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Colorado and Dakota Groups

Sandstone and shale with minor limestone; includes Carlile, Greenhorn, Graneros, and Dakota FMs. Full section present in only the extreme NW part of Iowa; in southern and easternmost areas only Dakota is present.
State Iowa
Name Colorado and Dakota Groups
Geologic age Phanerozoic | Mesozoic | Cretaceous-Early Cretaceous-Middle(?) Cretaceous-Late
Comments Colorado Group:Carlile Shale- thinly laminated, highly calcareous clay shale, fossiliferous in spots. Approximate 105 ft thick. Greenhorn Limestone- alternating light gray dense limestone and darker gray calcarous shale. approx. 130-200 ft thick. Graneros Shale- thinly laminated calcareous clay shale with some silty interbeds, some beds are fossiliferous, decreasingly calcarous downward. approx. 30 ft. thick Dakota Group: consists primarily of non-marine deposits formed in a series of fluvial enviroments, capped by seds deposited in a complex of marginal marine enviroments. The formation is split into: Lower Dakota -mostly coarse grain quartz sandstone with thin beds of shale and siltstone; Upper Dakota- dominant shale facies, subequal sand and shale units and some dominant sand. approx. 400 ft. thick.
Primary rock type sandstone
Secondary rock type shale
Other rock types limestone
Lithologic constituents
Sedimentary > Clastic > Sandstone (Bed)
Sedimentary > Clastic > Mudstone > Shale (Bed)
Sedimentary > Carbonate > Limestone (Bed)
Map references
Hershey H. G. 1969, Geologic Map of Iowa: Iowa Geological Survey, scale 1:500K
Unit references
Hershey H. G. 1969, Geologic Map of Iowa: Iowa Geological Survey, scale 1:500K
Brenner, R. L., 1981, Cretaceous Stratigraphy and Sedimentation in Northwestern Iowa, Northeastern Nebraska, & Southeast South Dakota: Iowa Geological Survey Guidebook Series No. 4
Counties Adair - Adams - Allamakee - Audubon - Buena Vista - Butler - Carroll - Cass - Cherokee - Clay - Crawford - Dickinson - Emmet - Floyd - Franklin - Guthrie - Hamilton - Hancock - Hardin - Harrison - Humboldt - Ida - Kossuth - Lyon - Mills - Mitchell - Montgomery - O'Brien - Osceola - Page - Palo Alto - Plymouth - Pocahontas - Pottawattamie - Sac - Sioux - Webster - Winnebago - Woodbury - Wright