The Duchesne River Formation is a Middle Eocene geologic formation in the Uinta Basin of northeastern Utah. It was originally named the Duchesne Formation by Carnegie Museum paleontologist O.A. Peterson,Peterson, O.A., 1932, New species from the Oligocene of the Uinta [Utah]: Carnegie Museum Annals, v. 21, no. 2, p. 61-78. and modified to the Duchesne River Formation by Carnegie Museum paleontologist J.L. Kay,Kay, J.L., 1934, The Tertiary formations of the Uinta basin, Utah: Carnegie Museum Annals, v. 23, p. 357-372. because the term Duschesne Formation was already in use for a Jurassic limestone. Peterson did not provide a type section when naming the formation, only noting that it is named for the Duchesne River, which flows roughly southeast and east to join the Green River.
The formation consists of over 3000 feet (915 m) of fluvial (river-carried) conglomerate, sandstone and fine-grained sedimentary rocks. It is subdivided into four members (below), whose sediments are thought to have eroded from the rising Uinta Mountains to the north and deposited in rivers, floodplains and small lakes.
Age Volcanic ash beds in the formation, sourced derived from volcanoes in the Wasatch Range, East Tintic, and Oquirrh Mountains to the west, have provided radiometric dates for the formation. Dates collected in 1990 range from 38.2 +/-1.8 Ma for the Brennan Basin Member to 28.7 +/-2.0 Ma for the Lapoint Member.Bryant, Bruce, Naeser, C.W., Marvin, R.F., and Mehnert, H.H., 1990, Upper Cretaceous and Paleogene sedimentary rocks and isotopic ages of Paleogene tuffs, Uinta basin, Utah, IN Evolution of sedimentary basins; Uinta and Piceance basins: U.S. Geological Survey Bulletin, 1787-J, p. J1-J22. These estimate turned out to be too young according to follow-up studies.
A 1992 survey dated an ash bed at the base of the Lapoint Member to 39.74 ± 0.07 Ma, and a 2012 study recalibrated this dating to 40.26 ± 0.08 Ma. Another recent (2020) dating of ash beds results in an age of 39.47 ± 0.16 Ma in the upper Dry Gulch member and 39.36 ± 0.15 Ma in the lower Lapoint Member.
Members The following members were defined by Andersen and Picard (1972): Prior to this more formal usage, Kay (1934) used a three-fold division at his fossil quarry: the Randlett, Halfway, and Lapoint horizons. The Randlett horizon is equivalent to most of the Brennan Basin Member, the Halfway horizon is equivalent to the uppermost Brennan Basin and entire Dry Gulch members, and the Lapoint horizon is equivalent to the Lapoint and Starr Flat member.
Brennan Basin Member Named for the Brennan Basin in Uintah County. Predominately sandstone that varies from yellow-gray to red brown, very fine, fine, medium, to coarse grained, thin to thick to very thick bedded, generally poorly sorted, and may be nodular, silty, pebbly, and cross stratified. Minor beds of green-gray to red brown, silty to sandy, thin to very thin bedded, poorly sorted claystone, red brown, thin-bedded, nodular, poorly sorted mudstone, yellow brown, very thin bedded, nodular, poorly sorted, horizontally stratified siltstone, and red-purple, sandy, clayey, thin-bedded, medium to coarse crystalline limestone. Contains a diverse late Eocene vertebrate fauna.Andersen, D.W., and Picard, M.D., 1972, Stratigraphy of the Duchesne River Formation (Eocene-Oligocene?), northern Uinta basin, northeastern Utah: Utah Geological and Mineral Survey Bulletin, no. 97, 29 p.
Dry Gulch Creek Member Named for Dry Gulch Creek in Uintah County. Consists of pale red, olive gray to yellow brown, coarse, medium, fine to very fine grained, thin-, thick to very thick bedded, massive, moderately to poorly sorted sandstone that may be horizontally or cross stratified and red, gray to brown, very thin, thin- to thick-bedded, moderately to poorly sorted claystone that may be nodular, micaceous, or laminated. Contains plant remains and a sparse vertebrate fauna.
