TheMississippi embayment is a physiographic feature in the south-centralUnited States, part of theMississippi Alluvial Plain. It is essentially a northward continuation of thefluvial sediments of theMississippi River Delta to its confluence with theOhio River atCairo, Illinois. The current sedimentary area was formed in theCretaceous and earlyCenozoic by the filling with sediment of a pre-existing basin. An explanation for the embayment's formation was put forward by Van Arsdale and Cox in 2007: movement of the Earth's crust brought this region over a volcanic "hotspot" in the Earth'smantle causing an upthrust ofmagma which formed the Appalachian-Ouachita range. Subsequent erosion caused a deep trough that was flooded by theGulf of Mexico and eventually filled with sediment from the Mississippi River.
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Theembayment is atopographically low-lyingbasin that is filled withCretaceous to recentsediments. The northern end of the embayment appears as an anomalous break in regionalgeologic structure withPaleozoicsedimentary rocks both to the east inKentucky andTennessee and to the west inMissouri andArkansas. The current sedimentary basin results from the filling of a Cretaceous tectonic basin and existed as a large bay in the Cretaceous through early Cenozoic shoreline.
TheNew Madrid Seismic Zone lies at the northern end of the embayment. It was the site of the largeNew Madrid earthquakes of 1811-12. The area is underlain by some anomalous geology. TheReelfoot Rift is an ancient failed continentalrift, anaulacogen, which dates back to thePrecambrianbreak-up of thesupercontinentRodinia. The more recent opening of theAtlantic Ocean andGulf of Mexico during latePaleozoic to earlyMesozoic break-up ofPangea no doubt affected and may have partially re-activated the old rift.
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The Mississippi embayment represents a break in what was once a single, continuous mountain range comprising the modernAppalachian range, which runs roughly on a north–south axis along the Atlantic coast of the United States, and theOuachita range, which runs on a rough east–west axis west of theMississippi River. The ancestral Appalachian-Ouachita range was thrust up when thetectonic plate carryingNorth America came into contact with the plates carryingSouth America andAfrica when all three became joined in the ancientsupercontinentPangaea about 300 million years ago.
As Pangaea began to break up about 200 million years ago, North America passed over a volcanic "hotspot" in the Earth'smantle (specifically, theBermuda hotspot) that was undergoing a period of intense activity. The upwelling ofmagma from the hotspot forced the further uplift to a height of perhaps 2–3 km of part of the Appalachian-Ouachita range, forming anarch. The uplifted land quickly eroded and, as North America moved away from the hot spot and as the hotspot's activity declined, the crust beneath the embayment region cooled, contracted and subsided to a depth of 2.6 km, forming atrough that was flooded by theGulf of Mexico. As sea levels dropped, the Mississippi and other rivers extended their courses into the embayment, which gradually became filled with sediment.[1]
Evidence for this explanation is found in the presence of the seismic zones centered on New Madrid, Missouri, andCharleston, South Carolina, each the source of devastating earthquakes in the 19th century, and in diamond-bearingkimberlite pipes inArkansas, which are products of volcanism.
33°45′04″N89°40′52″W / 33.751°N 89.681°W /33.751; -89.681
