Cretaceous seaway1

Western Interior Seaway during the mid-Cretaceous, about 100 million years before the present.

The Western Interior Seaway, also called the Cretaceous Seaway, the Niobraran Sea, and the North American Inland Sea, was a huge inland sea that split the continent of North America into two halves during most of the mid and late Cretaceous Period.

Origins and GeologyEdit

The Seaway was created as the Pacific and North American tectonic plates collided, causing the Rocky Mountains to form in western North America. With high eustatic sea levels existing worldwide during the Cretaceous, waters from the Arctic Ocean in the north and the Gulf of Mexico in the south met and flooded the central lowlands, forming a sea that transgressed (grew) and regressed (receded) over the course of the Cretaceous.

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, a characteristic rock formation that is rich in oil shale. In the south, the Gulf of Mexico was an extension of the Tethys Sea, which met with the Mowry Sea in the late Cretaceous, forming the "complete" Seaway.

At its largest, the Western Interior Seaway stretched from the Rockies to the Appalachians in the east, some 1000 km wide. At its deepest, it may have been only 800 or 900 meters 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, consists 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 algae. Rudy Slingerland of Penn State University has computer-modelled a counter-clockwise gyre for the Cretaceous Seaway, with cooler waters flowing south along the eastern seacoasts of Wyoming and Colorado.

At the end of the Cretaceous continuing uplift in a mountain-building episode called the Laramide orogeny hoisted the sandbanks (sandstone) and muddy brackish lagoons (shale), the 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, regressive phase of the Western Interior Seaway is sometimes called the Pierre Seaway.

During the early Paleocene, parts of the Western Interior Seaway (marine waters) 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.


The Western Interior Seaway was a shallow sea, filled with abundant marine life. Interior Seaway denizens included predatory marine reptiles, the largest animals in the Cretaceous seas: mosasaurs growing up to 18 meters long, ichthyosaurs (one specimen from Pink Mountain in British Columbia is currently the largest ichthyosaur specimen found to date[citation needed]), and plesiosaurs (an inspiration for the Loch Ness Monster). Other marine life included sharks, such as Squalicorax, and advanced bony fish including Pachyrhizodus, Enchodus, and the massive 5-meter long Xiphactinus, a fish 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.

The Western Interior Seaway was also home to early birds, including the flightless Hesperornis, which had stout legs for swimming through the water and small wing-like appendages used for marine steering rather than flight; and the tern-like Ichthyornis, an early avian with a toothy beak.

On the bottom the giant clam Inoceramus has left common fossilized shells in the Pierre Shale. The clam had a thick shell paved with "prisms" of calcite deposited perpendicular to the surface, which gave it a pearly luster in life. Paleontologists suggest that the giant size was an adaptation for life in the murky bottom waters, where a correspondingly large gill area would have allowed the animal to cope with oxygen-depleted waters.