ARE SOME GROUPS OF MIDWEST PIMPLE MOUNDS LINKED TO STRONG PALEOSEISMICITY?

Within the Midwest intraplate region there are unevenly distributed groups of soil mounds commonly called "pimple mounds," "Mima mounds" or "Mima-like mounds." Although many mechanisms have been proposed to explain these intriguing "pedomounds," those in the Midwest are most often attributed to bioturbational, erosional, and periglacial actions. Recently, Vaughn (2001) proposed that many Missouri pimple mounds may have been produced by strong seismic vibration of wet silty to sandy sediments. Typically, pimple mounds are on nearly level to gently sloping soil areas that have either perched water tables or high groundwater tables.

In Missouri, the largest and most numerous mounds south of the Missouri River are associated with major active faults and other major structural zones; north of the Missouri River, the few mounds that have been identified overlie the recently discovered Coon Creek fault. In Iowa, the main known swarm of mounds lies near the north rim of the large Manson Impact Structure. East of the Mississippi River, the few mounds that have been reported are associated with seismic liquefaction features and the Newbury fault in eastern Massachusetts.

These findings suggest that the origin of some--but certainly not all--mounds in the Midwest may be linked to strong paleoseismicity. In some cases, such as along the Commerce geophysical lineament in Missouri and Arkansas, the seismic source was nearby. In other cases, it is speculated that the energy source was more distant and that anomalous seismic focusing may have been involved in producing "seismomounds" and other topographic irregularities modified later by other processes.