Western Interior Seaway
Overview
 
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
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...

 of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 into two halves, Laramidia
Laramidia
Laramidia is a name coined by J. David Archibald in 1996 to describe an island continent that existed during the Late Cretaceous period, when the Western Interior Seaway split the continent of North America in two. Two landmasses existed: the eastern one is called Appalachia; Laramidia is the name...

 and Appalachia
Appalachia (Mesozoic)
In the Mesozoic Era, Appalachia was a land area which is now an eastern part of the USA and Canada, separated from Laramidia by the Western Interior Seaway, which shrank, divided across the Dakotas, retreated south towards the Gulf of Mexico and finally dried up.-Fauna:From the Turonian age of the...

, during most of the mid- and late-Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 Period. It was 2500 feet (762 m) deep, 600 miles (965.6 km) wide and over 2000 miles (3,218.7 km) long.
The Seaway was created as the Farallon
Farallon Plate
The Farallon Plate was an ancient oceanic plate, which began subducting under the west coast of the North American Plate— then located in modern Utah— as Pangaea broke apart during the Jurassic Period...

 tectonic plate subducted under the North American Plate
North American Plate
The North American Plate is a tectonic plate covering most of North America, Greenland, Cuba, Bahamas, and parts of Siberia, Japan and Iceland. It extends eastward to the Mid-Atlantic Ridge and westward to the Chersky Range in eastern Siberia. The plate includes both continental and oceanic crust...

 during Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 time.
Encyclopedia
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
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...

 of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 into two halves, Laramidia
Laramidia
Laramidia is a name coined by J. David Archibald in 1996 to describe an island continent that existed during the Late Cretaceous period, when the Western Interior Seaway split the continent of North America in two. Two landmasses existed: the eastern one is called Appalachia; Laramidia is the name...

 and Appalachia
Appalachia (Mesozoic)
In the Mesozoic Era, Appalachia was a land area which is now an eastern part of the USA and Canada, separated from Laramidia by the Western Interior Seaway, which shrank, divided across the Dakotas, retreated south towards the Gulf of Mexico and finally dried up.-Fauna:From the Turonian age of the...

, during most of the mid- and late-Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 Period. It was 2500 feet (762 m) deep, 600 miles (965.6 km) wide and over 2000 miles (3,218.7 km) long.

Origin and geology

The Seaway was created as the Farallon
Farallon Plate
The Farallon Plate was an ancient oceanic plate, which began subducting under the west coast of the North American Plate— then located in modern Utah— as Pangaea broke apart during the Jurassic Period...

 tectonic plate subducted under the North American Plate
North American Plate
The North American Plate is a tectonic plate covering most of North America, Greenland, Cuba, Bahamas, and parts of Siberia, Japan and Iceland. It extends eastward to the Mid-Atlantic Ridge and westward to the Chersky Range in eastern Siberia. The plate includes both continental and oceanic crust...

 during Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 time. As plate convergence proceeded, 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
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

, pulling it down and producing "dynamic topography
Dynamic topography
The term dynamic topography is used in geodynamics and oceanography to refer to elevation differences caused by the flow within the Earth's mantle and the ocean water, respectively.-Geodynamics:...

" at the surface that caused the opening of the Western Interior Seaway
Western Interior Seaway
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, Laramidia and Appalachia, during most of the mid- and late-Cretaceous Period...

. This depression and the high eustatic sea levels existing during the Cretaceous allowed waters from the Arctic Ocean
Arctic Ocean
The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions...

 in the north and the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...

 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
Mowry or Thermopolis Shale
The Mowry or Thermopolis Shale is a geological formation in Wyoming whose strata date back to the Early Cretaceous. Dinosaur remains are among the fossils that have been recovered from the formation....

, an organic-rich rock formation
Geologic formation
A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties...

. 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
Silt
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...

 that formed shifting delta systems along its low-lying coasts. There was little sedimentation
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

 on the eastern shores of the Seaway; the western boundary, however, consisted of a thick clastic wedge
Clastic wedge
In geology, clastic wedge usually refers to a thick assemblage of sediments--often lens-shaped in profile--eroded and deposited landward of a mountain chain; they begin at the mountain front, thicken considerably landwards of it to a peak depth, and progressively thin with increasing distance inland...

 eroded eastward from the Sevier orogenic belt
Sevier orogeny
The Sevier orogeny was a mountain-building event that affected western North America from Canada to the north to Mexico to the south. This orogeny was the result of convergent boundary tectonic activity between approximately 140 million years ago and 50 Ma. The Sevier River area of central Utah...

