Published by the American Geological Institute
Newsmagazine of the Earth Sciences
It is uncommon that a large alluvial river
and scarp cross paths, but when they do an interesting geomorphic scenario
occurs — riverbeds morph, streams reverse flow and prehistoric earthquake
records can be erased from the geologic record. The Mississippi River flood
plain provides a unique setting for studying the interaction of a large
river system and an active seismic zone.
Many alluvial flood plains are tectonically quiet regions where sedimentation depends on river flow. In the case of the Mississippi Alluvial Valley, the flood plain is large and the Mississippi River and its tributary streams create a variety of depositional patterns. At the Geological Society of America meeting in November, Margaret Guccione of the University of Arkansas and Karl Mueller of the University of Colorado reported how deformational events in the New Madrid seismic zone have altered the flow of the Mississippi River and its tributary streams during the past 2,400 years.
The mighty Mississippi River flows down through America’s heartland and over the Reelfoot Scarp — the scarp most recently deformed by the 1811-1812 New Madrid earthquakes. In the region upstream from the Reelfoot Scarp, the riverbed was relatively depressed 8 to 11 meters as a result of the New Madrid earthquakes. As the river flows across the lowered region, it floods into the basin and sediment accumulates along its banks, making the channel deeper. As the modern river passes the scarp and flows across the uplifted region, the channel shallows and the riverbed broadens to accommodate the high volume of water. “You can move the water through faster, or you can change the cross section,” Guccione says. “It appears that the river changed its cross section and the river is still trying to return to equilibrium after the 1811-1812 events.”
In other areas of the flood plain, Guccione takes sediment cores and looks for changes in depositional patterns. She looks at the overbank, or fine-grained flood deposits, to see how river flooding has changed. Abrupt changes in sedimentation rates, or lack of a sedimentary layer altogether, tell Guccione that the flood plain experienced some deformation and deposition rates have been locally increased or decreased. By studying sedimentation across the flood plain, Guccione has been able to narrow the age range of prehistoric earthquakes. She also found that the large quake that occurred sometime around 850 did not alter flood plain deposition, leading Guccione to believe that its epicenter was further from the Reelfoot scarp than the large earthquakes that occurred around 1450 and 1812.