AN UPDATE ON EARTH MAPPING RESOURCES INITIATIVE (EARTH MRI) AIRBORNE GEOPHYSICS IN THE SOUTHEASTERN U.S.

Earth MRI has sponsored multiple modern, high-resolution airborne geophysical surveys over a large portion of the southeastern U.S. (SEUS) with applications to critical mineral assessments, mine waste characterization, earthquake hazard studies, radon evaluation, and more. Magnetic and radiometric surveys, flown mostly with 250-m line spacing at 400-1000 ft above ground, cover parts of the Coastal Plain (AL, FL, GA, NC, SC, and VA), Piedmont and/or Blue Ridge (AL, MD, PA, NC, SC, and VA), Valley and Ridge (MD, PA, VA, and WV), and Appalachian Plateau (PA, WV). Here we explore initial results from these datasets, noting that interpretations depend heavily on ground-truth from mapping and sampling efforts by corresponding state geological surveys.

Compositional variations in Coastal Plain sediments are highlighted by radiometric data (magnetic data reflect the buried crystalline basement). Heavy mineral sand concentrations (Ti-Zr-REE) are highlighted by radiometric Th anomalies reflecting even low levels of monazite, which is common within these concentrations. Recently eroded sediments in streams and rivers can be traced as K anomalies reflecting K-feldspar and mica. The corresponding ages and geomorphologic context of these anomalies provide new insights into deposit formation. Phosphates in the SEUS, which may contain REEs, are known to also contain U. Both in situ phosphates and phosphate mining waste areas show elevated radiometric U or U/Th.

Within the Piedmont and Blue Ridge, radiometric and magnetic data delineate lithologic boundaries, e.g. schists typically show elevated Th and subdued magnetic anomalies, mafic volcanics may show low Th, U, and K but linear magnetic anomalies, and so on. Within the Valley and Ridge, radiometric data again show lithologic contrasts (e.g. carbonates show elevated K and shales show elevated Th), and subtle magnetic anomalies likely represent biogenic layers containing magnetic minerals; in map-view they highlight structural trends.

Jurassic dikes produce narrow, linear anomalies from SC northward. Rift basins containing intrusions or volcanics (e.g. Gettysburg and South Georgia) show prominent magnetic anomalies, whereas basins lacking igneous material show more subdued magnetic anomalies, reflecting deeper basement sources beneath sedimentary fill.