Mountain Building 2: the Paleozoic

Overlying the older rock of the Canadian Shield, much of the remaining geologic history of the Midwest records the presence of shallow seas, the formation of the supercontinent Pangaea, and—most recently—the modern ice age.

Cambrian deposits are recorded in Wisconsin, Illinois, Minnesota, and Iowa. During this time, shallow seas covered much of the Midwest, with several transgression and regression episodes recorded as well. Nearly all of what would become North America was located just south of the equator at the dawn of the Cambrian, and it drifted within the tropics for most of its existence.

During the middle of the Ordovician period, about 470 million years ago, the Iapetus Ocean began to close as Baltica (proto-Europe) approached the North American plate from the southeast (Figure 1.6). The intense pressure of the colliding plates and islands smashing into the side of North America caused its edge to crumple, crushing and folding it into mountains. This mountain-building event is called the Taconic Orogeny, and the resulting Taconic Mountains stretched from Newfoundland to Georgia, sharing roughly the same location and orientation of the Appalachian Mountains today, but with towering peaks in eastern Canada and New England.

Figure 1.6: Ordovician: 458 million years ago. Shaded areas represent land that 323
was above water.

Figure 1.6: Ordovician: 458 million years ago. Shaded areas represent land that 323 million years ago was above water.

This folding propagated far to the west, forming waves parallel to the Taconic Mountains themselves that ran roughly southwest to northeast. Nearest the mountains, the crust warped downwards from central New York to central North Carolina, creating the Appalachian Basin. From western Ohio to Alabama, it was warped up into the Cincinnati Arch, and, most distally, down again, creating the Michigan and Illinois basins. These formations are prominent features of Midwestern geology. The inland basins were flooded by the ocean for nearly all of their existence.

The mountain building ceased around the beginning of the Silurian, but the Taconic Mountains still played a major role in the formation of the Midwest. As sediment was eroded from the western side of the Taconic Mountains, deposits spread away, through the Northeast and Midwest, as far west as Wisconsin (Figure 1.7). Thousands of feet of this sediment built up on the floors of the seas that filled the Midwestern basins, and this sediment, along with the millions of generations of marine organisms that lived there, formed the Silurian and Devonian bedrock of much of the area.

Figure 1.7: Volcanic islands formed where the plates were forced together as the Iapetus Ocean closed. The compression crumpled the crust, forming the Taconic Mountains and shallow inland seas.

Figure 1.7: Volcanic islands formed where the plates were forced together as the Iapetus Ocean closed. The compression crumpled the crust, forming the Taconic Mountains and shallow inland seas.

For millions of years during the Paleozoic, an inland extension of the Iapetus Ocean covered the eastern half of North America, filling the basins formed by the mountain- building events. The inland ocean was separated from the main Iapetus Ocean by the Taconic and Acadian Mountains.

Sea level rose and fell in this inland sea during the Paleozoic, in part because the convergence of the plates carrying North America and Baltica continued to buckle the inland basin, deepening the ocean. Sediment eroded from the mountains, however, was also filling the inland ocean.

For about 60 million years, the eastern margin of North America was relatively quiet. The subduction of the oceanic Iapetus plate caused volcanic eruptions that occasionally spread ash over the Midwest, but for the most part, the Taconics were slowly eroding. Finally, Baltica collided with North America near the end of the Devonian period, around 380 million years ago, and mountain- building began again, creating the Acadian Mountains. The Acadian Mountains effectively replaced the Taconics, creating a massive range that was similar in location and extent (Figure 1.8). Just as in the Taconic mountain-building period, compression from the Acadian continental collision warped the crust downward, reinforcing the inland seas. Sediments eroding from the mountains formed the Catskill Delta, creating a new wedge of sediments stretching into a shallow inland sea;; the Devonian and Mississippian rocks of Ohio are evidence of this event.

At the time of the Acadian mountain building and subsequent erosion during the Devonian, the Midwest was located at the Equator and experienced a tropical climate (Figure 1.9). Baltica and North America were united as one larger landmass. Africa, South America, India, Australia, Antarctica, and Florida formed a second continent—Gondwana—in the southern hemisphere.

Figure 1.8: The collision of Baltica and North America, which led to the deposition of sediments in the shallow seas of the Midwest.

Figure 1.8: The collision of Baltica and North America, which led to the deposition of sediments in the shallow seas of the Midwest.

During the Carboniferous, the collision of North America with Gondwana (Figure 1.10) was the genesis of Pangaea. This event also resulted in the formation of both the Appalachian Mountains and, in the South Central region, the Interior Highlands. The interior seaways ultimately regressed from the Midwest.

Figure 1.9: The Devonian period globe.

Figure 1.9: The Devonian period globe.

Figure 1.10: Late Carboniferous.

Figure 1.10: Late Carboniferous.

In the Midwest, Mesozoic-aged rocks are preserved primarily in Minnesota, Iowa, and, to a lesser extent, in Illinois. For the remainder of the Midwest, the Mesozoic was a time of erosion and very little deposition. The rocks that do exist were deposited mainly during the Cretaceous. At that time, North America was roughly divided into thirds: a western portion, an eastern portion, and a vast seaway inundating the center. This seaway stretched from Utah to the western edge of the Midwest and connected the Gulf of Mexico to the Arctic Ocean (Figure 1.12). Deposits in western Minnesota and Iowa contain a variety of Cretaceous creatures that lived near the shores of the Western Interior Seaway. The early Cretaceous was the first time during the Phanerozoic that the Midwest was north of the tropics, approaching its current position.

Evidence for Pangaea

How do we know that Pangaea (Figure 1.11) 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, slowly moving toward their present-day positions over the following 150 million years.

Figure 1.11: Pangaea during the late Paleozoic Era.

Figure 1.11: Pangaea during the late Paleozoic Era.

Figure 1.12: Cretaceous continental seas over North America.

Figure 1.12: Cretaceous continental seas over North America.