. The western shore was thus highly variable, depending on variations in sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

 and sediment supply.

Widespread carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

 deposition suggests that the Seaway was warm and tropical, with abundant calcareous algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

.

At the end of the Cretaceous, a continuing uplift in a mountain-building episode called the Laramide orogeny
Laramide orogeny
The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute, as is the cause. The Laramide...

 hoisted the sandbanks (sandstone) and muddy brackish lagoons (shale) – the thick sequences of silt and sandstone still seen today as the Laramie Formation
Laramie Formation
The Laramie Formation is a geologic formation of Cretaceous age, named by Clarence King in 1876 for exposures in northeastern Colorado, in the United States....

 – while low-lying basins between them gradually subsided. The Western Interior Seaway divided across the Dakotas and retreated south towards the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...

. This shrunken, regressive phase of the Western Interior Seaway is sometimes called the Pierre Seaway.

During the early Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...

, parts of the Western Interior Seaway (marine waters) still occupied areas of the Mississippi Embayment
Mississippi embayment
The Mississippi Embayment is a physiographic feature in the south-central United States, part of the Mississippi Alluvial Plain. It is essentially a northward continuation of the fluvial sediments of the Mississippi River Delta to its confluence with the Ohio River at Cairo, Illinois. The embayment...

, submerging the site of present-day Memphis. Later transgression, however, was associated with the Cenozoic Tejas sequence
Tejas sequence
The Tejas sequence was the last major marine transgression across the North American craton. Following the late Cretaceous regression that ended the Zuñi sequence, the oceans advanced again early in the Cenozoic, peaking during the Paleocene and Eocene epochs...

, 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 reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s such as plesiosaurs, and mosasaur
Mosasaur
Mosasaurs are large extinct marine lizards. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764...

s that grew up to 18 meters long. Other marine life included shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s such as Squalicorax
Squalicorax
Squalicorax is a genus of extinct lamniform shark known to have lived during the Cretaceous period. A fully articulated 1.9 m long fossil skeleton of Squalicorax has been found in Kansas, evidence of its presence in the Western Interior Seaway...

and the giant shellfish-eating Ptychodus mortoni
Ptychodus mortoni
Ptychodus mortoni was a shark believed to be about 10 m long that probably crushed and ate large shelled animals such as giant clams....

(believed to be 10 meters long); and advanced bony fish including Pachyrhizodus
Pachyrhizodus
Pachyrhizodus is an extinct genus of bony fish.-Further reading:*Wildlife of Gondwana: Dinosaurs and Other Vertebrates from the Ancient Supercontinent by Pat Vickers Rich, Thomas Hewitt Rich, Francesco Coffa, and Steven Morton...

, Enchodus
Enchodus
Enchodus is an extinct genus of bony fish. It flourished during the Upper Cretaceous and was small to medium in size. One of the genus' most notable attributes are the large "fangs" at the front of the upper and lower jaws and on the palatine bones, leading to its misleading nickname among fossil...

, and also the massive 5-meter long Xiphactinus
Xiphactinus
Xiphactinus was a large, 4.5 to 6 m long predatory bony fish that lived in the Western Interior Sea, over what is now the middle of North America, during the Late Cretaceous. When alive, the fish would have resembled a gargantuan, fanged tarpon...

– a fish larger than any modern bony fish. Other sea life included invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s such as mollusks, ammonites, squid-like belemnites, and plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

 including coccolithophore
Coccolithophore
Coccolithophores are single-celled algae, protists, and phytoplankton belonging to the division of haptophytes. They are distinguished by special calcium carbonate plates of uncertain function called coccoliths , which are important microfossils...

s that secreted the chalky platelets that give the Cretaceous its name, foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...

ns and radiolarian
Radiolarian
Radiolarians are amoeboid protozoa that produce intricate mineral skeletons, typically with a central capsule dividing the cell into inner and outer portions, called endoplasm and ectoplasm. They are found as zooplankton throughout the ocean, and their skeletal remains cover large portions of the...

s.

