12.4: Geography and Tectonics of the Late Paleozoic

Overview

The Carboniferous and Permian Periods comprise the late Paleozoic, a chapter in Earth’s history that experienced the most intense ice age of the Paleozoic Era, followed by intense global warming and the greatest extinction of life our planet has experienced. High sea levels at the start of the Carboniferous created widespread shallow seas and extensive limestone deposits, and warm conditions in the equatorial regions allowed for coal forests to thrive. In these freshwater swamps, plants rapidly grew, died, and were buried, resulting in thick accumulations of peat. Over time, after burial, carbon-rich peat is transformed into coal. By the end of the Carboniferous, the coal forests died out in North America, and Pangaea was nearly fully assembled.

Figure. Configuration of the continents during the Carboniferous. Image from GeoPotinga, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

The Carboniferous Period was named in Britain for the coal deposits that define rocks from this time. However, in North America, rock in the early half of this period is limestone, as typified by strata in Mississippi; in Pennsylvania, coal is the dominant lithology in late Carboniferous deposits. Consequently, this period is subdivided into the Mississippian and Pennsylvanian Periods in North America.

Mountain Building

The third and final episode of Appalachian mountain building happened during this time, the Alleghenian Orogeny. This folded strata into mountains in the southern part of the range during the collision of Euramerica and Gondwanaland. This convergence deformed the crust along the southern margin of North America, where today is Texas and Oklahoma, building the Ouachita Mountains.

Along North America's western margin, the Sonoma Orogeny picked up where the Antler Orogeny left off, building up mountains through the accretion of island arcs and other fragments of continental crust. This event also thrust seafloor sediments onto the continent and added considerable volume to its western margin.

Figure. Image from BZenith, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons.

Ice Age

The most significant ice age of the Paleozoic Era started in the mid-Carboniferous. It lasted until the early Permian, when glaciers at low elevations were found even in equatorial regions, like during Snowball Earth. This happened because vegetation was rapidly buried in coal swamps, where bacterial decomposition couldn’t keep pace with burial. Consequently, carbon was being buried instead of returning to the atmosphere through decomposition processes (see Carbon cycle). CO2 levels fell, and oxygen levels rose to as high as 35% (today, the atmosphere has 21% oxygen) as global warming decreased. This resulted in global cooling by lowering the concentration of the greenhouse gas CO2. It also increased the size of insects, which took advantage of the higher oxygen levels in the air (Stanley and Luczaj, 2015).

Carbon and oxygen fluxes between plants, the atmosphere, and the geosphere. (Callan Bentley cartoon.)

Evidence for this ice includes a global unconformity that formed as the ice age locked up water in glaciers, lowering sea levels, and exposing continental rock to erosion. The unconformity is exposed in outcrops around the world. More evidence comes in the form of tree rings, which have seasonal growth patterns indicative of the significant temperature swings associated with ice ages.

The Carboniferous unconformity. Image from Gordon Hatton.

The ice age was followed by global warming and drying that persisted through the end of the Paleozoic and were the driving factors behind the greatest extinction of life our planet has experienced, the Permian-Triassic (P-T) extinction. The drying was caused by the assembly of Pangea, which cut off inland portions from ocean-derived moisture due to its immense size and mountain ranges.