Morrison Formation

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Morrison Formation
Stratigraphic range: Late Jurassic, 156.3–146.8 Ma
Green River UT 2005-10-14 2104.jpg
The distinctive banding of the Morrison Formation, a group of rock layers that occur throughout Dinosaur National Monument and the source of fossils like those found at the Dinosaur Quarry
TypeGeologic formation
UnderliesCedar Mountain Formation, Cloverly Formation, Lakota Formation
OverliesSummerville Formation, Curtis Formation, Bell Ranch Formation, Sundance Formation
ThicknessUp to 200 m
Lithology
PrimaryMudstone
OtherSandstone, siltstone, limestone
Location
RegionCentral North America:
 Arizona
 Colorado
 Idaho
 Kansas
 Montana
 Nebraska
 New Mexico
 North Dakota
 Oklahoma
 South Dakota
 Texas
 Utah
 Wyoming
Country United States[1]
Type section
Named forMorrison, Colorado

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, siltstone, and limestone and is light gray, greenish gray, or red. Most of the fossils occur in the green siltstone beds and lower sandstones, relics of the rivers and floodplains of the Jurassic period.

It is centered in Wyoming and Colorado, with outcrops in Montana, North Dakota, South Dakota, Nebraska, Kansas, the panhandles of Oklahoma and Texas, New Mexico, Arizona, Utah, and Idaho. Equivalent rocks under different names are found in Canada.[2] It covers an area of 1.5 million square kilometers (600,000 square miles), although only a tiny fraction is exposed and accessible to geologists and paleontologists. Over 75% is still buried under the prairie to the east, and much of its western paleogeographic extent was eroded during exhumation of the Rocky Mountains.

It was named after Morrison, Colorado, where the first fossils in the formation were discovered by Arthur Lakes in 1877. That same year, it became the center of the Bone Wars, a fossil-collecting rivalry between early paleontologists Othniel Charles Marsh and Edward Drinker Cope. In Colorado, New Mexico, and Utah, the Morrison Formation was a major source of uranium ore.

Description[edit]

Geologic history[edit]

According to radiometric dating, the Morrison Formation dates from 156.3 ± 2 million years old (Ma) at its base,[3] to 146.8 ± 1 million years old at the top,[4] which places it in the earliest Kimmeridgian, and early Tithonian stages of the late Jurassic. This is similar in age to the Solnhofen Limestone Formation in Germany and the Tendaguru Formation in Tanzania. The age and much of the fauna is similar to the Lourinhã Formation in Portugal.[5] Throughout the western United States, it variously overlies the Middle Jurassic Summerville, Sundance, Bell Ranch, Wanakah, and Stump Formations.

At the time, the supercontinent of Laurasia had recently split into the continents of North America and Eurasia, although they were still connected by land bridges. North America moved north and was passing through the subtropical regions.

The Morrison Basin, which stretched from New Mexico in the south to Alberta and Saskatchewan in the north, was formed during the Nevadan orogeny, a precursor event to later orogenic episodes that created the Rocky Mountains started pushing up to the west. The deposits from their east-facing drainage basins, carried by streams and rivers from the Elko Highlands (along the borders of present-day Nevada and Utah) and deposited in swampy lowlands, lakes, river channels and floodplains, became the Morrison Formation.

In the north, the Sundance Sea, an extension of the Arctic Ocean, stretched through Canada down to the United States. Coal is found in the Morrison Formation of Montana, which means that the northern part of the formation, along the shores of the sea, was wet and swampy, with more vegetation. Aeolian, or wind-deposited sandstones, are found in the southwestern part, which indicates it was much more arid—a desert, with sand dunes.