Lapoint Member Named for the town of Lapoint in Uintah County. Composed of olive, green-gray, dark gray, very thin to thin bedded, nodular, poorly sorted, horizontally stratified, laminated claystone; light gray, light brown, gray orange to red brown, thin-bedded, slabby, horizontally stratified, poorly sorted, medium-grained, to thick-bedded, blocky, poorly sorted, coarse-grained sandstone; gray-red to red-brown, thin-bedded, nodular, horizontally stratified, poorly sorted siltstone; and gray-orange, very thick bedded, massive, poorly sorted pebble conglomerate. Abundant late Eocene vertebrates.
Starr Flat Named for John Starr Flat in Duchesne County. Consists of: 1) red- to yellow-brown, light gray, gray- to yellow-orange, pale red to gray red sandstone that ranges from coarse, medium, fine to very fine grained, thin, thick to very thick bedded, sandy, silty, to clayey, cross-stratified to horizontally stratified, flaggy, blocky, massive, brown-gray, gray-red to olive, sandy to silty; 2) very thin bedded, poorly sorted, platy, laminated, nodular to massive claystone; and 3) gray red, sandy, poorly sorted, thin-bedded, nodular, massive siltstone. Sandstone may also be poorly sorted, pebbly, nodular and massive. No fossils found expect for indeterminate artiodactyl fragments.Kelly, T.S.; Murphey, P.C.; Walsh, S.L. (2012). "New records of small mammals from the middle Eocene Duchesne River Formation, Utah, and their implications for the Uintan–Duchesnean North American Land Mammal Age transition." Paludicola 8: 208–251.
Paleobiota The Randlett fauna (lower Brennan Basin Member) can safely be assigned to the Uintan 3 North American Land Mammal Age (NALMA). On the geomagnetic polarity time scale, it corresponds to the upper part of chron C19r or lower part of chron C19n.
The Halfway and Lapoint faunas (uppermost Brennan Basin, Dry Gulch Creek, and Lapoint members) belong to the Duchesnean NALMA, which is named after the Duchesne River Formation. They both correspond to the upper part of chron C18r.
Mammalia
Artiodactyla An indeterminate oromerycid is known from the Brennan Basin Member. Reports of Helohyus teeth are probably misidentifications. Artiodactyls of the Duchesne River FormationGenus / TaxonSpeciesMemberNotesImagesAgriochoerusA. maximusLapoint MemberAn agriochoerid (semi-arboreal oreodont relative)BrachyhyopsB. wyomingensisLapoint MemberAn entelodont (boar-like hippo relative)DiplobunopsD. crassusBrennan Basin MemberAn agriochoerid (semi-arboreal oreodont relative)Hypertragulidaesp."Randlett"An indeterminate hypertragulidLeptotragulusL. proavusBrennan Basin MemberA protoceratidPentacemylusP. progressusBrennan Basin MemberA homacodontidPoabromylusP. kayiDry Gulch Creek Member, Lapoint MemberA protoceratidProtoreodonP. primusBrennan Basin MemberAn agriochoerid (semi-arboreal oreodont relative)P. pumilusBrennan Basin MemberP. sp. cf. P. pumilusLapoint MemberSimimeryxS. minutusLapoint MemberAn early ruminant, possibly a hypertragulid
Carnivoramorphs A large indeterminate carnivoran, possibly a miacid or amphicyonid, is known from the Lapoint Member. Carnivoramorphs of the Duchesne River FormationGenus / TaxonSpeciesMemberNotesImagesEosictisE. avinoffiDry Gulch Creek Member or Lapoint MemberAn indeterminate carnivoranHyaenodonH. sp. cf. H. vetusLapoint MemberA hyaenodontine hyaenodontPleurocyon? or Uintacyon?sp."Randlett"An indeterminate carnivoranSimidectesS. sp. cf. S. magnusBrennan Basin MemberAn early carnivoramorph
Eulipotyphla Another indeterminate eulipotyphlan species is known from the Dry Gulch Creek Member. Eulipotyphlans of the Duchesne River FormationGenus / TaxonSpeciesMemberNotesImagesCentetodonC. sp.Lapoint MemberA geolabidid. Previously identified as Protictops alticuspidens.
