Application of aerial gamma-ray spectrometric and magnetic surveys in geologic mapping: a case study in southern Virginia and northern North Carolina

Aerial gamma-ray spectrometric and magnetic survey maps are valuable aids for geologic
mapping where rocks are poorly exposed in south-central Virginia and northernmost North Carolina.
Broad low areas on the potassium and thorium gamma-ray survey maps distinguish the Carolina,
Spring Hope, and Roanoke Rapids terranes from more highly radiogenic areas of the Raleigh and
Triplet terranes, reflecting differences in the compositions of residual soils. Granitic rocks are
delineated most clearly by potassium highs and less clearly by thorium highs. Nearly all the
thorium highs other than those related to granites are associated with amphibolite-facies rocks of
the Raleigh and Triplet terranes. Contrasting thorium lows within these terranes help to distinguish
the individual rock units. In the Carolina and Roanoke Rapids terranes, high-gradient magnetic
patterns delineate stratified metavolcanic and metasedimentary units that are not discernible from
the gamma-ray surveys. Circular magnetic highs coincide with gabbro plutons, and numerous
magnetic lineaments correspond to Jurassic diabase dikes. Magnetically uniform, low-gradient
areas coincide with less mafic plutons. A magnetic lineament (high) coincides with the Nutbush
Creek fault zone, and other faults are distinguished as boundaries between zones of contrasting
geophysical properties. The gamma-ray spectrometric and magnetic survey maps most effectively
indicate geologic features in the region if they are employed collectively, and if they are interpreted
in concert with simultaneous geologic field investigations.