Hazard Assessment Santa Fe Drive - I-25 to C-470 |year= 2020 |publisher= Shannon & Wilson |page = 3 |url=https://coloradogeologicalsurvey.org/publications/cretaceous-stratigraphy-colorado/ |access-date= 2023-11-22 |quote= Consists of Niobrara Formation (Kn) and either Benton Shale or Carlile, Greenhorn, and Graneros Formation (Kcg)}}</ref><ref>{{citation |author= Robert J. Raynolds |title= Cretaceous Stratigraphy of Colorado |year= 2022 |publisher=Colorado Geological Survey |url=https://coloradogeologicalsurvey.org/publications/cretaceous-stratigraphy-colorado/ |access-date= 2023-11-22 }}</ref><ref>{{citation |title= Classification of Rocks in Kansas (Kansas Stratigraphic Chart) |year= 2018 |publisher= [[Kansas Geological Survey]] |url=https://www.kgs.ku.edu/Publications/Bulletins/189/13_chart.html |access-date= 2023-11-22 }}</ref><ref name="Hattin209Strat" >{{cite book | author = Donald E. Hattin | author-link = Donald E. Hattin | title = Stratigraphy and Depositional Environment of Greenhorn Limestone (Upper Cretaceous) of Kansas, Kansas Geological Survey, Bulletin 209 | publisher = University of Kansas Publications, State Geological Survey of Kansas | date = 1975 | url = https://www.kgs.ku.edu/Publications/Bulletins/209/04_strat.html | page = Stratigraphy of the Greenhorn Limestone }}</ref>
|-
! [[Formation (stratigraphy)|Formation]] !! [[Stratigraphic unit#Member|Member]] !! [[Geochronology|Age]] !! Lithology
|-
|rowspan=2|[[Niobrara Formation|Niobrara]] (calcareous)
|[[Smoky Hill Chalk]] ||style="white-space: nowrap;"| [[Campanian|Early-Campanian]]- [[Coniacian|Late-Coniacian]] || repeating beds of buff chalk and chalky shale
|-
|[[Fort Hays Limestone Member|Fort Hays Limestone]] || [[Coniacian]] || massive chalky limestone
|-
|rowspan=4|[[Carlile Shale|Carlile]] (carbonaceous)
|style="white-space: nowrap;"| [[Juana Lopez Member|Juana Lopez Sandstone]] || [[Coniacian|Early-Coniacian]] || beds of [[calcarenite]] separated by intervals of [[mudstone]]
|-
|Codell Sandstone || [[Turonian|Late-Turonian]] || very fine-grained sandstone and siltstone, shale
|-
|Blue Hill Shale || [[Turonian]] || olive-black, carbonaceous shale; repeating thin [[bentonite]]s; septarians
|-
|Fairport Shale || [[Turonian]] || olive-black, carbonaceous shale to chalky shale with repeating thin chalks; repeating thin [[bentonite]]s
|-
|rowspan=4|[[Greenhorn Limestone|Greenhorn ]] (calcareous)
|Bridge Creek/ Pfeifer Shale || [[Turonian]] || bench-forming [[Fencepost limestone|Fencepost limestone marker bed]] olive-black, chalky shale with repeating, thin chalky limestone beds; repeating thin [[bentonite]]s
|-
|Bridge Creek/ Jetmore Chalk || [[Turonian]] || bench-forming Shellrock marker bed chalky shale with very rhythmic, thin limestone beds
|-
|Hartland Shale || [[Cenomanian|Late-Cenomanian]] || chalky shale almost devoid of limestone
|-
|Lincoln Limestone || [[Cenomanian|Late-Cenomanian]] || shaly chalk with many scattered, thin skeletal limestone beds; repeating thin bentonites [[X-bentonite|X-bentonite marker bed]] (Colorado)
|-
|[[Graneros Shale|Graneros]] (carbonaceous) || ([[Thatcher Limestone]]) || [[Cenomanian|Mid-Cenomanian]] || 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 [[coccolith]]ic debris.

