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 split the continent of North America into two landmasses. The ancient sea, which existed from the early Late Cretaceous (100 million years ago) to the earliest Paleocene (66 Ma), connected the Gulf of Mexico, through the United States and Canada, to the Arctic Ocean. The two land masses it created were Laramidia to the west and Appalachia to the east. At its largest extent, it was deep, wide and over long.
Origin and geology
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 continental crust ahead of the growing Laramide/Rockies mountain chain.
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. In the south, the Gulf of Mexico was originally an extension of the Tethys Sea. 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.
Relative sea levels fell multiple times, as a margin of land temporarily rose above the water along the ancestral Transcontinental Arch, each time rejoining the separated, divergent land populations, allowing a temporary mixing of newer species before again separating the populations.
At its largest, the Western Interior Seaway stretched from the Rockies east to the Appalachians, some wide. At its deepest, it may have been only deep, shallow in terms of seas. Two great continental watersheds drained into it from east and west, diluting its waters and bringing resources in eroded silt that formed shifting delta systems along its low-lying coasts. There was little sedimentation on the eastern shores of the Seaway; the western boundary, however, consisted of a thick clastic wedge eroded eastward from the Sevier orogenic belt. The western shore was thus highly variable, depending on variations in sea level and sediment supply.
Widespread carbonate deposition suggests that the Seaway was warm and tropical, with abundant calcareous planktonic algae. Remnants of these deposits are found in northwest Kansas. A prominent example is Monument Rocks, an exposed chalk formation towering over the surrounding range land. It is designated a National Natural Landmark and one of the Eight Wonders of Kansas. It is located south of Oakley, Kansas. The Western Interior Seaway is believed to have behaved similarly to a giant estuary in terms of water mass transport. Riverine inputs exited the seaway as coastal jets, while correspondingly drawing in Tethyan waters from the south and Boreal waters from the north. During the late Cretaceous, the Western Interior Seaway went through multiple periods of anoxia, when the bottom water was devoid of oxygen and the water column was stratified.
At the end of the Cretaceous, continued Laramide uplift hoisted the sandbanks (sandstone) and muddy brackish lagoons (shale), thick sequences of silt and sandstone still seen today as the Laramie Formation, while low-lying basins between them gradually subsided. The Western Interior Seaway divided across the Dakotas and retreated south towards the Gulf of Mexico. This shrunken, and final regressive phase is sometimes called the Pierre Seaway.
During the early Paleocene, parts of the Western Interior Seaway still occupied areas of the Mississippi Embayment, submerging the site of present-day Memphis. Later transgression, however, was associated with the Cenozoic Tejas sequence, rather than with the previous event responsible for the Seaway.
Fauna
The Western Interior Seaway was a shallow sea, filled with abundant marine life. Interior Seaway denizens included predatory marine reptiles such as plesiosaurs, and mosasaurs. Other marine life included sharks such as Squalicorax, Cretoxyrhina, and the giant shellfish-eating Ptychodus mortoni (believed to be long); and advanced bony fish including Pachyrhizodus, Enchodus, and the massive long Xiphactinus, larger than any modern bony fish. Other sea life included invertebrates such as mollusks, ammonites, squid-like belemnites, and plankton including coccolithophores that secreted the chalky platelets that give the Cretaceous its name, foraminiferans and radiolarians.