Prior to about 1974, most of the work in the Mississippi embayment area by members of the U.S. Geological Survey was motivated by interest in the embayment’s paleontologic aspects, stratigraphy, and economic resources, especially ground water. However, an excellent description of the effects of the New Madrid earthquake series was published on the centennial of that 1811-12 seismicity.
During World War II, combined efforts of the U.S. Geological Survey and the U.S. Bureau of Mines produced a wealth of information about the Little Rock pluton and the process of laterizing exposed nepheline syenite to form bauxite. That project, in a search for additional intrusive bodies at shallow depth, sponsored a reconnaissance aeromagnetic survey along the embayment edge from Little Rock, Arkansas, to Cairo, Illinois. Magnetic anomalies that were identified then are now known to be related to the series of buried plutons aligned along the northwestern margin of the upper Mississippi embayment graben. Later investigations assessed the geochemistry of the more mafic parts of the Little Rock pluton.
In 1974, U.S. Geological Survey effort, along with that of other federal agencies, state agencies, and academic institutions, was directed toward finding the cause of ongoing seismicity in the upper embayment and toward assessing the related potential effects on persons and property. Aeromagnetic, gravity, seismic reflection, seismic refraction, stress-field, surface-geologic, subsurface-geologic, geomorphic, and paleontologic investigations were completed, and many of the results have been published.
The purpose of this poster display is to summarize the more significant findings in this area related to (a) the rock sequence, lower crust to surface; (b) the major structural features, including a rift system; (c) the current state of stress; and (d) the present-day seismicity.