Western Interior Seaway

{{Short description|Prehistoric inland sea that split the continent of North America}}

File:Map of North America with the Western Interior Seaway during the Campanian (Upper Cretaceous).png]]

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 {{cvt|2500|ft|m}} deep, {{cvt|600|mi|km}} wide and over {{cvt|2000|mi|km}} long.

Origin and geology

Image:BrokenConcretion22.jpg inside; late Cretaceous Pierre Shale near Ekalaka, Montana.]]

File:Monument rocks view.jpg, located 25 miles south of Oakley.]]

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,{{cite journal |author= R.J. Weimer |year= 1984 |journal= AAPG Memoir |title= Relation of unconformities, tectonics, and sea-level changes, Cretaceous of Western Interior, U.S.A.; in |editor= J.S. Schlee |publisher= American Association of Petroleum Geologists |issue= Memoir 36, Interregional unconformities and hydrocarbon accumulation |page= 7-35 |url=https://terra.rice.edu/department/faculty/morganj/ESCI536/Readings/Weimer_CretaceousSeaway.pdf |quote= [The url is to a Rice University-hosted pdf of a book chapter adapted from the original Weimer 1984 paper.] |access-date= March 6, 2021 }} 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 {{Convert|1000|km|abbr=on}} wide. At its deepest, it may have been only {{Convert|800|or|900|m}} 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.{{cite book|last=Monroe |first=James S. |title=The Changing Earth: Exploring Geology and Evolution |url=https://archive.org/details/changingearthexp00monr |url-access=limited |year=2009 |publisher=Brooks/Cole, Cengage Learning |location=Belmont, CA |isbn=978-0495554806 |page=[https://archive.org/details/changingearthexp00monr/page/n621 605] |edition=5th |author2=Wicander, Reed }} 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 planktonic algae.{{cite web|title=Oceans of Kansas Paleontology |publisher=Mike Everhart |url=http://www.oceansofkansas.com/index2.html |access-date=2007-02-06}} Remnants of these deposits are found in northwest Kansas. A prominent example is Monument Rocks, an exposed chalk formation towering {{Convert|70|ft}} over the surrounding range land.{{cite web|url=http://www.kansastravel.org/monumentrocks.htm|title=Monument Rocks, the Chalk Pyramids - Kansas|first=Keith|last=Stokes|website=www.kansastravel.org|access-date=7 April 2018}} 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.{{cite journal |last1=Slingerland |first1=Rudy |last2=Kump |first2=Lee R. |last3=Arthur |first3=Michael A. |last4=Fawcett |first4=Peter J. |last5=Sageman |first5=Bradley B. |last6=Barron |first6=Eric J. |date=1 August 1996 |title=Estuarine circulation in the Turonian Western Interior seaway of North America |url=https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/108/8/941/183171/Estuarine-circulation-in-the-Turonian-Western?redirectedFrom=fulltext |journal=Geological Society of America Bulletin |volume=108 |issue=8 |pages=941–952 |doi=10.1130/0016-7606(1996)108<0941:ECITTW>2.3.CO;2 |access-date=5 April 2023|url-access=subscription }} 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.{{cite journal |last1=Lowery |first1=Christopher M. |last2=Leckie |first2=R. Mark |last3=Bryant |first3=Raquel |last4=Elderbak |first4=Khalifa |last5=Parker |first5=Amanda |last6=Polyak |first6=Desiree E. |last7=Schmidt |first7=Maxine |last8=Snoeyenbos-West |first8=Oona |last9=Sterzinare |first9=Ericfa |title=The Late Cretaceous Western Interior Seaway as a model for oxygenation change in epicontinental restricted basins |journal=Earth-Science Reviews |date=1 February 2018 |volume=177 |pages=545–564 |doi=10.1016/j.earscirev.2017.12.001|bibcode=2018ESRv..177..545L |url=https://findresearcher.sdu.dk:8443/ws/files/143467468/Manuscript_REVISED.pdf }}

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.{{cite book |last=Stanley |first=Steven M. |title=Earth System History |location=New York |publisher=W.H. Freeman and Company |year=1999 |isbn=0-7167-2882-6 |pages=487–489}}

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.{{cite book |last1=Stanley |first1=Steven M. |title=Earth system history |date=1998 |publisher=W.H. Freeman |location=New York |isbn=0716728826 |page=516}}{{cite book |last1=Monroe |first1=James S. |date=1997 |title=The changing earth: exploring geology and evolution |location=Belmont, Calif. |publisher=Wadsworth Pub |isbn=0314095772 |page=643 |edition=2nd}}{{cite book |last1=Frazier |first1=William J. |last2=Schwimmer |first2=David R. |title=Regional Stratigraphy of North America |chapter=The Tejas Sequence: Tertiary—Recent |date=1987 |pages=523–652 |doi=10.1007/978-1-4613-1795-1_9|isbn=978-1-4612-9005-6 }}

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. 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, Montana, 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.{{cite book|last1=Lillegraven|first1=J.A.|last2=Ostresh|first2=L.M. jr.|year=1990|chapter=Late Cretaceous (earliest Campanian/Maastrichtian) evolution of western shorelines of the North American Western Interior Seaway in relation to known mammalian faunas|editor-last=Bown|editor-first=T.M.|editor2-last=Rose|editor2-first=K.D.|title=Dawn of the Age of Mammals in the northern part of the Rocky Mountain Interior, North America|publisher=Geological Society of America Special Papers|pages=1-30|doi=10.1130/SPE243-p1}}

