The Cloverly Formation is a

geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

Early Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Early ...

and

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

age (

Valanginian In the geologic timescale, the Valanginian is an age or stage of the Early or Lower Cretaceous. It spans between 139.8 ± 3.0 Ma and 132.9 ± 2.0 Ma (million years ago). The Valanginian Stage succeeds the Berriasian Stage of the Lower Cretace ...

Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in ...

stage) that is present in parts of

Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columb ...

Wyoming Wyoming () is a state in the Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the southwest, and Colorado to t ...

Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of the ...

and

Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to its ...

in the western United States. It was named for a post office on the eastern side of the Bighorn Basin in Wyoming by N.H. Darton in 1904. The

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s of formation were deposited in

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...

environments and contain

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s, including a diverse assemblage of

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

remains. In 1973, the Cloverly Formation Site was designated as a National Natural Landmark by the National Park Service.

Stratigraphy

The Cloverly Formation rests disconformably on the

Morrison Formation The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltston ...

and is conformably overlain by the

Thermopolis Shale The Thermopolis Shale is a geologic formation which formed in west-central North America in the Albian age of the Late Cretaceous period. Surface outcroppings occur in central Canada, and the U.S. states of Montana and Wyoming. The rock formatio ...

. It is subdivided into a variety of members, depending on the location.Moberly, R.M., Jr., 1960, Morrison, Cloverly, and Sykes Mountain formations, northern Bighorn basin, Wyoming and Montana: Geological Society of America Bulletin, v. 71, no. 8, p. 1137-1176. In the Bighorn Basin along the Montana-Wyoming border, Moberly (1960) divided the Cloverly into the following three members: *The Pryor Conglomerate lies at the base and contains abundant black

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a ...

. It is named from thick beds exposed on the west side of the

Pryor Mountains The Pryor Mountains are a mountain range in Carbon and Big Horn counties of Montana, and Big Horn County, Wyoming. They are located on the Crow Indian Reservation and the Custer National Forest, and portions of them are on private land. The ...

. *The Little Sheep Member lies in the middle and is composed of pale-purple, gray to almost white, bentonitic

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology.' ...

. *The uppermost unit is the Himes Member, which contains some coarse-grained channel

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...

deposits, but consists primarily of brightly multicolored (variegated) mudstones. In contrast, Ostrom (1970) divided the formation into four units, which he named Units IV-VII: * ''Unit IV'' equates to the Pryor Conglomerate of Moberly and consists of a conglomerate or conglomeratic sandstone. * ''Unit V'', overlaying Unit IV, consists of a lower grey-to-purple claystone with abundant fist-sized

chalcedony Chalcedony ( , or ) is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monocli ...

and

barite Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate ( Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...

concretions; the unit is highly bentonitic and contains occasional channel sands. * ''Unit VI'' is a discontinuous "salt and pepper" cross-stratified channel sandstone with occasional conglomerate, considered by Moberly to be part of the Himes Member. * ''Unit VII'', the uppermost, is a maroon to orange claystone with occasional highly rounded and polished pebbles of

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

Age

A stratigraphic revision of the Cloverly Formation using new uranium lead dates reinterpret the formation as spanning the Valanginian-Cenomanian stages of the Cretaceous period. The individual ages of the members are listed below: * Pryor Conglomerate; 140-130 Ma (

Hauterivian The Hauterivian is, in the geologic timescale, an age in the Early Cretaceous Epoch or a stage in the Lower Cretaceous Series. It spans the time between 132.9 ± 2 Ma and 129.4 ± 1.5 Ma (million years ago). The Hauterivian is preceded by th ...

) * Little Sheep Member; 130-124 Ma (marine section) (Hauterivian-

Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 129.4 ± 1.5 Ma (million years ago) and 121.4 ± 1.0 Ma). It is a subdivision of the Early Cretaceous Epoch (or Lower Cretaceous Series). It is preceded ...

) and 124-109 Ma (terrestrial section) (Barremian-

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/ Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0 ...

) * Himes Member; 109-98 Ma (Albian-

)

Depositional environment

The sediments of the Cloverly Formation were deposited in

alluvial Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. ...

and

environments. The basal conglomerates probably represent

braided river A braided river, or braided channel, consists of a network of river channels separated by small, often temporary, islands called braid bars or, in English usage, ''aits'' or ''eyots''. Braided streams tend to occur in rivers with high sediment ...

deposits, while the sandstones were deposited in

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...

channels. The mudstones that contain most of the fossils represent overbank, lacustrine, and pedogenic deposits.May, M.T. 1992. Intra- and extrabasinal tectonism, climate and intrinsic threshold cycles as possible controls on Early Cretaceous fluvial architecture, Wind River basin, Wyoming. In: Sundell, K.A., and Anderson, T.C., eds., Rediscover the Rockies: Wyoming Geological Association Field Conference Guidebook, 43rd Annual Field Conference, Casper, WY, September 12–19, 1992, no. 43, p. 61-74.

