Morrison Formation

The Morrison Formation is a special layer of Upper Jurassic sedimentary rock found across many western states in the United States. It is famous because it holds the most dinosaur fossils discovered in North America. This rock layer is made of different types of stone, such as mudstone, sandstone, siltstone, and limestone, and it can be light gray, greenish gray, or red in color. Most of the important fossils are found in the green siltstone and lower sandstone parts, which remind us of the rivers and floodplains that existed millions of years ago during the Jurassic period.

The Morrison Formation is mainly located in Wyoming and Colorado, but it stretches into many other states including Montana, North Dakota, South Dakota, Nebraska, Kansas, Oklahoma, Texas, New Mexico, Arizona, Utah, and Idaho. Rocks similar to the Morrison Formation are also found in Canada. Although it covers a huge area of about 1.5 million square kilometers, most of it is hidden underground, with only a small part visible for geologists and paleontologists to study.

The formation was named after the town of Morrison, Colorado, where some of the first fossils were found by a man named Arthur Lakes in 1877. That same year, the area became important during the Bone Wars, a friendly competition between two early scientists, Othniel Charles Marsh and Edward Drinker Cope, who raced to discover new fossils. Besides fossils, parts of the Morrison Formation in Colorado, New Mexico, and Utah were once a major source of uranium ore.

Geologic history

The Morrison Formation is a layer of rock from between 156 to 147 million years ago, during the late Jurassic period. It is found in the western United States and is similar in age to rock layers in Germany, Tanzania, and Portugal. At this time, the land that would become North America was moving north through warm climates.

The Morrison Formation formed in various environments, from swampy lowlands and river channels to deserts with sand dunes. In the north, near an ancient sea, the land was wet and covered with plants, while in the southwest, it was dry and sandy. These different conditions created the mudstone, sandstone, siltstone, and limestone that make up the Morrison Formation today.

Stratigraphy

The Morrison Formation is divided into several parts, or members, which appear differently across the western United States. These members include the Bluff Sandstone, known for its light brown sandstone formed in ancient desert settings; the Brushy Basin, made of mudstone and conglomerate from rivers and lakes; and the Recapture Member, which sits at the bottom of the formation and includes clayey sandstone from river environments. Other notable members are the Tidwell, mainly composed of siltstone and shale from mudflats and rivers, and the Westwater Canyon, rich in organic matter and a source of uranium ore. Each member provides clues about the ancient landscapes and environments of the Jurassic period.

Main articles: Bluff Sandstone Member, Brushy Basin Member, Fiftymile Member, Jackpile Sandstone Member, Ralston Creek Member, Recapture Member, Salt Wash Member, Tidwell Member, Unkpapa Sandstone Member, Westwater Canyon Member, Windy Hill Member

Fossil content

Main article: Paleobiota of the Morrison Formation

Main article: List of dinosaurs of the Morrison Formation

The Morrison Formation is famous for its many dinosaur fossils. Even though many of these fossils are incomplete, they give us a good idea of what life was like during the Kimmeridgian time. The climate was dry, similar to a savanna, but without grasses, flowers, or certain trees. Instead, plants like conifers, ginkgos, cycads, tree ferns, and horsetail rushes grew along river floodplains.

Many different animals lived here, including fish, frogs, salamanders, lizards, crocodiles, turtles, pterosaurs, crayfish, clams, and early mammals. Dinosaurs were also common, especially along the rivers. Some of the dinosaurs found include Allosaurus, Ceratosaurus, Torvosaurus, Camptosaurus, Ornitholestes, stegosaurs like Stegosaurus and Hesperosaurus, early ankylosaurs such as Mymoorapelta and Gargoyleosaurus, and many large sauropods like Diplodocus, Camarasaurus, Brachiosaurus, Apatosaurus, Brontosaurus, Barosaurus, Haplocanthosaurus, and Supersaurus. Some of these dinosaurs, like Camptosaurus, are known to have nested in the area, showing that it was a good home for them.

Economic geology

The Morrison Formation has important uranium deposits. One notable discovery was the Jackpile uranium body near Grants, New Mexico in 1951. Mines in the Grants area produced a large amount of U₃O₈ from 1948 to 2002. The uranium was naturally trapped by plant materials and other substances in the sandstone layers.