Western Interior Seaway
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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 The Great Plains is a broad expanse of plain, flatland in North America. The region stretches east of the Rocky Mountains, much of it covered in prairie, steppe, and grassland. They are the western part of the Interior Plains, which include th ...
of
North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...
, splitting the continent into two landmasses, i.e.
Laramidia Laramidia was an island continent that existed during the Late Cretaceous period (99.6–66 Year#SI prefix multipliers, Ma), when the Western Interior Seaway split the continent of North America in two. In the Mesozoic era, Laramidia was an island ...
to the west and
Appalachia Appalachia ( ) is a geographic region located in the Appalachian Mountains#Regions, central and southern sections of the Appalachian Mountains in the east of North America. In the north, its boundaries stretch from the western Catskill Mountai ...
to the east. The ancient sea, which existed for 34 million years from the early
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...
(100 Ma) to the earliest
Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...
(66 Ma), connected the
Gulf of Mexico The Gulf of Mexico () is an oceanic basin and a marginal sea of the Atlantic Ocean, mostly surrounded by the North American continent. It is bounded on the northeast, north, and northwest by the Gulf Coast of the United States; on the southw ...
(then a
marginal sea This is a list of seas of the World Ocean, including marginal seas, areas of water, various gulfs, bights, bays, and straits. In many cases it is a matter of tradition for a body of water to be named a sea or a bay, etc., therefore all these ...
of the Central American Seaway) to the
Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five oceanic divisions. It spans an area of approximately and is the coldest of the world's oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, ...
. At its largest extent, the seaway was deep, wide and over long.


Origin and geology

By the late Cretaceous, Eurasia and the Americas had separated along the south Atlantic, and
subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...
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 Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in great-circle distance, straight-line distance from the northernmost part of Western Can ...
. The Western Interior Seaway may be seen as a downwarping of the
continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...
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 The Arctic Ocean is the smallest and shallowest of the world's five oceanic divisions. It spans an area of approximately and is the coldest of the world's oceans. The International Hydrographic Organization (IHO) recognizes it as an 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 The Appalachian Mountains, often called the Appalachians, are a mountain range in eastern to northeastern North America. The term "Appalachian" refers to several different regions associated with the mountain range, and its surrounding terrain ...
, 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 Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...
that formed shifting delta systems along its low-lying coasts. There was little
sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...
ation 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 Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...
and sediment supply. Widespread
carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...
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 Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Ch ...
formation towering 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 The Laramie Formation is a geologic formation (geology), formation of the Late Cretaceous (Maastrichtian) age, named by Clarence King in 1876 for exposures in northeastern Colorado, in the United States.King, C. 1876. Report of the Geological Exp ...
, while low-lying basins between them gradually subsided. The Western Interior Seaway divided across the Dakotas and retreated south towards the
Gulf of Mexico The Gulf of Mexico () is an oceanic basin and a marginal sea of the Atlantic Ocean, mostly surrounded by the North American continent. It is bounded on the northeast, north, and northwest by the Gulf Coast of the United States; on the southw ...
. This shrunken and final regressive phase is sometimes called the Pierre Seaway. During the early
Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...
, 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.


