Western Interior Seaway
The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea that existed during the mid- to late Cretaceous period as well as the very early Paleogene, splitting the continent of North America into two landmasses, Laramidia to the west and Appalachia to the east. The ancient sea stretched from the Gulf of Mexico and through the middle of the modern-day countries of the United States and Canada, meeting with the Arctic Ocean to the north. At its largest, it was 2,500 feet (760 m) deep, 600 miles (970 km) wide and over 2,000 miles (3,200 km) long.
The earliest phase of the Seaway began in the mid-Cretaceous period when an arm of the Arctic Ocean transgressed south over western North America; this formed the Mowry Sea, so named for the Mowry Shale, an organic-rich rock formation.[1] In the south, the Gulf of Mexico was originally an extension of the Tethys Sea. By Late-Cretaceous times, Eurasia and the Americas had separated along the south Atlantic and subduction on the west coast of the Americas had commenced, resulting in the Laramide orogeny, the early phase of growth of the modern Rocky Mountains. The Western Interior seaway may be seen as a downwarping of the US continental crust ahead of the growing Laramide/Rockies mountain chain. In time, the southern embayment merged with the Mowry Sea in the late Cretaceous, forming the "complete" Seaway, creating isolated environments for land animals and plants.[1]
Relative sea levels fell multiple times, as a margin of land temporarily rose above the water along the ancestral Transcontinental Arch,[2] each time rejoining the separated, divergent land populations, allowing a temporary mixing of newer species before again separating the populations.
