Morrison Formation

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

The Morrison Formation is mainly in Wyoming and Colorado, but it stretches into many other states. These states include Montana, North Dakota, South Dakota, Nebraska, Kansas, Oklahoma, Texas, New Mexico, Arizona, Utah, and Idaho. Rocks similar to the Morrison Formation are also in Canada. It covers a huge area, but most of it is hidden underground. Only a small part is visible for geologists and paleontologists to study.

The formation was named after the town of Morrison, Colorado. Some of the first fossils were found there by a man named Arthur Lakes in 1877. That year, the area became important during the Bone Wars. This was 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 and 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 different places, 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. 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, called members. These parts look different across the western United States. The Bluff Sandstone is light brown sandstone from an ancient desert. The Brushy Basin is made of mudstone and conglomerate from old rivers and lakes. The Recapture Member is at the bottom and has clayey sandstone from rivers.

Other important members are the Tidwell, made of siltstone and shale from mudflats and rivers, and the Westwater Canyon, which has lots of organic matter and is a source of uranium ore. Each member helps us learn 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 well known for its many dinosaur fossils. Even though many fossils are not complete, they help us learn about life during the Kimmeridgian time. The climate was dry, like a savanna, but without grasses, flowers, or some trees. Plants such as conifers, ginkgos, cycads, tree ferns, and horsetail rushes grew near rivers.

Many animals lived here, including fish, frogs, salamanders, lizards, crocodiles, turtles, pterosaurs, crayfish, clams, and early mammals. Dinosaurs were common, especially near rivers. Some dinosaurs found here are Allosaurus, Ceratosaurus, Torvosaurus, Camptosaurus, Ornitholestes, stegosaurs like Stegosaurus and Hesperosaurus, early ankylosaurs such as Mymoorapelta and Gargoyleosaurus, and 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 it was a good place for them.

Economic geology

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