The [[Geological Survey of Canada|Canadian Geological Survey]] classification of the Colorado Group includes the following sub-units, from top to bottom:
{|class="wikitable"
|+ Canadian classification of regional Colorado Group subunits
|-
! Subdivision !! [[Geological unit|Sub-unit]] !! [[Geochronology|Age]] !! Lithology !! Max Thickness !! Reference
|-
|rowspan=3|Upper (calcareous)
|[[First White Speckled Shale]] || [[Santonian]] || olive-black, [[chalk]]-speckled, calcareous and carbonaceous shale; minor shaly limestone || {{convert|157|m|ft|-1|abbr=on}} ||<ref>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}}.</ref>
|-
|[[Medicine Hat Sandstone]] || [[Santonian]] || muddy [[sandstone]] and [[siltstone]]|| {{convert|14|m|ft|-1|abbr=on}}
|<ref>{{Cite web|url=http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:009567|archive-url=https://archive.today/20120707220931/http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:009567|url-status=dead|archive-date=July 7, 2012|title=Medicine Hat Sandstone|author=Lexicon of Canadian Geologic Units|accessdate=2009-03-01}}</ref>
|-
|[[Martin Sandy Zone]] || [[Santonian]] || Siltstone, fine grained sandstone and calcareous shale || {{convert|60|m|ft|-1|abbr=on}}
|<ref>{{Cite web|url=http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:009262|archive-url=https://archive.today/20120707045411/http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:009262|url-status=dead|archive-date=July 7, 2012|title=Martin Sandy Zone|author=Lexicon of Canadian Geologic Units|accessdate=2009-03-01}}</ref>
|-
|rowspan=4|Lower (non-calcareous)
| [[Cardium Formation|Cardium Sandstone]] || [[Turonian]] to [[Coniacian]] || marine sandstone || {{convert|108|m|ft|-1|abbr=on}}
|<ref>{{Cite web|url=http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:002448 |archive-url=https://archive.today/20120707181825/http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:002448 |url-status=dead |archive-date=2012-07-07 |title=Cardium Sandstone |author=Lexicon of Canadian Geologic Units |accessdate=2009-03-01 }}</ref>
|-
|[[Bowdoin Sandstone]] || [[Turonian]] || very fine-grained sandstone and siltstone, shale || {{convert|61|m|ft|-1|abbr=on}}
|<ref>{{Cite web|url=http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:001739|archive-url=https://archive.today/20120716121748/http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:001739|url-status=dead|archive-date=July 16, 2012|title=Bowdoin Sandstone|author=Lexicon of Canadian Geologic Units|accessdate=2009-03-01}}</ref>
|-
|[[Second White Speckled Shale]] || [[Turonian]] || olive-black, chalk-speckled, calcareous and carbonaceous shale; minor shaly limestone || {{convert|70|m|ft|-1|abbr=on}} ||<ref>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}}.</ref>
|-
|[[Phillips Sandstone]] || [[Turonian]] || very fine grained sandstone and siltstone, dark calcareous shale || {{convert|38|m|ft|-1|abbr=on}}
|<ref>{{Cite web|url=http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:011796|archive-url=https://archive.today/20120710100248/http://cgkn1.cgkn.net/weblex/weblex_litho_detail_e.pl?00053:011796|url-status=dead|archive-date=July 10, 2012|title=Phillips Sandstone|author=Lexicon of Canadian Geologic Units|accessdate=2009-03-01}}</ref>
|-
|}

==Further reading==
* {{cite journal |journal= Open File Report |issue= 534 |author= Karen W. Porter, Jennie Ridgley |title= Field Trip # 1 Marine Cretaceous Reservoirs in Central and Northern Montana: Road Log for Day 1 |year= 2006 |url= http://www.mbmg.mtech.edu/pdf-open-files/mbmg534-roadlog.pdf |publisher= Montana Bureau of Mines and Geology |pages= |access-date= 2022-07-03 }} - Correlation chart for the field trip over outcrops of the original Colorado classification.

==References==
{{Reflist}}

{{Colorado}}
{{Western Canadian Sedimentary Basin| South AB=yes|Central_Plains=yes}}

[[Category:Geologic groups of Saskatchewan]]
[[Category:Geologic groups of Alberta]]
[[Category:Geologic groups of British Columbia]]
[[Category:Western Canadian Sedimentary Basin]]
[[Category:Cretaceous Alberta]]
[[Category:Cretaceous British Columbia]]
[[Category:Lower Cretaceous Series of North America]]
[[Category:Upper Cretaceous Series of North America]]
[[Category:Geologic groups of Montana]]
[[Category:Geologic groups of Wyoming]]
[[Category:Geologic groups of Colorado]]

Colorado Group - Revision history

imported>Volcanoguy: MOS:INFOBOXFLAG