The Mesozoic

For much of the Mesozoic, large sections of the Western States were underwater. Around 300 million years ago, the Western States were part of the continental margin surrounding the supercontinent Pangaea. However, that changed once Pangaea began splitting apart 250 million years ago. As the supercontinent rifted apart, subduction, volcanism, and accretion in the Western States accelerated, adding more land to the continental margin (Figure 1.8). Land did not build up continuously—accretion in these states delivered packages of rock known as accreted or exotic terranes. Each terrane consists of sedimentary rock made from former seafloor sediment, slabs of oceanic crust (ophiolites, Figure 1.9), the remains of volcanic islands, and, in some instances, shards of continental crust. The terrane is pressed against the edge of the continent in a process sometimes described as “docking.”

Figure 1.8: The Western United States at 135 million years ago. The terrane contains an island arc in the process of docking with the Western States. This arc will eventually become part of Alaska and British Columbia.

In general, accretion was accompanied by volcanism, which resulted in several major orogenies (episodes of mountain building). During the Mesozoic, three such episodes were particularly important in the development of western North America. The Nevadan, Siever, and Laramide orogenies took place between 180 and 50 million years ago. The remnants of these mountain-building events can still be seen as bodies of intrusive rock embedded in the West’s present-day ranges.

Figure 1.9: Formation of an ophiolite.

While the addition of land to the continental margin was a general theme throughout the Mesozoic, it was by no means a consistent process. Sedimentary rock found in the Midwest indicates that shallow seas episodically inundated the interior of North America, turning major parts of the Western States into a broad isthmus (Figures 1.10 and 1.11). The exact reasons for this periodic flooding are not known, but there are two possibilities: sea level rise or a change in the elevation of the continental plain. Although sea level change is often associated with climate change, it can also be driven by changes in the bathymetry of the ocean floor. Increased volcanic activity at mid-ocean ridges could have increased the size of the ridges, displacing seawater and causing the sea level to rise. Additionally, as Pangaea began to drift apart, the crust underlying the Interior Plains most likely stretched, causing it to thin and making the land surface drop below sea level.

Figure 1.10: North America 90 million years ago. During this time, a shallow seaway covered much of central North America.

Figure 1.11: Cretaceous continental seas over North America.