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Three Western Interior Seaway anoxic events occurred during the Cretaceous in the shallow inland seaway that divided North America in two island continents, Appalachia and Laramidia (''see map''). During these anoxic events much of the water column was depleted in dissolved oxygen. While anoxic events impact the world's oceans, Western Interior Seaway anoxic events exhibit a unique paleonenvironment compared to other basins. The notable Cretaceous anoxic events in the Western Interior Seaway mark the boundaries at the Aptian-Albian, Cenomanian-Turonian, and Coniacian-Santonian stages, and are identified as Oceanic Anoxic Events I, II, and III respectively. The episodes of anoxia came about at times when very high sea levels coincided with the nearby Sevier orogeny that affected Laramidia to the west and Caribbean large igneous province to the south, which delivered nutrients and oxygen-adsorbing compounds into the water column. Most anoxic events are recognized using the 13C isotope as a proxy to indicate total organic carbon preserved in sedimentary rocks. If there is very little oxygen, then organic material that settles to the bottom of the water column will not be degraded as readily compared to normal oxygen settings and can be incorporated into the rock. 13Corganic is calculated by comparing the amount of 13C to a carbon isotope standard, and using multiple samples can track changes (δ) in organic carbon content through rocks over time, forming a δ13Corganic curve. The δ13Corganic, as a result, serves as a benthic oxygen curve. The excellent organic carbon preservation brought about by these successive anoxic events makes Western Interior Seaway strata some of the richest source rocks for oil and gas. ==Western Interior Seaway tectonics and geography== During the Cretaceous Period, along the western shore of the Western Interior Seaway there was active volcanism and foreland subsidence brought about by the Sevier orogeny, formed by the convergence of the oceanic Farallon and Kula plates with the North American plate.〔Shurr, G.W., Ludvigson, G.A., Hammond, R.H. 1994. Perspectives on the Eastern Margin of the Cretaceous Western Interior Basin. Geological Society of America, Boulder: Special Paper #287, 264 p.〕 Active volcanism during the Sevier orogeny was the product of partial melting of the subducting Farallon and Kula plates: that resulting melt traveled up through the overlying North American plate, creating a belt of active volcanos. Most active volcanism occurred in the extreme northern and southern portions of the western shoreline of the Western Interior Seaway.〔 To the east of the orogeny, a back-arc basin formed due to the warping of the North American plate in response to the horizontal stress of the subducting oceanic plates. The low-lying area was under water throughout the Cretaceous due to the warm climate causing the planet's ocean waters to expand and flood the continent's interior. Sea level during Oceanic Anoxic Event II at the Cenomanian-Turonian boundary was at its highest of the Cretaceous due to high global temperatures. At that time, the Western Interior Seaway stretched from the Boreal Sea (present Arctic Sea) to the Tethys Sea (present Gulf of Mexico), making it 6000 km long and 2000 km wide.〔Slingerland , R.L., Kump, L.R, Arthur, M.A., Fawcett, P.J., Sageman, B.B., and Barron, E.J. 1996. ''Geological Society of America Bulletin'', 108, 941-952.〕〔Bowman, A.R. and Gale, A.S., Hardenbol, J., Hathaway, B., Kennedy, W.J., Young, J.R., and Phansalkar, V. 2002. Global correlation of Cenomanian (Upper Cretaceous) sequences: Evidence for Milankovitch control on sea level. ''Geology'', 30, 291-294.〕〔Bralower, T.J. 2005. Paleoceanographic significance of high-resolution carbon isotope records across the Cenomanian-Turonian boundary in the Western Interior and New Jersey coastal plain, USA. ''Marine Geology'', 217, 305-321.〕 The deepest portions were around 500 m deep.〔 Formation of the Caribbean Plate in the Tethys Sea near the southern region of the Western Interior Seaway created a large igneous province (called the Caribbean Plateau) that produced underwater lava flows from 95-87 million years ago.〔Bralower, T.J. 2008. Volcanic cause of catastrophe. ''Nature'', 454, 285-287.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Western Interior Seaway anoxia」の詳細全文を読む スポンサード リンク
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