12.3: Geography and Tectonics of the Middle Paleozoic
- Page ID
- 41049
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The Silurian and Devonian periods comprise the middle Paleozoic. Sea levels rose during this time as the Ordovician glaciers melted, and sediment was deposited across most continents which helped record the history we are about to discuss.
Mountain Building
Eastern North America
In the late Silurian through the Devonian, Laurentia experienced two mountain-building events, the Caledonian and Acadian orogenies. The Caledonian effected the region that is today northern Laurentia, Greenland, the British Isle, and Norway, while the Acadian Orogeny happened further south along the eastern margin of current North America resulting in another wave of Appalachian mountain-building. One of the most recognizable unconformities on Earth was formed from the crustal deformation accompanying these events.
We know it was relatively warm in the Silurian through the middle Devonian, with extensive shallow seas as evidenced by the widespread coral reef deposits. Evaporite deposits reflect arid conditions and an episode of sea level regression near the end of the Silurian.
Figure \(\PageIndex{1}\): World map during the Silurian modified from Scotese, Christopher R.; Vérard, Christian; Burgener, Landon; Elling, Reece P.; Kocsis, Ádám T., CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons.
Figure \(\PageIndex{2}\): Angular unconformity with nearly verical Silurian strata overlain by gently dipping Devonian strata. Image from dave souza, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons.
In addition to unconformities and deformed crust, sedimentary sequences record the mountain-building of the Middle Paleozoic. They tell a story of prolonged erosion during the first part of the Silurian that established a temporary passive margin along which coral-stromatolite reef communities flourished. These deposits are abruptly overlain by deep-water flysch deposits, signaling the subsidence of the shallow sea, as Avalonia approached and caused the downward deformation of the foreland basin containing the Iapetus Sea. As mountain-building progressed, erosion of the highlands filled this basin, and the flysch deposits were replaced with coarser-grained terrestrial and shallow sea molasse sediments. This sequence of events was similar to what happened during the Taconian Orogeny and produced a depositional sequence that can be seen throughout the eastern US today.
Figure \(\PageIndex{3}\): Image modified from USGS, Public domain, via Wikimedia Commons.
Western North America
Near the end of the Devonian, the western margin of Laurentia became active due to the collision with an island arc. This caused the Antler Orogeny, the first mountain-building event of the Cordilleran.
Figure \(\PageIndex{4}\): Map of the American Cordillera. Image from Knightoftheswords281, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons.
Mass Extinction Event
The late Devonian extinction event demonstrates how biology, geology, and the atmosphere are interconnected. The spread of plants across the continents removed CO2 from the atmosphere, reducing the greenhouse effect and leading to global cooling. Plants also grew taller during the Devonian, and consequently, their root grew deeper. This caused the weathering of rock, which also used atmospheric CO2 and released elements, like Phosphorus, that made their way into the oceans. Phosphorus stimulated the growth of marine algae, thereby increasing the activity of bacteria that break down dead algae. The decay process required oxygen, and the oceans eventually became too deficient to support life, leading to widespread extinctions.
- Acadian Orogeny - a mountain-building event affecting a more southern part of the east coast of Laurentia occurring during the late Silurian through the Devonian resulting in the second wave of Appalachian mountain-building
- Antler Orogeny - first mountain-building event of the Cordilleran occurring at the end of the Devonian
- Caledonian Orogeny - a mountain-building event affecting the northern part of the east coast of Laurentia occurring during the late Silurian through the Devonian
- late Devonian extinction event - an extinction event at the end of the Devonian occurring as the oceans became too deficient in oxygen to support life