Vertebrate fauna

Animals recovered include the dinosaurs '' Deinonychus'', ''

Microvenator ''Microvenator'' (meaning "small hunter") is a genus of dinosaur from the Early Cretaceous Cloverly Formation in what is now south central Montana. ''Microvenator'' was an oviraptorosaurian theropod. The holotype fossil is an incomplete skeleton, ...

'', '' Tenontosaurus'', '' Zephyrosaurus'' and ''

Sauropelta ''Sauropelta'' ( ; meaning 'lizard shield') is a genus of nodosaurid dinosaur that existed in the Early Cretaceous Period of North America. One species (''S. edwardsorum'') has been named although others may have existed. Anatomically, ''Saurope ...

'' as well as fragmentary remains of Titanosaurs,

Ankylosaurs Ankylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms, similar to turtles. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. ...

and Ornithomimids. As well, two genera of turtle '' Naomichelys'' and ''

Glyptops ''Glyptops'' (Greek for "grooved face") is an extinct genus of pleurosternid freshwater turtle known from the Late Jurassic of North America. Taxonomy The type species, ''Glyptops plicatulus'', was first described as ''Compsemys plicatulus'' by ...

'' and the lungfish ''

Ceratodus ''Ceratodus'' (from el, κέρας , 'horn' and el, ὀδούς 'tooth') was a wide-ranging genus of extinct lungfish. Fossil evidence dates back to the Early Triassic. A wide range of fossil species from different time periods have been found ...

''. Dinosaur eggs have been found in Montana. References for data: Ostrom 1970; Cifelli et al. 1998; Cifelli 1999; Nydam and Cifelli 2002. Possible

goniopholidid Goniopholididae is an extinct family of moderate-sized semi-aquatic neosuchian crocodyliformes. Their bodyplan and morphology are convergent on living crocodilians. They lived across Laurasia (Asia, Europe and North America) between the Middle Ju ...

remains are known from the formation.

Ornithischians

Saurischians

Theropod eggshell fragments are known from the formation. Unidentifiable ornithomimid remains are present and most commonly represented by toe bones. Indeterminate allosauroid remains are known from the formation. Remains identified by John Ostrom as ''Ornithomimus'' are suspected by Jack Horner to be of a new ornithomimid genus. Possible remains of a microraptorian, a troodontid, and a basal tyrannosauroid similar to ''Moros'' have also been found here as well.

Mammals

Turtles

Bony fish

Indeterminate amiiformes are known from the formation.

Footnotes

References

* Burton, D., Greenhalgh, B.W., Britt, B.B., Kowallis, B.J., Elliott, W.S., and Barrick, R. 2006. New radiometric ages from the Cedar Mountain Formation, Utah and the Cloverly Formation, Wyoming: implications for contained dinosaur faunas. Geological Society of America Abstracts with Programs 38(7): 52. * Chen, Z.-Q. and Lubin, S. 1997. A fission track study of the terrigenous sedimentary sequences of the Morrison and Cloverly Formations in northeastern Bighorn Basin, Wyoming. The Mountain Geologist 34:51-62. * Cifelli, R.L. 1999. Tribosphenic mammal from the North American Early Cretaceous. Nature 401:363-366. * Cifelli, R.L., Wible, J.R., and Jenkins, F.A. 1998. Triconodont mammals from the Cloverly Formation (Lower Cretaceous), Montana and Wyoming. Journal of Vertebrate Paleontology 18: 237-241. * Horner, John R. ''Dinosaurs Under the Big Sky'' (Cloverly Formation). Mountain Press Publishing Company. pp. 93–100. . * Nydam, R.L., and Cifelli, R.L. 2002. Lizards from the Lower Cretaceous (Aptian-Albian) Antlers and Cloverly Formations. Journal of Vertebrate Paleontology 22: 286-298. * Ostrom, J. H. 1970. Stratigraphy and paleontology of the Cloverly Formation (Lower Cretaceous) of the Bighorn Basin area, Wyoming and Montana. Peabody Museum Bulletin 35:1-234 * Varricchio, D. J. 2001. Late Cretaceous oviraptorosaur (Theropoda) dinosaurs from Montana. pp. 42–57 in D. H. Tanke and K. Carpenter (eds.), Mesozoic Vertebrate Life. Indiana University Press, Indianapolis, Indiana. * Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. 861 pp. . {{authority control Geologic formations of Colorado Geologic formations of Montana Geologic formations of Wyoming Geologic formations of Utah Lower Cretaceous Series of North America Cretaceous Colorado Cretaceous Montana Cretaceous geology of Wyoming Cretaceous geology of Utah Aptian Stage Albian Stage Mudstone formations Conglomerate formations Sandstone formations of the United States Fluvial deposits Lacustrine deposits Fossiliferous stratigraphic units of North America Paleontology in Montana Paleontology in Wyoming Geology of the Rocky Mountains National Natural Landmarks in Montana