The
Morrison Formation is a distinctive sequence of Late
Jurassic sedimentary rock that is 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 grey, 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.
It covers an area of 1.5 million square km (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 the rest was destroyed by erosion as the
Rocky Mountains rose to the west.
It was named after
Morrison, Colorado, where the first fossils 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.
Morrison at the type locality (the location that defines the formation) at Dinosaur Ridge, west of Denver, Colorado.
Close-up of the Morrison at the type locality.
In
Colorado,
New Mexico, and
Utah, the Morrison Formation was a major source of
uranium ore. (See
Uranium mining in the United States.)
Geologic History
According to
radiometric dating, the Morrison Formation dates from 156.3 ± 2 million years old (
Ma) at its base,
to 146.8 ± 1 million years old at the top,
which places it in the latest
Oxfordian,
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. Throughout the western
USA, 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 Saskatchewan in the north, was formed when the precursors to the
Front Range of 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.
Eolian, or wind-deposited sandstones are found in the southwestern part, which indicates it was much more arid — a
desert, with sand
dunes.
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:
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.
- 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.
- Tidwell Member: The Sundance Sea receded to Wyoming during this member and was replaced by lakes and mudflats.
- Salt Wash Member: The first purely terrestrial member. The basin was a semi-arid alluvial plain, with seasonal mudflats.
- 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' and extensive wetlands that were located just west of the modern Uncompahgre Plateau.
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 finds
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.
Insects were very similar to modern
species, with
termites building 30 m (100 ft.) tall nests. Along the rivers, there were
fish,
frogs,
salamanders,
lizards,
crocodiles,
turtles,
pterosaurs,
crayfish,
clams, and
monotremes (
prototherian
mammals, the largest of which was about the size of a
rat).
The
dinosaurs were most likely riparian, as well. Hundreds of dinosaur fossils have been discovered, such as
Allosaurus,
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 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 the
Diplodocus (most famously, the first nearly-complete specimen of
D. carnegiei, which is now exhibited at the
Carnegie Museum of Natural History, in Pittsburgh, Pennsylvania),
Camarasaurus (the most commonly found sauropod),
Brachiosaurus,
Apatosaurus (also wrongly known as
Brontosaurus),
Barosaurus, the uncommon
Haplocanthosaurus and the
Seismosaurus. 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
Locations where significant Morrison Formation fossil discoveries have been made include:
- Garden Park, Colorado: One of the three major sites excavated by the paleontologists Othniel Charles Marsh and Edward Drinker Cope during the Bone Wars in 1877, though most of the specimens were too incomplete to classify (nomina dubia). In 1992, a specimen of Stegosaurus stenops was discovered with its armor still in place, which confirmed that the dinosaur had two rows of plates on its back.
- Cleveland-Lloyd Quarry, Utah: First excavated by Lee Stokes in 1937. In the Jurassic, the quarry was a mudhole where several enormous sauropods got stuck and apparently caused a feeding frenzy that lured and trapped many carnivorous dinosaurs. Most of the allosaurs are from this site, as well as the unique Stokesosaurus and Marshosaurus.
- Como Bluff, Wyoming: One of the most renowned fossil sites in North America. It was first worked by Cope and particularly Marsh in 1877 and has been the source of many different sauropods and non-dinosaur species. The Cloverly Formation from the Cretaceous and some Triassic strata are also exposed at this location.
- Fruita Paleontological Resource Area: Badlands sites located south of Fruita, Colorado, were actively worked by George Callison from the University of California, Los Angeles. Numerous specimens of mammals, lizards, and crocodiles were found. Most recently, Fruitafossor windscheffelia was described from the area.
Fruita Paleontological Resource Area. One of the sites is denoted by the arrow.
Site comparisons
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.
See also
