Geologic Time and Fossil Formation

Utah’s geology has changed over time. Notice where North is in this picture. 550 Million Years Ago Present Day

The Early Years: Paleozoic. Utah was at the edge of North America. The eastern part of the state was flat and low, close to sea level. Coral reefs became thick limestones. These helped form the Wasatch Mountains. Shallow seas led to deep oceans in the west. 500 million years ago Utah was near the equator and at about sea level.

Early Fossils 500 mya – 248 mya Trilobites Giant Insects

Early Jurassic: Utah was cut off from the ocean winds by rising mountains. Desert sands blew into Utah from the north. These blowing sands formed dunes. The dunes turned into rock and are preserved in what is now called the Navajo Sandstone. You can see them in Zion National Park and on the San Rafael Swell. 200 million years ago Utah was no longer at the edge of the ocean.

Fossils 206 mya – 180 mya Allosaurus Tanystropheidae Captorhinus

Hot and swampy. A great place for dinosaurs! Famous Dinosaurs: Late Jurassic. At this time Utah was a hot, swampy lowland with mountains and volcanoes to the west and northwest. Meandering rivers and lakes abounded, while dinosaurs roamed the land. Their fossilized bones are preserved and can be seen at famous sites such as the Cleveland-Lloyd Dinosaur Quarry and Dinosaur National Monument.

Late Jurassic Period Fossils Stegosaurus Camarasaurus

Late Cretaceous: Coal forms. The continent ran into the Pacific Plate. This created high mountains in the west. In the east, an inland sea stretched from the Gulf of Mexico to the Arctic. Sediment filled the sea and the basin sank. Later, this turned into sandstone. Dinosaurs continued to live in huge, wet forests. These forests turned into coal deposits. 100 million years ago, plants grew that became coal deposits.

Late Cretaceous Period Fossils Coal Ferns Tyrannosaurus

An inland sea formed, and the land was pushed up. Utah Starts to Come Up in the World: Paleocene. Erosion wore down mountain in the west. Sediments filled the inland sea to the east. Pressure from the Pacific Plate pushed the ground up. This is called uplift. It created the Uinta mountains and the Colorado Plateau. The pressure created the swells and depressions now found in Utah. One of these is the San Rafael Swell. A large freshwater body, called Lake Flagstaff, formed in southern Utah.

Paleocene Fossils Triceratops Ernanodon (early mammal)

Organic matter becomes oil shale. Oil Shale and Fossil Fish: Eocene. Utah spent nearly 500 million years near sea level. Now it rose to become nearly a mile high. The Colorado Plateau warped even more. It produced basins for lakes such as Lake Uinta. Organic-rich sediments include fish fossils and oil shales. The western mountains were nearly eroded away.

Eocene Fossils Fish in mudstone Amber with insects Seahorse Oil shale with fossil leaf.

The land uplifted and modern rivers began to form. Uplift and Volcanoes: Oligocene. Lakes on the Colorado Plateau filled up with sediment. Wide plains sat between mountain uplifts. Modern rivers began to run across these plains. The land in northeastern Utah was divided by mountains. This caused the Green River to drain into the Mississippi River. Today, the Green River drains into the Colorado River. The land in western Utah began to stretch. Later, this would create the Basin and Range.

Oligocene Fossils Mesohippus (ancient horse ) Palaelodus (swimming flamingo)

Basin and range forms. Metals are deposited in Utah’s mountains. Precious Metals Form: Miocene. Land near what is now San Francisco had been moving closer to Salt Lake City. Now, the land stretched and they moved apart. This stretching left tall mountains divided by wide plains. This formed the Basin and Range. Volcanic activity formed three areas with metalic minerals. They are: Park City-Oquirrh, Deer Creek-Tintic, and Wah Wah-Tushar. The Colorado Plateau continued to rise and tilt northeastward.

Miocene Fossils Livyatan melvillei (toothed whale)  Australopithecine (hominid)

Finally, Lake Bonneville! Water and Ice: Pleistocene. The geography of Utah was very close to what it is now. Mountains, canyons, and rivers were all well in place. The climate at this time was wetter and colder. Glaciers formed in the mountains. They carved canyons in the Uinta Mountains and other mountain ranges throughout the state. A giant fresh-water body called Lake Bonneville also formed. It stretched from the Wasatch Mountains to Nevada and from the Utah-Idaho border nearly to Cedar City in southern Utah.

Pleistocene Fossils Bison latifronsSmilodon fatalis saber- toothed cat Mammoth (frozen in ice)

It’s all about the past. Our ancient geologic history explains the present. These are the Places: Present. Our geologic informs us about Utah's present. We see why the rocks to the east are brightly colored while the rocks to the west are duller. We see why huge canyons formed on the Colorado Plateau while much of the Basin and Range has no external drainage. It explains why a high mountain chain, the Wasatch, runs down the middle of the state. This history helps us understand why settlements, industry, and recreation sites are located where they are.

Summing Up Land on Earth’s surface moves over time (plate tectonics) Utah once sat near the equator. Over time it moved north. Utah’s land has uplifted throughout geologic history. Stretching and plate movement created ranges and basins. Earth material and fossils are deposited based on the events of an era. Older fossils are generally found below younger ones. The fossil record informs geology and geology informs fossil dating. Fossils are created in different ways, depending on location and circumstances.