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WINDS OFF THE RIDGE
March 2008

Mesozoic Moments

MeMo #6

Morrison Formation – “Jurassic West”

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            The Morrison Formation, while having a local type section, is found over much of the Rocky Mountain Region with an extent of over one million km².  The general depositional environment for this huge area, is savannah-like; it would contain river channel, sand splays, both damp and dry floodplains, ponds, lakes, wetlands and sand dunes. ⁽¹⁾  Other interpretations exist; perhaps there was a sea of ferns and cycads as shown in “Ancient Denvers”. ⁽²⁾  Although the prevailing lithology is comprised of the typical red and green sandstones and mudstones, there are minor amounts of limestone, coal and ash. ⁽¹⁾ 

            Although we don’t see plant fossils at Dinosaur Ridge, elsewhere in the Morrison Formation, plant fossils include algae, mosses, ferns, horsetails, tree ferns, cycadophytes, seed ferns, ginkgoes and conifers.  Although best known for its dinosaur fauna, many other vertebrate fossils have been found, including turtles, crocodiles, fish, mammals, lizards, frogs, pterosaurs and salamanders. Invertebrate fossils include crayfish, snails, clams, arthropods and trace fossils for others. ⁽¹⁾  Termite mounds have even been found.⁽³⁾

            Although dinosaur fossils had been found earlier in the Morrison Formation, its golden era began in 1877 when major finds were discovered at Dinosaur Ridge, Garden Park and Como Bluffs.  The Morrison Formation contains at least eleven genera of theropods, seven sauropods, a few stegosaurs, ankylosaurs, and a number of ornithopods. The most common dinosaurs are the ones that we talk about on our tours; Camarasaurus (the most abundant basin-wide, but maybe not present at Dinosaur Ridge), Allosaurus, Stegosaurus, Apatosaurus and Diplodocus. ⁽¹⁾   In addition to body fossils for these animals, Morrison trackways have been found in over 30 locations, including the Purgatoire River Rancho Del Rio sites.  ⁽⁴⁾ 

            The Morrison Formation contains widespread uranium deposits, including the significant reserves in the Grants uranium region of New Mexico. ⁽⁵⁾  Recent analysis of the Morrison dinosaur bones at Dinosaur Ridge with an X Ray Fluorescence Spectrometer indicate the presence of Uranium from 286 - 608 ppm.  This study also indicates the presences of a large percentage of iron (17,912 – 274,476 ppm) as well lesser quantities of strontium, arsenic and lead. ⁽⁶⁾

            John Foster’s new book, “Jurassic West”, is a fascinating look at many aspects of the Morrison world, including information on many fossil locations and descriptions of over 90 species of vertebrate fossils; depositional environments and statistical and population studies; and a story about a back-packing trip through the Morrison Basin in Jurassic times.  I highly recommend that you take a look at this volume.

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Visit the MeMo repository in the Dinosaur Ridge Volunteer Room.

KES
February, 2008

REFERENCES

⁽¹⁾ Foster, John, 2007 Jurassic West: The Dinosaurs of the Morrison Formation and their World. 389 pp.
⁽²⁾ Johnson, Kirk, and Raynolds, Robert, 2003, Ancient Denvers.
⁽³⁾ Steven Hasiotis in http://rainbow.ldeo.columbia.edu/courses/v1001/morisson14.html
⁽⁴⁾ Lockley, Martin and Hunt, Adrian, 1995, Dinosaur Tracks and other Fossil Footprints of the Western United States, p 164.
⁽⁵⁾ AAPG Studies in Geology #22, 1986, A Basin Analysis Case Study: The Morrison Formation, Grants Uranium Region, New Mexico
⁽⁶⁾ Erik Hunter, Colorado School of Mines, personal communication, 2008.

ADDITIONAL REFERENCES

Tidwell, Virginia, and Carpenter, Kenneth, 2005, Thunder Lizards: The Sauropodomorph Dinosaurs.
Curry Rogers, Kristina A., and Wilson, Jeffrey A., 2005, The Sauropods: Evolution and Paleobiology.
Weishampel, D.B., Dodson,p., and Osmolska, H., 2004,  The Dinosauria, Second Edition.

15e. The Morrison Formation (Jurassic)

Distribution of the Morrison Formation. From the Town of Morrison Server.