Lagomorpha Lagomorphs of the Duchesne River FormationGenus / TaxonSpeciesMemberNotesImagesMytonolagusM. sp.Brennan Basin Member, Dry Gulch Creek MemberAn early leporid (rabbit)
Mesonychia Mesonychians of the Duchesne River FormationGenus / TaxonSpeciesMemberNotesImagesHessolestesH. ultimusLapoint MemberA mesonychid
Metatheria Metatherians of the Duchesne River FormationGenus / TaxonSpeciesMemberNotesImagesCopedelphysC. sp.Dry Gulch Creek MemberA herpetotheriid
Perissodactyla Perissodactyls of the Duchesne River FormationGenus / TaxonSpeciesMemberNotesImagesColodonC. sp.Lapoint MemberA helaletid (tapir relative)DilophodonD. leotanusBrennan Basin MemberA helaletid (tapir relative). Sometimes considered a species of Heteraletes.cf. Diplacodoncf. D. elatusBrennan Basin MemberA brontothereDuchesneodusD. uintensisBrennan Basin Member, Dry Gulch Creek Member, Lapoint MemberA brontothere, previously identified as "Teleodus" or Protitanotherium.Epihippus (Duchesnehippus)E. (D.) intermediusDry Gulch Creek Member, Lapoint MemberAn early equid (horse)EpitriplopusE. mediusBrennan Basin MemberA hyracodontid ("running rhino")HyracodonH. mediusLapoint MemberA hyracodontid ("running rhino")H. primusLapoint MemberMegalamynodonM. regalisBrennan Basin MemberAn amynodontid (hippo-like rhino)"Mesamynodon""M." mediusBrennan Basin MemberA hyracodontid ("running rhino"), possibly synonymous with Epitriplopus medius.UintacerasU. sp.Brennan Basin MemberA rhinocerotoid (rhino relative)
Rodentia An indeterminate sciuravid or myomorph is known from the Brennan Basin Member, and an indeterminate myomorph is known from the Dry Gulch Creek Member. Rodents of the Duchesne River FormationGenus / TaxonSpeciesMemberNotesImagesAdjidaumoA. sp. cf. A. craigiDry Gulch Creek MemberAn eomyidEutypomysE. sp.Dry Gulch Creek MemberA eutypomyidGriphomysG. sp.Dry Gulch Creek MemberAn indeterminate rodent.HeliscomysH. sp.Dry Gulch Creek MemberA heliscomyidMetanoiamysM. lacusDry Gulch Creek MemberAn eomyidM. sp. cf. M. korthiDry Gulch Creek MemberMicroeutypomysM. sp.Dry Gulch Creek MemberA eutypomyidMytonomysM. robustusBrennan Basin MemberAn ailuravine ischyromyid. Leptotomus kayi is a synonym of this species.PareumysP. guensbergiDry Gulch Creek Member, Lapoint MemberA cylindrodontidP. sp. aff. P. milleriBrennan Basin MemberParadjidaumoP. sp.Dry Gulch Creek MemberAn eomyidPassaliscomysP. sp.Dry Gulch Creek MemberA heliscomyidProtadjidaumoP. sp. cf. P. typusDry Gulch Creek MemberAn eomyidP. typusDry Gulch Creek Member, Lapoint MemberSimiacritomysS. sp.Dry Gulch Creek MemberAn eomyid
Reptilia Reptiles of the Duchesne River FormationGenus / TaxonSpeciesMemberNotesImages"Crocodylus""C." acerBrennan Basin MemberA crocodyloidCymatholcusC. longus"Halfway"A tortoise
See also
List of fossiliferous stratigraphic units in Utah Paleontology in Utah
References