The Carboniferous Period

Map of North America in the early Mississipian sub-period
North America 345 million years ago
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The Carboniferous Period in North America is generally divided into two eras: the Mississippian and the Pennsylvanian. In the early Mississippian, the length of a day was 22.4 hours. That faster spin with a thicker atmosphere made for much stronger winds and contributed to the thick aeolian deposits in the interior of Pangaea. The heavier atmosphere and high humidity contributed to the evolution of giant insects and arthropods. Dragonfly-like creatures grew to 30 inch wingspans while millipedes grew to 8 foot lengths.

It was a time when much of the planet's land surface was covered in dense forests. It was those forests that laid down much of the coal under the United States and Canada. The Mississippian Era lasted from about 359 million years ago to about 318 million years ago. In the rest of the world the Carboniferous is broken into other series, dependent on local geological stratigraphy. But generally, everything more or less coincides with the Upper and Lower Mississippian and Upper and Lower Pennsylvanian.

Sea levels dropped precipitously at the end of the Devonian but recovered quickly in the early Carboniferous. That contributed to the wide dispersion of carbonate sediments on the continental shelves of the time. South polar temperatures dropped about the same time and the southern reaches of Gondwana were glaciated for much of the Period. The deep tropics of the time harbored flourishing coal swamps.

During the Carboniferous Period, the Laurentia craton and Gondwana slowly approached each other and closed the Rheic ocean. Initial contacts were at promontories of what is now Africa and the US East Coast. Pressure from that approach began the Lackawanna phase of the Alleghenian orogeny. The initial collision of Africa and peri-Gondwanan terranes with Laurentia began about 325 million years ago. The major collision with the Africa plate began about 5 million years later, near Mississippian-Pennsylvanian boundary.

North America during the late Mississippian
North America 325 million years ago
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Considerable transform motions along the Hun superterrane shortened and stacked its elements, pushing up Iberia. Western fragments of the Hun superterrane collided with what is now the southern US to begin the Ouachita orogeny. That collision (and subsequent terrane accretion) was well underway by the end of the Mississippian. There was widespread and dominant carbonate deposition across much of Laurentia except in those areas adjacent to orogenic activity.

On what is now the West Coast of North America, the Greater Ancestral Rocky Mountains, the Quesnell and the Stikine Terranes became the leading edge of the Laurentia craton along an east-dipping subduction zone as the early phases of the Cordilleran margin evolved. Back arc spreading and transform processes transported the Stikine outboard of the Quesnell Terrane to eventually form a double east-dipping arc complex (the McCloud arc). The Havallah backarc basin formed between the Quesnell Terrane and Laurentia. Laurentia cratonic shelf sediments formed throughout the interval but none of the basins that would dominate the Late Paleozoic were evident yet.

The boundary of the Mississippian-Pennsylvanian is dated to about 323 million years ago when sea levels again dropped precipitously and triggered a massive marine species extinction. That was also the beginning of the Permo-Carboniferous Glaciation, a time when a complex center of glaciation migrated across South America, Antarctica and South Africa between 350 and 240 million years ago (generally the time of the supercontinent Pangaea which straddled the South Polar latitudes at the time). At its greatest extent, the ice sheet covered everything from the South Pole up to 50°S latitude.

Map of North America 315  million years ago
North America 315 million years ago
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The Pennsylvanian Era lasted from about 318 million years ago to about 299 million years ago.

The Pennsylvanian is the time when western Pangaea formed with Gondwana suturing to what is now the eastern US and the Appalachian-Ouachitan orogenies neared their climax Extensive foreland basins formed in both of what are now the Old and New Worlds. Juxtaposition of Hun superterrane proceeded along Southern Europe while east of that area, the Tethyian Ocean still separated Gondwana from the land mass of Asia.

Through the Pennsylvanian there was a dramatic change in sedimentation: from carbonate to silisiclastic in the orogenic areas. Carbonate sedimentation continued in some basins and early phases of wind-blown sedimentation began in parts of western Laurentia due to the land mass being so large. Cyclic sedimentation obvious in all sedimentary facies from the time reflect glacial cycles across the Gondwana land mass. Near the end of the Pennsylvanian there was a general drying across western areas of the supercontinent and thick salt deposits formed in the Western Interior basin. Western peri-Gondwanan terranes closed on what is now west Texas and caused the Marathon orogeny.

Along what was North America's Pacific Coast the Caborca block moved southeast as the southern McCloud arc (Stikine terrane) drifted northwest in a side-slip grating along the edge of the greater land mass. The Quesnell fragment of the McCloud arc was separated from Laurentia by the backarc Havallah basin.

Map of North America in the late Pennsylvanian
North America 300 million years ago
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About 305 million years ago, near the end of the Pennsylvanian, came the Carboniferous Rainforest Collapse, a minor extinction event occurring simultaneously with a rapid increase in glaciation across southern Gondwana. During the collapse, the lush, thick rainforests that carpeted Euramerica (on the equator at the time) were broken into islands as huge ferns replaced trees. With the drastic change in food resources many animal species died off. At the same time, species that had been spread throughout Pangaea became isolated in the various islands and began evolving in disparate ways.

At the time of the Collapse, the global climate became cooler and drier, aridification being the order of the day at the Permo-Carboniferous Boundary. Many amphibious species didn't make it while many reptilians did. From the late Pennsylvanian into the mid-Permian atmospheric levels of carbon dioxide crashed to a historic low and the coal forests were replaced with grassy plains and stands of tree ferns while huge sections in the interior of Pangaea were covered with wind-blown sands.

A fossilized Lycopsid
A Lycopsid fossil found in Nova Scotia
Upper image courtesy of Nobu Tamura, CCA-by-SA 3.0 License
Lower photo courtesy of Michael C. Rygel, CCA-by-SA 3.0 License
Maps © Ron Blakey, NAU Geology,