Everything about the Mississippi Embayment totally explained
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
fluvial sediments of the
Mississippi River Delta to its confluence with the
Ohio River at
Cairo, Illinois. The
embayment is a
topographically low-lying
basin that's filled with
Cretaceous to recent
sediments. The northern end of the embayment appears as an anomalous break in regional
geologic structure with
Paleozoic sedimentary rocks both to the east in
Kentucky and
Tennessee and to the west in
Missouri and
Arkansas. The current sedimentary basin results from the filling of a Cretaceous tectonic basin and existed as a large bay in the Cretaceous through early Tertiary shoreline.
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 -
1812. The area is underlain by some anomalous geology. The
Reelfoot Rift is an ancient failed continental
rift which dates back to the
Precambrian break-up of the
supercontinent Rodinia. The relatively 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.
Formation of the embayment: the Van Arsdale-Cox explanation
The Mississippi embayment represents a break in what was once a single, continuous mountain range comprising the modern
Appalachian range, which runs roughly on a north-south axis along the Atlantic coast of the United States, and the
Ouachita range, which runs on a rough east-west axis west of the
Mississippi River. The ancestral Appalachian-Ouachita range was thrust up when the
tectonic plate carrying
North America came into contact with the plates carrying
South America and
Africa when all three became joined in the ancient
supercontinent Pangaea about 300 million years ago. Explaining the formation of the embayment requires explaining how part of a mountain range became a basin.
Writing in the January
2007 issue of
Scientific American,
Roy B. Van Arsdale and
Randel Cox of the
University of Memphis offered the following explanation of the embayment's complex origin:
As Pangaea began to break up about 95 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
arch. 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 the Van Arsdale-Cox 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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