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1969; Doar and Kendall, 2014). Within the decay series, so measurements are typically Drapes over lidar elevation data (Fig. 3)
survey area, detailed geologic mapping and referred to as equivalent uranium (eU) and show the eTh values are generally higher
geochronological studies (see footnote 1, thorium (eTh). The surveys were flown along along the shoreface and lower along the
item S1) defined a series of Quaternary allo- NW-SE traverses at a nominal height of 100 m backbarrier, similar to deposits mapped via
formations that lie unconformably upon above ground, although areas above the city sampling in Georgia, Florida, and eastern
Neogene and Paleogene sediments. Most riv- of Charleston were flown at >300 m above Australia (Roy, 1999; Pirkle et al., 2013).
ers in the study area drain from upland areas ground as per safety regulations. Data and The highest eTh values occur mainly near
within the Atlantic Coastal Plain. The pri- details regarding contractor processing are the tips of the barrier islands at inlets where
mary exception is the Santee River, with provided by Shah (2020). tidal activity is increased and near cross-
headwaters originating in the Piedmont and Previous geologic maps (see footnote 1, cutting features visible in lidar data that
traversing metamorphic and igneous ter- item S1) (Fig. 1) were used to calculate basic suggest multiple episodes of sand rework-
ranes for more than 100 km. The wide flood- statistical measures (mean, median, stan- ing (marked in Fig. 3).
plain of the Santee River is now exposed dard deviation, and skewness) for K and High eTh values are also observed over the
downriver of the 1941 Santee dam. eTh over the total area of various geologic width of the upper Santee River floodplain
Heavy mineral sands, eroded from the alloformations and sediment facies. For and immediately to the north, where a mix of
neighboring metamorphic and igneous consistency between map units, we terrace, eolian, and marine sediments has
Piedmont Province, have been observed restricted the statistical calculations to the been observed. In the lower Santee River
throughout the study area via auger samples subset of the survey area mapped by Weems floodplain, eTh anomalies are focused along
collected by R. Weems, E. Force, and others et al. (2014). Lidar elevation shaded relief eolian sand ridges. The transition between
(see footnote 1, items S2 and S3). These stud- maps (South Carolina lidar data: https:// these different anomaly styles occurs near
ies found heavy mineral concentrations from www.dnr.sc.gov/GIS/lidar.html, accessed the head of an Illinoian (>130 ka) paleochan-
0% to more than 25% in layers up to several Feb. 2020) were used to examine the geo- nel (Colquhoun et al., 1972), along which a
meters thick within Quaternary sediments. morphologic context of the airborne data. broad eTh low is present (Fig. 2A).
They contain ilmenite, epidote, and silli- For ground truth, we used previously pub-
manite with smaller amounts of rutile, lished visual estimates of heavy mineral Potassium
monazite, leucoxene, and other minerals. content from more than 1000 auger samples The radiometric K map (Fig. 4A) is domi-
Aeroradioactivity total count surveys (see footnote 1, item S2) and detailed miner- nated by highs over the Charleston metro-
were flown in the study area with 1.6-km alogical analyses on several dozen samples politan area, smaller towns, and along major
line spacing during the 1960s and early collected over high total count anomalies, roads, attributed to K-rich materials in con-
1970s. Using these data to guide ground including weight percent heavy minerals crete. High K values are also observed over
gamma spectrometry measurements and and mineralogy (see footnote 1, item S3). the Santee River floodplain and along
sample mineralogical analyses, Force et al. Holocene distributary channels up to 15 km
(1982) found direct correlations between RESULTS from the floodplain (Fig. 4C). Elevated K
radiometric Th and heavy mineral concen- over the Santee floodplain are consistent
trations, attributable to Th in monazite. Thorium with previous observations of potassium
Radiometric K correlated well with potas- Radiometric eTh shows a reasonable cor- feldspar or mica (Force et al., 1982).
sium feldspars and micas, which are consid- respondence with previous observations Radiometric K is also elevated for surfi-
ered immature because leaching of potas- of heavy minerals in shallow samples cial/shallow sediments that are younger
sium typically leads to alteration. They up to variability within the resolution of than ca. 130 ka, and mean values of K
estimated grades of economic Ti-Zr-REE the airborne survey footprint (Fig. 2A). within the study area (Fig. 4B) steadily
minerals as high as 2% and tonnage up to Comparisons to sample data (see footnote 1, decrease with age (except for Holocene
70,000 metric tons at some locales (see items S2 and S3) support heavy mineral sand clays). This is not a local phenomenon:
footnote 1, item S3). They also found the concentrations near the surface as the pri- Regional radiometric data (Duval et al.,
mineralogy varies locally, and some heavy mary source of radiometric Th anomalies. 2005) show elevated K for most sediments
mineral concentrations contained large The anomalies are highest over 3–12-km- within 20–50 km of the coast from northern
amounts of immature minerals with little long and 400–1200-m-wide portions of sand South Carolina near the Pee Dee River
economic value. ridges (Fig. 2B), similar to some of the shore- mouth to northern Florida (Fig. 4D) and for
line deposits located near the actively mined other rivers with headwaters in the
METHODS Trail Ridge/Folkston system in Georgia and Piedmont. In the survey area, K highs over
The 2019 airborne magnetic and radiomet- Florida (Pirkle et al., 2013). These anomalies younger sediments are discontinuous with
ric data were collected over a 134 km × 90 km contrast broader, rounded, and lower anoma- those over the Santee River floodplain and
area surrounding the city of Charleston, South lies over clay/mud estuarine areas (Fig. 2C). are not focused near the Santee River.
Carolina, by contract for the U.S. Geological Statistical measures (Figs. 2D–2E) mostly
Survey (Figs. 2–4). This method provides sta- show higher eTh skewness values for sands DISCUSSION
tistical estimates of K, Th, and U concentra- even though the mean values for sands and
tions within the upper 1 m of the surface and clays/muds are similar. Holocene clays, The Distribution of Heavy Minerals
several hundred meters in each horizontal which are subject to tidal flooding, show The new airborne radiometric data pro-
direction (International Atomic Energy lower values because gamma rays are attenu- vide an unprecedented view of geochemical
Agency, 2003). Th and U involve multiple ated by the fluid medium. variations within the Quaternary Atlantic

6 GSA Today | November 2021