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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 River delta of the Mississippi River Delta to its confluence with the Ohio River at Cairo, Illinois. The current sedimentary area was formed in the Cretaceous and early Cenozoic 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's mantle causing an upthrust of magma which formed the Appalachian-Ouachita range. Subsequent erosion caused a deep trough that was flooded by the Gulf of Mexico and eventually filled with sediment from the Mississippi River.
The New Madrid Seismic Zone lies at the northern end of the embayment. It was the site of the large New Madrid earthquakes of 1811-12. The area is underlain by some anomalous geology. The Reelfoot Rift is an ancient failed continental rift, an aulacogen, which dates back to the Precambrian plate tectonics of the supercontinent Rodinia. The more recent opening of the Atlantic Ocean and Gulf of Mexico during late Paleozoic to early Mesozoic break-up of Pangea no doubt affected and may have partially re-activated the old rift.
As Pangaea began to break up about 200 million years ago, North America passed over a volcanic "hotspot" in the Earth's mantle (specifically, the Bermuda hotspot) that was undergoing a period of intense activity. The upwelling of magma from the hotspot forced the further uplift to a height of perhaps 2–3 km of part of the Appalachian-Ouachita range, forming an anticline. 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 a trough that was flooded by the Gulf of Mexico. As sea levels dropped, the Mississippi and other rivers extended their courses into the embayment, which gradually became filled with sediment.
Evidence for this explanation is found in the presence of the seismic zones centered on New Madrid, Missouri, and Charleston, South Carolina, each the source of devastating earthquakes in the 19th century, and in diamond-bearing kimberlite pipes in Arkansas, which are products of volcanism.
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