Fauna

The Western Interior Seaway was a shallow sea, with abundant marine life. Interior seaway denizens included predatory marine reptiles such as plesiosaurs, and mosasaurs. Other marine life included sharks such as Squalicorax, Cretoxyrhina, and the giant durophagous Ptychodus mortoni (believed to be {{Convert|10|m}} long);{{cite news|last=Walker |first=Matt |url=http://news.bbc.co.uk/earth/hi/earth_news/newsid_8530000/8530995.stm |title=Giant predatory shark fossil unearthed in Kansas |newspaper=BBC Earth News |date=24 February 2010 |access-date=16 April 2013}} and advanced bony fish including Pachyrhizodus,{{cite web |title=Pachyrhizodus. A Large Predatory Fish from the Late Cretaceous Western Interior Sea |author=Mike Everhart |date=February 2, 2010 |work=Oceans of Kansas Paleontology |url=http://www.oceansofkansas.com/pachyrhi.html |access-date=May 5, 2011}} Enchodus, and the massive {{Convert|4|to|5|m|adj=on}} long Xiphactinus, larger than any modern bony fish.{{cite journal |last1=Cumbaa |first1=Stephen L. |last2=Tokaryk |first2=Tim T. |title=Recent Discoveries of Cretaceous Marine Vertebrates on the Eastern Margins of the Western Interior Seaway |journal=Saskatchewan Geological Survey Summary of Investigations |date=1999 |volume=1 |pages=57–63 |url=http://publications.gov.sk.ca/documents/310/88504-Cumbaa-Tokaryk_1999_volume1_MiscRep99-4.1.pdf |access-date=27 August 2021}} 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.{{cite journal |last1=Boyles |first1=M.J. |last2= Scott |first2=A.J. |year=1982 |title=Comparison of Wave-Dominated Deltaic Deposits and Associated Sand-Rich Strand Plains, Mesaverde Group, Northwest Colorado |journal=AAPG Bulletin |volume=66 |number=5 |pages=551–552}}{{cite book |last1=Kauffman |first1=E.G. |year=1984 |chapter=Paleobiogeography and evolutionary response dynamic in the Cretaceous Western Interior Seaway of North America |title=Jurassic-Cretaceous biochronology and paleogeography of North America |volume=27 |pages=273–306 |publisher=Geological Association of Canada |url=http://mmtk.ginras.ru/pdf/Kauffman,1984_K2_N_America.pdf |access-date=27 August 2021 |archive-date=27 August 2021 |archive-url=https://web.archive.org/web/20210827163525/http://mmtk.ginras.ru/pdf/Kauffman,1984_K2_N_America.pdf |url-status=dead }}

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 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.{{cite journal |last1=Da Gama |first1=Rui O.B.P. |last2=Lutz |first2=Brendan |last3=Desjardins |first3=Patricio |last4=Thompson |first4=Michelle |last5=Prince |first5=Iain |last6=Espejo |first6=Irene |title=Integrated paleoenvironmental analysis of the Niobrara Formation: Cretaceous Western Interior Seaway, northern Colorado |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=November 2014 |volume=413 |pages=66–80 |doi=10.1016/j.palaeo.2014.05.005|bibcode=2014PPP...413...66D }} 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{{Cite journal |last=Moss |first=Rycroft G. |date=May 2004 |orig-year=1 December 1932 |title=Bulletin 19: The Geology of Ness and Hodgeman Counties, Kansas |url=http://www.kgs.ku.edu/General/Geology/Ness/04_expos.html |journal=Bulletin of the University of Kansas—Lawrence |volume=33 |issue=18 |page=Stratigraphy: Rocks Exposed |access-date=2020-11-17}} could be well over a meter in diameter. Entire schools of fish sometimes sought shelter within the shell of the giant Platyceramus.{{cite book |last1=Prothero |first1=Donald R. |title=Bringing fossils to life : an introduction to paleobiology |date=2013 |publisher=Columbia University Press |location=New York |isbn=9780231158930 |edition=Third |page=172}} 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.{{cite book |last1=Ludvigsen |first1=Rolf |last2=Beard |first2=Graham |year=1997 |title=West Coast Fossils: A Guide to the Ancient Life of Vancouver Island |url=https://archive.org/details/westcoastfossils0000ludv |url-access=registration |pages=[https://archive.org/details/westcoastfossils0000ludv/page/102 102–103]|publisher=Harbour Pub. |isbn=9781550171792 }}

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|Borealopelta's holotype specimen, an exceptionally well-preserved specimen that was fossilized in the Western Interior Seaway.

See also

{{Portal|Oceans}}

  • {{Annotated link|Geology of the Bryce Canyon area}}
  • {{Annotated link|Hudson Seaway}}
  • {{Annotated link|Lake Agassiz}}
  • {{Annotated link|Sundance Sea}}
  • {{Annotated link|Zuñi sequence}}
  • {{Annotated link|Tanis (fossil site)}}

References

{{Reflist|2}}

Further reading

  • {{Cite book |last1=Kauffman |first1=Erle G. |last2=Caldwell |first2=W.G.E. |date=1993 |chapter=The Western Interior Basin in Space and Time |editor-last1=Caldwell |editor-first1=W.G.E. |editor-last2=Kauffman |editor-first2=Erle G. |title=Evolution of the Western Interior Basin |url=https://www.researchgate.net/publication/248065566 |series=Volume 39 of Geological Association of Canada Special Paper |location=St. John's, NL |publisher=Geological Association of Canada |access-date=2022-02-13}}