Stratigraphy[edit]

In the Colorado Plateau region, the Morrison Formation is further broken into four sub-divisions, or members. From the oldest to the most recent, they are:

Bluish beds of the Brushy Basin Member containing alkali minerals deposited in Lake T'oo'dichi'
Reddish mudstones of the Tidwell Member underlying the whitish sandstones of the Saltwash Member, south of Cisco, Utah.
Brushy Basin Member on the Colorado Plateau
"Popcorn" texture due to bentonite, formed from volcanic ash, characterizes the Brushy Basin Member
  1. Windy Hill Member: The oldest member. At the time, the Morrison basin was characterized by shallow marine and tidal flat deposition along the southern shore of the Sundance Sea.
  2. Tidwell Member: The Sundance Sea receded to Wyoming during this member and was replaced by lakes and mudflats.
  3. Salt Wash Member: The first purely terrestrial member. The basin was a semi-arid alluvial plain, with seasonal mudflats.
  4. Brushy Basin Member: Much finer-grained than the Salt Wash Member, the Brushy Basin Member is dominated by mudstone rich in volcanic ash. Rivers flowed from the west into a basin that contained a giant, saline alkaline lake called Lake T'oo'dichi'[6] and extensive wetlands that were located just west of the modern Uncompahgre Plateau.

In the San Juan Basin, there is an additional Jackpile Sandstone Member above the Brushy Basin Member. This is primarily a whitish crossbedded subarkose sandstone with a clay matrix. It is interbedded with variegated, pale-green to red, bentonitic mudstone lenses.[7]

Deposition in the Morrison Formation ended about 147 Ma. The latest Morrison strata are followed by a thirty-million-year gap in the geologic record. The overlying units are the Lower Cretaceous Cedar Mountain, Burro Canyon, Lytle, and Cloverly Formations.

Fossil content[edit]

Though many of the Morrison Formation fossils are fragmentary, they are sufficient to provide a good picture of the flora and fauna in the Morrison Basin during the Kimmeridgian. Overall, the climate was dry, similar to a savanna but, since there were no angiosperms (grasses, flowers, and some trees), the flora was quite different. Conifers, the dominant plants of the time, were to be found with ginkgos, cycads, tree ferns, and horsetail rushes. Much of the fossilized vegetation was riparian, living along the river flood plains. Along the rivers, there were fish, frogs, salamanders, lizards, crocodiles, turtles, pterosaurs, crayfish, clams, and mammaliforms.

The dinosaurs were most likely riparian, as well. Hundreds of dinosaur fossils have been discovered, such as Allosaurus, Ceratosaurus, Torvosaurus, Saurophaganax, Camptosaurus, Ornitholestes, several stegosaurs comprising at least two species of Stegosaurus and the slightly older Hesperosaurus, and the early ankylosaurs, Mymoorapelta and Gargoyleosaurus, most notably a very broad range of sauropods (the giants of the Mesozoic era). Since at least some of these species are known to have nested in the area (Camptosaurus embryoes have been discovered), there are indications that it was a good environment for dinosaurs and not just home to migratory, seasonal populations.

Sauropods that have been discovered include Diplodocus (most famously, the first nearly complete specimen of D. carnegii, which is now exhibited at the Carnegie Museum of Natural History, in Pittsburgh, Pennsylvania), Camarasaurus (the most commonly found sauropod), Brachiosaurus, Apatosaurus, Brontosaurus, Barosaurus, the uncommon Haplocanthosaurus and Supersaurus. The very diversity of the sauropods has raised some questions about how they could all co-exist. While their body shapes are very similar (long neck, long tail, huge elephant-like body), they are assumed to have had very different feeding strategies, in order for all to have existed in the same time frame and similar environment.

Sites and quarries[edit]

Workers inside the Dinosaur Quarry building, at the Dinosaur National Monument

Locations where significant Morrison Formation fossil discoveries have been made include:

Colorado[edit]

Fruita Paleontological Resource Area. One of the sites is denoted by the arrow.

Utah[edit]

Wyoming[edit]

Site comparisons[edit]

The Morrison Formation is comparable to the Tendaguru beds in Tanzania and Lourinhã Formation in Portugal. In age it is also comparable with the Solnhofen Limestone in Germany.

