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80°30ʹW 80°W 79°30ʹW Coastal Plain, with elevated eTh highlighting
A concentrations of heavy mineral sands. An
interpretation of highest-grade heavy miner-
33°30ʹN N C als residing primarily in sands along the
shoreface, especially in areas that have been
reworked during multiple episodes or by

based on extensive sampling within the
33°N multiple processes, matches observations
Quaternary coastal plain settings of Australia
and the U.S. (Roy, 1999; Pirkle et al., 2013).
For most finer-grained estuarine sediments,

32°30ʹN Mean K(%) similar average eTh values combined with
lower skewness values suggest that heavy
Increasing Age minerals are present but dispersed over
broad areas with lower local concentrations.
These differences are attributed to greater
B reworking by waves and fluvial, tidal, or
marine currents in the higher-energy, sand-
C dominated environments.
T
T Mineral Pathways and Provenance
The mineralogy of the heavy mineral
T T assemblage can provide key insights into sedi-
T mentary provenance. The importance of ero-
+ sion and transport by fluvial processes to
33°10’N
79°45’W coastal locales was recognized by Colquhoun
et al. (1972) and Neiheisel (1976), who
observed immature minerals in the lower
Santee River and other floodplains that must
have been recently eroded from the Piedmont.
N However, away from major rivers, heavy min-
10 km eral concentrations less than ~50 km from the
Piedmont Province show mineralogy similar
82°W 80°W 78°W to the adjacent Piedmont rock, attributed to
D marine processes eroding a rocky coast fol-
lowing opening of the Atlantic Ocean (Shah et
NC al., 2017). This study also showed that heavy
34°N SC mineral concentrations closer to the modern
coast have more varied compositions and thus
32°N GA N cific source regions poorly known.
more complex delivery pathways, with spe-
The presence of elevated K, representing
immature minerals (Force et al., 1982),
requires sediments that were recently
eroded from igneous and/or metamorphic
30°N Trailridge rocks in the Piedmont. The most likely
transport mechanism from the Piedmont to
placer
the coast is via major rivers such as the
deposits
Santee, consistent with high K anomalies
observed over its floodplain. However,
FL
prominent K anomalies are also observed
more than 100 km from the floodplain,
requiring additional transport. The few dis-
Figure 4. (A) Airborne K over the 2019 survey area. Shaded area with green outline delineates urban tributary channels from the Santee River
areas; yellow box shows location of close-up draped over lidar near the Santee River floodplain (C),
where highs along tributaries are observed only up to ~15 km from the floodplain (“T”). (B) Mean value showing elevated K are less than 15 km
of K (%) for the various units. (D) Regional radiometric K over the Lower Coastal Plain of the southeast- long, making them an unlikely transport
ern U.S. (Duval et al., 2005). White box shows the location of the 2019 survey area. Potassium is ele-
vated for younger sediments. Arrows show the suggested path of heavy minerals along major rivers to route. Additionally, the high K values over
offshore areas, transported parallel to the coast, and then redeposited onshore. H—Holocene; marine sediments are discontinuous with
SB—Silver Bluff Beds; Wd—Wando; TM—Ten Mile Hill; Ld—Ladson; P—Penholoway; W—Waccamaw;
NC—North Carolina; SC—South Carolina; GA—Georgia; FL—Florida. those in the Santee River floodplain.

8 GSA Today | November 2021