Phases

The Western Interior Seaway experienced multiple sequences of transgression and regression as the sea level rose and lowered, respectively. Over at least the last 20 million years of the Cretaceous, the seaway generally regressed, but period of transgression over time have been given different names relative to their
cyclothem In geology, cyclothems are alternating stratigraphy, stratigraphic sequences of marine and non-marine Sedimentary structures, sediments, sometimes interbedded with coal seams. The cyclothems consist of repeated sequences, each typically several m ...
. The Niobrara sea was formed by the first sea level rise expanding the seaway westward, which then regressed through the Aquilian Land-vertebrate age. Following this, the sea level rose again in the "Clagette transgression", named for the Clagette Shale, before regressing again in the Judithian. The sea level then rose again to form the shallow Bearpaw Sea of the "Bearpaw transgression" (named after the Bearpaw Shale), which expanded over much of
Wyoming Wyoming ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States, Western United States. It borders Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho t ...
,
Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...
, and the prairie provinces, before regressing in the Edmontonian. Following this was a localized "Lewis" sea separated from the Bearpaw Sea to the north, before more regression of the Western Interior 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 The Plesiosauria or plesiosaurs are an Order (biology), order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period (geology), Period, possibly in the Rhaetian st ...
, and
mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') are an extinct group of large aquatic reptiles within the family Mosasauridae that lived during the Late Cretaceous. Their first fossil remains wer ...
s. Other marine life included sharks such as '' Squalicorax, Cretoxyrhina,'' and the giant durophagous '' Ptychodus mortoni'' (believed to be  long); and advanced bony fish including '' Pachyrhizodus'', '' Enchodus'', and the massive long '' Xiphactinus'', larger than any modern
bony fish Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...
. Other sea life included invertebrates such as mollusks,
ammonites Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...
, squid-like belemnites, and
plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...
including
coccolithophore Coccolithophores, or coccolithophorids, are single-celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingdom ...
s that secreted the chalky platelets that give the Cretaceous its name,
foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...
ns and radiolarians. The seaway was home to early birds, including the flightless '' Hesperornis'' that had stout legs for swimming through water and tiny wings used for marine steering rather than flight; and the tern-like '' Ichthyornis'', an early avian with a toothy beak. ''Ichthyornis'' shared the sky with large pterosaurs such as '' Nyctosaurus'' and ''
Pteranodon ''Pteranodon'' (; from and ) is a genus of pterosaur that included some of the largest known flying reptiles, with ''P. longiceps'' having a wingspan of over . They lived during the late Cretaceous geological period of North America in presen ...
''. ''Pteranodon'' fossils are very common; it was probably a major participant in the surface ecosystem, though it was found in only the southern reaches of the seaway.Benton, S.C. (1994). "The Pterosaurs of the Niobrara Chalk." ''The Earth Scientist'', 11(1): 22-25. Inoceramids (oyster-like bivalve molluscs) were well-adapted to life in the oxygen-poor bottom mud of the seaway. These left abundant fossils in the Kiowa, Greenhorn, Niobrara, Mancos, and Pierre formations. There is great variety in the shells and the many distinct species have been dated and can be used to identify specific beds in those rock formations of the seaway. Many species can easily fit in the palm of the hand, while some like ''Inoceramus (Haploscapha) grandis'' could be well over a meter in diameter. Entire schools of fish sometimes sought shelter within the shell of the giant ''
Platyceramus ''Platyceramus'' was a genus of Cretaceous bivalve molluscs belonging to the extinct inoceramid lineage. It is sometimes classified as a subgenus of ''Inoceramus''. Size The largest and best known species is ''P. platinus''. Individuals of this ...
''. The shells of the genus are known for being composed of prismatic calcitic crystals that grew perpendicular to the surface, and fossils often retain a pearly luster. Image:Kansas sea2DB.jpg, Artist's impression of a '' Cretoxyrhina'' and two '' Squalicorax'' circling a dead '' Claosaurus'' in the Western Interior Seaway File:Elasomosaurus Face Clean.png, '' Elasmosaurus platyurus'' in the Rocky Mountain Dinosaur Resource Center in Woodland Park, Colorado Image:InoceramusCretaceousSouthDakota.jpg, '' Inoceramus'', an ancient bivalve from the Cretaceous of South Dakota. File:Nodosaur.jpg, '' Borealopeltas
holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...
specimen, an exceptionally well-preserved specimen that was fossilized in the Western Interior Seaway.


See also

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References


Further reading

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External links

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Marine Reptiles of South Dakota

Paleo Map Project


{{Authority control Cretaceous paleogeography Geology of Canada Geology of North America Geology of the United States Historical oceans Paleocene paleogeography