The Chugwater Formation is a mapped bedrock unit consisting primarily of red sandstone, in the states of Wyoming, Montana, and Colorado in the United States. It is recognized as a geologic formation in Colorado and Montana, but as a Group (set of formations), the Chugwater Group, in Wyoming. Despite its presence below the highly studied Morrison Formation, the Chugwater receives little attention.
The most noticeable feature on a large scale is the brick-red color, caused by oxidation of iron minerals in the rock. This color is periodically interrupted by streaks and spots of reduced iron, a light bluish-gray shade. Near the top of the formation is a thick layer of gypsum of very high quality. The whole rock is interrupted by gypsum veins as well as having a disrupted texture because of the precipitation of gypsum crystals after deposition of the rock.
The Chugwater consists mainly of siltstone and shales with interspersed sandstones. While this composition will lend itself to breaking easily and not outcropping, the shear size of the formation causes there to be large outcrops. Though most sedimentary structures are not visible due to the gypsum interruption, there are a few examples of ripple marks and occasional crossbedding. Examples of infilled mudcracks of irregular shape, are common.
The main constituent of the Chugwater is quartz, which helps to account for its unusual hardness. Feldspars are also present, though in lesser amounts, as are hematite and gypsum. Small flecks of a dark, opaque mineral have been tentatively identified as biotite. The minerals are cemented by a non-carbonate matrix, probably siliceous.
The Chugwater is a prime example of red beds, commonly deposited during the Permian and Triassic periods.
^Darton, N.H., 1904, Comparison of the stratigraphy of the Black Hills, Bighorn Mountains, and Rocky Mountain Front Range: Geological Society of America Bulletin, v. 15, p. 379-448. abstract
^Pipiringos, G.N., 1968, Correlation and nomenclature of some Triassic and Jurassic rocks in south-central Wyoming, IN Shorter contributions to general geology, 1967: U.S. Geological Survey Professional Paper, 594-D, p. D1-D26. link to report
^High, L.R., Jr., and Picard, M.D., 1969, Stratigraphic relations within upper Chugwater Group (Triassic), Wyoming: American Association of Petroleum Geologists Bulletin, v. 53, no. 5, p. 1091-1104. abstract
^Cavaroc, V.V., and Flores, R.M., 1991, Red beds of the Triassic Chugwater Group, southwestern Powder River basin, Wyoming, IN Evolution of sedimentary basins; Powder River basin: U.S. Geological Survey Bulletin, 1917-E, p. E1-E17. link to report
October 27, 2023
chugwater, formation, mapped, bedrock, unit, consisting, primarily, sandstone, states, wyoming, montana, colorado, united, states, recognized, geologic, formation, colorado, montana, group, formations, chugwater, group, wyoming, despite, presence, below, highl. The Chugwater Formation is a mapped bedrock unit consisting primarily of red sandstone in the states of Wyoming Montana and Colorado in the United States It is recognized as a geologic formation in Colorado and Montana but as a Group set of formations the Chugwater Group in Wyoming Despite its presence below the highly studied Morrison Formation the Chugwater receives little attention Chugwater Formation Group Stratigraphic range TriassicOutcrop of Chugwater Group near Thermopolis WyomingTypesedimentaryUnderliesNugget Sandstone Morrison FormationOverliesDinwoody Formation Goose Egg FormationLithologyPrimarysandstoneOthershale gypsum limestoneLocationExtentWyoming Colorado MontanaType sectionNamed forChugwater WyomingNamed byN H Darton 1904 1 Gypsum veins in the Chugwater same outcrop as above Contents 1 Description 1 1 Stratigraphy 2 Age 3 ReferencesDescription editThe most noticeable feature on a large scale is the brick red color caused by oxidation of iron minerals in the rock This color is periodically interrupted by streaks and spots of reduced iron a light bluish gray shade Near the top of the formation is a thick layer of gypsum of very high quality The whole rock is interrupted by gypsum veins as well as having a disrupted texture because of the precipitation of gypsum crystals after deposition of the rock The Chugwater consists mainly of siltstone and shales with interspersed sandstones While this composition will lend itself to breaking easily and not outcropping the shear size of the formation causes there to be large outcrops Though most sedimentary structures are not visible due to the gypsum interruption there are a few examples of ripple marks and occasional crossbedding Examples of infilled mudcracks of irregular shape are common The main constituent of the Chugwater is quartz which helps to account for its unusual hardness Feldspars are also present though in lesser amounts as are hematite and gypsum Small flecks of a dark opaque mineral have been tentatively identified as biotite The minerals are cemented by a non carbonate matrix probably siliceous The Chugwater is a prime example of red beds commonly deposited during the Permian and Triassic periods Stratigraphy edit The Chugwater Group 2 is divided into various formations depending on the basin in which it occurs It is mapped in the following basins within the Rocky Mountains Denver Basin Wind River Basin Bighorn Basin Powder River Basin Greater Green River Basin Laramie Basin For example in the Wind River and Greater Green River basins the formations in ascending stratigraphic order are Red Peak Formation Crow Mountain Formation Jelm Formation and Popo Agie Formation 3 However in the Powder River Basin the formations are Red Peak Formation Alcova Limestone Crow Mountain Sandstone unnamed red beds and Popo Agie Formation 4 Age editRelative age dating of the Chugwater places it in the Triassic Period References edit nbsp Geology portal Darton N H 1904 Comparison of the stratigraphy of the Black Hills Bighorn Mountains and Rocky Mountain Front Range Geological Society of America Bulletin v 15 p 379 448 abstract Pipiringos G N 1968 Correlation and nomenclature of some Triassic and Jurassic rocks in south central Wyoming IN Shorter contributions to general geology 1967 U S Geological Survey Professional Paper 594 D p D1 D26 link to report High L R Jr and Picard M D 1969 Stratigraphic relations within upper Chugwater Group Triassic Wyoming American Association of Petroleum Geologists Bulletin v 53 no 5 p 1091 1104 abstract Cavaroc V V and Flores R M 1991 Red beds of the Triassic Chugwater Group southwestern Powder River basin Wyoming IN Evolution of sedimentary basins Powder River basin U S Geological Survey Bulletin 1917 E p E1 E17 link to report Retrieved from https en wikipedia org w index php title Chugwater Formation amp oldid 1149712296, wikipedia, wiki, book, books, library,