Economic geology[edit]

The Morrison Formation contains uranium deposits, including the Jackpile uranium body discovered near Grants, New Mexico in 1951.[7]

See also[edit]

References[edit]

  1. ^ "Morrison Formation" (PDF). CGKN. Retrieved 25 May 2013.[permanent dead link]
  2. ^ Parrish, J.T.; Peterson, F.; Turner, C.E. (2004). "Jurassic "savannah"-plant taphonomy and climate of the Morrison Formation (Upper Jurassic, Western USA)". Sedimentary Geology. 167 (3–4): 137–162. Bibcode:2004SedG..167..137P. doi:10.1016/j.sedgeo.2004.01.004.
  3. ^ Trujillo, K.C.; Chamberlain, K.R.; Strickland, A. (2006). "Oxfordian U/Pb ages from SHRIMP analysis for the Upper Jurassic Morrison Formation of southeastern Wyoming with implications for biostratigraphic correlations". Geological Society of America Abstracts with Programs. 38 (6): 7.
  4. ^ Bilbey, S.A. (1998). "Cleveland-Lloyd Dinosaur Quarry - age, stratigraphy and depositional environments". In Carpenter, K.; Chure, D.; Kirkland, J.I. (eds.). The Morrison Formation: An Interdisciplinary Study. Modern Geology. 22. Taylor and Francis Group. pp. 87–120. ISSN 0026-7775.
  5. ^ Mateus, O. 2006. Late Jurassic dinosaurs from the Morrison Formation, the Lourinhã and Alcobaça Formations (Portugal), and the Tendaguru Beds (Tanzania): a comparison. in Foster, J.R. and Lucas, S. G. R.M., eds., 2006, Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin 36: 223-231.
  6. ^ Turner, Christine E.; Fishman, Neil S. (April 1991). "Jurassic Lake T'oo'dichi': A large alkaline, saline lake, Morrison Formation, eastern Colorado Plateau". Geological Society of America Bulletin. 103 (4): 538–558. doi:10.1130/0016-7606(1991)103<0538:JLTODA>2.3.CO;2.
  7. ^ a b Owen, Donald E.; Walters, Lester J., Jr.; Beck, Ronald G. (August 1984). "The Jackpile Sandstone Member 0f the Morrison Formation in west-central New Mexicoby Donald E. jwen, Consulting Geologist, Tulsa, 0K 74152, and Lester J.Walters, Jr. and Ronald G. Beck, ARCO Oil and Gas Co., Dallas, IX75221 a formal definiti" (PDF). New Mexico Geology. 6 (3). Retrieved 4 June 2020.
  8. ^ Saleiro, A., & Mateus O. (2017). Upper Jurassic bonebeds around Ten Sleep, Wyoming, USA: overview and stratigraphy. Abstract book of the XV Encuentro de Jóvenes Investigadores en Paleontología/XV Encontro de Jovens Investigadores em Paleontologia, Lisboa, 428 pp.. 357-361.

Further reading[edit]

  • Foster, J. 2007. Jurassic West: The Dinosaurs of the Morrison Formation and Their World. Indiana University Press. 389pp.
  • Foster, J.R. 2003. Paleoecological Analysis of the Vertebrate Fauna of the Morrison Formation (Upper Jurassic), Rocky Mountain Region, U.S.A. Albuquerque, New Mexico: New Mexico Museum of Natural History and Science. Bulletin 23.
  • Jenkins, J.T. and J.L. Jenkins. 1993. Colorado's Dinosaurs. Denver, Colorado: Colorado Geologic Survey. Special Publication 35.
  • Mateus, O. 2006. Late Jurassic dinosaurs from the Morrison Formation, the Lourinhã and Alcobaça Formations (Portugal), and the Tendaguru Beds (Tanzania): a comparison. in Foster, J.R. and Lucas, S. G. R.M., eds., 2006, Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin 36: 223-231.

External links[edit]