The Western Interior Seaway was home to early birds also, including the flightless Hesperornis
Hesperornis
Hesperornis is a genus of flightless aquatic birds that spanned the first half of the Campanian age of the Late Cretaceous period . One of the lesser-known discoveries of the paleontologist O. C. Marsh in the late 19th century Bone Wars, it was an important early find in the history of avian...

which had stout legs for swimming through water and small wing-like appendages used for marine steering rather than flight; and the tern
Tern
Terns are seabirds in the family Sternidae, previously considered a subfamily of the gull family Laridae . They form a lineage with the gulls and skimmers which in turn is related to skuas and auks...

-like Ichthyornis
Ichthyornis
Ichthyornis is a genus of toothed seabirds from the Late Cretaceous of North America. Its fossil remains are known from the chalks of Alberta, Alabama, Kansas, New Mexico, Saskatchewan, and Texas, in strata that were laid down in the Western Interior Seaway during the Turonian-Campanian ages,...

, an early avian with a toothy beak. Icthyornis shared the sky with large pterosaur
Pterosaur
Pterosaurs were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period . Pterosaurs are the earliest vertebrates known to have evolved powered flight...

s such as Nyctosaurus
Nyctosaurus
Nyctosaurus is a genus of pterodactyloid pterosaur, the remains of which have been found in the Niobrara Formation of the mid-western United States, which, during the late Cretaceous Period, was covered in an extensive shallow sea. The genus Nyctosaurus has had numerous species referred to it,...

and Pteranodon
Pteranodon
Pteranodon , from the Late Cretaceous geological period of North America in present day Kansas, Alabama, Nebraska, Wyoming, and South Dakota, was one of the largest pterosaur genera and had a maximum wingspan of over...

. Pteranodon fossils are very common and it was likely a major component of the surface ecosystem, though it was only found in the southern reaches of the Seaway.

On the bottom, the giant clam Inoceramus
Inoceramus
Inoceramus is an extinct genus of fossil marine pteriomorphian bivalves that superficially resembled the related winged pearly oysters of the extant genus Pteria....

left common fossilized shells in the Pierre Shale
Pierre Shale
The Pierre Shale is a geologic formation or series in the Upper Cretaceous which occurs east of the Rocky Mountains in the Great Plains, from North Dakota to New Mexico....

. This clam had a thick shell paved with "prisms" of calcite deposited perpendicular to the surface, giving it it a pearly luster in life. Paleontologists suggest that its 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.

See also

  • Geology of the Bryce Canyon area
    Geology of the Bryce Canyon area
    The exposed geology of the Bryce Canyon area in Utah shows a record of deposition that covers the last part of the Cretaceous Period and the first half of the Cenozoic era in that part of North America...

  • Zuñi sequence
    Zuñi sequence
    The Zuñi sequence was the major cratonic sequence after the Absaroka sequence that began in the latest Jurassic, peaked in the late Cretaceous, and ended by the start of the following Paleocene...

  • Sundance Sea
    Sundance Sea
    The Sundance Sea was an epeiric sea that existed in North America during the mid to late Jurassic Period of the Mesozoic Era. It was an arm of what is now the Arctic Ocean, and extended through what is now western Canada into the central western United States...

     - a separate seaway of the Jurassic
    Jurassic
    The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

  • Pierre Shale
    Pierre Shale
    The Pierre Shale is a geologic formation or series in the Upper Cretaceous which occurs east of the Rocky Mountains in the Great Plains, from North Dakota to New Mexico....

  • Hudson Seaway
    Hudson Seaway
    The Hudson Seaway was made up of mostly the present-day Hudson Bay some 75 million years ago. Along with the Western Interior Seaway, the Hudson Seaway split the western and eastern parts of modern-day North America, Laramidia and Appalachia, respectively. Greenland was separated by the foundered...

  • Lake Agassiz
    Lake Agassiz
    Lake Agassiz was an immense glacial lake located in the center of North America. Fed by glacial runoff at the end of the last glacial period, its area was larger than all of the modern Great Lakes combined, and it held more water than contained by all lakes in the world today.-Conception:First...

    , a later glacial lake covering some of the same area

External links

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