The Morrison Formation is laterally very extensive (about 1 million km2), and has been found over a large area of the western states, including South Dakota, Idaho, Arizona, New Mexico, Wyoming, Texas and Oklahoma. The Morrison contains massive amounts of dinosaur fossils, and has been described as a bone yard. Several members are recognized, the most prominent being the Salt Wash member, the Recapture Shale Member, the Westwater Canyon Sandstone Member, and the Brushy Basin Shale Member. Along the west side of Dinosaur Ridge, the Morrison Formation is 280 to 320 feet thick. In the area of Arches National Park, the Morrison is approximately 600ft thick. Layers of volcanic ash just above the formation have been radiometrically dated to 147 million years (Chronic, 1990, p. 90 Chronic, Halka (1987). Roadside Geology of New Mexico. Mountain Press Publishing.).

The Morrison Formation is incompatible with the flood model in several ways. First of all, there is abundant evidence that the Morrison deposits are continental, lake and stream deposits, rather than open marine deposits. Some of the thin limestone beds contain abundant charophyte fossils, a type of algae that lives only in fresh-water lakes (Baar, p. 184). The Morrison preserves well-developed paleosols, complete with: abundant carbonate nodules (Turner and Peterson, 1998), "pseudo-microkarst and microkarst features with vadose and internal sediment fills; root traces, columnar and stacked rhizoconcretions, and Microcodium; circumgranular, desiccation, horizontal, and septarian cracks; brecciation and grainification; and rare blackened pebbles" (Dunagan, p. 24). Some sections also contain bedded gypsum. Lastly, the Morrison is unconformable with the strata above and below it (Morales, p. 247).

The Morrison Formation is certainly one of the more interesting formations in terms of its fossil content. Over much of that area, it has yielded a rich trove of dinosaur fossils, beginning with dinosaur discoveries made near Morrison in 1877. These include several species of Diplodocus, Brachiosaurus, Stegosaurus, Seismosaurus, Allosaurus, Ceratosaurus, Camarasaurus, Camptisaurus, Ornitholestes, and others. Other fossils include crocodiles, lizards, frogs, tiny primitive mammals, freshwater clams, fish, ferns, trees, and even large termite nests [see also: Termite tenacity]! The termite nests are especially interesting significant, since they are apparently presrved in situ, and would have been destroyed by flood conditions.

Tracks are found in the Morrison at least thirty locations (Lockley and Hunt, p. 164), and include those of sauropods, theropods, some ornithisicians, pterosaurs, bird-like theropod, and possibly the only known stegosaur track. Whereas underlying formations of the San Rafael are dominated by small and medium theropods tracks, with a small precentage of sauropods, the Morrison track fauna contain numerous tracks of large sauropods, a smaller proportion but a wider variety of theropod tracks, and a wider size range of ornithopod tracks. The largest sauropod tracks are nearly 1 meter long. The smallest tracks Lockley and Hunt note that dinosaur fossils in the Morrison "are preserved at the base of river channel deposits or in fossil soils where tracks are rarely found, whereas some of the best tracks occur along the shores of alkaline lakes or at the top of river channel sequences where bones are not usually found" (p. 177).

The Purgatoire Valley tracksite contains tracks on 4 seperate beds. Over 1300 tracks have been mapped at this location, from one outcrop of one of the beds. Some layers at Purgatoire Valley are in fact heavily trampled or 'dinoturbated,' a phenomenon which becomes widespread in late Mesozoic deposits (p. 173). The site has been dubbed dinosaur lake, for the tracks appear to have been made on the shores of a freshwater lake, based on the associated fossils of plants, algae, snails, clams, crustaceans, and fish. Sometime the tracks contain crushed clam shells and flattened plant stems created perhaps when dinosaurs came to the lake shore for a drink. This sight also contains evidence of gragarious behavior in the form of 5 evenly spaced, parallel brontosaur tracks, which extend roughly 100 meters, virtually the whole length of the outcrop. In Cretaceous deposits, parallel trackways of dozens of individuals are known. Apparently these dinosaurs had no problems gathering themselves into herds and taking long walks, even during the final stages of the global flood.

See also Morrison Research Initiative, Brief Description of the Morrison, The Real Jurassic Park, Late Jurassic Ecosystem Reconstruction During Deposition of the Morrison Formation, Dinosaur National Monument Homepage, DINODATA - Fossils from the Morrison Formation.