REGIONAL GRAVITY SURVEY INVESTIGATING POORLY STUDIED AREAS OF THE NORTHWEST ST. FRANCOIS TERRANE, SOUTHEAST MISSOURI

The exposed Precambrian St. Francois Mountains in southeast Missouri are a well-studied region of rhyolites, granites, and basaltic dikes, but much of the buried basement lithology west of the exposed region is still poorly delineated. To aid in delineating the structures in the subsurface aeromagnetic surveys were conducted in the 1950s. To the east of the St. Francois Mountains is the Reelfoot Rift and New Madrid Seismic Zone which has garnered most of the time and attention of researchers in the region. Aided by the first aeromagnetic investigations, there have been some detailed localized geophysical studies west of the St. Francois Mountains; however there are still large areas where the basement geology is largely unknown. These detailed studies have previously located large lead, zinc, and iron deposits. Within some of the iron deposits, and specifically in the Pea Ridge Mine in the northern part of the study area, there are known rare earth element deposits. In order to further investigate the Precambrian basement lithologies and the location of additional rare earth element deposits a gravity survey was conducted during the summer of 2013. Previous regional gravity studies have mostly had wide, greater than one-mile, station spacing, whereas this study has constrained station spacing to within half-a-mile giving a much greater spatial resolution of the data. Preliminary Bouguer gravity anomaly maps of the 384 gravity stations recorded merged with previous existing data have shown anomalies that may prove promising with further processing (e.g., wavelength filtering and derivative analysis) and 2.5- and 3-D computer modeling.