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 Southwestern 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. In the Southwest, the shoreline withdrew from central Utah to western Nevada. During the Triassic, Earth’s climate was much warmer than today, with an average global temperature of about 25°C. The Southwest was now largely dry land, and rocks from this period record complex and varied deposition in rivers and deserts. Subduction along the western margin of the Americas generated the Cordilleran volcanic arc, which was active throughout much of the Mesozoic. Triassic rocks of the Grand Canyon record an influx of volcanic material from the adjacent arc, as well as sediment from adjacent highlands composed of sedimentary and plutonic rocks. Rocks from the Cordilleran arc are still present today in the Andes, Central America, the Cascades, and the Aleutians.

Pangaea began to break up during the early Jurassic, and sea levels began to rise again. By the middle Jurassic, a shallow arm of the sea reached from Canada south through Montana and parts of Wyoming, Utah, and the Dakotas (Figure 1.14), depositing thin layers of limestone, mudstone, and sand. During the Jurassic, mudstone and sandstones were also deposited in lowland areas and river channels throughout the Rockies and Colorado Plateau; these formed the Morrison Formation, which is famous for its abundant dinosaur fossils.

Figure 1.14: The Southwestern US during the Jurassic, approximately 170 million years ago.

During the Cretaceous, Pangaea entered its final stages of breakup (Figure 1.15). 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. As oceanic crust was subducted beneath the continent, a new volcanic arc formed the Sierra Nevada of California. The new Atlantic Ocean widened, and sea levels began to rise. The Farallon plate began to subduct at an unusually shallow angle, sliding farther inland beneath western North America before finally sinking into the asthenosphere (Figure 1.16). 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 1.17). This shallow inland sea inundated a 1000-kilometer (620-mile) wide swath from Mexico to Alaska, separating North America into an Appalachian island to the east and a mountainous Cordilleran island to the west. Erosion from these western mountains resulted in deposition of thick layers of sediment throughout the seaway. 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 1.15: 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) formed an extensive belt of and folds, also known as the “Overthrust Belt,” extending from the Sierra Nevada in California to the edge of the Colorado Plateau in western Utah. The second event, the Laramide Orogeny, peaked around 68– 65 million years ago when the angle of the subducting plate became shallower. This formed mountains farther inland than would normally be expected above a subduction zone, uplifting the Rocky Mountains in Wyoming, Colorado, and New Mexico (see Figure 1.16). These mountain ranges are bounded by thrust faults. 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 to the present day.

Evidence for Pangaea

How do we know that Pangaea existed 250 million years ago? Long before the discovery of plate tectonics in the 1960s and early 1970s, fossils and mountain belts provided evidence that the continents had not always been in their current positions. For example, the Permian-aged 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 1.16: The Sevier and Laramide orogenies.

Figure 1.17: The Western Interior Seaway.

Understanding Plate Boundaries

Active plate margins are the boundaries between two plates of the Earth’s crust that are colliding, pulling apart, or moving past each other as they move over the mantle. Some of these plates move as fast as 10 centimeters/ year (4 inches/year). The processes of plate movement, spreading, subduction, and mountain building are collectively called plate tectonics.

When one plate slides beneath another, it is called a convergent boundary or subduction zone. When two plates pull apart from each other, it is called a divergent boundary or rift margin. When the plates slip past each other in opposite directions, it is known as a transform boundary.

The Cretaceous-Paleogene (K-Pg) boundary (previously known as the Cretaceous-Tertiary [K-T] boundary) marks one of the most significant physical and biological events in Earth history. The boundary marks the contact between the Mesozoic and Cenozoic eras at around 65 million years ago, representing a time during which a large proportion (perhaps 50–70%) of all species of animals and plants (both marine and terrestrial, from microscopic one-celled organisms to massive dinosaurs) abruptly became extinct. Most geologists and paleontologists think these extinctions resulted from the impact of a large comet or asteroid, perhaps associated with an impact crater in the subsurface of Mexico’s Yucatan Peninsula. There is also evidence for the occurrence of extensive volcanism at the K-Pg boundary, indicated by large basaltic lava flows in India called the Deccan Traps. The end-Cretaceous event greatly altered the history of life on land and in the sea, and these changes are clearly visible in the Southwest’s fossil record. The boundary itself is rarely preserved in the geologic record, due to incomplete sedimentation and widespread erosion. However, there are several Southwestern localities, especially in northern New Mexico, that preserve the K-Pg boundary layer.