Mesozoic History

Triassic (252–201 Ma)

• Life recovered from the end-Permian extinction.

• Pangaea remained largely assembled, with extensive arid continental interiors.

• Rifting began to fragment Pangaea, especially in eastern North America.

• Basaltic eruptions associated with the Central Atlantic Magmatic Province (CAMP) occurred around 201 Ma near the Triassic–Jurassic boundary.

• Rifting formed fault-bounded basins where rivers and lakes deposited sediment. These deposits include the Newark basin succession.

• The Palisades Sill formed when magma intruded existing rocks.

• The Triassic–Jurassic mass extinction was associated with CAMP volcanism and major environmental changes.

Jurassic (201–145 Ma)

• Continued rifting and seafloor spreading widened the Atlantic and separated parts of Pangaea.

• The Cordilleran subduction system developed along western North America, generating volcanic arcs and mountain-building events.

• The Nevadan Orogeny affected the western Cordillera; the Sevier Orogeny began later and continued into the Cretaceous.

• The Navajo Sandstone records extensive desert dunes, recognizable through large-scale cross-bedding.

• The Morrison Formation accumulated in river and floodplain environments and is famous for dinosaur fossils.

• Marine incursions deposited sediments in several regions, while evaporites and salt accumulated in parts of the Gulf of Mexico region as basin development continued.

Cretaceous (145–66 Ma)

• Continued plate separation opened more of the Atlantic and changed global ocean circulation.

• Sea level reached exceptionally high levels during parts of the Cretaceous, creating extensive shallow epicontinental seas.

• The Western Interior Seaway covered much of central North America, including large portions of the region that is now Colorado.

• Marine shale, chalk, sandstone, and other sediments accumulated in these environments.

• The Sevier Orogeny involved thin-skinned thrust faulting, while the Laramide Orogeny began during the Late Cretaceous and continued into the early Cenozoic.

• Laramide deformation uplifted basement blocks that contributed to the formation of the modern Rocky Mountain ranges.

• Coal accumulated in coastal plains and foreland-basin environments.

• Flowering plants diversified, while dinosaurs and other reptiles remained prominent.

• The end-Cretaceous extinction at 66 Ma followed the Chicxulub asteroid impact and eliminated all non-avian dinosaurs, along with many other groups.

Pangaea's breakup: the essential chain

• Continental rifting stretched and faulted the crust.

• Rift basins filled with sediment and sometimes volcanic rocks.

• Continued separation allowed magma to form new oceanic crust.

• Seafloor spreading widened the Atlantic Ocean.

• Continents moved toward their modern positions.