Western Interior Seaway

The Western Interior Seaway was a shallow Cretaceous sea that split North America into eastern and western landmasses and connected the Arctic region with the Gulf of Mexico.

Cretaceous: The geological period from about 145 to 66 million years ago, marked by warm climates and the breakup of Pangaea. The seaway formed and persisted during this period.
Western Interior Basin: A long-lived sedimentary basin beneath central North America, filled with thick Paleozoic through Cenozoic rock sequences. Its Cretaceous flooding created the seaway’s marine basin.
Niobrara Formation: A Late Cretaceous chalk and shale formation deposited in the Western Interior Seaway, known for abundant marine fossils. Its rocks preserve organisms from the seaway’s open-water ecosystems.
Laramidia: The western landmass formed when the Western Interior Seaway divided North America during the Cretaceous. It was one of the two continents created by the seaway’s division.
Cretaceous eustatic sea-level rise: A global rise in sea level during the Cretaceous, driven chiefly by increased ocean-crust volume and limited polar ice. High global sea levels helped flood the continent’s interior.
Western Interior Seaway transgression: The Cretaceous advance of marine waters across central North America as sea level rose and the basin subsided. This inundation established the seaway across the continent.
Hesperornis: An extinct, flightless diving bird that lived in Cretaceous seas and had toothed jaws. Its fossils show that specialized diving birds inhabited the seaway.
Appalachia: The eastern landmass isolated from western North America by the Western Interior Seaway during the Cretaceous. It formed the seaway’s eastern counterpart to Laramidia.
Laramide orogeny: A mountain-building episode that raised the Rocky Mountains and affected western North America during the Late Cretaceous and early Paleogene. Uplift along the western margin shaped the seaway’s western basin and sediment supply.
Marine transgression: The landward advance of the sea over previously exposed land, commonly caused by rising sea level or subsiding land. The seaway expanded through repeated marine advances.

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