Western Interior Seaway

The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea that existed roughly over the present-day Great Plains of North America, splitting the continent into two landmasses, Laramidia to the west and Appalachia to the east. The ancient sea, which existed for 34 million years from the early Late Cretaceous (100 Ma) to the earliest Paleocene (66 Ma), connected the Gulf of Mexico (then a marginal sea of the Central American Seaway) to the Arctic Ocean. At its largest extent, the seaway was 2,500 ft (760 m) deep, 600 mi (970 km) wide and over 2,000 mi (3,200 km) long.

By the late Cretaceous, 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 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 Ocean. In time, the southern embayment merged with the Mowry Sea in the late Cretaceous, forming a completed 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 Appalachian Mountains, some 1,000 km (620 mi) wide. At its deepest, it may have been only 800 or 900 metres (2,600 or 3,000 ft) 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. Biostratigraphy suggests that average sea surface temperatures around this time (the Turonian) were about 32 °C (90 °F) (foraminiferal records), and (based on ammonite-based biostratigraphy) could debatably reach as high as 44 °C (111 °F).

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 70 feet (21 m) over the surrounding range land. 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 water from the Tethys in 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. Well-preserved fossil otoliths of the marine catfish Vorhisia suggest that around this time, following a major cooling trend from the highs of the Cretaceous Thermal Maximum, average water temperatures in the Western Interior Seaway were about 18 °C (64 °F). Ammonite-based biostratigarphy suggests that the lowest average temperatures reached by the Western Interior Seaway were during the early Maastrichtian, about 16 °C (61 °F), with a slight warming towards the end of the Maastrichtian to 22 °C (72 °F).

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.