Paleozoic History

Cambrian (539–485 Ma)

• The Cambrian explosion involved rapid diversification of animal life, including many organisms with mineralized shells and skeletons.

• Trilobites and other marine animals became common in the fossil record.

• Shallow marine environments deposited sandstone, shale, and limestone.

• Sea-level rise flooded continental interiors, producing widespread shallow seas and sedimentary deposits.

Ordovician (485–444 Ma)

• Marine life diversified further, including brachiopods, corals, and other invertebrates.

• Continental collisions and volcanic island-arc activity contributed to the Taconic Orogeny in eastern North America, approximately 480–460 Ma.

• The Taconic mountain belt supplied sediment to adjacent basins, forming the Queenston Delta or clastic wedge.

• Late Ordovician glaciation and associated sea-level fall contributed to a major mass extinction.

Silurian (444–419 Ma)

• Marine environments continued to deposit carbonate rocks, including limestone.

• The Salinic and related Appalachian tectonic events, along with the Caledonian Orogeny in the broader North Atlantic region, reflected collisions and convergence among continental blocks.

• Sea levels rose after the Ordovician glaciation, producing widespread marine deposition.

• Early vascular plants and terrestrial ecosystems expanded.

Devonian (419–359 Ma)

• Often called the Age of Fishes, the Devonian saw major diversification of fish.

• Forests and more complex land plants expanded; the first tetrapods evolved near the end of the period.

• The Acadian Orogeny affected eastern North America, contributing sediment to adjacent basins.

• The Antler Orogeny affected the western margin of North America, associated with tectonic convergence and deformation.

• Marine environments deposited limestone and other sediments; reef systems were widespread.

Carboniferous (359–299 Ma)

• Extensive tropical forests and swamps produced organic-rich deposits that eventually became major coal beds.

• Repeated sea-level changes caused cyclothems: recurring sequences of marine and nonmarine sedimentary rocks.

• The Alleghanian Orogeny began during the later Paleozoic as Africa and other Gondwanan landmasses converged with Laurentia.

• The ancestral Rocky Mountain region began experiencing tectonic changes later associated with the Ancestral Rocky Mountains.

Permian (299–252 Ma)

• Continued collisions completed the assembly of Pangaea, surrounded by the Panthalassa Ocean.

• The Alleghanian Orogeny continued, building the Appalachian Mountains.

• The Ancestral Rocky Mountains developed through tectonic deformation associated with regional plate interactions.

• Arid conditions became widespread in many continental interiors.

• Marine environments deposited evaporites, reef carbonates, and basin sediments, including deposits in parts of West Texas and New Mexico.

• The end-Permian mass extinction, approximately 252 Ma, was Earth's most severe known mass extinction, associated largely with massive Siberian Traps volcanism and its climatic and oceanic consequences.

Paleozoic concepts

Wilson cycle

Rifting → new ocean basin → seafloor spreading → subduction → ocean closure → continental collision and mountain building.

Transgression and regression

Transgression occurs when the sea advances over land; regression occurs when the shoreline moves seaward. These shifts create changing sedimentary environments and rock sequences.

Orogeny

A mountain-building episode caused by tectonic processes, commonly involving convergence, collision, deformation, metamorphism, and sometimes magmatism.