Western Interior Seaway - Origin and Geology

Origin and Geology

The Seaway was created as the Farallon tectonic plate subducted under the North American Plate during Cretaceous time. As plate convergence proceeded, the younger and more buoyant lithosphere of the Farallon Plate started to become subducted. This caused it to subduct at a much more shallow angle, in what is known as a "flat slab". This shallowly-subducting slab exerted a traction on the base of the lithosphere, pulling it down and producing "dynamic topography" at the surface that caused the opening of the Western Interior Seaway. This depression and the high eustatic sea levels existing during the Cretaceous allowed waters from the Arctic Ocean in the north and the Gulf of Mexico in the south to meet and flood 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, an organic-rich rock formation. 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, 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 algae.

At a few times during the Late Cretaceous the Western Interior seaway went through periods of anoxia, where the bottom water was devoid of oxygen and the water column was stratified.

At the end of the Cretaceous, a 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.