The Mesozoic: A Story of Mountains and Seas

The Mesozoic era is frequently known as the Age of the Dinosaurs or Age of Reptiles, but many other life forms evolved and thrived during this time, including marine invertebrates, flowering plants, birds, and mammals. The Mesozoic was also a time of major geologic change during which great thicknesses of rocks were deposited across the western US.

The supercontinent Pangaea was in place by the end of the Permian period, and global sea level was probably at its lowest of any time during the past 600 million years. During the Triassic and Jurassic, sea levels rose, and a shallow arm of the sea reached from Canada through Montana and parts of Wyoming (Figure 1.10). Iron-rich limestones, sandstones, and mudstones laid down in this sea were oxidized, giving a distinctive and characteristic red color to the rocks, which are appropriately called “red beds.” During the Jurassic, mudstone and sandstones were also deposited in lowland areas and river channels throughout the Rocky Mountains and Colorado Plateau; these formed the Morrison Formation, which is famous for its abundant dinosaur fossils.

Figure 1.10: The Northwest Central US during the Jurassic, approximately 170 million years ago.

During the early Cretaceous, Pangaea entered its final stages of breakup (Figure 1.11). Far to the west, oceanic crust (the Farallon plate) had been subducting under western North America for tens of millions of years, causing a series of volcanic island complexes to collide with and become accreted to that margin of the continent, forming the Sierra Nevada of California. As the new Atlantic Ocean widened, sea levels began to rise. Around 85 million years ago, when the Farallon plate began to subduct at an unusually shallow angle, it slid farther inland beneath western North America before finally sinking into the asthenosphere. This downwarped the center of the continent and created a basin that allowed the waters of the Gulf of Mexico to meet with those in the north, forming the Western Interior Seaway (Figure 1.12), which inundated a 1000-kilometer (620-mile) wide swath from Mexico to Alaska. During the very latest stages of the Cretaceous period, around 70 million years ago, the Western Interior Seaway was displaced by slow uplift of the continent.

Figure 1.11: Landmasses following the breakup of Pangaea.

The Farallon plate continued to collide with western North America, thrusting layers of rock up over each other and causing increasing volcanism to the west of the Western Interior Seaway. The compressional forces of subduction faulted the crustal rocks of western North America and uplifted the Rocky Mountains in two major pulses. The Sevier Orogeny (100 - 72 million years ago) raised the portion of the Rocky Mountains in Montana, Wyoming, and Utah known as the “Overthrust Belt.” The second event, the Laramide Orogeny, peaked around 68 - 65 million years ago, when the angle of the subducting plate became shallower, uplifting the Rocky Mountains in Colorado and New Mexico. While most of the magmatic activity at this time occurred on the western edge of the continent in the volcanic arc of the Sierra Nevada, some did take place farther inland. The largest and most important evidence of this is the Idaho Batholith— three major lobes of granitic material intruded beneath large areas of Idaho between 100 and 65 million years ago. The rising ancestral Rocky Mountains provided sediment that filled the seaway, and uplift from the ongoing orogeny finally caused the water to split in the Dakotas and retreat south.

Evidence for Pangaea

How do we know that Pangaea existed 250 million years ago? Fossil evidence and mountain belts provide some of the clues. For example, the Permian-age fossil plant Glossopteris had seeds too heavy to be blown across an ocean. Yet Glossopteris fossils are found in South America, Africa, Australia, India, and Antarctica! The mountain belts along the margins of North America, Africa, and Europe line up as well and have similar rock types, an indication that the continents at one time were joined as Pangaea. Despite the discovery of Glossopteris and other geologic evidence, the theory of continental drift was not accepted for decades, until the mechanisms of continental movement were discovered and reformulated under the modern theory of plate tectonics. The supercontinent Pangaea existed for approximately 100 million years, reaching its largest size during the Triassic period. During the Jurassic, the landmass began to fragment into the modern continents, which slowly moved toward their present-day positions over the following 150 million years.

Pangaea during the late Paleozoic era

Figure 1.12: The Western Interior Seaway.

Because the crust flexes or breaks under compression, several inland basins formed between the mountain ranges, and the eroding mountains shed thick layers of sediment into these basins, forming conglomerates, sandstones, and mudstones. The Colorado Plateau remained stable during this time of compression, and persisted during the subsequent episode of extension that followed from the Paleogene period to the present day.